sail aerodynamics [Archive] - Page 2

sharpii2

03-10-2008, 07:39 PM

Not being any kind of an aerodynamic expert myself, I see two possibilities for this performance enhancement.

1.) that the projections are creating vortexes, which break away from the upper surface later, or

2.) they break the wing up into separate lifting sections at the leading edge, but let them join as one, further back, thus delaying upper surface separation which seems to start pretty far forward.

A good way to see which one is correct would be to build a wing with cut water like, fence projections, which only extend forward, but have the same spacing, while keeping the rest of the wing shape exactly the same.

It would be interesting if the enhanced performance goes or stays, or is simply reduced.

Such an experiment, IMHO, could go a long way toward sorting out what is really going on.

The reason I suspect that the second theory might be right is that I heard of an Experimental Aircraft Association lecture about stalls and flat spins in airplanes.

The lecturer was a test pilot who would try to deliberately put a planes into flat spins (he had a way to escape if the spin got unstoppable) . Most of the light planes he tested, he was able to.

He then tried to see if he could create a spin proof wing. He did this by creating a solid leading edge projection near the tip of each wing that was a significant portion of its span in length along the wing. It, of course created an ugly notch where it ended, but it worked. He could not get the plane into a flat spin.

He then tried to fair this projection into the rest of the leading edge. It didn't work. The plane was able to get into a flat spin again, just as before.

My take on this story is that the wing ended up with two distinct lifting surfaces at the leading edge. This way either all the wing was producing lift or at least part of it was. It would be the projection part of the span or the inward part. And this would all happen during at least part of each turn of the spin, making the spin wobbly and unstable.

I imagine that once he fared out the notch, created by the projection, he created an unbroken leading edge. This way, if the separation started anywhere along the span at all, it would end up encompassing the entire span under these more extreme conditions.

This, I imagine, is because fluid molecules tend to play 'follow the leader'. Once part of the flow breaks away, the rest seems inclined to follow. Maybe this is why weather patterns seem to be heavily influenced by mountain ranges of even modest heights.

To make a safe airplane, you need only to make the outer end of the wing have its separation last. This way, if the plane stalls, the wings will be more inclined to stay level. This is done by making the pitch of the outer span less than the pitch of the inner span, creating a slight twist in the wing. This does not prevent a stall, or even delay it, but only tries to keep the airplane from tipping sideways once it happens.

ivor Bittle

03-15-2008, 04:53 AM

Try reading my website at www.ivorbittle.co.uk

Paul Scott

03-15-2008, 12:44 PM

Two things-

1) the bumps on the whale fin and the idealized bumps on the 'man made fin' are not the same- the 'man made fin' has it's bumps at 90 degrees to the whale's bumps(?).

2) as long as wing sails have surfaced, here's something you might be interested in, if you haven't seen it before- "An Autonomous Wing Sailed Catamaran" Phd thesis by Gabriel H. Elkhaim (the Atlantis project):

http://www.soe.ucsc.edu/~elkaim/Documents/Catalyst_BoatArticle.pdf

Paul

yipster

03-15-2008, 03:33 PM

seen that interesting ucsc doc before but now i was thinking that was the santa cruz boardwalk i saw in the opening pictures background but no, this world IS changing and i appologise for bluring this perfect thread, i'm gonna reread it again!

ivor Bittle

03-17-2008, 02:49 AM

Paul,
I am new to forums. I looked for experience with one and now realise that I should not have taken part at all. Forgive me. I shall not make the same mistake again. Re Gabriel Elkaim-I would like to see a history of all the revolutionary devices that have been promoted for aero- and hydro- dynamic devices over the years.
Ivor Bittle

Paul Scott

03-17-2008, 10:05 AM

Ivor, please take part. You are thinking about this stuff, and no one is going to jump on you for it. No one has asked me to keep it down, and Lord knows, I have deserved it at times.

As far as devices, start Googling! You never know where they lurk, and a lot of them are not even on the Web. I found Elkaim's piece while Googling 'reynolds numbers of hulls', so there you go. As far as Elkaim's sort of thing, I don't know of a lot out there that are modern, outside of speed record stuff, and some of the open canoe, and older IC guys. You might try wing sails, etc. Sites come and go...... Print them out, as they do tend to dissappear forever. There are academic pages too.

The list I have tends to revolve around flow re numbers from 30,000 to 3 or 4 million, which usually gets me into the UAV, MAV, RC, and wind turbine sites.

My feeling on this forum is that the only sin is not trying.

Paul

ivor Bittle

03-18-2008, 11:44 AM

Paul,
Please read attachment,
Ivor Bittle

Paul Scott

03-18-2008, 09:53 PM

Ivor, being a retired professor of music, I have some idea of theses tomfoolery. The thesis has holes you could run a truck through, but given the paucity of this sort of stuff for sailboats outside of this site, it is somewhat instructive. The section he uses does have a lot in common with some other mid low re sections (?? ~ 250,000) I've run across, and I was particularly interested in the discussion of flow separation towards the nose, and the use of the reverse curve towards the trailing edge, which in certain circles is kind of controversial. It would have been interesting to see some closeup pics of flow around the section while sailing. They may have been reinventing the wheel, but they did build and sail it, and it interested me that the wing really only started working (if I remember right) at 8K or so, which would be about the right windspeed for a 3d foil to start performing better than a curved plate (according to other authors), i.e. sails on a catamaran. I thought this might stimulate some responses concerning modern wing mast theory vs sails vs 3d sections for different re.

If you really want to get into the politics of higher learning, I suppose you could try Sailing Anarchy, but I'm really more interested in the aero part of the design, as the control system seemed a bit dodgy to me. (This all reminds me of the old joke- why are the politics in academia so vicious? because the stakes are so low........)

To sum up, the thesis is interesting to me because of re regime explored. (And the poetics involved.)

Paul

Guest625101138

03-19-2008, 02:19 AM

....... I thought this might stimulate some responses concerning modern wing mast theory vs sails vs 3d sections for different re.

Paul

Paul
I am not sure if it has come up on this thread before but it could be worthwhile playing with JavaFoil if you want to get into low speed foil design. It is nice software to use and gives good results. I have checked its output against test data from Selig et al and it is favourable.

Martin Herpperle also has some of his own foils that are used for wings and he gives their design working range.

I have used JavaFoil quite a lot to develop shapes for low Re# water propellers and the results have been good.

Rick W.

Guillermo

03-19-2008, 03:21 AM

Ivor,
I wanted to give you rep points because of your answer, but it happens I had given you already. So, from here my applause.

Cheers.

brian eiland

03-19-2008, 12:22 PM

Paul
I am not sure if it has come up on this thread before but it could be worthwhile playing with JavaFoil if you want to get into low speed foil design. It is nice software to use and gives good results. I have checked its output against test data from Selig et al and it is favourable.
I have used JavaFoil quite a lot to develop shapes for low Re# water propellers and the results have been good. Rick W.
Here are a few JavaFoil presentations on this subject thread addressing my 'sail interaction' (slot effect) contentions:

http://boatdesign.net/forums/showpost.php?p=89766&postcount=148

http://www.boatdesign.net/forums/showpost.php?p=89921&postcount=151
_______________________________________________________________

Here's another posting referencing some of these images...from a mast-aft subject thread (http://www.boatdesign.net/forums/showthread.php?t=623)

Nobody yet in this thread has offered up any quantitative estimate as to the performance advantages of their design over a conventional sloop or cutter rig designed to the same requirements, or shown how there's any weight savings in the structure. I can understand the desire to simplify sail handling with roller furling staysails. But any improvement in performance is by definition a quantitative issue, not a qualitative one.

Hello again Tom. Once again I must beg off on a ‘quantitative’ reply to the advantages of the headsail arrangement on my mast aft rig concept. I believe just as with the rigging load questions (http://boatdesign.net/forums/showthread.php?t=2293), a straight-forward quantitative calculation as to the superiority of the efficiency of the headsail over the mainsail may well be an incalculable quantity from a purely mathematical standpoint. So I have to rely on a variety of observations gathered from history, from pass designer’s applications, from current theorist, from real time sailor’s experiences, etc.

So I will present a few of those observations:
1) Lets begin with a ‘pictorial’ example. Rick Loheed recently posted these images here (http://boatdesign.net/forums/showpost.php?p=89766&postcount=148), “I found a post that had a reference to JavaFoil, Martin Hepperle's 'relatively simple' inviscid foil analysis program that will do multi-element airfoils. It is great for illustration purposes here. Clearly it shows the affect of the whole system- actually, as a cascade of foils. Further Aft foils must have more incidence- but when incidence is added, they help increase circulation around the whole system, increasing the forward foils effectiveness by inducing more incidence and accelerating more mass about the whole mess".
Which sail looks most effective at driving you forward.

2) Another JavaFoil analysis by Rick here (http://boatdesign.net/forums/showpost.php?p=89921&postcount=151), “Here is a comparison using Javafoil of a simple 15% camber (pretty high lift) 40% max camber location Jib/main combination arbitrarily loaded to near Max CL, and a 20% Clark 'Y' wingmast shape based on methods from Tom Speer's wingmast paper. For an input Aspect ratio of 10 for each case, this simulation shows a Max Cl of 2.11 for the main jib combo readily achieved for the combination, whereas the wingmast gets to a fairly typical Cl of 1.2 max”

3) Excerpted from aerodynamicist and North Sails consultant Paul Bogataj’s paper, “How Sails Work” http://www.northsailsod.com/articles/article6-1.html
‘Sails in Combination’, “Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other. The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone.
The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses”

4) Its been some 40 years ago that I did extensive study of some of the classic sailing boat design books, but I can distinctly remember how there was applied a ‘performance factor’ to the headsail of 1,3 to 1.5 more effective than its actual sail area when computing the CE of the sail plan. So even though the theory was not thoroughly understood at the time, real time observations came into the equation.

5) Redcooprs was talking of the subject under the ‘Fraction Rig’ discussion (www), “However, the design of sailforms is very much a practical nature. In terms of sailing, the feel is that our jib supplies the driving power - and wind tunnel tests also show that it has a very large Cl compared to the main. The main on the other hand, is very responsible for the righting moment and general tuning of the boat.”
...item #24
http://www.boatdesign.net/forums/showpost.php?p=110983&postcount=24

Guest625101138

03-19-2008, 04:07 PM

Brian
There is some nice work there - thank you for the references. The comment of Rick Loheed about aspect ratio correction in JavaFoil is of interest. I use my own empirical method for induced drag for my props but I often just use the JavaFoil information for rudders and sails. It looks OK but I should check to see how it compares. I expect it assumes semi-elliptical shape.

Rick W.

markdrela

03-19-2008, 08:55 PM

The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses
All these discussions don't address the main function of the slot: It's primarily a boundary layer control device, which allows the overall slotted section to tolerate a larger AoA and hence a larger CL before stall, compared to a single sail of the same area. If the AoA isn't increased, then the slot has essentially no effect on the lift.

The attached inviscid panel calculation plots show this convincingly. The first case is a typical AC jib+mainsail airfoil, at 18 degrees AoA. The second case is the same, except the jib has been translated so as to almost close off the slot. The CL is nearly identical at the same 18 degree AoA. The jib translation has been done perpendicular to the freestream, so the projected chord is the same, for a fair comparison.

RHough

03-19-2008, 09:26 PM

All these discussions don't address the main function of the slot: It's primarily a boundary layer control device, which allows the overall slotted section to tolerate a larger AoA and hence a larger CL before stall, compared to a single sail of the same area. If the AoA isn't increased, then the slot has essentially no effect on the lift.

The attached inviscid panel calculation plots show this convincingly. The first case is a typical AC jib+mainsail airfoil, at 18 degrees AoA. The second case is the same, except the jib has been translated so as to almost close off the slot. The CL is nearly identical at the same 18 degree AoA. The jib translation has been done perpendicular to the freestream, so the projected chord is the same, for a fair comparison.

What happens to the curve if the same jib area is used and the overlap is varied? In other words, how much overlap is needed to produce the high CL? Any?

What does the L/D curve look like for same area and AoA with varied overlap?

If the performance is related to projected chord, I would guess that the overlapped area is next to useless?

Paul Scott

03-19-2008, 09:55 PM

Paul Scott

03-19-2008, 10:08 PM

What happens to the curve if the same jib area is used and the overlap is varied? In other words, how much overlap is needed to produce the high CL? Any?

What does the L/D curve look like for same area and AoA with varied overlap?

If the performance is related to projected chord, I would guess that the overlapped area is next to useless?

But if jib overlap (leach) ends at the point on the lee of the mainsail where the mainsail's lee flow is at it's max velocity, doesn't that help influence the Kutta condition at the leach of the jib, decreasing the deceleration of flow to the leach on the lee side of the jib? That would be good, wouldn't it?

Paul

tspeer

03-19-2008, 10:23 PM

But if jib overlap (leach) ends at the point on the lee of the mainsail where the mainsail's lee flow is at it's max velocity, doesn't that help influence the Kutta condition at the leach of the jib, decreasing the deceleration of flow to the leach on the lee side of the jib? That would be good, wouldn't it?

Yes, it does, and yes, it is. This is what A.M.O. Smith calls the "dumping effect". There doesn't have to be as much deceleration toward the leech of the jib, so the boundary layer isn't as stressed and is less prone to separation.

For the Kutta condition itself, it's the normal velocity at the leech that requires circulation on the jib to balance it out.

markdrela

03-19-2008, 11:20 PM

For the Kutta condition itself, it's the normal velocity at the leech that requires circulation on the jib to balance it out.
Actually, not so much. The "normal-velocity at slat TE" effect described by AMO Smith actually has little effect on the lift. See the attached panel solution. Whether the jib's leech is just under the mast or just above the mast doesn't affect the lift very much, despite a very large change in the normal velocity seen by the leech.

In light of classical thin-airfoil theory, none of this is very surprising. This theory indicates that for a given AoA, lift is almost entirely determined by the camber at the rear of the airfoil. Modifying the shape near front like in this case has almost no effect. In the context of this theory, a small slot between the jib and mainsail is just a tiny interval where the modeling vortex sheet has zero strength. This small interval will only affect the flow in its immediate vicinity, but it cannot influence the overall circulation and lift.

Guest625101138

03-19-2008, 11:24 PM

......

Do you run different re's for indivual propellers?

Paul

Paul
Yes - Also I adjust Re# over the length of the blade using a segmented approach. I have a neat way to take it into account for a blade I can explain if you are interested.

I believe my prop optimising model gives better results, and is more flexible, than JavaProp but I use JavaFoil to generate the polar curves. I did buy Volume 2 of Selig's low speed foil data but have found JavaFoil is so close it does not matter.

It would be really nice to have a 3D version of JavaFoil but have to do with 2D for now and use empirical additions to get something that relates to 3D.

Have put this question to Leo Lazauskas but he said it is way too complex for him to fit into his current work program.

Rick W.

Paul Scott

03-19-2008, 11:35 PM

Scarburgh (1709 AD) "To which therefore t'is said to be a Normal Line."

Drela (2008 AD) "a small slot between the jib and mainsail is just a tiny interval where the modeling vortex sheet has zero strength."

It seems to me this argues for overlap? But what would it argue if the main were a symmetrical foil, and the jib a curved plate? Low (80,000) re? 3,000,000 re? The same?

Paul

markdrela

03-19-2008, 11:36 PM

If the performance is related to projected chord, I would guess that the overlapped area is next to useless?
Multielement airfoils on aircraft have nearly zero overlap --- maybe 1% chord at most. It doesn't help to increase lift.

One possible good reason for using a healthy overlap on a jib/mainsail is to push the mast farther forward into the low-velocity region on the windward side of the jib. The profile-drag penalty of the mast is proportional to the local velocity cubed, so even a modest velocity decrease at the mast can be very beneficial. I can see how this might easily overcome the skin-friction penalty of the overlap itself.

markdrela

03-19-2008, 11:47 PM

It seems to me this argues for overlap?
No. The point I'm trying to make is that designing a slot geometry based on inviscid lift arguments is utterly pointless. The slot should be designed by viscous flow considerations. Specifically:

1) To relieve the adverse pressure gradients on the jib and mainsail boundary layers. This will increase CLmax, but only if you also crank up the AoA.

2) To reduce the velocity at the mast. This will decrease profile drag.

RHough

03-20-2008, 12:09 AM

No. The point I'm trying to make is that designing a slot geometry based on inviscid lift arguments is utterly pointless. The slot should be designed by viscous flow considerations. Specifically:

1) To relieve the adverse pressure gradients on the jib and mainsail boundary layers. This will increase CLmax, but only if you also crank up the AoA.

2) To reduce the velocity at the mast. This will decrease profile drag.

That would fit with area limited classes ending up with small, non overlapping jibs.

Does it also argue for rotating the mast to reduce profile drag rather than going for overlap and hoping for a net gain?

Once the boat is powered up and max C/L is not needed, are the considerations still the same? ie Overlap is good only if the reduction in profile drag is greater than the increase in skin friction?

Paul Scott

03-20-2008, 09:28 AM

No. The point I'm trying to make is that designing a slot geometry based on inviscid lift arguments is utterly pointless. The slot should be designed by viscous flow considerations. Specifically:

1) To relieve the adverse pressure gradients on the jib and mainsail boundary layers. This will increase CLmax, but only if you also crank up the AoA.

2) To reduce the velocity at the mast. This will decrease profile drag.

Mark, do your points at least partly explain why Dennis Conner's AC cat had the jib (and it's leach) so far ahead of it's wing sail (vs it's conventional rig?)? And if so, the jib would tend to migrate closer to the main (even to overlap) the more lumpy things get at the leading edge of the main? If this is true, does the importance of inflencing Kutta condition at the leach of the jib become more or less important on a sliding scale depending on the profile drag around the leading edge of the main? Or do you consider the Kutta condition/ jib leach arguement too much on the inviscid end of things?

markdrela

03-20-2008, 10:32 AM

On an airplane, the shaping of the slot geometry --- slot width, overlap, contraction angle, etc --- is entirely driven by the requirement of having "nice" (non-spiky) pressure distributions on the leading edge of the rear element, and also to prevent the merging of the slat wake and rear-element BL farther downstream. In other words, it's a BL management problem. It's not a lift manipulation problem, via Kutta arguments or whatever, mainly because the slot geometry doesn't affect overall lift at a given AoA.

On a jib/mainsail, there is an additional consideration of reducing the overall velocity over the leading edge, i.e. the mast, not just in reducing localized Cp spikes. And any overlap rules in sail rating may also come into play. The AC cat had the obvious requirement that the rigid jib's leech had to clear the mainsail's leading edge when changing tacks. That would produce a very open-looking slot.

RHough

04-01-2008, 11:27 AM

I'm looking at a mutlihull design ...

For a given RM limit, what would the design steps be to arrive at the optimum rig?

Is there a sail area/righting moment ratio sort of formula that is used to find a starting point? SA/D upwind of 40+ is not unusual and SA/D downwind of 70+ is also not unusual from some of the boats I've looked at.

I know that tspeer has done a lot of work to optimize planforms for high L/D while limiting heeling moment.

Before I try to hammer this out from a clean sheet of paper, I thought I'd ask if there are some rules of thumb to get the starting point close.

It seems to me that a flexible sailplan that allows the crew to change area to use the available RM to best effect is the goal. This would lead me to look at multiple head sail choices on a bowsprit of some sort (not a new idea). At some point there must be diminishing returns when adding area this way. How would you deal with large changes in the longitudinal location of the centre of effort?

Can anyone point me in the right direction here?

Erwan

04-01-2008, 11:59 AM

Hi

Look at this link:http://www.tspeer.com/
If you look at "Optimum planform" I guess you will get all your answers

Best regards

Erwan

RHough

04-01-2008, 03:24 PM

Hi

Look at this link:http://www.tspeer.com/
If you look at "Optimum planform" I guess you will get all your answers

Best regards

Erwan

Thanks, I'm as familiar with Tom's work as my level of understanding allows. I'm looking for a little simpler explanation perhaps ... to reduce the amount of time I have to spend relearning long forgotten math and perhaps having to struggle through all the theory that I once used more regularly when I was designing RC sailplanes ... :(

The problem of root gaps did not have to be considered since in aircraft design you can just eliminate the gap. It would be easier for me to relate to and understand Tom's work if there were some examples to relate real world sail plans to his theory. I may have missed those.

tspeer

04-02-2008, 12:18 AM

...Is there a sail area/righting moment ratio sort of formula that is used to find a starting point? SA/D upwind of 40+ is not unusual and SA/D downwind of 70+ is also not unusual from some of the boats I've looked at....

Shuttleworth has stability indices (http://www.john-shuttleworth.com/Articles/NESTalk.html) for tri's and cat's. He has indices for sideways capsize, pitchpoling and diagonal capsize.

Another good source of starting points are catalogs by noted designers, such as Kurt Hughes and John Shuttleworth. Kurt's catalog, in particular, has a wide range of designs from small day-sailers to large yachts. You're sure to find a design that's close to the sort of boat you want, and go from there.

I find the multihull footprint plot (http://www.tspeer.com/temp/hullbalance.gif) to be very useful. The idea is to compute a virtual c.g. location that provides the same heeling and pitching moment as the applied loads (black lines). The virtual c.g. will correspond to the combined center of buoyancy of the hulls (blue lines). By plotting the c.b. for different combinations of heel and trim, you can quickly see what the solution will be for any wind condition or point of sail.

Shuttleworth's stability indices can be presented in the same manner (http://www.tspeer.com/temp/stabilityindex.gif). The black lines in the previous plot were based on a modified Hazen sail model, and Shuttleworth's indices are quite comparable.

CT 249

04-03-2008, 07:07 PM

Fascinating thread. Particular thanks to Mark and Tom.

Mark, re the fact that that the profile-drag penalty of the mast is proportional to the local velocity cubed. Does this mean that the comparative advantage of a lower-drag mast is reduced in slower boats, where the apparent wind is lower and therefore the local velocity is less?

Does this explain the (comparative) lack of advantage of wing masts in medium-performance boats where the apparent wind is lower?

In craft like the Tasar and NS14 and the other dinghies and skiffs that have tried wing masts, experience has not shown the sort of speed increase that many theories would indicate. This is despite development involving 1900+ boats (in the NS14s alone) over several decades.

This fact (and gust response which is also related to the inherent speed and stability of the craft) neatly ties in with the observed fact that wingmasts have a great advantage in fast una-rig cats, but no huge advantage in many other craft.

It's nice to see that the theory we get from such impeccable sources is always in accord with practise, whereas the theory we get from lesser sources is often wildly at odds with what is proven to work on the water.

RHough

04-03-2008, 08:06 PM

Paul Scott

04-03-2008, 08:09 PM

Although Mark's statement that slot geometry doesn't change lift at a given AoA has me doodling some funky ideas...........

Paul :P

Don't Panic.

Petros

04-03-2008, 08:57 PM

RHough

04-03-2008, 10:28 PM

slots allow higher Cl at higher angle of attack (it delays the stall to a higher AOA), at the expense of higher drag. It seems to me what is missing in this discussion is a consideration of total drag in the rig design. Would not the best performance come from the best L/D, not merely from a high Cl? The sail/wing configuration that gives the best L/D is very different than one that gives the best Cl max. ISTM that when the whole rig has poor L/D, you would get higher healing moment, and higher leeway, both affecting performance.

I think it is wind speed dependant. On a displacement mono, below hull speed, the greatest lift that the rig can produce, is not enough to bring the boat to it's limits. There is some fixed amount of power (lift) needed to drive the boat. As soon as the rig can produce that power at less than maximum C/L, then high L/D trim provides the best performance?

CT 249

04-04-2008, 06:25 AM

RE "Would not the best performance come from the best L/D, not merely from a high Cl? The sail/wing configuration that gives the best L/D is very different than one that gives the best Cl max."

I'm no expert, but Mr Drela certainly is. He has pointed out here something that accords about 100% with reality (as I have seen it) - that it is not the L/D of the rig itself that is the important thing, but the L/D of the rig and drag package.

As I understand it, if you are sailing an A Class cat, a very low-drag platform and a high-powered comparatively stable package, then increasing the L/D of the rig will create a performance advantage. The drag and required power of the whole kit 'n kaboodle is so low that reducing the rig's L/D is vital.

If, on the other hand,you are sailing an IRC racer/cuiser like a Benny 40.7, the hull drag is so high that maximising L is the vital thing; improving the L/D of the rig itself will result in a highly "efficient" rig (in L/D terms) that is so feeble that the boat will not be able to sail to the end of its slip.

If I understand Messr Drela correctly, it's not L over D of the rig; it's L of the rig over D of the hull/rig package; or is that L of the package over drag of the package?

This approach, which seems to be an elegant statement of common sense and experience, seems to be borne out in reality. It's easily seen on windsurfers, where you can shift high L rigs onto boards designed for high L/D and vice versa. After all, for all the stuff spoken about improving L/D how many of the "improved" rigs for the typical boat have ever proven to be vastly superior on the water? Very, very few - or is it none?

PS good point RHoough, I was just thinking in the limited terms of 'performance' rigs.

RHough

04-04-2008, 09:30 AM

CT ... you have it right ...

It is the L/D of the complete vessel that maters.

The L/D of the rig is only one part of it. The L/D of the hull/foils in the water cannot be ignored. Above the water, only the sails produce lift, everything else is drag.

Putting the very best, highest L/D rig on a barge is not going to make it sail very well. ;)

Have the fast windsurf guys started to experiment with drag reduction of the sailor? High L/D rigs with low CE are nice, then Bubba stands next to it in a baggy outfit and a PFD. How much "free" performance is to be had by experimenting with sailing suits and postures that reduce drag? The mast has to be a very small part of it.

It cracks me up to see $$$$ in new sails on a boat that has had NO attention paid to the hull and keel.

sharpii2

04-04-2008, 11:19 AM

...The profile-drag penalty of the mast is proportional to the local velocity cubed, so even a modest velocity decrease at the mast can be very beneficial. I can see how this might easily overcome the skin-friction penalty of the overlap itself.

Isn't the real number 'increases in wind velocity SQUARED'. That's how frontal drag is calculated. I can't see how this would be any different with the mast rather than, say, a sheet of plywood sticking up with an equal frontal area to the mast.

Bob

markdrela

04-04-2008, 02:34 PM

Isn't the real number 'increases in wind velocity SQUARED'. That's how frontal drag is calculated. I can't see how this would be any different with the mast rather than, say, a sheet of plywood sticking up with an equal frontal area to the mast. This view does not account for the significant pressure-drag forces which the mast, jib, and mainsail impose on each other. Once the additional pressure drag induced by the mast is added, its contribution to the total drag increases from a velocity^2 scaling to a velocity^3 scaling.

The velocity^3 dependence also shows up when total drag is computed by summing the dissipated power over all the wetted area elements dA of the wing or sail system and its wake:

V_inf * D_profile = Sum [ rho V^3 Cdiss ] dA

Where Cdiss is the dissipation coefficient and V is the speed over the area element dA. Note the V^3 factor, and the absence of pressure forces which don't explicitly contribute to viscous power dissipation.

Cdiss is similar to the more familiar skin-friction coefficient Cf, except that it's much less weakly dependent on pressure gradients than Cf. In particular, Cdiss is strictly positive, even in separated reverse flow, so unlike Cf it also accounts for pressure drag due to separation.

tspeer

04-04-2008, 07:42 PM

RHough

04-04-2008, 08:18 PM

I think an important point that's often lost is one of the functions of the jib is to reduce the drag of the mast.

In the absence of the jib, there would be a much larger separation bubble behind the mast, or just outright separated flow all the way to the leech. The favorable pressure gradient in the slot leads to quicker reattachment of the separation behind the mast and a significant reduction in the drag.

I don't believe you can sum the drag of the mast alone with the skin friction of the sails and get a very accurate estimate of the profile drag of the whole configuration.

I'm sure I've seen a wind tunnel test that shows the comparison of a sail vs a sail with mast ... all that is lacking the test of the mast with no sail. I doubt that mast + sail = mast with sail ... the sail would prevent the mast from shedding vortexes.

Paul Scott

04-05-2008, 04:08 PM

I think an important point that's often lost is one of the functions of the jib is to reduce the drag of the mast.

In the absence of the jib, there would be a much larger separation bubble behind the mast, or just outright separated flow all the way to the leech. The favorable pressure gradient in the slot leads to quicker reattachment of the separation behind the mast and a significant reduction in the drag.

I don't believe you can sum the drag of the mast alone with the skin friction of the sails and get a very accurate estimate of the profile drag of the whole configuration.

But couldn't the jib slow things down enough at a low enough speed to get the flow behind the mast into the re region where once detached, the flow (in any form) doesn't reattach? And at least from Gentry's streamline/speed figures (like fig 17) the amount of overlap then matters, at least in the sense that the boundary layer on the lee side of the mast is being stressed by different wind speeds at different points on the jib in the slot?

I'm not sure this is english.

Paul :P

Don't panic.

brian eiland

04-27-2008, 06:35 PM

Fascinating thread. Particular thanks to Mark and Tom.
This is indead a great thread.

I see there is renewed technical discussion of the headsail/mainsail 'slot' interaction , and some new participants with respectable technical backgrounds.

Over the past number of years I have attempted to make a positive case for my mast-aft sailing rig configuration, both within, and aside of the technical discussions of these sail aerodynamics. I’m going to refrain from re-entering these technical discussions at this moment, and watch where they go.

Rather I will choose to do a brief review of why I pursued my alternative rig configuration based upon the real-time observable phenomena that we experience as sailors.

Because my submission is more ‘rig specific’ and not of the technical ‘sail aerodynamics’, I will make it here under the aftmast subject thread

http://www.boatdesign.net/forums/showpost.php?p=198605&postcount=98

I also thought I would add in some other materials I've collected in the past.

PI Design

04-28-2008, 03:11 AM

This is a great thread - many thanks to all the knowledgable contributors.
I have a challenge for anyone willing to take it up - design a NS14 rig. NS14s are very simple boats but with very free rules on the rig design. Essentially, sail area is limited to 9.3sq m (100sq ft), which can be split in any configuration provided there is only one mast and it has a maximum length of 5.5m (18ft). So you can mainsail only, jib only, 6.9/2.4 main/jib split etc, etc. The mast section must pass through a 100mm ring and over-rotating wing masts are the norm. Hull length is 4.3m (14ft), waterline beam is 1.2m (4ft) and max beam is 1.83m (6ft). I'd be fascinated to see what you come up with. For info, these are typical:

Mikko Brummer

04-30-2008, 08:51 AM

I'm sure I've seen a wind tunnel test that shows the comparison of a sail vs a sail with mast ... all that is lacking the test of the mast with no sail. I doubt that mast + sail = mast with sail ... the sail would prevent the mast from shedding vortexes.

Yes... it seems that the mast is not just a drag-device but contributes significantly to the driving force of the sails. In a recent CFD study on Star sails, the mainsail drive was 10,2 kgf, jib drive 6,7 kgf and the mast drive 1,2 kgf (that's in the positive, forward direction for the mast as well). This was in 6 m/s apparent wind and at AWA 27 deg.

The mainsail behind the mast cuts down the drag remarkably, compared to a bare mast. The Star mast is really slim and refined, though.

tspeer

05-02-2008, 09:53 PM

What kind of calculation are the Star results from, Mikko? A Navier Stokes code?

BOATMIK

05-02-2008, 10:04 PM

Milgram did some stuff on mast sail interactions in the '70s.

He made a statement like "significant amounts of lift are developed by the mast, which is probably why big old fat IOR masts are never quite as bad as we expect them to be"

I can't remember whether it was tunnel or beginning of CFD applications to sails stuff.

Best wishes
Michael Storer

BOATMIK

05-02-2008, 10:17 PM

RE "Would not the best performance come from the best L/D, not merely from a high Cl? The sail/wing configuration that gives the best L/D is very different than one that gives the best Cl max."

Isn't part of the problem that for most boats the heading of the boat redefines the importance of lift vs drag.

Dead downwind in a slow boat I am trying to get drag to the max. I'll get my crew to wear their pants on their head if it will help.

I have had a bit of a browse through the past few pages and it seems to be focussed on solutions to the upwind problem. For fast boats this may be acceptable approach, but the results will be less useful for slower ones.

So doesn't the problem become a whole set of optimum L/Ds for different headings? Or isn't that different apparent wind angles?

??

Best wishes
Michael Storer

Mikko Brummer

05-04-2008, 08:02 AM

What kind of calculation are the Star results from, Mikko? A Navier Stokes code?

Yes, N-S code. The model is rather elaborate, with boat hull, sails, rig and crew included. But the mesh is coarser than one would hope for, due to memory limitations on my desktop PC. You can look at more pictures from last year at http://web.mac.com/mbrummer/Mikko.mac/CFDpics.mac.html

At least according to this analysis, it would appear the mast in front of the mainsail is doing a good job in capturing some of the "suction force" at the leading edge of the sail, a problem well known for thin, plate-like airfoils.

The viscous 3D-analysis reveals many things that won't show in an inviscid (panel method) analysis, or even a 2D N-S run. Separation bubbles (that play such an important role in sail aerodynamics) appear rather as "separation vortices" than "bubbles"... Heel angle, and the hull under the jib have a major effect on separation at the tack on the leeward side at larger angles.

Also, wind gradient and the associated twist in the apparent wind (inflow) have a significant effect on the absolute forces produced by sails... it is understandable, when you think that you loose both wind speed & angle of attack in the foot part of the sails, where the most of the (triangular sails) sail area lies.

In this particular Star case, for an AWS= 6 m/s and AWA= 27 deg, you only have 4,9 m/s and 23,5 degt at 1 m above sea level. If you refer drive and heel coefficients to the apparent wind at 10 m height (as you want them for a VPP, for instance), they tend to be much smaller (some 25% if I recall) than for a uniform flow.

brian eiland

06-24-2008, 08:13 PM

Erik wrote:
I am a bit confused with the explanations of how the sail actually pushes the boat forward. Do anyone have a good scientific explanation? Not just "The lower pressure on the leeward side sucks the sail forward".
/ErikW
_______________________________________
I would encourage anyone interested in this subject, and that is looking for a simplified explanation of 'lift' to read these two articles:

1) Taking Flight, New Scientist magazine 5 May 2001
http://www.steamradio.com/pipermail/multihulls/2001-June/003583.html

2) The Physics of Airplanes---Why We Go Up, Discover magazine Apr 2001
http://www.allstar.fiu.edu/aero/airflylvl3.htm

Enjoy, I did, Brian
Back in 2003 I posted a reference to these two sites explaining 'lift'. Well they no longer are reachable, but that's okay because they were substantially lacking.

I was going back thru a quick review of some of this subject, and ran across this excellent reference site submitted by Tom Speer on another forum. So I felt obligated to bring the references up to date:

...per Tom
This is an excellent article - one of the best you'll find.

How Airplanes Fly: A Physical Description of Lift
http://www.allstar.fiu.edu/aero/airflylvl3.htm

Doug Lord

06-24-2008, 08:21 PM

Brian, it appears the link isn't working.....

markdrela

06-24-2008, 08:43 PM

Try this:
http://www.allstar.fiu.edu/aero/airflylvl3.htm

BTW, a lot of the explanations in that article aren't quite as airtight as they might seem.
For example, all the "momentum" arguments ignore one crucial fact: a lifting wing does not change the vertical momentum of the entire airmass. There is indeed downward motion between the tip vortices, but there's also upward motion outboard of the tip vortices. The net vertical momentum behind the wing is zero.

The article also invokes the Coanda effect to explain why fluid sticks to the upper surface of an airfoil. This is sketchy at best, since the Coanda effect has to do with enhanced turbulent mixing of an excess-speed jet adjacent to a curved surface. A conventional airfoil does not have an excess-speed jet on it.

Doug Lord

06-24-2008, 08:57 PM

Thanks, Mark!

Petros

06-25-2008, 08:16 PM

Allow me to try and make this very simple (I worked in aerodynamics for many years, and have studied it even longer).

When ever you accelerate any mass you get a reaction. Newton discovered the well known relationship F=ma. That is the force (or lift in this case) is equal to the mass times the acceleration. And the first law of motion, for every action there is a reaction. Accelerate the mass of the air over the curved surface of the sail, and you get thrust (or lift).

Anytime you curve a mass you get an acceleration. A simple example would be a weight at the end of a string, when you swing it around in a circle you get tension on the string. The weight is "curved" into a circle the radius of the string, and the reaction "F" is outward on the string. You can also see this with a garden hose on the lawn: lay it in a curve over the lawn, and turn on the water. As the mass of the water curves around the hose, it will react outward, pushing the hose over the grass away from the curved path of the hose (effectively the mass of the water wants to go strait).

Similarly, when the mass of the moving air (i.e. the wind) is curved or accelerated off of its strait path by the shape of the sails, you get a reaction that pushes the boat through the water.

There are lot of complicating details of course on how to do this efficiently (getting the most amount of thrust from a given amount of wind), but the principles are the same for all fluids. You have to minimize drag, turbulence and keep the flow attached over the sail surface. But the thrust off the sails comes from curving or accelerating the air mass, or F=ma.

This BTW is also how the keel and rudder works, a propeller (a curved surface going around in a circle through a fluid), aircraft wings, a paddle blade on an oar, etc.

brian eiland

06-26-2008, 05:00 PM

How Airplanes Fly: A Physical Description of Lift
http://www.allstar.fiu.edu/aero/airflylvl3.htm
Under the subheading, “Wing Efficiency”, the explanation begins with these sentences:

“At cruise, a non-negligible amount of the drag of a modern wing is induced drag. Parasitic drag, which dominates at cruise, of a Boeing 747 wing is only equivalent to that of a 1/2-inch cable of the same length.”

My question, is there a grammatical mistake here? It appears to me they are saying both the parasitic drag and the induced drag are significant factors at cruise speed? Should the “non-negligible” wording really be ‘negligible’??

On another subject, Powered Flight verses Sailing;
...would someone venture to express in similar terms these same aero observations in this paper from the viewpoint of sailing rather than powered flight….in sailing we are extracting energy from the airflow verses power flight were we are injecting energy to fly??

markdrela

06-26-2008, 06:38 PM

My question, is there a grammatical mistake here? It appears to me they are saying both the parasitic drag and the induced drag are significant factors at cruise speed? Should the “non-negligible” wording really be ‘negligible’??
For an airplane which is well-optimized for cruise, the induced and parasitic drags are comparable in magnitude.

markdrela

06-26-2008, 06:56 PM

...would someone venture to express in similar terms these same aero observations in this paper from the viewpoint of sailing rather than powered flight….in sailing we are extracting energy from the airflow verses power flight were we are injecting energy to fly?? Yes. But where the sailing power comes from and where it goes depends on the observer.

1) To a stationary fish, or to the sailor:
The sail reduces the magnitude of the apparent wind behind the sail, via its tip and root vortices. So the sail extracts Kinetic Energy from the moving air mass. Some of this energy goes into the keel vortex wake motion, some goes into wave motion, and the rest heats the air and water.

2) To a hovering bird or an observer in a balloon stationary with respect to the air:
The keel reduces the magnitude of the water speed behind the keel, via its tip and root vortices. So the keel extracts Kinetic Energy from the moving water mass. Some of this energy goes into the sail vortex wake motion, some goes into wave motion, and the rest heats the air and water.

Petros

06-26-2008, 10:07 PM

This is a non-trivial issue to keep strait. You extract energy from moving air, which must be moving relative to your craft. A sailboat can not move in still air, but a powered airplane can.

Consider a sail plane, in still air it can only fall and take energy out of the fall to move forward and overcome drag. They can only stay up if there is upward moving air from the heat of the sun, or from ridge riding where the moving air is forced upward by a mountain (taking energy out of the forward movement of the mass of air). But the powered aircraft overcomes the drag of forward movement by adding power the system.

If the water surface and the air were moving together at the same speed, a sailboat can not make any headway relative to the water surface. So it depends on you being able to have relative movement between the two to extract energy out of it to make the boat move relative to the water.

This is actually a good issue to always keep in mind. I have gotten into pointless on-line "discussions" with a NA who should have known better. He clearly did not have this understanding of the relationship clear. And apparently his ego did not allow him come around to understanding this relationship, no matter how patiently I tried to explain it to him. He wanted to argue rather than to learn.

Petros

06-26-2008, 10:20 PM

Under the subheading, “Wing Efficiency”, the explanation begins with these sentences:

“At cruise, a non-negligible amount of the drag of a modern wing is induced drag. Parasitic drag, which dominates at cruise, of a Boeing 747 wing is only equivalent to that of a 1/2-inch cable of the same length.”

My question, is there a grammatical mistake here?

No grammatical mistake, just obfuscation. This is collage professor speak trying to say: "you have two large components to drag, parasitic drag and induced drag on a wing that generates lift. Both are significant. On the 747 parasitic drag is larger at cruise speed".

Note this may not be true of other designs or even at different speeds. At lower speeds for example you can have induced drag being much larger than the parasitic drag with the same aircraft.

The faster you go, parasitic drag goes up exponentially, but you need less angle of attack (lower Cl) with the higher airflow over the wings, so the induced drag goes down (the weight, and the total lift requirement, of the aircraft is presumably the same at both speeds). At low speeds you need a high angle of attack (or higher Cl) to keep the aircraft in the air, so induced drag then dominates. The "induced drag" is the "cost" of generating lift.

gggGuest

06-27-2008, 02:32 AM

This is a non-trivial issue to keep strait. You extract energy from moving air, which must be moving relative to your craft.
Whilst not in any way disagreeing with the content I think its better to say that you extract energy from the relative motion of wind and water. A lot of muddled thinking comes from not realising that the sailing craft can only operate when there is a difference in velocity between these two media, and the result of its passing, as well as heat, is that this velocity difference is reduced.

tspeer

06-27-2008, 03:28 AM

...Consider a sail plane, in still air it can only fall and take energy out of the fall to move forward and overcome drag. They can only stay up if there is upward moving air from the heat of the sun, or from ridge riding where the moving air is forced upward by a mountain (taking energy out of the forward movement of the mass of air). But the powered aircraft overcomes the drag of forward movement by adding power the system.

If the water surface and the air were moving together at the same speed, a sailboat can not make any headway relative to the water surface. So it depends on you being able to have relative movement between the two to extract energy out of it to make the boat move relative to the water.
....

What you say is true, however, you're confounding several different things, here. The sailplane has two forms of energy - potential energy due to its height, and kinetic energy due to its speed. When a sailplane does a loop, it is exchanging kinetic and potential energy as it rises and falls. However, it is also continuously losing some of its energy to drag - its specific excess power is always negative, although varying in magnitude. In a steady glide, it is bleeding potential energy into work done on, and heating of, the atmosphere.

The powered airplane also has both kinetic and potential energy. However, its excess power can be positive, zero (unaccelerated flight), or negative. It has the chemical energy of the fuel that adds to the potential and kinetic energy.

A power boat is much like the powered airplane, except that it only has one form of energy - kinetic - because it travels on an equipotential surface. Plus the chemical energy of its fuel. A submarine has both potential energy (by virtue of its variable buoyancy and ability to rise or sink) and kinetic energy.

A sailboat also only has kinetic energy, which is used, for example, when shooting for a mark, or landing at a dock with the sails lowered. However, unlike the sailplane, powered plane, or power boat, its thrust is dependent on its direction of travel. You are right in that it depends on the difference in velocity at the boundary between the two fluids, and cannot achieve any thrust if there's no velocity difference.

As Mark Drela points out, which medium is considered the donor and which the recipient is very much a matter of one's point of view. This is especially true when one considers wind-powered craft that can sail directly downwind faster than the wind. An example is a land-based craft with a propeller in the air geared to wheels. (Yes, such craft have actually been built and shown to work.) When traveling slower than the wind, the momentum of the air leaving the propeller disk is lower than the air entering it, but when traveling faster than the wind, the momentum of the air leaving the disk is higher than the air entering it. So one might reasonably say when traveling slower than the wind the craft is extracting energy from the air and transferring it to the ground, but when traveling faster than the wind, it is transferring energy from the ground to the air. But both air and ground get hotter in either case!

As a practical matter, I haven't found it very useful to use energy- or power-balance approaches for sailcraft. The reason is the thrust horsepower available is not fixed, as it is with powered craft, but depends on the speed of the boat. A high-performance sailcraft generates more power as it accelerates because the apparent wind increases.

I find a force-balance approach is much more useful. For example, even if the drag of the "hull" is zero, the speed of a sailcraft is still finite because the rotation of the apparent wind means there is less and less driving force component available, until the speed is limited by the aerodynamic drag angle.

The fundamental sailing performance equations can be expressed in terms of lift/drag ratios. These are largely self-compensating and don't vary so much as the speed and direction change, compared to the individual magnitudes of lift, drag, or exchange of energy between the fluids. So the force-balance approach is easier to use for engineering the boat's design.

But arguments about where energy is extracted and deposited certainly keep things animated at the bar!

yipster

06-27-2008, 10:48 AM

i'm trying to keep myself animated installing vista 64 over 32
do still read this tho and want to trow a motor sailing cat into the equasion as beeing relevant here as well
yet anyone who has done that vista 64 upgrade in a breeze plz pm me...

=========================================================

edit, free updating the oem vista 32 to 64 bit in the microsoft method two is hard to do
you may also delete the 32 boot partion, vista still runs and now does accept a 64 bit instal

http://support.microsoft.com/kb/932795/en-us

Installing a 64-bit version of Windows Vista on computer that is running a 32-bit version of Windows Vista
If you have purchased an Upgrade license together with a Windows Vista DVD, you must use one of the following methods.
Method 1
Purchase a full version of the 64-bit version of Windows Vista.
Method 2
1. Remove the 32-bit version of Windows Vista.
2. Install Windows XP.
3. Install the 64-bit version of Windows Vista by using an installation method that is listed earlier in this article.
If you have purchased a full license together with a Windows Vista DVD, follow these steps:1. Back up all the data and settings by using Windows Easy Transfer. Windows Easy Transfer is available on the Windows Vista DVD. However, you must use the version that is on the Windows Vista DVD for your currently installed 32-bit version of Windows Vista.
2. Insert the 64-bit version of Windows Vista into the system DVD drive, and then restart the computer.
3. Start Windows Vista Setup from the DVD when you are prompted.

Note You must start Windows Vista Setup by starting the computer from the Windows Vista 64-bit DVD. The installation package will not run on a 32-bit operating system.
4. When you are prompted during Windows Vista Setup, remember to select Custom as your installation choice.
5. When the installation is complete, you can restore the data from its backup location.

brian eiland

07-13-2008, 01:44 PM

Does anyone have a 'digitial file' of this paper, A.M.O Smith's 1975 Wright Brother's Lecture, "High Lift Aerodynamics", AIAA Journal of Aircraft??

http://boatdesign.net/forums/showpost.php?p=9987&postcount=15
http://boatdesign.net/forums/showpost.php?p=9987&postcount=24
http://boatdesign.net/forums/showpost.php?p=9987&postcount=60

tspeer

07-13-2008, 04:01 PM

You should buy a copy from AIAA (http://www.aiaa.org).

However, if you'd like to see what it'll look like when you do, see the attached file.

brian eiland

07-13-2008, 04:44 PM

That's a large file isn't it !!

Actually I had a zerox copy, but I was looking to utilize it in reference to a posting I wanted to make on this subject thread, and was trying to avoid scanning it into a digital format.

Thanks

brian eiland

08-12-2008, 09:46 PM

ivor Bittle

08-15-2008, 05:01 AM

I am grateful to Brian Eiland and the forum for drawing my attention to this paper. The author has used CFD to find a flow pattern round a single sail and round a Bermuda rig when close hauled.

I first drew these flow patterns about 15 years ago when I drew them in chalk on the square paving under a bedroom window so that I could rub out the chalk and change the pattern and see what it looked like from overhead. They were not guesses. There are rules for drawing flow patterns. These patterns are shown in the first section of my website at www.ivorbittle.co.uk. This Italian paper is the first time that I have seen anyone else with essentially the same flow pattern and the first time that I have seen anyone accept that sails work when stalled in the aerofoil sense. I think that sailing people are so keen to be able to claim their sails to be efficient that they become blinded to the tractability of real sails that comes
from working in the stalled condition.

The author of this paper is wedded to the idea that the jib is the counterpart of a slat on a wing. In my website I suggest that it is better to treat the jib as operating in the air that has been diverted upwards and over the luff of the mainsail. Then, when close hauled, the jib drives the boat and the main sail makes no contribution. This is why having the main in front of the jib is not successful. The little sail cannot divert enough air to affect the much larger main in front of it. I see that sheeting the main to windward is becoming more popular.

yipster

08-15-2008, 09:37 AM

The author of this paper is wedded to the idea that the jib is the counterpart of a slat on a wing.
well wedded i aint but glad you sayd that. in my thinking also a jib works indeed as leading edge extension like on a wing and is the reason the combination preforms better than the sum of the parts
like to design my dreamboat (http://www.boatdesign.net/gallery/showgallery.php/cat/500/ppuser/690) better, note the LEX but installing a wifi connection with two 54 lan sticks should be easy they say, but dont you belive it, not in vista, no free rides nowhere :mad:

RHough

08-17-2008, 09:48 PM

... This Italian paper is the first time that I have seen anyone else with essentially the same flow pattern and the first time that I have seen anyone accept that sails work when stalled in the aerofoil sense. I think that sailing people are so keen to be able to claim their sails to be efficient that they become blinded to the tractability of real sails that comes from working in the stalled condition.

The author of this paper is wedded to the idea that the jib is the counterpart of a slat on a wing. In my website I suggest that it is better to treat the jib as operating in the air that has been diverted upwards and over the luff of the mainsail. Then, when close hauled, the jib drives the boat and the main sail makes no contribution. This is why having the main in front of the jib is not successful. The little sail cannot divert enough air to affect the much larger main in front of it. I see that sheeting the main to windward is becoming more popular.

Wow ... am I the only one to see the glaring flaw in the diagrams?

Yes, it is possible to trim the sails so the jib has a large separation bubble, and above the head of the jib the main is stalled. That does not make it the optimum trim condition. Note that the AoA of the upper part of the main in the diagram is exactly the same as the AoA of the lower part behind the jib ... twist? Anyone ever hear of twist or wash out in aerodynamic terms?

The trim shown in the diagrams is a near maximum lift condition and not very good trim at that. Drawing the conclusion that good sail trim leaves sails with large separation bubbles and nearly complete flow separation above the jib cannot possibly lead to a conclusion that sails are efficient when stalled.

Two sails are not 'seen' as separate entities by the air. The total force generated is purely a matter of mass of air deflected and how much it is deflected ... F = MA^2

It does not matter one little bit if the area is split into two surfaces or what the distribution of that area is. If you want high lift per unit area, all you have to do is design the foils to produce the flow pattern you need to get it. At high lift per unit area, induced drag is predominate, the shape you use to get the lift is a very small part of it. What keeps the main from stalling is the increased pressure at the trailing edge of the jib.

What does trimming the main boom have to do with anything? At high sailing angles (close to the wind) an easily driven hull does not need much of the force from the sails to have a forward vector. At the point where the boat has constant velocity (drive = drag) Maintaining attached flow on the main may well require the main boom be carried above the centreline. The sail twists away from the boom to the head and the sail does NOT have an average 'to weather' trim and the sail is NOT stalled (when sailing upwind).

The high lift trim also produces a turning moment, on an airplane it is a pitch moment (nose up/down) on a sailboat it is a a yaw moment (bow up/down or weather/lee moment) Some 'weather helm' is required to force the keel and rudder to both lift to windward. In general increased camber (the same as trimming the main boom to centre or above) increases the moment and any small sacrifice in drive or drag from the sails, is more than made up by the reduced leeway from the water foils.

Once a boat is fully powered up ... when all the available righting moment is used, high lift per unit area is no longer required. Gains upwind are made from drag reduction, not increased force. Now the trim is far from the near stall high lift trim that was needed, the trim goal is to minimize drag ... since drag induced from lift is the major component, *reducing* lift reduces drag. As Angle of Attack is reduced, so is lift. At some point, a profile that gave high lift per unit area (high CL) starts to have profile drag = or greater than induced drag and further gain must come from changing the profile. For soft sails, that means reducing camber and twist and keeping the flow attached.

For an aircraft wing, during take off and landing high CL is required. During cruise, there is no need for high CL, low drag is the design goal. In order to achieve this, the area is split by combinations of leading and trailing flaps/slats or other flow control devices. Moving the leading and trailing edges to increase the effective chord increases the effective area. Lift *always* equals weight in level flight. The requirement for change is to allow slow speed take off and landing while at the same time allowing efficient high speed cruise. For displacement hull boats, once the sails produce enough power to drive the hull to 'hull speed' plus a bit, any extra force just goes to increase heel, the sails get trimmed close to the airplane's cruise profile. For upwind planing boats and multihulls, the trim change comes when sail heeling force equals righting moment, then the sails go from high lift (and high drag) to more moderate lift and drag trim.

Going up wind with the sails stalled is NOT a normal condition. Going downwind with the sails stalled is only acceptable for slow boats. Fast planing boats and multihulls sail downwind with the apparent wind well forward of the beam ... stalled sails downwind don't work well for them either.

Real sails on real boats are not intentionally trimmed to the stalled condition except on slow heavy boats that cannot easily exceed the 1.34 S/L ratio that is their hull speed.

I think that the idea that the jib and main of a sloop act as separate foils is what keeps sailors from understanding sail trim. They are not separate, they are but parts of a single system, that cannot be considered separately to good result. When sailing upwind or with the apparent wind well forward, changing the trim of one sail much be matched with a change in trim of the other, and the effect of the sail trim on the water foils cannot be ignored. It is MUCH more complex than 'the jib drives the boat upwind and the main does nothing'.

Guest625101138

08-18-2008, 06:09 AM

There could be confusion here over flow separation and stall.

There is a good discussion here on stall:
http://igitur-archive.library.uu.nl/dissertations/1950226/c2.pdf
go to page 27.

Once again I can recommend JavaFoil if you want to do your own quantitative analysis with particular sails. I have attached a sample from JavaFoil similar to that provided in the paper.

Rick W.

yipster

08-19-2008, 11:35 AM

installing a wifi connection with two 54 lan sticks should be easy they say, but dont you belive it, not in vista, no free rides nowhere :mad:
appears a vista error? manual IP adress instead of auto works and working on it..
RHough, thanks for the drag and Rick for the javafoil link, thats next

brian eiland

08-19-2008, 12:32 PM

brian eiland

08-19-2008, 12:46 PM

I haven't had time to review this paper...just ran across it while looking for something else. But it sure looked like a paper that belonged in this subject thread:

AERODYNAMIC SIMULATION OF UPWIND AND DOWNWIND SAILS
http://www.nautica.it/superyacht/517/tecnica/sail.htm
Upon reflection, I'm sorry I posted this paper. I really don't see it as useful information. It seems to rely on a lot CFD study.

I might refer you back to my posting #44 (http://boatdesign.net/forums/showpost.php?p=22183&postcount=44) in this subject thread;

What I am finding in my review of some of this computational methods of investigating sail forces and flow analysis (CFA, CFD, vortex lattice models, etc) is that generally there are so many assumptions made upfront in order to simplify the equations so the computer can solve them, that the results get skewed quite a bit from reality:

.....ie, a quote from one of the annalist;
"CFD = Computer Fluid Dynamics. CFD is a great tool for visualizing and explaining flow phenomena. While the latest flow software is very powerful and capable of calculating amazing things at astonishing accuracy, the old saying "garbage in, garbage out" is more true than ever. Besides of presenting the problem in a meaningful way, one needs lots of knowledge and experience to interpret the results correctly. Simulation through CFD is especially useful at giving qualitative information - when it comes to quantitative results or hard numbers, you have to be even more cautious when drawing conclusions about the merits of one design over another. Wind tunnel tests are needed to calibrate and validate the CFD code before reliable results are obtained."

"With the power of modern CFD at the desktop, it is too easy to produce beautiful pictures with little connection to reality. Often these pictures are produced by flow experts with little sail-specific knowledge, and then interpreted by sail designers without sufficient understanding of the CFD tool, and as a result you get just that - pretty pictures."

RHough

08-19-2008, 06:45 PM

We think differently here.

And I believe a few other folks see this differently also, including Avery Gentry, AMO Smith, and Tom Speer

I'll bring this subject up in a new posting.

I've not seen anything from any of those gentlemen that says that two or more elements don't produce a single flow pattern. A single circulation pattern. Changing the trim of any element effects the whole system.

The argument for multi-element systems is that they provide a solution to flow separation at high AoA (High CL). Maximum CL is only needed on a sailing boat when the area is too small to provide the force needed at moderate CL.

I the real word, that means in the windspeed range of 0-10 knts or so ... under canvased boats might need maximum CL from 0-14 ...

The fact is that even heavy boats in the 300 D/L area ... (very heavy by modern standards) the sails are only trimmed to maximum CL when reaching, going upwind they are *not* at maximum CL.

I happen to have the polars for my heavy old boat ... I wanted to know what the difference was between a 13.2 LP Jib (100%) and a 19.8 LP Genoa (150%).

In true wind speeds up to 12-14 knots the 100% jib/main has a maximum CL=1.83, however the best upwind performance is not at max CL.
Best upwind:
6 knts CL=1.64
8 knts CL=1.66
10 knts CL=1.66
12 knts CL=1.57
16 knts CL=1.19

Below 12 knts true wind, the sails are trimmed to full chamber, above 12 knots they are flattened to reduce camber and reduce drag.

The 150% Genoa/main has a maximum CL=2.346 (this shows the benefit of overlap). Again, best performance upwind is *not* at maximum CL.
Best upwind:
6 knts CL=2.07
8 knts CL=2.12
10 knts CL=1.87
12 knts CL=1.61
16 knts CL=1.21

With the greater area, the sails start getting flattened at 10 knots compared to the 12 knots of the smaller sails.

The point of this is, that even heavy old boats don't sail to weather at the sailplan's maximum CL. As the SA-D goes up, the requirement for high CL sailplans goes down.

It makes more sense to add area than to add elements. Other constraints like RM, rating rules, construction difficulty, and ease of sail handling place effective limits on area. If the hull is a brick, the extra power from high CL multiple element sailplans can be the best solution.

A wing sail with great area that would drive the boat at low CL would be a much better solution if you could reduce the area and lower the height of the CE when the RM limits are reached.

There is no magic to multiple elements, they are a soft foil solution to a design requirement for high CL where flow separation is a problem. If you could build a boat with high RM, high SA/D and low D/L ... like a fast catamaran, there is no need for CL over 1 or so. CL=1 does not require high AoA and multiple elements.

tspeer

08-19-2008, 11:16 PM

I think that the idea that the jib and main of a sloop act as separate foils is what keeps sailors from understanding sail trim. They are not separate, they are but parts of a single system, that cannot be considered separately to good result.
We think differently here.

And I believe a few other folks see this differently also, including Avery Gentry, AMO Smith, and Tom Speer

Excuse me, please do not put words in my mouth. Nor should you speak for Arvel Gentry or A.M.O. Smith.

I agree with RHough. The main and jib do have to be considered together, rather than as separate entities. I do not subscribe to the notion that the main is but an inefficient appendage to the jib.

brian eiland

08-25-2008, 10:18 AM

Dear Randy and Tom,
I will be answering both of your recent postings very soon here, however I wish to try and clear up a misconception I seem to detect that you may have about my concept to legitimize the jib/mainsail configuration.

It appears to me that you think I believe the two sail (jib/mainsail) system should be considered two sail entities unto themselves, and that the jib-genoa is the more powerful of the two, thus deserving a more prominent position in any sail rig design, and particularly in my aftmast concept. Or that I would even consider that headsails are so superior that they can act on their own.

And maybe a portion of this misconception results from the fact that I emphasize that my aftmast rig is 'mainless' (no mainsail), or that I appear to have only two headsails. This might infer that I do not see the necessity for a mainsail (like that quote above by Ivor, "Then, when close hauled, the jib drives the boat and the main sail makes no contribution).

Absolutely, I do NOT believe this to be the case. I have long recognized and expressed in numerous postings that it requires a trailing sail to generously impart to the leading sail (two elements) the special gifts of extra power and better pointing capabilities.
I've just chosen a 'different configuration' for my trailing sail. I call it a mainstaysail rather than what normally would be the traditional mainsail.

Regardless of what you think about the rest of my aftmast rig concept as far as practicality, engineering, extra drag, etc, I think it might be said 'that from a purely aerodynamic viewpoint my mainstaysail foil, in the two-element foil system, is more favorably disposed to help the headsail foil than is the traditional mainsail foil.'

brian eiland

08-25-2008, 10:56 AM

Excuse me, please do not put words in my mouth. Nor should you speak for Arvel Gentry or A.M.O. Smith.

I agree with RHough. The main and jib do have to be considered together, rather than as separate entities. I do not subscribe to the notion that the main is but an inefficient appendage to the jib.
Hello Tom,
First let me say that I very highly respect your opinion, and I have learned a lot from you...so don't give up on a total conversion of me yet. ;)

I was not trying to 'put words in your mouth'. I will quote some of your previous postings that led me to believe, that like myself, you see the interaction of the two-element sail system to be advantagous to the headsail.

It doesn't appear that the forward quarter of the mainsail is directly contributing a great deal, but you can get a sense of how the jib benefits from the presence of the main, making the combination more powerful. http://www.boatdesign.net/forums/showpost.php?p=143209&postcount=8

In upwind sailing, if the maximum aerodynamic L/D is below stall, it may pay to trim the rig for maximum lift instead of maximum L/D, even though the aerodynamic drag angle will increase. The added lift can improve the hull's L/D and reduce the hydrodynamic drag angle more than the loss in the aerodynamic drag angle, and improve performance.

In general, for the kinds of boats we're talking about, the windage is so high that the rig will stall before it reaches maximum L/D. So there's benefit in improving the maximum lift as well as the L/D of the rig alone. But the really big gain may come from reducing windage.
http://www.boatdesign.net/forums/showpost.php?p=111366&postcount=94 (http://www.boatdesign.net/forums/showpost.php?p=111366&postcount=94)

The theoretical effectiveness of the headsail is not due to its cleaner leading edge. It's due to its interaction with the mainsail. The mainsail gives a boost to the head sail, and the headsail reduces the lift of the mainsail. It's exactly the same as lee-bowing your competitor - the jib gets a lift from the main, and the main gets a header from the jib. But the effectiveness of the whole combination is increased, so there's a net gain.
http://www.boatdesign.net/forums/showpost.php?p=90396&postcount=152

…. Most boats have so much windage that the maximum L/D occurs above stall onset. So that's a big reason why you want to raise the maximum lift as high as possible.

… Multiple surfaces properly shaped and arranged can produce a higher maximum lift than any single surface of the same chord. For a more in-depth discussion, search this forum for A.M.O Smith's "High Lift Aerodynamics".

… I think you ought to consider the planform area of the main and jib together when talking about aspect ratio. They really act more like one multi-element airfoil than separate foils.
Brian added: key word…multi-element airfoil, not just single airfoil

.. Exactly. The main does indeed make the jib act like it's got a flap deflected. This is why people find that jibs are more powerful than mainsails of the same area.

… The main really does transfer lift to the jib. The combination of the two is more effective than the same area allocated to either one separately. This is true even if both of the elements are good performers in their own right.

Any study of rig effectivness points out that on a per-area basis, jibs are more effective than mainsails. That leads to the conclusion that rigs would better if they were all jib - hence the mast aft rig. What these studies often fail to point out is the effectiveness of the jib comes from its interaction with the main. On its own, it doesn't perform as well. And theoretically the main suffers in the exchange, making it look worse. But the combination is more than either sail acting in isolation.

...and finally excerpted from aerodynamicist and North Sails consultant Paul Bogataj’s paper, “How Sails Work” http://www.northsailsod.com/articles/article6-1.html
‘Sails in Combination’, “Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other. The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone.
The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses”

Paul Scott

08-25-2008, 11:03 AM

brian eiland

08-25-2008, 11:30 AM

Brian, this is not on the particular point above, but ever since I've been reading about your mast aft concept, I've wondered- Why not configure your aft mast to be a largish rotating wingmast (4 or 5 ft chord on a 60 ft mast, for example), and at least get some of the benefits of a trailing airfoil out of the mast?

If you have objections to this approach, are they practical? Structural?

Paul
I'd have to give that idea more thought, but two items pop into my mind at the moment.
1) A largish rotating wingmast would likely close off the slot between the headsail and mast mounted mainsail to such a degree that it would present problems.
2) An aft mounted largish wingmast could present real steering problems downwind.

But how about a little less rake forward, and mount the mizzen sail to the mast that is stayed somewhat akin to a B&R :?:

RHough

08-25-2008, 02:42 PM

Hello Tom,
First let me say that I very highly respect your opinion, and I have learned a lot from you...so don't give up on a total conversion of me yet. ;)

I was not trying to 'put words in your mouth'. I will quote some of your previous postings that led me to believe, that like myself, you see the interaction of the two-element sail system to be advantagous to the headsail.

Brian,

None of the quotes support the advantageous to the headsail idea.

The total projected lateral area of a sail plan is the effective area. Overlap does NOT increase projected area.

The total lift of the projected area falls at about the 1/4 chord point of the total area. That happens to be forward of the mast for many boats and leads one to conclude the headsail is doing all the work. Ain't so. ;)

A wing, a single element, can generate a Cl of 1.6 or so with the corresponding high induced drag. If some performance goal requires higher lift force (not higher Cl) and for some reason area cannot be added, then the design must produce the lift force by operating at a Cl higher than 1.6.

The way to do this is to use a multi-element configuration to control separation and prevent stall. Cl of over 2.0 can be reached, it might seem reasonable to think that two foils acting together should be able to reach Cl = 3.6 (twice 1.6), but I have not seen a multi-element foil reach that number:

Multiple Element Airfoils Optimized for Maximum Lift Coefficient
ALLEN I. ORMSBEE* AND ALLEN W. CHEN|
University of Illinois at Urbana-Champaign, Urbana, III.
Optimum airfoils in the sense of maximum lift coefficient are obtained for incompressible fluid flow at large Reynolds number. The maximum lift coefficient is achieved by requiring that the turbulent skin friction be zero in the pressure rise region on the airfoil upper surface. Under this constraint, the pressure distribution is optimized. The optimum pressure distribution is a function of Reynolds number and the trailing edge velocity. Geometries of those airfoils which will generate these optimum pressure distributions are obtained using a direct-iterative method which is developed in this study. This method can be used to design airfoils consisting of any number of elements. Numerical examples of one-and two-element airfoils are given. The CLmax values obtained range from 2 to 2.5.

Now, the 2D lift line slope is very close to .1 Cl per degree of Angle.
Cl = 1 is a 10 deg AoA
Cl = 1.6 is 16 deg
Cl = 2.5 is 25 deg

It is NOT possible for the high lift foil to operate a low AoA. A Cl = 1.0 foil will "point" 15 deg higher than the multiple element Cl = 2.5 foil.

Multiple element foils do not point higher than single element foils.

Tom makes a point that I have not fully researched, that increased drive and thus increased speed can allow the hydro forces to compensate for the increased angle and drag of a high Cl multiple element foil, the result might be a course closer to the wind ... "pointing higher" ... I don't know that I'm ready to agree or not. ;)

If you set the drive requirement to equal the drag at a target speed, you obtain that force with a combination of area and Cl. Increasing area reduces the Cl required, increasing Cl reduces the area required.

With greater area and lower Cl the induced drag is lower ... boat 'points' higher. With greater Cl (multiple element foil) and lower area the induced drag is higher ... boat 'points' lower.

If there is a limit to area, the only way to increase drive is to increase lift by using higher Cl. What is the limiting factor for area on a sailboat?

Area is limited by the boat's resistance to heel. In an extreme case you could have a 100 sqft foil with a 100 ft span and a 1 ft chord or the same area in a foil with a 10 ft span and a 10 ft chord.

For simplicity assume the lift acts at the geometric center of each foil. If the drive force is x, the 100 ft foil has a 50x heeling moment and the 10ft foil has a 5x heeling moment. If the boat has a righting moment of 10x, it cannot use the drive of the 100 ft span, the maximum heeling moment the boat can use is 10x.

If the righting moment limits the area so high Cl is required, and the required Cl is greater than 1.6 or so, only then does a multiple element foil become an optimum solution.

On your cruising cat, what is the RM? How tall would a single foil have to be to drive it at a target cruise speed? What Cl is needed produce the required force? Does longitudinal stability limit the height of the rig? Why are you convinced you need a multiple element sail plan?

If it turns out that some factor like bridge clearance or stabilty limits rig height, factors other than simple aerodynamics have to be considered.

From a purely theoretical point of view, the case that multiple element foils are superior (high L/D) cannot be made. It cannot be done, other limits must be applied to justify their use.

markdrela

08-25-2008, 03:14 PM

Multiple element foils do not point higher than single element foils. The "pointing angle" of a sail airfoil is mostly irrelevant. All that matters is its Cl and Cd. The AoA can always be set to reach whatever Cl is needed, short of stall.

With greater area and lower Cl the induced drag is lower ... boat 'points' higher. With greater Cl (multiple element foil) and lower area the induced drag is higher ... boat 'points' lower. Not so. If you double the CL and halve the chord, the lift and the induced drag are not affected, even though the AR has been doubled.
CL' = 2 CL
c' = 0.5 c
S' = 0.5 S
AR' = 2 AR
CDi' = 2 CDi
L' = q S' CL' = q S CL = L
Di' = q S' CDi' = q S CDi = Di

As I've said before, the slot between the jib and mainsail is purely a boundary layer control device. It does not significantly affect the sail's overall potential-flow characteristics:
* it has little or no effect on the sail's CL(alpha) curve
* it has little or no effect on the induced drag at a given speed and lift

Any improvement of L/D caused by the slot will likely be due to a reduction of the mast drag.

RHough

08-25-2008, 04:35 PM

Not so. If you double the CL and halve the chord, the lift and the induced drag are not affected, even though the AR has been doubled.
CL' = 2 CL
c' = 0.5 c
S' = 0.5 S
AR' = 2 AR
CDi' = 2 CDi
L' = q S' CL' = q S CL = L
Di' = q S' CDi' = q S CDi = Di

Any improvement of L/D caused by the slot will likely be due to a reduction of the mast drag.

You and Tom crack me up. ;)

Increasing span without changing chord, increases area, increases AR, reduces required Cl and and thus reduces induced drag and increases form drag.

Increasing span without changing area , reduces chord, increases AR, the Cl remains the same, but the induced drag is reduced and the form drag remains the same.

Increasing AR without changing span, reduces area, increases required Cl and the induced drag remains the same and the form drag is reduced.

Is that about right? :)

I have no idea why the two of you insist on considering span independent of area (and thus AR). It is misleading to an extreme.

Changing span and increasing area at the same time is reducing one drag producing factor while increasing another.

Changing AR (span) with fixed area changes only one drag producing factor.

Yes, you nit pickers will point out the obvious error ... at some speeds Re limits gains from increased AR.

Can we at least agree that L/D max occurs when lift exceeds drag by the greatest margin?

That leads to the conclusion that when form drag (shape and area of both surfaces) and induced drag are equal, maximum L/D is reached.

Increasing span alone increases form or profile drag in two ways. The obvious increase in wetted surface due to the increase in effective area x 2 *and* the increase in drag due to the increased thickness/chord ratio required to keep the foil from breaking at the root. At some point increasing span increases drag. What you have pointed out is that AR does not always reduce induced drag, while failing to note that the drag force in your halve the chord example *is* decreased ... thus I am quite correct; increasing AR increases L/D :P

To start a reply with a smug "no so" and then to proceed to ignore the fact that I very clearly qualified the increased AR/Induced Drag/Area relationship is offensive. :mad:

I have worked on the L/D problem with a span limit constraint and with an area limit constraint. I cannot think of span as different or independant from AR. I think I understand the realtionships pretty well. ;)

I agree completely that any increase in L/D is probably due to the mast being in the slot ... a benefit that the aftmast rig does not enjoy. ;)

Cheers,

Randy

PS. The AoA on a sail cannot always be increased to provide the required lift ... unless you don't expect to sail to weather very well. ;)

markdrela

08-25-2008, 05:08 PM

I have no idea why the two of you insist on considering span independent of area (and thus AR). It is misleading to an extreme. Because when considering sail airfoils, it makes the most sense to consider the span fixed, and just let the chord vary. That way you nearly decouple both the heeling moment and the induced drag from the sail airfoil problem.

Paul Scott

08-25-2008, 06:21 PM

I'd have to give that idea more thought, but two items pop into my mind at the moment.
1) A largish rotating wingmast would likely close off the slot between the headsail and mast mounted mainsail to such a degree that it would present problems.
2) An aft mounted largish wingmast could present real steering problems downwind.

But how about a little less rake forward, and mount the mizzen sail to the mast that is stayed somewhat akin to a B&R :?:

I guess I should be more specific about which rig of I'm talking about- I'm thinking about a rig with a vertical mast in back, and 2-3 jibs in front of it. I'm assuming storm ji(b)s to balance things in the stronger stuff. Advantages I see for the wing mast are

Lift at small aoa
less drag when feathered (or near feathered)
adding to system lift when needed.

I'm trying to imagine your #1 criticism-

the cut of the mainsil could be different
the overlap could be different
what degree of choke of the slot would be too much for you? (Dr. Smith is trying to say something to me through Dark Matter, but it's too faint to hear clearly...)

I'm wondering at what size a smaller wing mast becomes useful, but the wierdness of the sub 100,000 re region seems to me like it would to come into play. In a most delightful way, nae doot.

Paul :cool:

RHough

08-25-2008, 06:28 PM

Because when considering sail airfoils, it makes the most sense to consider the span fixed, and just let the chord vary. That way you nearly decouple both the heeling moment and the induced drag from the sail airfoil problem.

I agree. But the way you put it makes it sound like changing span is a good drag reduction method.

Increasing the span of either or both foils (aero and hydro) tends to increase the heeling moment so keeping the heeling moment constant pretty much removes adding span to reduce drag as an option does it not?

For a fixed span solution span loading becomes problem to solve, You and Tom have probably been looking at this for years. ;)

Back 20 some years ago I met Tom Selig (sp?) when I was designing low Re RC sailplanes and had the honour of building a few wings with his profiles. The last I heard he was still doing low Re research ... I wish I was still in contact and could infect him with the sailing bug. :)

One thing seems pretty certain to me. A masthead rig with sails bound by the forestay and backstay might be a bad choice. No design I'm aware of has evolved to that planform.

Thanks for the reply,

Randy

Paul Scott

08-25-2008, 06:42 PM

Randy- there was a high altitude variant of the the Spitfire close to the end of WWII that had long pointy (pinhead in sailing parlance) wings, very much (to my musical eyes) like a sail. Probably different because it was a wing. But I've always wondered why they did this, as it seems to go against the grain, so to speak.

Paul

markdrela

08-25-2008, 07:03 PM

Randy- there was a high altitude variant of the the Spitfire close to the end of WWII that had long pointy (pinhead in sailing parlance) wings, very much (to my musical eyes) like a sail. Probably different because it was a wing. But I've always wondered why they did this It was to primarily to reduce the wing loading while leaving most of the wing unchanged. Lower wing loading gives greater max altitude capability.

RHough

08-25-2008, 07:11 PM

It was to primarily to reduce the wing loading while leaving most of the wing unchanged. Lower wing loading gives greater max altitude capability.

Is this where I jump in and say ... but lift = weight how does increasing area let the plane fly higher? :D

Just kidding!

Paul Scott

08-25-2008, 07:16 PM

Does it count as evolution or desperation?

:eek:

RHough

08-25-2008, 10:55 PM

Back 20 some years ago I met Tom Selig (sp?) when I was designing low Re RC sailplanes

Age is a rotten thing ... Michael Selig

:(

gggGuest

08-26-2008, 01:49 AM

But I've always wondered why they did this, as it seems to go against the grain, so to speak.
Engineering issues. The wingtip on the Spitfire was attached to the main structure. Any change of the main structure would be a very major redesign, but making the tips smaller or larger was a reasonably straightforward exercise. The most practical way to add a little more area to a Spitfire wing was to make larger wingtips, and the pointy end about the only solution that wouldn't be totally bizarre...

This page seems good
http://spitfiresite.com/reference/variants-technology/2008/04/spitfire-wings-02.htm

RHough

08-26-2008, 02:06 AM

Engineering issues. The wingtip on the Spitfire was attached to the main structure. Any change of the main structure would be a very major redesign, but making the tips smaller or larger was a reasonably straightforward exercise. The most practical way to add a little more area to a Spitfire wing was to make larger wingtips, and the pointy end about the only solution that wouldn't be totally bizarre...

This page seems good
http://spitfiresite.com/reference/variants-technology/2008/04/spitfire-wings-02.htm

Very nice! Thanks for digging it up.

Paul Scott

08-26-2008, 10:36 AM

Petros

08-26-2008, 02:35 PM

There is another issue at play with the wing tip I think. Increasing the area outboard would raise the stress on the main spare at the root, meaning a major redesign of primary structure. But if you increased the total span only, without increasing the area you will not increase the wing root loads as much, and you greatly improve the climbing performance. With aircraft the span loading (total weight over total wing span) is directly related to climbing capacity with any given engine power. I think this was a way of improving the climb performance of the Spitfire without having to redesign the wing or re-engine the aircraft. If you increase span without increasing the wing area you end up with pointy wing tips.

Something missing on sail design I think is consideration of L/D. Though it has been mentioned here a few times, there tends to be little consideration for looking at the total L/D of the rig design. There is always consideration for max CL and max AOA before stall (best pointing) perhaps because these are important in a sailboat when at certain extreme edges of the envalope (and in racing that would give you an advantage when sailing there), but I wonder if better overall performance will be improved with better L/D, not just Clmax. All of the CFD models appear to ignor L/D max, yet in an aircraft or sail plane, the best speed and best distance is acheived at best L/D. It seems to me this would be an advante in a sail boat as well.

Most sailing rigs, both modern and traditional are very draggy, and only in a few instances have I seen any consideration for reducing the total drag of the rig. All of the focus is always on max Cl and max AOA. Multiple element foils (and sail plans) achieve much higher Cl max, but ALWAYS at the expense of L/D max. On an aircraft Clmax is only used for landing or in a max g manuver (not your typical flight condition), and extra drag on landing is helpful in controling your flight path angle. I suspect the thinking is that in a sail boat you always want the max speed for any given wind speed, which means achieving the max CL under all conditions (unlike an aircraft where the total life at any given speed or flight condition has to equal the weight of the aircraft). So drag is ignored on a sail.

But on a sailboat when you are at CLmax, you will be fighting against the higher drag. In all points of sail except going down wind high drag kill your performance. It will slow the boat, it causes more heeling moment (reducing the efficiency of both the hull/keel and the sail) and increases the slip or leeway. It seems to me if you have a rig design that gives say 9:1 L/D instead of the more typical 3:1, you would have 2/3rds less drag for the same amount of lift, that means less heeling moment and better windward performance. Going down wind of course the drag direction is the same as your lift direction, so it is the only place where the drag is beneficial. But even then if you generate more lift for any given amount of drag, even your down wind performance could be improved.

Best L/D is a complex relationship with the lift curve and the induced and parasitic drag relationship and there is no simple rule of thumb for determining where it occurs. The best L/d doesn't occur at minimum drag, nor when induced and parasitic drag are equal as someone speculated. It just all depends on where each of the curves land on a plot of total drag and total life vs. angle of attack. It is usually plotted and the ratio calculated at each point of lift (or AOA).

I am planing on experimenting some day with a small rig that would optimize L/D rather than clmax and see what I get. Needless to say, it seems to me that minimizing all the drag on the rigging, the deck, the mast and the sails all will improve performance, and having the highest practical aspect ratio (to reduce induced drag) all will be useful. I suspect that better being able to control both camber and twist of the sail will also yield better performance. This means a single element "foil" or sail.

Any thoughts on the L/D performance of the rig deign vs. CLmax?

RHough

08-26-2008, 04:17 PM

I am planing on experimenting some day with a small rig that would optimize L/D rather than clmax and see what I get. Needless to say, it seems to me that minimizing all the drag on the rigging, the deck, the mast and the sails all will improve performance, and having the highest practical aspect ratio (to reduce induced drag) all will be useful. I suspect that better being able to control both camber and twist of the sail will also yield better performance. This means a single element "foil" or sail.

Any thoughts on the L/D performance of the rig deign vs. CLmax?

Put your flame suit on. :)

Sailing upwind (anytime the apparent wind is forward of the beam) is a Max L/D *of the entire system* problem.

TSpeer and others have been looking at L/D max for years. At the risk of misquoting, it may be that L/D max of the sail plan cannot be used.

On an aircraft wing, span loading and root chord load do not produce a roll moment, the forces are mirrored by the other wing.

On a sailing vessel the sail plan produces a roll moment (heel) that must be balanced by righting moment. The hydro foil acts to *increase* the heel moment. Thus sail rigs must maximize L/D of the sail plan without increasing the heel or roll moment. The lift force available at L/D max with the roll moment constraint is not enough to drive the boat to Vmax.

You cannot just double the span (height of rig) and keep the boat from tipping over. As you point out, sailing boats are draggy things. All the deck gear, rigging, people on deck, lifelines etc add to drag. Max L/D of the aero half of the problem must include all the aero drag, not just that of the sails. It may well be that on many boats that the parasitic drag is so high that even near Clmax for the sails, the induced drag is lower than the parasitic drag, and L/D max cannot be reached at all.

:)

Cheers

Randy

Paul Scott

08-26-2008, 09:04 PM

At the risk of sounding atavistic, L. Francis Herrescoff, on page 21 & 22 of The Common Sense of Yacht Design describes an experiment (in 1925) with 3 identical hulls of the Developmental Class, 19' Length, 125 sq ft of sail. Smooth seam Spanish cedar planking like a rowing shell. "One had a conventional sloop rig, round mast , shrouds and all. One was cat rigged with a double luff sail: the third was a cat yawl with both sails double luffed.... from what little racing we did it seemed apparent that the sloop with the round mast was fastest in moderate wind and a choppy sea. The cat was fastest in light wind and a smooth sea. The cat yawl seemed much the best in strong wind, particularly to windward, and won the most points in the races.."

Apart from some multis (and you might argue that the slim hulls smooth out motion and chop resistance), how much has changed?

L. Francis was working for Starling Burgess at the time, who had been an "aeroplane designer" and they were "wind resistant conscious" which would make sense, as minimizing drag was big at that time. He then goes on to talk about double luff sails on an iceboat and "tacking to leeward".

The problem with maxing L/D has been messed with for a long time, with some progress, but the sloop keeps on keeping on... Maybe we are now getting to a point where L/D max is important from a practical standpoint?
Although round masts are still hanging in there.

Paul

Petros

08-26-2008, 09:27 PM

Put your flame suit on. :)
.... It may well be that on many boats that the parasitic drag is so high that even near Clmax for the sails, the induced drag is lower than the parasitic drag, and L/D max cannot be reached at all.

I do not see the need for a flame suit yet... I presume we are having a civil discussion here. I know a lot of aircraft aerodynamics, and have owned and been studying sail boats for years, but I am trying to apply the acquired knowledge across an industry gap. I do not presume to know very much about sail boat dynamics (not just the air, but you have to include the effects of the water on the hull, combined it appears to be a very complex interaction). So all of my previous posting was along the lines of my musing and speculation, I was not arguing a point. So tell me if I am misunderstanding anything about effects of the sail on a boat.

I do not think your last statement is correct BTW, all devices that generate lift in a fluid (be it liquid or gas, as in air) have an L/Dmax. Trying to sail at it at all points of the wind may be another issue of course, which may be what you are referring to. You may not have enough power or thrust from the sails to reach the speed of where L/Dmax occurs. But even with very high parasitic drag you will still have a L/D max, look at the old wire braced biplanes like the Jenny and others, they were very draggy, worse than modern ultralite aircraft. But they do have an L/D max, but sometimes you may not have enough power to fly level at it (it means you will need to be in a slight dive to reach the L/D max speed). The use of a sail is quite different than a wing, even if they work the same way. A wing generates lift upward to counteract gravity, a sail generates lift to propel a sailboat at all points to the wind ideally.

Yes, you would have to consider L/D of the whole system, not just the sail. I suppose you also have to included the hull/keel interaction as well. For example if a very high aspect ratio sail causes too much heel, the hull drag will go up, so your best L/D may drop off. My test mule was going to be a catamaran so I would not expect much change in heel anyway, and to limit the heeling variable hopefully enough to ignore it.

Drag would have to be a consideration if you are trying to break a new speed sailing record, at around 50 knots all the drag, parasitic and induced, are too large to ignore. At those speeds, a one inch by three inch teardrop shaped strut for example will have less drag than an 1/8" diameter steel cable. So bracing the mast with struts rather cable will have less drag. In a recreational mono-hull that only moves along at 6 to 8 knots at best, parasitic drag is not particularly large, and cable bracing is much simpler and less expensive.

One test I could do is build my little catamaran to maximize L/D, and go out and try it with my single element sail. And install a jib or a leading edge slot to create Clmax, at the expense of L/D max. And see if it any faster at a give wind speed or around a give course. It may be we need L/D optimized for certain legs of the course, and Clmax for others. I just have never seen a complete discussion of the relevance of L/D to sailing in any of my NA books nor on this forum. It does come up occasionally, I just do not have a clear picture of its relevance to sailing performance. I suspect it may be more important than it is considered by most.

A sail plane or hang glider goes furthest when flying at L/D max, a powered aircraft consumes less fuel when flying at L/D max. So why not should a sail boat move fastest when the whole system is operating at L/D max? The power used to drive the boat is that which can be extracted from the moving air relative to the boat. The most lift that can be extracted from the wind at the least drag, it seems to me, should yield the best speed. IF you can get more total lift at the expense of even higher drag, you will not be moving as fast. Conversely, if you sail for minimum drag, you sacrifice lift and speed since that would occur at a much lower speed. So it has to be that the best speed occurs where the L/D is best because that is where the most amount of excess power is available to drive the boat. You would have to have both the best L/d of the sail rig occur at the same speed as the best L/d of the hull/keel, which may be difficult to get to occur at all points to the wind.

Or am I missing something? Perhaps I am.

tspeer

08-27-2008, 12:30 AM

...Increasing the span of either or both foils (aero and hydro) tends to increase the heeling moment so keeping the heeling moment constant pretty much removes adding span to reduce drag as an option does it not?...

You can optimize on the basis of a fixed heeling moment. That's a reason for considering the dimensional lift to be fixed in the previous discussions - it's largely dictated by the stability limits of the craft if there's enough wind.

Say you have a given mast height, and you've optimized the planform of the rig to give you the minimum induced drag for that mast height. You can reduce the induced drag while keeping the same heeling moment by using a taller, more tapered planform. The taller rig won't have the minimum drag for its height, but it will have less drag than the rig optimized for the smaller span.

And yes, its aspect ratio will be higher if the sail area is kept constant. But the induced drag will still be lower even if there's more sail area to keep the same aspect ratio, and trimmed to maintain the same heeling moment.

This figure (http://www.tspeer.com/Planforms/Fig20.gif) shows quantitatively what such a tradeoff would look like. Each point plotted is a different design, optimized for a given span and gap at the foot (dashed lines), or a given heeling moment and gap (solid lines). For the example shown, the more tapered rig can be about 17% taller and reduce the induced drag by about 20% for the same heeling moment.

RHough

08-27-2008, 02:03 AM

You can optimize on the basis of a fixed heeling moment. That's a reason for considering the dimensional lift to be fixed in the previous discussions - it's largely dictated by the stability limits of the craft if there's enough wind.

Say you have a given mast height, and you've optimized the planform of the rig to give you the minimum induced drag for that mast height. You can reduce the induced drag while keeping the same heeling moment by using a taller, more tapered planform. The taller rig won't have the minimum drag for its height, but it will have less drag than the rig optimized for the smaller span.

And yes, its aspect ratio will be higher if the sail area is kept constant. But the induced drag will still be lower even if there's more sail area to keep the same aspect ratio, and trimmed to maintain the same heeling moment.

This figure (http://www.tspeer.com/Planforms/Fig20.gif) shows quantitatively what such a tradeoff would look like. Each point plotted is a different design, optimized for a given span and gap at the foot (dashed lines), or a given heeling moment and gap (solid lines). For the example shown, the more tapered rig can be about 17% taller and reduce the induced drag by about 20% for the same heeling moment.

What we are saying is that if the heeling moment is kept fixed at HM = RM, an increase in effective span reduces induced drag. Right?

The lift force (not Cl) is constant for both rigs, so the added span is effectively a tip loss device added to the shorter rig.

Quick sketch ... Green is optimized area for short mast. Yellow is added area for greater span. Guesstimated "CE" does not change if yellow area is washed out to zero lift. Leads me to think that twisted off main would be better than reefed main?

RHough

08-27-2008, 03:16 AM

I do not see the need for a flame suit yet... I presume we are having a civil discussion here.

I do not think your last statement is correct BTW, all devices that generate lift in a fluid (be it liquid or gas, as in air) have an L/Dmax.

LOL ... Just trying to tell you that the one thing most of us agree on is that L/D max is the fastest way to sail.

You didn't understand the statement. I know that every lifting body has an L/D max. ;)

I too learned aero theory enough to design successful RC Sailplanes. Good thing ... my early design errors didn't kill me. :)

If you define L/Dmax as the point where Lift/Total Drag is highest, you can easily see that Di can be no greater than the sum of all other drag. Theoretical L/Dmax is when Di equals the sum of all other drag. Correct?

To reach L/Dmax you increase lift until Di equals Dp. Even at Lmax for the sails, the Di is lower than Dp for many boats. Thus the practical L/Dmax never reaches theoretical L/Dmax.

To help with the transition from aircraft to sailing vessels, I will be so bold as to suggest two books to you. Frank Bethwaite's "High Performance Sailing" (he also has designed RC sailplanes and speaks aircraft language) and C A Marchaj's "Sail Performance".

Bethwaite explains the limits of a boats ability to carry sail very well. I have a feeling that you are on the right track, but you have yet to grasp how limiting righting moment is. There is a very good reason that 4-5:1 is considered a high AR for sails. Above 6:1 is where aero theory works well, at extremely low AR like 1.5 - 2 (keels on monohulls) aero theory is not so great.

It is great to see someone else join the discussion with the firm conviction that starting with a sail plan that produces the required drive at it's L/D max is the way to go, I once thought that too. ;)

I think you will find that a sailboat is much like an airplane that is has such a high wing loading that it can barely fly, even at Clmax.

Cheers

R

tspeer

08-27-2008, 10:40 AM

What we are saying is that if the heeling moment is kept fixed at HM = RM, an increase in effective span reduces induced drag. Right?

The lift force (not Cl) is constant for both rigs, so the added span is effectively a tip loss device added to the shorter rig....

Yes, that's it. The optimum dihedral angle for a winglet is zero, unless the span is constrained for some reason.

The rest of the planform has area shifted farther down to make up for the moment due to the extension in span, so as to maintain the same height of the center of effort.

tspeer

08-27-2008, 10:52 AM

... Just trying to tell you that the one thing most of us agree on is that L/D max is the fastest way to sail....

That's what I thought, too, until I started running a simple landyacht VPP. That's when I found out L/D max is not necessarily the fastest way to trim the sails. Max lift turned out to be faster for that configuration, even though it was past the point of best L/D.

The reason turned out to be that speed depends on both the aerodynamic L/D and the hydrodynamic L/D, but the two are not independent of each other. The hydrodynamic lift has to match the aerodynamic load. So unless the sail rig provides a side force, the hydrodynamic L/D is zero.

What was happening was by trimming at maximum lift, I was loading up the chassis, so its L/D improved. The improvement in "hydrodynamic" L/D was more than the decrease in aerodynamic L/D, so the net performance improved.

[Note that tires on a landyacht operate very similarly to the hydrodynamics of a board. There's a linear range where side force is proportional to leeward drift (this is not skidding, but comes from the flexibility of the tires), followed by a nonlinear region where the side force does not increase appreciably and can decrease as the drift becomes large (this is skidding), similar to hydrodynamic stall. The sideways drift angles are not that much different than a boat's leeway angles, either]

RHough

08-27-2008, 02:25 PM

That's what I thought, too, until I started running a simple landyacht VPP. That's when I found out L/D max is not necessarily the fastest way to trim the sails. Max lift turned out to be faster for that configuration, even though it was past the point of best L/D.

The reason turned out to be that speed depends on both the aerodynamic L/D and the hydrodynamic L/D, but the two are not independent of each other. The hydrodynamic lift has to match the aerodynamic load. So unless the sail rig provides a side force, the hydrodynamic L/D is zero.

What was happening was by trimming at maximum lift, I was loading up the chassis, so its L/D improved. The improvement in "hydrodynamic" L/D was more than the decrease in aerodynamic L/D, so the net performance improved.

[Note that tires on a landyacht operate very similarly to the hydrodynamics of a board. There's a linear range where side force is proportional to leeward drift (this is not skidding, but comes from the flexibility of the tires), followed by a nonlinear region where the side force does not increase appreciably and can decrease as the drift becomes large (this is skidding), similar to hydrodynamic stall. The sideways drift angles are not that much different than a boat's leeway angles, either]

LOL ...

Did you ever get down to the point of finding out what the tyre slip angle was? And did you discover the fact that a small difference in tyre tread speed to ground speed is the point of maximum traction? :)

In one of my many past lives, road racing was a passion for me. Years ago the drag racing guys looked at the relationship between wheel slip and acceleration. Intuition says that a spinning tyre has less traction than a tyre that is not spinning in relation to the surface. What they discovered at the time is that a 12% slip produced the maximum traction. They then proceeded to adjust the clutch systems to try to maintain 12% overspeed for the entire 1/4 mile.

We found the same thing to be true road racing, the tyres are 'skidding' at maximum accelerations, the linear part of the traction curve includes a small amount of 'skid' or drift.

It sounds like the best net performance was at L/D max for the entire vehicle, although the sail was operating at the max lift the chassis could handle and at lift greater than L/D max for the sail alone.

Am I close?

tspeer

08-27-2008, 04:35 PM

...Did you ever get down to the point of finding out what the tyre slip angle was? And did you discover the fact that a small difference in tyre tread speed to ground speed is the point of maximum traction?...
Yes, in fact it was quite easy to measure. Landyachts are three-wheel vehicles, and even on a hard, dry lakebed, there are tracks left by their passage. If the slip angle is zero, the track of the front wheel will be equidistant from the tracks of the two rear wheels. But with slip, the front wheel's track is closer to the windward track than the leeward track. It's quite obvious that there's a significant slip angle.

The slip is due to flexibility of the tires. Imagine that you made a wheel by cutting the tread into small squares and mounted each square on a rod sticking out from the hub, so that the wheel was made up of a whole collection of individual feet. Now track the motion of one foot as the wheel rotates under a side load. As the foot comes around, the rod is undeflected. When the weight comes onto that foot, the rod deflects under the side load, allowing the hub to move in the direction of the applied load by the amount of the rod's deflection. When the foot comes free of the surface, the rod springs back. In this way the tire under side load walks its way sideways as each patch of tread is planted and the sidewalls flex. Each tread patch is firmly planted, with no skidding, yet the wheel has a velocity component normal to the plane of rotation.

The slip velocity due to traction or braking is due to the same flexibility. It's just that the equivalent of the rod is bending in the plane of the tire instead of at right angles to it. Of course, with a landyacht, there is no slip velocity in the plane of the wheel, because there's no torque being applied to the tire.
It sounds like the best net performance was at L/D max for the entire vehicle, although the sail was operating at the max lift the chassis could handle and at lift greater than L/D max for the sail alone.

Am I close?

Yes. Actually, the sail was operating at its maximum lift, and the chassis was still in its linear range. I had deliberately arranged the aerodynamic parasite drag so that max L/D occurred below stall. With more parasite drag, the L/D would have been increasing right up to the point where the drag started to increase due to separation and stall.

RHough

08-27-2008, 08:03 PM

Yes, in fact it was quite easy to measure. Landyachts are three-wheel vehicles, and even on a hard, dry lakebed, there are tracks left by their passage. If the slip angle is zero, the track of the front wheel will be equidistant from the tracks of the two rear wheels. But with slip, the front wheel's track is closer to the windward track than the leeward track. It's quite obvious that there's a significant slip angle.

Also obvious is that since the 'bow' of the yacht has one tyre and the 'transom' has two tyres the 'centre of lateral resistance' is 2/3 of LOA aft of the 'bow' at rest (assuming equal load on each tyre). At RM max, the load should be on the leeward tyres and the 'CLR' moves foward to the midpoint. Max L/D should see both leeward tyres loaded equally at their traction limit.

I think I captured the idea in the attached sketch.

The slip is due to flexibility of the tires. Imagine that you made a wheel by cutting the tread into small squares and mounted each square on a rod sticking out from the hub, so that the wheel was made up of a whole collection of individual feet.

The slip velocity due to traction or braking is due to the same flexibility. It's just that the equivalent of the rod is bending in the plane of the tire instead of at right angles to it. Of course, with a landyacht, there is no slip velocity in the plane of the wheel, because there's no torque being applied to the tire.

Here we disagree. What you describe are the normal forces that allow tyres to accelerate the vehicle they support. What has been found true is that some slip at the traction surface produces the greatest acceleration. The latest value is about 10%, a bit lower than the 12% the drag race guys found years ago.

Street traction and ABS systems attempt to keep this slip to 0, racing systems are set to 5-10% slip number. Yes, you can accelerate and turn and stop within the elastic 'slip' limits of the tyres. However the marks on the pavement that are left by heavy accelerations are evidence of the slip I refer to. Within the elastic limits, there is no transfer of tyre rubber to the surface. At the limit of traction there is.

More info Here (http://www.pdm-racing.com/products/racelogic.html)

http://www.pdm-racing.com/products/imag/slipgraph.gif

tspeer

08-28-2008, 01:27 AM

Also obvious is that since the 'bow' of the yacht has one tyre and the 'transom' has two tyres the 'centre of lateral resistance' is 2/3 of LOA aft of the 'bow' at rest (assuming equal load on each tyre). At RM max, the load should be on the leeward tyres and the 'CLR' moves foward to the midpoint. Max L/D should see both leeward tyres loaded equally at their traction limit.

I think I captured the idea in the attached sketch.

Well, we're drifting away from the topic of this thread, but perhaps it's not so obvious that the center of lateral resistance is nowhere near 2/3 LOA.

The c.g. of the yacht is much farther back, and so is the center of lateral resistance. In fact, when sailing, the vast majority of the weight is taken by the leeward wheel - ideally, all of it would be. This was the big advance in performance with the "Slingshot" configuration, in which the cockpit was located behind the axle, thus moving the c.g. back relative to the axle. The windward wheel is unloaded and just skimming the ground, just like flying a hull on a beach cat. The load on the front wheel is always a compromise between having enough to steer the yacht vs giving up some righting moment.

The side force per degree of sideslip is proportional to the vertical load applied to the wheel. This tends to make the chassis somewhat self-compensating. If the c.g. is moved forward, the forward wheel also picks up proportionately more of the sideforce, moving the CLR forward, too. This is why you don't have big changes in the handling qualities of a truck when you load it up in the back.

You have the relationship of the slip angle to the distance between the tracks correct.

Here we disagree. What you describe are the normal forces that allow tyres to accelerate the vehicle they support. ...

No, I'm talking about the side forces, not the acceleration forces. This would be equivalent to cornering with the transmission in neutral. It's what's needed to resist the side forces from the sail rig. A landyacht's tires are loaded as though the yacht is cornering even when it is traveling in a straight line. It operates on a different part of the friction circle than a dragster.

Say the curve you posted is equally valid for side force as it is for fore-aft traction. A 5% sideways slip velocity would be 3 degrees of sideslip angle. Not that different from the leeway angle of a decent keel.

The anti-skid systems I'm used to dealing with - those on aircraft - aim to operate at the peak of the curve. When the wheel starts to skid, it decelerates rapidly, leading to the large slip velocity you've shown. The anti-skid compares the deceleration of the wheel with the maximum deceleration that should be achievable on dry pavement. If the wheel's deceleration exceeds that amount, the anti-skid system then releases the brake pressure.

Early anti-skid systems dumped all the brake pressure, causing the operating point to traverse all the way to the origin, then re-applied the brakes. This led to a pulsing of the brakes that caused the operating point to run back and forth over the curve. More modern systems have a proportional valve that smoothly dumps brake pressure to get back to the front side, but don't go all the way to zero and provide better performance.

You wouldn't want to reduce the slip velocity to zero because that means zero braking - it would be worse than a skid. Instead, you want to operate at the peak of the curve, which is above the flat zone that corresponds to full skidding. That's the advantage of ABS over manual braking.

I suspect the tire starts leaving rubber on the surface when the curve starts to become nonlinear. That means some part of the contact patch is starting to slide, while other parts are still experiencing increased resistance in their local linear range. Once more of the patch is sliding than is increasing its resistance, then the curve has rounded the corner and started downhill.

But we're getting off of sail aerodynamics, so let's take any more discussion of tires to a new thread. I only broached the subject to point out the similarity between the landyacht VPP I was using and a watercraft VPP.

RHough

08-28-2008, 02:31 AM

Well, we're drifting away from the topic of this thread, but perhaps it's not so obvious that the center of lateral resistance is nowhere near 2/3 LOA.

You have the relationship of the slip angle to the distance between the tracks correct.

No, I'm talking about the side forces, not the acceleration forces. This would be equivalent to cornering with the transmission in neutral.

But we're getting off of sail aerodynamics, so let's take any more discussion of tires to a new thread. I only broached the subject to point out the similarity between the landyacht VPP I was using and a watercraft VPP.

:)

It was a good detour.

I'll stop after this comment.

In practice, it is not possible to turn a racing car if the front tyres are at the limit of traction under braking. The braking torque must be reduced so lateral force can be developed. The magnitude of the traction vector does not change, only the direction. It includes slip at the contact surface.

Back on topic ... Sail Aerodynamics ...

With a gap at the foot of the main, I have a clear picture of the spanwise lift distribution.

Since the fordeck of a modern tri looks like it lacks the ability to seal a genoa or jib at deck level, can I assume that the lift distribution is similar to that of any other sail with a gap at the foot? ie, the centre is about 40% of the span above the foot?

Can I safely use the total projected area and assume the AC is close to the 1/4 chord line of the sailplan at about 40% of span? That would surely make guestimates easier.

Paul Scott

08-28-2008, 09:22 AM

Tom, what happens to the airflow around a landsailor's wing when the craft experiences roughness on the ground of the racecourse?

Paul

Mikko Brummer

10-13-2008, 04:21 AM

Yes... it seems that the mast is not just a drag-device but contributes significantly to the driving force of the sails. In a recent CFD study on Star sails, the mainsail drive was 10,2 kgf, jib drive 6,7 kgf and the mast drive 1,2 kgf (that's in the positive, forward direction for the mast as well). This was in 6 m/s apparent wind and at AWA 27 deg.

The mainsail behind the mast cuts down the drag remarkably, compared to a bare mast. The Star mast is really slim and refined, though.

About the parasitic drag of the mast ... I was a bit surprised (dissapointed?) my insert did not raise any discussion.

I looked recently at the sails of a classic 6 meter, which has a rather large and blunt mast. In case of the 6 meter, CFD shows a 2,8 kgf drive for the mast, which amounts to 4,5% of the total drive of the sails (rig). Using the IMS (nowadays called ORCi) VPP parasitic drag coefficients, they would predict a negative drive in the order of -3,8 kgf... So the IMS VPP (like most commercial VPPs which are based on the iMS), would underpredict drive in the 6 meter case by some 6.6 kgf - that is 10,5% of the total (CFD predicted) drive. If my CFD is right, that is.

A similar error would be introduced in windtunnel measurements, where common practice is to measure the bare pole drag without sails and then substract it from the measured total drag, to get the sail drag coefficients.

It seems to me the mast should be considered as part of the mainsail profile, rather than a necessary drag device. There could be benefits in trying to shape the mast for max. drive in conjunction with the sail behind it, rather than try to minimize the drag of the bare mast, as seems to have been the practice earlier on.

tspeer

10-13-2008, 07:26 AM

...It seems to me the mast should be considered as part of the mainsail profile, rather than a necessary drag device. There could be benefits in trying to shape the mast for max. drive in conjunction with the sail behind it, rather than try to minimize the drag of the bare mast, as seems to have been the practice earlier on.

I agree completely. The leading edge suction on the mast is a significant contributor to the drive of the whole sail rig. And I'll take it a step farther.

With a rotating wingmast, it's possible to achieve fully attached flow on both sides of the mast, at least at high-ish Reynolds numbers. For such a mast, its drag is mostly due to skin friction and the pressure drag of boundary layer growth, without the drag of separation bubbles between the mast and mainsail.

One can also apply the same principles to the design of headfoils, although I've not yet tried to tackle that particular problem.

RHough

10-13-2008, 10:26 AM

About the parasitic drag of the mast ... I was a bit surprised (dissapointed?) my insert did not raise any discussion.
...

It seems to me the mast should be considered as part of the mainsail profile, rather than a necessary drag device. There could be benefits in trying to shape the mast for max. drive in conjunction with the sail behind it, rather than try to minimize the drag of the bare mast, as seems to have been the practice earlier on.

This is obvious, the only people that would debate it are proponents of rigs that claim to sail better because the sails are not set behind the mast. The aft-masts, the a-frames, and the Mainstay(tm) rigs come to mind. ;)

The rating rules for fast boats include the mast area as part of the total sail area for a reason. The mast helps drive the boat.

Mikko Brummer

10-14-2008, 02:19 AM

I agree completely. The leading edge suction on the mast is a significant contributor to the drive of the whole sail rig. And I'll take it a step farther.

With a rotating wingmast, it's possible to achieve fully attached flow on both sides of the mast, at least at high-ish Reynolds numbers. For such a mast, its drag is mostly due to skin friction and the pressure drag of boundary layer growth, without the drag of separation bubbles between the mast and mainsail.

Yes... I have done an analysis on the A-Cat with a wingmast. Maybe a bit to my surprise, the wing mast does not contribute here more than 5% of the total drive. It was downwind, and there is a huge wind shear then, 25 degrees from foot to top. While the sail part can be twisted, the mast cannot, so the mast is over rotated in the lower part when the upper part is OK.

One can also apply the same principles to the design of headfoils, although I've not yet tried to tackle that particular problem.

Rule makers have spotted this and penalize you by adding double the foil width to your sail area.

Mikko Brummer

10-14-2008, 02:22 AM

This is obvious, the only people that would debate it are proponents of rigs that claim to sail better because the sails are not set behind the mast. The aft-masts, the a-frames, and the Mainstay(tm) rigs come to mind. ;)

The rating rules for fast boats include the mast area as part of the total sail area for a reason. The mast helps drive the boat.

Well, like a said earlier, the IMS-rule does include the mast, but in the wrong way: they give you a better rating for a big mast, based on the old drag device thnking.

Paul Scott

10-14-2008, 01:04 PM

Here is an interesting piece that might have some contribution to make to Mikko's point, esp. as the re numbers are interesting:

" Effect of Regular Surface Perturbations on Flow Over an Airfoil"

Santhanakrishnan and Jacob

Dept. of Mechanical Engineering, U of Kentucky

I think if you Google it, you will get it. I got it off the net, but my printout doesn't have the address.

I know there are those here who don't find trips elegant, but if the mast was leading edge and trip of the rig et al, would it be more efficient? How close could it come to a wingmast in performance, given that twist and bend viz wingmasts are somewhat problematic? The Tasar and Finn masts come to mind.

Paul

Design_1

10-14-2008, 01:33 PM

I am not sure if this is on topic, but I need a little input. I am designing a
6.2m sailcat for myself and need help with mast location. Can anyone point me in the direction of the formulas used to locate the mast origin of placement?

Thanks,
Chris

GEDaggett

10-15-2008, 09:59 PM

So after reading most of this thread my brain hurts....badly. I know that I trim my sails try and pay attention to the wind and let God do the rest. I guess I should probably learn a bit more about it but this works fine for now.

Mikko Brummer

10-17-2008, 05:50 AM

Joakim

10-17-2008, 06:33 AM

I looked recently at the sails of a classic 6 meter, which has a rather large and blunt mast. In case of the 6 meter, CFD shows a 2,8 kgf drive for the mast, which amounts to 4,5% of the total drive of the sails (rig). Using the IMS (nowadays called ORCi) VPP parasitic drag coefficients, they would predict a negative drive in the order of -3,8 kgf... So the IMS VPP (like most commercial VPPs which are based on the iMS), would underpredict drive in the 6 meter case by some 6.6 kgf - that is 10,5% of the total (CFD predicted) drive. If my CFD is right, that is.

Could you be more specific on what you have actually calculated? I assume you have calculated the whole sail plan with mast using CFD (OpenFoam?, with full N-S? or a panel method?) and the calculated the drive of each component using pressure (and friction) integrals on surfaces. This tells the forces on the objects, but it does not tell the reason for these forces and you can not say from these if the mast is beneficial or not or would a bigger profile even be better. You have to compare to identical calculations with a bigger mast and no mast at all. Then you can calculate the contribution of the mast to total drive as a difference of total drive between these calculation.

The problem with rating rules is that they have to try to be able to deal with all the possible loop holes, sometimes at the price of accuracy. If the current model would be clearly wrong, I think there would have already been huge mast profiles. But I think there is no real difference between IMS/ORCi profiles, IRC and modern OD profiles. Instead there is a clear difference to the old IOR profiles which were very thin, which is probably due to not taking into account the mast profile in the rule.

Joakim

Erwan

10-17-2008, 08:06 AM

Hi Mikko,

Congratulations for CFD modelization an A-Cat sailing the "wild thing"

I would like to provide a more "rock-bottom" perspective about mast's twist issue.

A lond tile ago I used a carbon Italian mast, with a Greg Goodall sail on my A-Cat.

I made my batten myself with 5.5 mm pultruded carbon-rod.
In order to achieve a consistent sail section including the mast and a theorical 17° twist all along the mast, I took a pencil to design the mast section on the floor, and each batten with its twist.

Doing that I discover how well each of the batten fits perfectly the mast, providing a perfectly smooth "wing section" (leeward side) at any level, without mast's twist of course.

And the reason is straigtforward: As you go up, you have less chord (shorter battens) and twist.

At the lower levels, mast account for 6.5% of total chord, at the square top, it accounts for nearly 20% for moderate Square top.

At the same time the radius of the batten gets flatter, and fits perfectly the mast as the twist increase at the same time.

In the A-Cat case it seems to me difficult to consider separetly mast and sail. Instead I think Tom Speer XFOIL workpaper about wing mast support this view for a 2D analysis.

Regards

EK

schakel

10-17-2008, 08:20 AM

Hi,

To obtain the most effective sloteffect I had always wanted to apply a kind of Hoyt boom to a genua. When I sail Half wind the genua is bended towards the ship in such a way that in my intuition the airflow is bended to much and a lot of turbulences is leeward of the genua. Esspecially in the sheetcorner.

The hoyt boom is a self tacking system and is to short for a genua nor can it be swiveled up or down. It might be appicable on the long run for ocean racers. I have a feeling this might work.

Does anyone have experience with this?

Mikko Brummer

10-17-2008, 08:21 AM

Could you be more specific on what you have actually calculated? I assume you have calculated the whole sail plan with mast using CFD (OpenFoam?, with full N-S? or a panel method?) and the calculated the drive of each component using pressure (and friction) integrals on surfaces. This tells the forces on the objects, but it does not tell the reason for these forces and you can not say from these if the mast is beneficial or not or would a bigger profile even be better. You have to compare to identical calculations with a bigger mast and no mast at all. Then you can calculate the contribution of the mast to total drive as a difference of total drive between these calculation.Joakim

Yes, the whole sail plan including the hull and mast with spreaders, and, in case of the Star for instance, also the crew. Full N-S, and extracting forces on separate objects (like sails, hull, mast etc.). Yes, what I am suggesting here more work should be done with different mast sizes and shapes. For the Star, in a 2D-model, the sails completely without a mast were slightly better than those with the mast, but I have not tried a 3D mastless model, since that would not be very realistic.

The problem with rating rules is that they have to try to be able to deal with all the possible loop holes, sometimes at the price of accuracy. If the current model would be clearly wrong, I think there would have already been huge mast profiles. But I think there is no real difference between IMS/ORCi profiles, IRC and modern OD profiles. Instead there is a clear difference to the old IOR profiles which were very thin, which is probably due to not taking into account the mast profile in the rule.
Joakim

Hmm.. I believe that masts have grown in size under IMS, because the rule interprets the mast as a drag device and gives you a beneficial rating for a bigger mast. To me the current mast profiles are huge ;-).

The AC-masts have grown into the max. permitted size within the rule, so that would suggest a bigger mast is beneficial. The same happened in the Finn, when they permitted a (small) wing mast in mid 90's.

However, I still think that the small IOR profiles were best for all round performance. The mast also has a supportive job to do, and bending is beneficial for sail trim. Under IOR we learned that the ability to bend the mast a lot is a real bonus, and small profiles were needed for that.

The IMS has in practice forbidden running backstays. So the permanent backstay has taken the role of the runners, for keeping the forestay tight. This (and the IMS aeromodel) has led to the full-width, back-swept spreaders, the almost masthead-, large profile jib-rigs that we see today. This could be another reason for the big masts. The one-design classes have copied the IMS, because that's trendy and also because they have to buy the same profiles from the mastmakers.

Paul Scott

10-17-2008, 11:11 AM

Interesting report, yes. Sails certainly are often "bumpy". However, do I sence here a little bit of the "old school" thinking? My point was that the mast is not a trip wire, but rather part of the sail area. In case of the Finn, the mast contribution to drive is up to 8,5%.

Mikko, sorry about the cryptic nature of the following post, but my anti virus program decided to restart my computer just as I was finishing my more stylish initial missive and I lost it, I lost it, and had to re do it-

1-If twist and side bend in wing masts is generally problematic, and therefore upwash is discouraged in a wingmast rig through lack of rake and little, if any taper in planform (I'm not talking about a Finn mast here), and additional upwash through rake and taper is beneficial for an Una rig, then a more conventional mast may be more useful that a wingmast, esp. at re numbers under say 200,000.

2-Windsurfing rigs with smaller diameter masts are finding some problems with detached flow, vs bigger windsurfing masts, which are not (as much).

3- The mast is part of the sail area (you agree with Herreschoff, who reasoned this out in the 1920's, trying to explain why square masts worked well).

4- Most of the forward drive of a sail is generated around the leading edge

5- So what shape could a more conventional mast take, and still have fore and aft bend, easily allow twist, generate a lot of forward lift, somehow reattach turbulent flow back to the sail.

6- I found this report that looked at bumps and flow (and in the re of smaller sailboats, at least). In one case, one bump.

7- A mast can be thought of as a bump (?). But the report, if I understand it correctly, also seems to imply that a bump is, in some sense, a trip.

Seems to beg the question- what shape and size can the bump be (viz the cross section of a mast) and still allow easy twist of the sail, fore and aft mast bend, forward drive, and flow getting back to stay on the sail?

This is very very very very very old school. Back to the future, as it were. :P

Joakim

10-17-2008, 03:24 PM

The IMS has in practice forbidden running backstays. So the permanent backstay has taken the role of the runners, for keeping the forestay tight. This (and the IMS aeromodel) has led to the full-width, back-swept spreaders, the almost masthead-, large profile jib-rigs that we see today. This could be another reason for the big masts. The one-design classes have copied the IMS, because that's trendy and also because they have to buy the same profiles from the mastmakers.

I don't think you can blame IMS for that anymore, although it might have been the initial reason 15 years ago. There are other rules and lot of new OD classes, but still all new designs use bigger profiles and no running backstays. I guess that is what sailors want. Large jibs are more due to IRC than IMS.

Joakim

schakel

10-20-2008, 02:36 AM

Mikko, sorry about the cryptic nature of the following post, but my anti virus program decided to restart my computer just as I was finishing my more stylish initial missive and I lost it, I lost it, and had to re do it-

1-If twist and side bend in wing masts is generally problematic, and therefore upwash is discouraged in a wingmast rig through lack of rake and little, if any taper in planform (I'm not talking about a Finn mast here), and additional upwash through rake and taper is beneficial for an Una rig, then a more conventional mast may be more useful that a wingmast, esp. at re numbers under say 200,000.

2-Windsurfing rigs with smaller diameter masts are finding some problems with detached flow, vs bigger windsurfing masts, which are not (as much).

3- The mast is part of the sail area (you agree with Herreschoff, who reasoned this out in the 1920's, trying to explain why square masts worked well).

4- Most of the forward drive of a sail is generated around the leading edge

5- So what shape could a more conventional mast take, and still have fore and aft bend, easily allow twist, generate a lot of forward lift, somehow reattach turbulent flow back to the sail.

6- I found this report that looked at bumps and flow (and in the re of smaller sailboats, at least). In one case, one bump.

7- A mast can be thought of as a bump (?). But the report, if I understand it correctly, also seems to imply that a bump is, in some sense, a trip.

Seems to beg the question- what shape and size can the bump be (viz the cross section of a mast) and still allow easy twist of the sail, fore and aft mast bend, forward drive, and flow getting back to stay on the sail?

This is very very very very very old school. Back to the future, as it were. :P

Up wash as in where the wind leaves the sail not backwards but upwards is a loss in forwarding power. If you apply canting masts that tilt windward you gain forwarding power and up warding momentum although I have never seen them and I think they are forbidden in most classes except C-cat of course but there it is not beneficial.

Mikko Brummer

10-20-2008, 07:23 AM

I don't think you can blame IMS for that anymore, although it might have been the initial reason 15 years ago. There are other rules and lot of new OD classes, but still all new designs use bigger profiles and no running backstays. I guess that is what sailors want. Large jibs are more due to IRC than IMS.
Joakim

I must be getting old and cynical... but I also "blame IMS" for:

- the plumb stem

- vertical topsides with no flare

- the flat bottom canoe body, excessive mid section coefficient

- favoring too much rocker in the aft run

- a rig completely lacking automatic gust response

- a sailplan with insufficient power control possibilities

None of these features, I feel, have really taken yacht design forward... but are rather, in my opinion, the reason for the demise of offshore racing. With today's breed of boats, people only want to sail round the buoys, windward-leeward, never overnight in open waters.

Motion comfort & ability to cope with louzy weather is so bad in modern boats that people don't want to race them offshore - or how else do you explain the nearly 50% rate of retirement during the first 24 hours of the 2007 Fastnet race, for instance. The weather report was bad, yes, and so was the weather itself, but in the IOR times boats (people?) coped much better with this kind of weather. Fastnet is raced under IRC, but today's boats carry their inheritage to the IMS. IRC is more permissive towards seaworthiness, but things change slowly.

I'm sorry for the negative attitude in my reply, it is not meant to be, pls. prove me I'm wrong. I also apologise for wandering off of the subject, sail aerodynamics.

yipster

10-20-2008, 07:59 AM

Up wash as in where the wind leaves the sail not backwards but upwards is a loss in forwarding power. If you apply canting masts that tilt windward you gain forwarding power and up warding momentum although I have never seen them and I think they are forbidden in most classes except C-cat of course but there it is not beneficial.
i too am wondering about these flows, as a rig heels how much loss it makes, mast rake, and what if when battens or a top boom would twist more windward

schakel

10-20-2008, 09:28 AM

i too am wondering about these flows, as a rig heels how much loss it makes, mast rake, and what if when battens or a top boom would twist more windward

If you use an outrigger for a stay running from the top of the mast to the boom at the side of the ship you have what you want because it will bent the mast windward and you make the upwash effect even worse because you bend it even more towards the sky and therefore create more downward momentum in the mast. On th other hand it is just a small part of the upwash since most wind leaves at the backend of the sail.

I heard these outrigger boom were forbiden in open 60.

Paul Scott

10-20-2008, 10:13 AM

schakel

10-20-2008, 11:47 AM

Schakel- I was trying to remember a piece by Bogotaj on sails where he points out that sweep and taper effect the amount of twist needed for a sail. Which seems to imply that more powered up twist (not luffing) equals more forward power, as the top of the sail is pointed more downwind than the bottom. Can you quantify how much power loss upwash causes?
Yipster, FWIW, Stradivarious, the new AC Tri, cants it's mast all over the place. If you haven't seen it yet, get on Youtube and search BMW Oracle Castleswreath. My wife and I video'd this, and it seemed to me that in addition to using cant to steer, they seemed to be playing with flow direction.
Some have speculated that mast rake is part of what makes windsurfers go so fast. A lot of classes allow a lot of rake- Contender comes to mind. I think I saw somewhere that the more rake made the boat faster.

Anyway, rake plus taper seems to equal more twist. And according to Bogotaj, fractional rigs too.

Paul

EDIT- I found the Bogotaj piece- 'How do sails work?' article by Paul Bogotaj. My printout does not have the web address on it. I hope it can be found via Google.

Loss of the main sail when under heel, but this is just high school physics, is the loss of wind compared to what you have with no heel. So the loss = cos*(angle of heel). in percentages of forwarding power.

Loss by the angle of twist is university physics and it takes me al night to figure that one out. If I do you be overwhelmed by the differential equations and Integrals and I do think you are waiting for that.

Main effect is what aeronautical engineers call down-wash which means the mass of air that is displaced by which angle per second.

Do you have Aero hydrodynamics of sailing, by C.A. Marchai in your library. I have so I know where to find it. (The academic answer)
But I do not have it here. If you are really interested I'll give you his equations as well.

Paul B

10-20-2008, 11:57 AM

EDIT- I found the Bogotaj piece- 'How do sails work?' article by Paul Bogotaj. My printout does not have the web address on it. I hope it can be found via Google.

Bogotaj works for North Sails and his article is on their website:

http://www.northsailsod.com/articles/article6-1.html

schakel

10-21-2008, 04:31 AM

Paul Scott

10-21-2008, 09:45 AM

I never knew why fractional rigs were used on my boats (a schakel and a dragon) Neither why the three top races have fractional rigs. (VOR, AC and Vendee Globe) I found it in the article from Paul Bogotai.

Thanks

Also why my ULDB Cruising sled (designed 1998) has a 3/4 rig.

I'm wondering how the Musto rig compares to an A Class(ish) rig, given the taper and rake of the Musto, vs much less taper and very little rake for the cat. IC typical sloop rig too.

Paul

schakel

10-21-2008, 10:27 AM

I ask this before but nobody could answer.

Since you know something about it: How come the Hoyt boom was never used on fractional rigs. And then I mean a genua that goes halfway the jib of the main.

What is the big benefit. the torque of the genua is controlled in a way that easily follows or prepares the main for optimal half wind forwarding power.

Here is an nice picture of what I had in mind. But with an genua instead and a Hoyt boom that you can apply on long half wind legs.

The yard is K&M who are terrific. Designer is van der Stadt.
but Hoeke (Pilgim Project by Wally)
and Gerard Dijkstra (Maltese Falcon by Perinni Navi)

are also house designer.

http://www.kmy.nl/

schakel

10-22-2008, 06:55 AM

[QUOTE=schakel;233700]

Loss of the main sail when under heel, but this is just high school physics, is the loss of wind compared to what you have with no heel. So the loss = cos*(angle of heel). in percentages of forwarding power.
Loss by the angle of twist is university physics and it takes me all night to figure that one out. If I do you be overwhelmed by the differential equations and Integrals and I do think you are waiting for that
Main effect is what aeronautical engineers call down-wash which means the mass of air that is displaced by which angle per second.
Do you have Aero hydrodynamics of sailing, by C.A. Marchai in your library. I have so I know where to find it. (The academic answer)
Page 306 and 307 of Marchaj gives an impression of how the angle of up wash is related to the lift and drag ratio. The power loss of the up wash is related to the forwarding power (in aerodynamics these are similar to what they call Lift in wings) so The L/D ratio in the graphs give the overall gain. If L/D is greater than 1 the overall effect is positive.
In figure 2.61 page 306 the optimum of the sail is somewhere around 7 degrees to the apparent wind.
But in Fig 2.62 the influence of the cambered sail is shown as well as the influence of the velocity of the wind.
Even more interesting is the stunning effect of a wing sail which provides almost 2 times the forwarding power. I have a concept for a wing sail on the drawing board but it is confidential.

yipster

10-22-2008, 10:12 AM

"Loss of the main sail when under heel, but this is just high school physics, is the loss of wind compared to what you have with no heel. So the loss = cos*(angle of heel). in percentages of forwarding power."

trying to get that in the calculator but playing with a degree arc i see 5% surface loss at 20 deg heel, 25% surface loss at 45deg heel and about 75% loss at 75 deg heel, is that basicly the same idea?

i like to have some idea but these calcs must be rough figers as sailcup, 3d wash or a winglet play a role as well

schakel

10-24-2008, 10:45 AM

Mikko Brummer

11-14-2008, 07:34 AM

For those interested, we've posted a new article about sail aerodynamics at http://www.wb-sails.fi/news/Ad_aerodynamics

tspeer

11-14-2008, 09:47 AM

Fantastic article, Mikko!

I especially liked the jet that impacts the lee side of the spinnaker - I'd not expected that.

yipster

03-01-2009, 07:20 AM

Paul Scott

03-01-2009, 01:48 PM

And later, Marchaj displays the amazing superiority of a high AR Gunter, which has left me totally obsessed......

Paul

brian eiland

03-01-2009, 10:47 PM

For those interested, we've posted a new article about sail aerodynamics at http://www.wb-sails.fi/news/Ad_aerodynamics

Hello Mikko,
You might go back and edit your posted reference to the website. I believe it should be
http://www.wb-sails.fi/news/Ad_aerodynamics/index.htm

sigurd

03-02-2009, 12:33 AM

the first one worked too. great article.

brian eiland

06-09-2009, 10:34 AM

Traditional cargo boat being sailed in Martinique. These things can move! Thanks to anarchist Jon from Dockwise Yacht Transport.

...or maybe this is the utilmate square top main (and jib);)

john schroeder

06-09-2009, 07:43 PM

Guest625101138

06-09-2009, 07:59 PM

Brian , Leave it up to you to drop a new idea on us or a very old being used well. I like the movie Water world with the catamaran and the mast that extends up I always though a telescoping mast bowsprit and boom .

Waterworld is my favourite movie with my favourite movie star - the mariners trimaran. I loved the way it transformed into a speed machine.

schakel

06-10-2009, 04:53 AM

Waterworld is my favourite movie with my favourite movie star - the mariners trimaran. I loved the way it transformed into a speed machine.

Rick W

You are right about the tri in Waterworld it is awesome. It was build by Henri Jeaunneau for Beneteau. Here is his resume:
http://www.sailboatdata.com/view_BUILDER.asp?Builder_ID=48

Notice he also build some of the AC boats. Great!

But a telescopic mast is something I only saw in that movie. The speed the tri makes while he stands in the crow nest looks very, very cool. That most of the crew was under deck while filming is to be guest.

Milan

06-10-2009, 10:45 AM

… my favourite movie star - the mariners trimaran. I loved the way it transformed into a speed machine….

Yes, it does look very nice in the film, unfortunately, it can’t work in reality. They actually had two trimarans in the film, one in the sailing, other in the windmill mode.

http://www.geocities.com/mariner767/index.html

schakel

06-13-2009, 08:31 AM

To be very precise the yard was Jeaunneau and the designers were Marc VAN PETEGHEM and Vincent LAURIOT-PREVOST Thanks for the nice site about the mariners’ catamaran Milan

brian eiland

01-18-2010, 10:55 PM

Just saw this interesting comment submitted to Scuttlebutt by an aero associate of the North loft

* From Paul Bogataj:
Since the Deed of Gift provides the Challenger a "match for this Cup with a
yacht or vessel propelled by sails only and constructed in the country to
which the challenging Club belongs ...", isn't there another question before
one even gets to the issue regarding where the yacht was constructed? Is the
wing a sail? What defines a sail? Is it anything that pulls a boat through
the water using force extracted from the wind (like a kite)?

Why would there be different words for sail, wing, and kite? They may
perform similar aerodynamic functions, but they have different
characteristics that define them as different things. Perhaps a sail is
something that is flexible and is capable of being raised and lowered along
the mast. It seems that there is more potential legal arguing, unless they
do not intend to race with the wing.

Curmudgeon's Comment:
Here is the definition from Wikipedia: "A sail is any type of surface intended to generate thrust by being placed in a wind...in essence a vertically-oriented wing. Sails are used in sailing." And, of course, there is the precedent from the 1988 Match when Stars & Stripes (USA) successfully defended the 27th Match with a wing-powered catamaran.

CT 249

01-19-2010, 03:40 AM

If a sail is only something that can be raised and lowered along with mast, what do Lasers, windsurfers, Moths, and 12, 16 and 18 Foot Skiffs have?

RHough

02-04-2010, 05:53 PM

Doug Lord

02-04-2010, 05:59 PM

And why wouldn't a "ribbon jib" that has a very short chord and covers the entire luff of the main be fast?

sharpii2

02-04-2010, 11:51 PM

sharpii2

02-05-2010, 12:07 AM

http://en.wikipedia.org/wiki/Crab_claw_sail
re-reading Marchaj sail performance i'm impressed again with the crab claw
properly set it easy outperformes a bermuda or whatever other rig for that matter
the crab claw was only briefly discussed in this thread i see so i bring it up again

1.) How do you reef it?
2.) Wouldn't comparing it to a well vanged, low aspect ratio Bermuda cat rig be a more fair comparison.

It seems that the big advantage the Crab Claw has is that the boom and the yard are connected ahead of the mast, allowing next to no twist.

I wonder how well it would compare to a low aspect ratio, Boomed Lateen sail, which can come close to matching both advantages?

Paul B

02-05-2010, 12:45 AM

For a change in pace ...

Why are overlapping head sails faster?

Seems that the two sails on a sloop are considered as one aerodynamic unit with one circulation pattern.

Just how effective is the overlapping area of large Genoa when sailing upwind.

I get that on a reach the two sails are not a single element, but when trimmed for sailing upwind why (how?) does an overlapping Genoa increase drive?

I can tell you that about 30 years ago North Sails was doing a lot of development in the J24 class. They had designers on both coasts working on all sorts of shapes.

I was invited along on a testing session when the guys from back East came out to test shapes against the West coast designs. We traded boats, sails, trimmers, and drivers for a couple of days in a variety of conditions, two boat testing.

The first morning it was very light, maybe 5 knots, as we went out. We were sailing in perfectly flat water, and we wanted to put up a jib that had not been out of the bag at the dock. So we put it up to take a look at fit, even though it was too light. The other boat put up one of their genoas.

We came together and started straight lining. We were surprised how well we could hang in with only a jib up. Everyone was laughing, and some fun was being poked at the designer of the genoa on the other boat.

As soon as we hit the first of the lump we went out the back door pretty quickly.

That day makes me think that the difference has a lot to do with stall. The higher aspect headsail stalls more easily than the overlapping, lower aspect headsail. So the genoa is more forgiving, with a wider groove. In fact, in J24s we used to hang onto the genoa as long as possible if it was lumpy. If you changed down in big lump you would be blown out the back. Ugly overpowered worked better than flatter and feeling in control.

Also, for the same chord thickness the longer sail is deeper, so more powerful. It should generate more lift.

We have just changed from sailing a OD48 (genoas) to sailing a Farr 40 (jibs). The driver is telling me it is harder to keep in the groove.

One other thing to consider is the ACC boats kept getting more overlap with each generation. I think the location of the vent in the overall foil (both sails together) is critical. It needs to be in a location to help the flow stay attached where it wants to break away. I don't think that location is so far forward in the overall foil to make a non-overlapper make sense.

I'll bet a smart sail designer could come on here and make it vary simple to understand. I'll bet there are some nice photos of wind tunnel testing that shows the best location for the vent.

Paul B

02-05-2010, 12:51 AM

This is why, when sail area was heavily counted, the mast head rig came into prominence. It had the bigger jib. Later, the mast was moved aft, making the main a ghost of its former self, and making the jib an even greater portion of the sail plan.

When Sail Area became less heavily counted, it made more sense to go with a bigger main, because there are real limits on how large you can make the jib.

If you are talking about the end of the CCA rule/early IOR rule era this is not the reason.

The shift to larger headsails was due to rating advantage and the limitations of available sail material of the time.

CT 249

02-05-2010, 12:58 AM

Paul Scott

02-05-2010, 10:04 AM

yipster

02-05-2010, 10:47 AM

1.) How do you reef it?
2.) Wouldn't comparing it to a well vanged, low aspect ratio Bermuda cat rig be a more fair comparison.

It seems that the big advantage the Crab Claw has is that the boom and the yard are connected ahead of the mast, allowing next to no twist.

I wonder how well it would compare to a low aspect ratio, Boomed Lateen sail, which can come close to matching both advantages?
yes there are planty questions, still that were some serious results.
was reading the new multihull magazine on jib vs genoa on cats
bought and just received Marchaj revised edition optimising sail
so can compare with some 100 fotocopy whats reised, i'll be back
was thinking lateen too but Marchaj's tunnel tests were not so good on the latin
i understood it was the vortex lift of he crab claw that made it score so high
as with many things, the rite setup makes a big difference

RHough

02-05-2010, 10:59 AM

One thing that might help a genoa is pointed out by Gentry- If the leach of the Genoa is located at the point of maximum flow (airspeed) of the main, the Kutta condition at the leach of the jib is satisfied at a higher windspeed than free flow, for example, which means that the flow over the suction side of the genoa doesn't have to recover to as low a speed, and as a result is not stressed as much, which would tend to keep the reattached turbulent flow closer to the leeward surface the Genoa. So stall is harder to achieve.

The genoa is bigger in chord, so the flow has more time to reattach.

At lower speeds, the re of the genoa is larger, so it's easier to get into, or retain, a more reliable flow regime than a blade.

Hope I got those right.

Thanks! That makes some sense.

The longer chord of a Genoa means it takes longer to establish lift than a Jib.

In Classes where total sail area is measured the sail plans have tended to evolve into fractional rigs with non-overlapping jibs. It was the rules that did not rate overlapping sails at their full area that spawned the big Genoas.

I don't think the AR of the sail itself has much to do with it. The projected area remains the same for a any amount of overlap. So the only thing that changes is the location of the vent at the leech of the Jib or Genoa.

Paul B's comment about a Jib being about as fast as a Genoa during J24 testing agrees with my experience. At one time I tried a "rule beater" light #3 to get a rating credit and still have a very powerful light air sail. It worked well some days and not so well other days, sea state might have been the difference.

The reason I asked the question is because most sail makers just build #1's at LP = 150-155%. I suspect there is a "too much overlap" size depending not on the percentage of the Genoa that overlaps, but the percentage of the Main that is overlapped. The same "150" might be at 40% of main chord or 60% of main chord depending on the relation ship between "J" and "E" ...

I've never seen this relationship explained very well.

Thanks for the responses!

Randy

Paul B

02-05-2010, 01:26 PM

I don't think the AR of the sail itself has much to do with it. The projected area remains the same for a any amount of overlap. So the only thing that changes is the location of the vent at the leech of the Jib or Genoa.

Perhaps in steady-state conditions we have perfect interaction between the sails, but probably in lump the flow is interrupted and the sails have to act alone for periods until the overall flow is re-established. In this case the aspect ratio might be a factor.

The reason I asked the question is because most sail makers just build #1's at LP = 150-155%. I suspect there is a "too much overlap" size depending not on the percentage of the Genoa that overlaps, but the percentage of the Main that is overlapped. The same "150" might be at 40% of main chord or 60% of main chord depending on the relation ship between "J" and "E" ...

I think you are correct about the relationship of J to E. The other thing to keep in mind is for IRC many people are sailing with smaller genoas, what we might have called #2s in the old days. If the shroud base is sufficiently narrow to allow the same sheeting angle for a 140% sail as a 150% sail, and if this puts the vent in the optimal place, it might be as fast or faster than the 150% sail.

Paul Scott

02-05-2010, 08:35 PM

If a genoa alone works well in smooth water, but genoa and main better in bumps, do the two act as 2 individual sails in bumps/gusts, or does the system as a whole with a genoa and main have enough flow hysteresis to average out the bumps, and/or gusts.

I'm wondering if some of the small jibs that the 30 sq m class tried out that would have been called blades if they hadn't been overlapping to the point of the jib leach being located at the point of max camber of the main. The striking thing about these jibs was how little the forstay was separated from the mast. Wasn't there a thread here somewhere about what the minimum separation was necessary from forestay to mast for a jib to work in circulation?

Paul

Doug Lord

02-05-2010, 09:19 PM

On page 638 of the Aero-Hydrodynamics of Sailing Marchaj says:" The powerful effect on the character of the flow exerted by a relatively small auxilliary foil situated in proximity to the leading edge of a much bigger foil(roughly 20times the chord of the small foil-dl) is dramaticaly exposed in Photo 3.29(I will try to reproduce below). It will be seen that at an angle of incidence of about 25 degrees, the full separation and stall is already developed on the main foil alone, while with the help of a small auxilliary foil, the rear separation just begins at an incidence angle of 31 degrees. As, a matter of fact, such a configuration reflects in princible the tall-boy type of sail, a short-footed sail set between genoa and mainsail for beating in light winds, or set across the foredeck for running.
A range of positions of the auxilliary foil with respect to the main foil was tested to find out which one of them gave the most substantial gain in terms of maximum lift. It was found that the best efficiency was achieved when the auxilliary foil was located(in relation to its trailing edge) 15% of the main foil chord ahead of the leading edge of the main foil and 12 % above the main chord line, both chords being parallel to each other. In the case of the Clark Y main foil section shape the maximum lift coefficient was about 1.8, which is about 40% greater than for the foil alone."
-----------------
Below is a picture of the above illustration taken directly from the book and a couple of poor pictures of an experimental version of this rig on a model catamaran. Compared to a "normal" rig the results with this rig in light air were astonishing-simply holding the rig and moving it with and without the ribbon jib showed a greatly increased lift with the ribbon jib. I came close to trying this on my first foiler but chickened out though the rig is based loosely on the idea. I think the concept has enormous potential at least for experimentation. The downside besides a little complication at the top and bottom of the "jib" is that ,as best I can tell, the mast must be very straight.....

Paul Scott

02-06-2010, 10:56 AM

There was a piece in Seahose a n issue or so ago about some wind tunnel work on tall boys and jibs for downwind work that showed a real increase in performance, but my qusetions about the Marchaj photo that Doug uses have always been whether there's more lift developed as a system, or whether there's less drag because the flow is smoothed out (a small difference I know): is there less lift from the main, or more lift from the slat? Didn't one of the German aero firms mess with this on their wings before WW2?

BobBill

02-07-2010, 06:05 AM

Mssrs. Speer and Eiland, and one or two others, taught me something. (Old school on slot, I was), rest is mostly banal chatter. I noted they likely ceased adding (reading) to this thread, by and large. Still, amazing what one simple question can whip up.

It is good, nevertheless.

One question lingers, for me and I will ask here:

America's Cup BOR American Challenger runs wing and headsail, it seems thus far; so,

when the gun booms, will BOR pull out the stops and run wing only, or run with both?

If so, why so; if not, why not? Keep it simple.

Paul Scott

02-07-2010, 12:57 PM

A wing can generate lift at small flow angle, which is advantageous when that wing is the mainsail in a tandem foil or main/jib system. The jib can then develop lift in the usual way.

The question to me is the C E of the sailplan, and the l/d of the wing/jib in higher winds.

Edit- We saw BOR from about 30' away the 2nd day of testing, and she really looked like a sloop. I suppose it's possible that she could go wing only, but I kind of doubt it.

Paul B

02-07-2010, 04:23 PM

There was a piece in Seahose a n issue or so ago about some wind tunnel work on tall boys and jibs for downwind work that showed a real increase in performance,

It seems Marchaj was under the impression that the tall boy would help upwind in light air as well as downwind.

What we learned back in the 1970s was the use of a tall boy upwind was a losing proposition. I had to opportunity to sail with a couple of guys who still beieved this after everyone else figured it out. It was not fun to get slaughtered on the first leg of the day.

People like Marchaj did what they could with the tools available at the time. Marchaj also had some points of view that I think colored some of his results. I doubt you could go into a modern wind tunnel test and conclude a crab claw sail would be the best choice on any point of sail, in any condition, or conclude the use of a tall boy when beating in light air would be a benefit.

Of course this will never stop people who have no real-life experience from quoting these archaic texts as if the words had been etched on stone tablets by some diety.

RHough

02-07-2010, 10:52 PM

People like Marchaj did what they could with the tools available at the time. Marchaj also had some points of view that I think colored some of his results. I doubt you could go into a modern wind tunnel test and conclude a crab claw sail would be the best choice on any point of sail, in any condition, or conclude the use of a tall boy when beating in light air would be a benefit.

The crab claw is an interesting idea, based on the observations of the flow on delta wings that hold on to high CL values at extreme AoA. Trying to duplicate that on sail boats has been ... um ... problematic? :D

The two luffs acting as the leading edges of a delta wing on landing approach has not been duplicated anywhere but in the pages of books AFAIK. But not for lack of trying.

Perm Stress

02-26-2010, 02:31 PM

If the Marchaj test results are right, then everyone - Moth foiler sailor, America's Cup tri racer, round the world racer, windsurfer, speed record canting-rig proa sailor, all of them - is wrong. With due respect to Marchaj, maybe we should go with everyone else.

We have seen people try flip rigs, assymetric rigs, kites, captive kites, delta rigs, jib only rigs, cat rigs, rigs with endplates, side-by-side rigs in cats, boomless rigs, bi-pole mast rigs, lattice-mast rigs, gollywobbler-carting schooners, staysail-setting cutters, wishbone-flailing schooners, lunjstrom rigs, dyna rigs, aero rigs, lateen rigs, every other sort of rig imaginable - and time and time again, the bermudan sloop proves to be superior in many ways.

Considering the ingenuity that has gone into design of sailboats, it's a big call to say that the crab claw sail is the best, but that no one has ever been able to show it to perform better before a wide audience. It's an especially big call when boats and boards with crab-claw type sails have been tried and have failed to perform. I can recall a pic of one at a speed week in the late '80s, and boards with similar rig were tried (by De Vries, IIRC) at speed weeks. Performance was definitely lacking.

Interesting to see that according to the Proafiles site, "Other experimentors have been unable to duplicate Marchaj's test results".

One reason often used to support the prevalence of bermudan sloops and bermudan cats is (as the Proafile says) "the modern sport of yacht racing has developed around the triangular race course, a course that highly favors windward sailing efficiency." However, that doesn't apply to many ocean races (which is why we've had ketches win the Round the World race) or in speed records, or in many windsurfing events - so why would they have stuck to bermudan-type rigs if they were inferior?

The thing that makes me dubious about many rigs is that the claims are so over the top. For example, there's been some talk here about the test that 'showed' that a sprit rig gave something like a 30% increase in windward performance over a sloop.

If that was true, you could re-rig a Snipe with a sprit rig and it would go upwind with a 49er (which is around 30% quicker than a Snipe). Or you could re-rig a Hobie 16 with a sprit rig and it would be about as quick as a 25' wing-masted C Class cat upwind. Or you could re-rig a Catalina 22 with a sprit rig and it would beat a J/44 upwind and leave a Farrier F 27 trimaran astern at the rate of 30 seconds per mile. Or you could sort of chuck a spritsail on an original Windsurfer, and it would beat or scare the latest Formula Windsurfers (with twice the sail area and 1/3 the weight) every beat.

Call me a conservative, but somehow I can't see a simple re-rigging job turning a Hobie 14 into something that can wipe a 6' longer Olympic Tornado off the planet upwind. So it seems more logical to think that the 'problems' with the bermudan rig are greatly exaggerated, and that other rigs tests that are not replicated on the water are also incorrect.

Did not paid much attention to this post until recently I did recall one obvious and well known thing:

Depending entirely on crew skill and desire to squeeze every bit of speed, passage times of identical sailboats are different up to 30% (compare how you trim, steer, change sails in a race and when cruising just for pleasure with friends :) )

Than we could remember and put together few more facts:
1) Highly skilled sailors rarely experiment with exotic rigs (crab claw, junk, whatever... ) -they are busy enough with conventional and recognized boats. It is mostly the enthusiasts or true originals, ho care to bother with out of the mainstream boats.
2) In wind tunnel every model tested is, so to say, equally well designed and equally well "handled", so any possible pluses and minuses are generally same for all models and cancel each other
3) Current bermuda rig with all it's efficiency and adjust-ability is a result of about 100 years of continuous development and optimization, driven by desire to win races
4) The opinion of Americas' Cup winner is much more valued by any one, not deeply involved in making unortodox rig, as that of the autor

When we put together all the 4 observations, it pretty clear, that:
1) 30% increase in efficiency of any unortodox rig will be not enough to give it wide acceptance and recognition, because up 30% of it could be easily lost due to lack of skill, desire to push and accumulated knowledge of generations of racing sailors
2) To gain instant recognition it is necessary to have near to 50-80% gain in efficiency, with same simplicity and robustness in operation; it is highly unlikely for for all of this to happen in one time
3) Even if the superiority is clear, this would mean a necessity to rearrange all the well established sailboats industry (new style of masts, sails, hulls, design philosophy, etc...) -who will care to do it?

To sum up, up to 30 % gain could be realistic, provided equal skill in design, build and handling is available. at the moment, all these advantages are on the side of bermuda rig...

ancient kayaker

02-26-2010, 07:24 PM

Haven't read the entire thread so I may repeat something here, but there are sound aerodynamic reasons why tall high aspect rigs work so well that they are all alone in open racing. Exotic rigs like crab claw and junk may have some advantrages in certain conditions but do not win races. The junk is designed to make optimum use of scarce materials and minimize crew, and it does it very well. The closer a sail gets to a wing the better it gets until it IS a wing then the taller it gets the better it gets ... of course if the hull is not good enough to let the rig show its stuff it's all moot. For a heavy, clumsy hull the biggest rag that will fit on it is the way to go. And all other things being equal the best crew/skip will win most if not all of the time.

Doug Lord

02-26-2010, 07:35 PM

This is from post # 7 in the "Why so Stable?" thread under "multihulls" by Tom Speer. I thought it was extremely relevant to the discussion of sail aerodynamics,particularly, when you realize he is talking about THE WING. The comment highlighted in red is worth some serious thought!

"Really, the wing is not that much different from the soft sail rig. There's a rotating mast and a mainsail attached to that. For the wing, the mast is larger in chord and the mainsail is thicker. But the cross section topology is the same for both rigs - a teardrop shaped mast, a small gap, and a much thinner mainsail that is articulated to be cambered relative to the mast. The hinge points where the flaps attach to the main element/mast are just like batten cars that are fixed in position instead of being on a track. The flap ribs are just thicker battens. The covering is also a woven material, so USA 17 could be said to have a more traditional sail material than A5's molded 3DLs!

Despite its size, the main (forward) element of the wing can be considered to be a leading edge device for the mainsail, just as the mast is for the soft sail rig. Deflecting the flap one degree while holding the main element fixed relative to the apparent wind will produce more than 80% of the lift one would get by rotating the whole wing one degree while holding the flap fixed relative to the main element. Which also means that rotating the main element while holding the same orientation of the flap relative to the apparent wind will only change the lift by less than 20% compared to rotating the whole wing. So qualitatively, it's where the flap points that matters.

That's one reason why twist control on USA 17 is all done with the flap instead of by twisting the main element as was done with Cogito. It was a simpler and lighter way to go for a wing that size. This is another way that the wing is similar to the soft sail, where twist is all in the mainsail, too.

It may look different, but it really is a sloop."
__________________
Tom Speer

peterraymond

03-02-2010, 09:14 PM

I have only gotten to page 19 of this thread, but since wings are a current topic I thought I'd jump in. Apologies if I'm repeating recent content.

In message 104 of this thread Tom Speer has three charts dealing with the optimum plan form for a sail. You can always reduce induced drag by making the sail taller, but at some point there is too much heeling moment. Also, the classic elliptical plan form assumes no gap at the base of the sail. Using lifting line theory and with some other assumptions, Tom showed the plan forms that will give you the lowest induced drag for fixed heeling moment and he did this for different gaps at the bottomand different relative luff lengths. Even for the case of no gap, the elliptical plan form is beaten by a taller shape that's more tapered.

If everyone remembers post 104, this is nothing new. But, when I think about how you would design a C-Class wing, I'm not sure if I understand all of the assumptions, or the format isn't quite what seems easiest to me.

A C-Class obviously has fixed sail area and the righting moment you can generate is based on boat and crew weight and that's fixed too. In the design process what can you control? You can change the gap at the foot and you can change the hoist.

If you increase the gap there is more flow under the boom and therefore more induced drag. Also, you lose power at the foot, so you need to sheet in to get back to the same heeling moment.

As you make the sail taller, induced drag goes down while the center of effort goes up. For both reasons you need to sheet out to get back to the same heeling moment.

Tom's graph looks at performance at different gaps at the foot and different luff lengths. What you read off the chart is the induced drag and the height of the center of effort for each optimized solution. I think the relative induced drag on the horizontal axis is exactly what you want, but on the vertical axis, I think it would be nice to see the lift you generate for constant righting moment. It seems like this gets you one step closer to having the numbers you need to figure out boat speed.

However, this is really just the start of the process. In the real world you have restrictions on the wing shapes you can generate. If you twist the wing to minimize induced drag, how do you then twist it to account for different apparent wind angles with height? You can also design this lovely plan form, but if you have a three element wing like Cogito, don't you want the trailing edge of the second element (the flap) and the leading edge of the third element to be straight?

I see how you could have a whole team just designing and modeling the wing.

Schoonertack

03-02-2010, 09:49 PM

I don't think Marchaj's results are wrong, I think what we see with the Crab claw sail in particular is the effects of scale.for example why a Dragonfly flies. the air is relatively thicker and more viscous for small scale items than large. So things don't really scale up. Like sail area wetted surface ratios, good when comparing the same rig less so for comparing different rigs. I would like to say thanks to everyone for their inputs. Wind abhors a vacuum.:) :)

peterraymond

03-03-2010, 08:55 PM

I'm actually caught up now, but it's hard to go through the posts without looking up some of the terms, and as a result I came across some interesting sites.

The first is a paper on a model for sail performance. It's by Krebber-Hochkirch and extends the work of Hazen and Jackson. It looks like it could be used as the sail model in a VP program. Here it is: http://www.futureship.net/downloads/KrebberHochkirchHPYD06.pdf

Also, I wandered to a site out of AU that has a ton of information on foil and hull performance. Here is that: http://www.cyberiad.net/science.htm

I will admit that I haven't spent a ton of time at either site, but they both look good.

yipster

03-17-2011, 09:59 PM

coming back on the crabclaw aka deltawing i dont think Marchaj's measurements were wrong eighter Schoonertack and you got a point worth checking. perhaps the research was not complete enough in translating and sure the delta has a low lift aspect and drag in attack angle, but many scientific papers confirm the extra vortex lift measured was about rite, even in so called low speed deltas. its intriquing and frustrating for many as i now read up Bernard Slotboom who investigated the crab claw as low aspect camber sail and belives Marchaj made an error somewere witch can be also possible. Martin Hielkema who raced crab claws mentions in his blog somewhere Marchaj's tunnel tests were at 12knt (i think and will check again) witch i missed in Marchaj's book where not much on speeds was said, have to reread again but there too may be a scaling or so. many way's to solve the mystery but looking at deltawing planes they come in with a high angle of attack still faster as regular wings, i imagine that also the delta sail probably only works well under speed and slow sailing it powers only the low aspect, without enough vortexis developping. guess it also needs a very fast, maybe planing platform than to maybe perform like a "flying" proa. proof it, hm, allready took my sleeping pil and probably forget mentioning more points but wanted to share my thoughts, let me hear yours

edit: not beeing a real mathwizzard i did learn to second check my calculator to avoid making obvious mistakes, now only checking windspeeds i see Martin Hielkema' (http://harmenhielkema.blogspot.com/2008/04/takapu-proa-dissertation.html)s 12knt but on page 153 of Marchaj's sail airodynamics 15.7knt or 29.3ft/sec compared to 8m/sec hmm.. am i still sleeping or are we all? still beliving in vortex lift i'm checking for example a delta wing kite drag/lift at the i-net here http://www.grc.nasa.gov/WWW/K-12/airplane/kiteprog.html and reading up some more

Pericles

03-18-2011, 03:09 AM

Here is a sail that deserves wider recognition. The Vela Latina Canaria. It's tacked like a dipping lug.

http://www.youtube.com/watch?v=8TtcZjAuF4A

BobBill

03-18-2011, 07:17 AM

Here is a sail that deserves wider recognition. The Vela Latina Canaria. It's tacked like a dipping lug.

http://www.youtube.com/watch?v=8TtcZjAuF4A

Crazy fun, looks simple. Also on page 12...this thread.

Some of the material regarding wings I have read, mostly on another site,reminds me of chili, pizza and meatloaf recipes ... every one can be a chef ... not to denigrate those efforts and improvements.

However, I think there is definitely going to be a sail plan that will either replace wings or partially replace them and allow boat to be moored etc. With the more durable materials, seems near certainty. Alas, am too old to dabble.

That said, I may mess around with the "humpback" bumps on my flexible spar to see if my catboat pointing will benefit...

Good stuff, good enough.

BobBill

03-18-2011, 07:32 AM

The slot effect is a myth. Don't get confused by all the aerodynamic models people try explain to you - most of them are degree level or beyond. If you want to sail fast then get a fast sailor to show you how to trim sails and if you really want to learn about aerodynamics or CFD then talk to students/graduates/lecturers from a university that does aeronautical engineering. Lastly if you want great advice on sail set up, talk to a competitive sail loft like North or Hyde.

Huw

I think "lee bow" stuff is mostly conjecture, if not myth on a practical level.

The informed and thoughtful comments above notwithstanding, it may have been mentioned or implied, but when I learned to sail - when boats were on buoys in Montrose Harbor in Chicago - I learned one rule I still use today for sails, trim and all around generally:

"When it looks good, it is good; when it looks ugly, make it look good."

tspeer

03-19-2011, 03:03 PM

...Tom's graph looks at performance at different gaps at the foot and different luff lengths. What you read off the chart is the induced drag and the height of the center of effort for each optimized solution. I think the relative induced drag on the horizontal axis is exactly what you want, but on the vertical axis, I think it would be nice to see the lift you generate for constant righting moment. ....

That is basically what the vertical axis is. The height of the center of effort is the heeling moment divided by the lift. The lower it is, the more lift for the same heeling moment. Plotting the lift for a given heeling moment would go to infinity as the height of the center of effort went to zero. It's cleaner to avoid plotting something that has a singularity.

BobBill

03-23-2011, 06:50 AM

The info above on Humpback fin bumps was intriguing. I did some minor research on the matter. Basically, what I found was not much and that the bumps assist the Humpback's manoeuvrings and not speed. The whale's flukes are smooth as they are on most fish tails and other sea mammal flukes.

Now, seems the idea might have some application to rudder foils and rotating masts...including aircraft.

Or, am I simply out of my league, as usual?

daiquiri

03-23-2011, 07:19 AM

The info above on Humpback fin bumps was intriguing. I did some minor research on the matter. Basically, what I found was not much and that the bumps assist the Humpback's manoeuvrings and not speed.
...
Now, seems the idea might have some application to rudder foils and rotating masts...including aircraft.
Following your post I did a research on "Humpback fin bumps", imagining that some kind of revolution might have been going on in aerodynamics in these years. What I got was tons of pages with bombastic articles like this one: http://www.gizmag.com/bumpy-whale-fins-set-to-spark-a-revolution-in-aerodynamics/9020/

Now I am really starting to believe that industries might have a serious problem with communication.

Leading-edge vortex generators have been used on airplane wings for decades, with the same purpose and results as those claimed in the said articles. And are much cheaper than complex shapes like humpback fin bumps.

http://3.bp.blogspot.com/_wI-DdPSXymk/TUTVPHl8p8I/AAAAAAAABBg/9XJIloU8FrU/s1600/A4D+Vortex+Generators.jpg

BobBill

03-23-2011, 07:44 AM

Daiquiri, Great stuff. I missed that one. Can see all sorts of possibilities. Kind of stuff, besides sex and sailing fast cats, that makes one want to be younger...

In fact, I am making a new dinghy rudder foil (blade) and may add "turbercles" to see if they will increase efficiency...

May see these things showing up on wings and making soft sails nearly as efficient at wings, without having to remove for mooring. Whoa, Steve Clark...

Thanks.

Pericles

03-23-2011, 10:08 AM

Great minds think alike.

http://diy-solar-and-wind-energy.co.uk/wind-turbine-blade-whale-fin-ridges-or-cutting-edge/

Guillermo

03-23-2011, 10:15 AM

Humpbacks bumps have been discussed already quite thoroughly in these forums:

http://www.boatdesign.net/forums/search.php?searchid=2080327

Cheers.

BobBill

03-23-2011, 01:43 PM

Is dead, Guillermo.

Guillermo

03-23-2011, 04:55 PM

Ooops!
Well, just search the forums for "humpback bumps"
Cheers.

Pericles

03-23-2011, 05:27 PM

http://www.boatdesign.net/forums/sailboats/newport-27-rudder-2886-2.html#post31559 #27

tspeer

03-23-2011, 11:45 PM

...Leading-edge vortex generators have been used on airplane wings for decades, with the same purpose and results as those claimed in the said articles. And are much cheaper than complex shapes like humpback fin bumps.

http://3.bp.blogspot.com/_wI-DdPSXymk/TUTVPHl8p8I/AAAAAAAABBg/9XJIloU8FrU/s1600/A4D+Vortex+Generators.jpg

Ah, the "horns of ignorance" as Jimmy Doolittle III (test pilot & grandson of the same) calls vortex generators.

I have two reservations with regard to the papers on humpback tubercles. The first is the wind tunnel tests were done at low-ish Reynolds numbers, and the benefits may not be the same at high Reynolds numbers. Britt Chance could probably identify with that.

The second is the tubercles may not have a hydrodynamic function at all! The researchers assume that because the turbercles are on the fin leading edges, they must be hydrodynamic. In the photos I've seen many of them are encrusted with barnacles, which are hardly clean hydrodynamic configuration. What if humpbacks evolved tubercles because other humpbacks found them sexy?

Leo Lazauskas

03-24-2011, 12:09 AM

Here's a reference to a very recent contribution from the Uni. of Adelaide researchers.

"Performance Variations of Leading-Edge Tubercles for Distinct Airfoil Profiles"
Kristy L. Hansen, Richard M. Kelso, and Bassam B. Dally,
AIAA JOURNAL, Vol. 49, No. 1, January 2011

ABSTRACT
An experimental investigation has been undertaken to determine the influence of sinusoidal leading-edge
protrusions on the performance of two NACA airfoils with different aerodynamic characteristics. Force
measurements on full-span airfoils with various combinations of tubercle amplitude and wavelength reveal that
when compared to the unmodified equivalent, tubercles are more beneficial for the NACA 65-021 airfoil than the
NACA 0021 airfoil. It was also found that for both airfoil profiles, reducing the tubercle amplitude leads to a higher
maximum lift coefficient and larger stall angle. In the poststall regime, however, the performance with largeramplitude
tubercles is more favorable. Reducing the wavelength leads to improvements in all aspects of lift
performance, including maximum lift coefficient, stall angle, and poststall characteristics. Nevertheless, there is a
certain point at which further reduction in wavelength has a negative impact on performance. The results also
suggest that tubercles act in a manner similar to conventional vortex generators.

BobBill

03-24-2011, 05:51 AM

Ah, the "horns of ignorance" as Jimmy Doolittle III (test pilot & grandson of the same) calls vortex generators.

I have two reservations with regard to the papers on humpback tubercles. The first is the wind tunnel tests were done at low-ish Reynolds numbers, and the benefits may not be the same at high Reynolds numbers. Britt Chance could probably identify with that.

The second is the tubercles may not have a hydrodynamic function at all! The researchers assume that because the turbercles are on the fin leading edges, they must be hydrodynamic. In the photos I've seen many of them are encrusted with barnacles, which are hardly clean hydrodynamic configuration. What if humpbacks evolved tubercles because other humpbacks found them sexy?

"Hey mate, she's got nice tubercles."...sounds plausible.

Still, it seems the tubercles have to offer a pragmatic benefit, as someone suggested, to become part of the evolutionary trend, at least in the sense the bumps do not detract from survival, the mating/propagation issue notwithstanding.

And, from the literature, looks to be the case.

Big boobs may be in now, but seem to have little to do with our survival, and most gals I know, prefer the lighter variety.

daiquiri

03-24-2011, 01:25 PM

Ah, the "horns of ignorance" as Jimmy Doolittle III (test pilot & grandson of the same) calls vortex generators.
Fortunately, the ignorance is a built-in feature of the mankind. It tempts us with an infinite possibility of discoveries.
Fortunately, the mankind cannot resist temptations. :)

The term "Horns of ignorance" reminds me of the so-called "Coefficient of Safety" or "Factor of Safety" (FoS).
This coefficient is also called by some (and I can't agree more) "Coefficient of Ignorance", because it is directly proportional to:
- our ignorance of variables which might screw up our most carefully performed calculations.
- our ignorance of the underlying physics.
- our general ignorance (and this one is big).
:)

I have two reservations with regard to the papers on humpback tubercles.
(...)
The second is the tubercles may not have a hydrodynamic function at all! The researchers assume that because the turbercles are on the fin leading edges, they must be hydrodynamic. (...) What if humpbacks evolved tubercles because other humpbacks found them sexy?
Could be, but I vote for hydrodynamic reasons. I have read that they have a very particular feeding technique (http://www.enchantedlearning.com/subjects/whales/species/Humpbackwhale.shtml) which requires a good low-speed maneuverability.

And besides that, how can anyone find pimples sexy? :P

BobBill

03-24-2011, 02:39 PM

I guess there is only one fun way to find out, we gotta grab the humps...by the horns :]

My dinghy is small, so it is also slow...I might just see some improved slow action...

Boston

03-24-2011, 10:54 PM

been reading along with this one for a while now, T has a good point about those tubercles being just some odd ball form of sexual dimorphism, happens all the time and just about every species has an example or two. Speech development is thought by some to be a form of phylogeny as well.

The barnacle whale relationship is an interesting aside from the conversation but does have some pertinent aspects. For instance most barnacle species are specific to a particular species of whale but more on topic is the fact that the larvae, once they find a whale, walk around on the whale till they find the preferred spot. then dig in. So if there tubercles are a common resting place for these barnacles then they might be a form of bio-symbiotic hosting not previously recognized. From what I can remember the relationship between barnacle and whale is considered obligate comunalism or something like that, which is about one step short of parasitism but the host neither gains nor looses anything in the deal. Point being that if it does in some way benefit the whale to host the barnacles then there's another reasonable explanation for the tubercles that doesn't involve hydrodynamics.

lots of various reasons for physical structures that don't have much to do with efficiency in the animal world.

good point T
cheers
B

BobBill

03-25-2011, 03:26 AM

The tubercles could be simply a means to attract, hug or hold on to a mate and control krill and/or maneuver...but I have to defer to those with more research and science.

Peeping Toms need to get to work.

CT 249

03-27-2011, 03:26 AM

Did not paid much attention to this post until recently I did recall one obvious and well known thing:

Depending entirely on crew skill and desire to squeeze every bit of speed, passage times of identical sailboats are different up to 30% (compare how you trim, steer, change sails in a race and when cruising just for pleasure with friends :) )

Than we could remember and put together few more facts:
1) Highly skilled sailors rarely experiment with exotic rigs (crab claw, junk, whatever... ) -they are busy enough with conventional and recognized boats. It is mostly the enthusiasts or true originals, ho care to bother with out of the mainstream boats.
2) In wind tunnel every model tested is, so to say, equally well designed and equally well "handled", so any possible pluses and minuses are generally same for all models and cancel each other
3) Current bermuda rig with all it's efficiency and adjust-ability is a result of about 100 years of continuous development and optimization, driven by desire to win races
4) The opinion of Americas' Cup winner is much more valued by any one, not deeply involved in making unortodox rig, as that of the autor

When we put together all the 4 observations, it pretty clear, that:
1) 30% increase in efficiency of any unortodox rig will be not enough to give it wide acceptance and recognition, because up 30% of it could be easily lost due to lack of skill, desire to push and accumulated knowledge of generations of racing sailors
2) To gain instant recognition it is necessary to have near to 50-80% gain in efficiency, with same simplicity and robustness in operation; it is highly unlikely for for all of this to happen in one time
3) Even if the superiority is clear, this would mean a necessity to rearrange all the well established sailboats industry (new style of masts, sails, hulls, design philosophy, etc...) -who will care to do it?

To sum up, up to 30 % gain could be realistic, provided equal skill in design, build and handling is available. at the moment, all these advantages are on the side of bermuda rig...

I hope it's okay, but since the revival of this thread lead me to note Perm Stress' post I thought I'd reply.

PS, I understand the point but I think there's a fair bit of evidence to say that a 30% increase in basic performance potential will lead to instantly noticeable results.

A few examples;

The modern assymetric spinnaker has nothing like a 30% speed increase (all round) over a symmetrical kite (I think it was around 3% from memory IF the spinnaker size was increased), but from the very first time the assy was used it was extremely competitive in both 18s and 14s.

The first Windsurfers were a vast step into unknown territory, but there's some evidence that they were very quickly sailing to about 90% of their performance potential.

The early trapezes, which were quite difficult to use, lead to an instant noticeable performance increase although they only add something in the region of around 2% to overall speed.

The first solid wing sails and wing masts would surely have required at least as much re-thinking as a crab-claw sail would require, yet their performance potential (which was slow to prove itself) lead to many experiments from "mainstream" sailors.

Many early multis proved their performance potential very quickly despite their novelty.

So while conventional rigs are better optimised, with respect, if there is any rig that can create a 30% improvement it has never turned up in races, and much smaller improvements are picked up avidly by racing sailors. The advent of the totally novel form of sailing (and rig) that was windsurfing is surely proof of the fact that good sailors can get to grips with very novel rigs very, very quickly.

Finally, there are claims that pacific sailors actually switched to sprit rigs after seeing Europeans sailing them, which would indicate that the crab claw may not have been superior.

Pericles

03-27-2011, 03:33 AM

CT 249

Ilan replied to you at #23

http://www.boatdesign.net/forums/multihulls/wing-sails-36306-2.html#post451440

P

brian eiland

08-28-2011, 08:32 AM

There are some interesting observations over here on triangular, vs fat-head, vs square-top mainsails.

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/aspect-ratio-headsail-vs-shroud-angle-39379-4.html#post483474

brian eiland

09-21-2011, 10:22 AM

I must be getting old and cynical... but I also "blame IMS" for:

- the plumb stem

- vertical topsides with no flare

- the flat bottom canoe body, excessive mid section coefficient

- favoring too much rocker in the aft run

- a rig completely lacking automatic gust response

- a sailplan with insufficient power control possibilities

None of these features, I feel, have really taken yacht design forward... but are rather, in my opinion, the reason for the demise of offshore racing. With today's breed of boats, people only want to sail round the buoys, windward-leeward, never overnight in open waters.

Motion comfort & ability to cope with louzy weather is so bad in modern boats that people don't want to race them offshore - or how else do you explain the nearly 50% rate of retirement during the first 24 hours of the 2007 Fastnet race, for instance. The weather report was bad, yes, and so was the weather itself, but in the IOR times boats (people?) coped much better with this kind of weather. Fastnet is raced under IRC, but today's boats carry their inheritage to the IMS. IRC is more permissive towards seaworthiness, but things change slowly....
I was just going back thru this subject thread and ran across this series of postings #370 to #374

I've often expressed this same siniment about handicap racing rules designing our vessels rather than mother ocean.

I want to revisit these discussions when I have a bit more time.

daiquiri

09-21-2011, 10:54 AM

I've often expressed this same siniment about handicap racing rules designing our vessels rather than mother ocean.
It's a fact and has been been so ever since boat races have been organized. See the link in this old post of mine: http://www.boatdesign.net/forums/boat-design/buttock-angle-37434.html#post453579

And when it wasn't handicap rules, then the hull shape (of both sail or motor boats) was historically often determined by the need to (pick your choices): maximize cargo or fish storage volume, allow access to shoal waters, make the boat beachable or groundable during low tides, ease the lifting of fishing nets, increase maneuverability, increase the initial stability, make the construction easier and cheaper, keep the CoG low etc. etc. Many things in the list besides (or before?) hydrodynamics and seakeeping.

Today we often tend to see boats as objects designed for leisure, relax, fun, speed, competition, exposition etc. But throughout centuries boats have been, above all, working tools. The concept of yachting has appeared no more than 300 years ago, so the work-related issues were historically much more important than powering, comfort or aesthetics.

Resuming, all this is to say that hullforms have always been dominated by different types of constraints, many of which have nothing to do with sailing or hydrodynamic qualities in a strict sense.

Cheers

P.S.
Looks like the above-mentioned link no more contains the article about history o handicap rules. That's a pitty, it was a nice reading. :(

blisspacket

10-30-2011, 09:15 AM

Not to corrupt a thread, but this is close enough: whatisit with the new Boeing 787 and those pointy wingtips. I've just gotten my two sailboats into the flathead main config, to minimize tip vortex stuff, etc etc, and now Boeing does this to me. OK, so we're only sub-sub sonic: I'll assume the practice is still worth pursuing: flathead mains.

Leo Lazauskas

10-31-2011, 03:36 AM

Not to corrupt a thread, but this is close enough: whatisit with the new Boeing 787 and those pointy wingtips.

Raked wingtips are found on a variety of fast birds and fish, e.g. swallows and tuna. (Their wing shapes are usually known as "lunate" or crescent-shaped).
The rake increases the effective aspect ratio, as do endplates and winglets, but without increasing the bending moment as much at the wing/fuselage intersection.

Leo.

blisspacket

10-31-2011, 06:43 AM

Clever Leo, thank you. Anytime we can draw parallels from nature, there's pretty solid backing to the design.

markdrela

10-31-2011, 09:52 AM

Raked wingtips are found on a variety of fast birds and fish, e.g. swallows and tuna. (Their wing shapes are usually known as "lunate" or crescent-shaped). The rake increases the effective aspect ratio, as do endplates and winglets, but without increasing the bending moment as much at the wing/fuselage intersection.
It's not so simple.
First of all, raking the tip on a swept wing while keeping the spanwise projected loading the same increases the magnitude of the root bending moment. The reason is that this moves the raked portion farther from the wing root. Note: To calculate the structural moments at any location (e.g. the wing root) due to a load at some distance, what counts is the load's distance projected onto the spanwise spar axis at the bending-moment location. This spar axis is swept on a swept wing, and includes a streamwise component. The lateral distance as seen from the front is only part of it.

But what really counts on a transport airplane is the bending moments at the critical max-G load case, which is set by regulations. The rake adds static washout twist into the outer wing under excess G's. This washout in turn moves the load distribution inward and thus momentarily reduces bending moments over the inner wing where most of the structural mass is. the passive washout effect allows the wing to be made lighter.

Clearly, using a raked tip on a wingsail because "the 787 has it so it must be good" is a bit naive. Any benefits are way more complicated than that.
The closest wingsail analogue might be a tip which passively unloads the upper sail when it's hit by a gust. In strong gusty conditions this would allow you to load up the sail more for a given capsize risk.

PS.
On tuna tails and swallow wings, the crescent tip provides some twist over the span during a power stroke, which is necessary to keep the local Cl's from getting too excessive near the tip. A similar effect happens on raptor wings, but here the individual tip feathers twist.

Mikko Brummer

10-31-2011, 10:16 AM

It's not so simple.
The closest wingsail analogue might be a tip which passively unloads the upper sail when it's hit by a gust. In strong gusty conditions this would allow you to load up the sail more for a given capsize risk.

On sailboats, this has long been achieved by bendy topmasts on a fractional rig.

Leo Lazauskas

10-31-2011, 10:40 AM

It's not so simple...
PS.
On tuna tails and swallow wings, the crescent tip provides some twist over the span during a power stroke, which is necessary to keep the local Cl's from getting too excessive near the tip. A similar effect happens on raptor wings, but here the individual tip feathers twist.

Thanks for the clarification, Mark.
I had a cursory look into the aerodynamics a long time ago when van Dam and his colleagues made some rather strong claims concerning (rigid) planar lunate wings. IIRC, the claims weren't supported by experiments and their claim of "optimality" was never very convincing. I'm not sure if Ilan Kroo made any progress with them after that.

Do you know if there are any animals that have endplate-like structures (i.e. ones extending above and below) the ends of the wings or other lifting surfaces?

All the best,
Leo.

Doug Lord

10-31-2011, 11:00 AM

This paper is interesting: http://www-scf.usc.edu/~tchklovs/Proposal.htm

Leo Lazauskas

10-31-2011, 02:45 PM

This paper is interesting: http://www-scf.usc.edu/~tchklovs/Proposal.htm

There are also several references to curved planforms and wing tip studies in the biblio at:
http://www.cyberiad.net/wingbib.htm

brian eiland

11-23-2011, 04:54 PM

What do you suppose Chris White has in mind with the aerodynamics of this new rig??

http://www.chriswhitedesigns.com/atlantic_cats/mastfoil/index.shtml

blisspacket

11-23-2011, 05:29 PM

It's too early to tell, for me. a roller furl jib and a roller furl mizzen staysail, but there's not enough detail to understand the subtleties.

sharpii2

11-23-2011, 08:50 PM

Looks like a staysail ketch with a balanced aft staysail and a mizzen on the aft mast.

I suppose he's looking for lower Center of Area (CA), over a more traditional cutter or mast aft rig.

The balanced aft staysail is interesting. It doesn't need quite as much back stay tension to stand well (I know. I had one on my first boat as the jib) It also has its luff slightly to the weather of the jib.

Another potential advantage is the mast for the large forward jib can have a better back stay angle than a mast aft rig and possibly have a tighter fore stay and stand better.

This rig eliminates one of the worst problems of the mast aft rig: there is no place to hang a sail aft the mast and the mast has to be somewhat forward of the transom to get any kind of effective back stay angle.

Oops. Looks like the aft stay sail is not balanced but just club footed.

brian eiland

11-24-2011, 09:43 AM

Looks like a staysail ketch with a balanced aft staysail and a mizzen on the aft mast.
I don't see a mizzen on the aft mast,...but rather the MastFoil itself might be the mizzen substitute?

I suppose he's looking for lower Center of Area (CA), over a more traditional cutter or mast aft rig.
Precisely, he is reverted back to the ketch concept to 'spread' the sail area out along the lenght of the vessel rather than ever-taller sloop rigs.

The balanced aft staysail is interesting. It doesn't need quite as much back stay tension to stand well (I know. I had one on my first boat as the jib) It also has its luff slightly to the weather of the jib.
True

Another potential advantage is the mast for the large forward jib can have a better back stay angle than a mast aft rig and possibly have a tighter fore stay and stand better.
True

This rig eliminates one of the worst problems of the mast aft rig: there is no place to hang a sail aft the mast and the mast has to be somewhat forward of the transom to get any kind of effective back stay angle.
Don't know that I understand what you are saying here?

Oops. Looks like the aft stay sail is not balanced but just club footed.
Added my comments in between the quotes above.
Brian

brian eiland

11-24-2011, 09:44 AM

Back to Chris White's new rig, the subject of this lastest posting. Did all of you take notice that his 'mastfoils' were two element foils themselves??
"Why does the foil have an articulating trailing edge flap?
Because a flap adds lots of power to the foil with very little additional weight and complication. Reaching and running the flap is set at a significant angle (approx 40 degrees) to the main foil which increases the overall camber of the foil and can nearly double its power. Sailing upwind only a small amount of flap angle is used but it helps create additional lift with very little drag"

This is an important and distinguishing feature of his design. This imparts a much greater contribution to their effectiveness, both alone, and in support of that headsail located in front of each MastFoil.

This unique design deserves more analysis before being tossed to the trash pile by the 'traditionalist'.

blisspacket

11-24-2011, 12:36 PM

The mastfoils not having spreaders, even on a beamy cat, limit the upper mastfoil area. It'd be nice to see the details up there. And the mastfoil doesn't rotate fully, so I'm puzzled how it can feather, if for example, boat is tied dockside and wind is off a stern quarter or fully astern. And I purely can't fathom that Chris White penned the optimistic lines that would have the cat sailing sideways off the dock with an on-dock breeze, by setting each mastfoil to lift to windward.

sharpii2

11-24-2011, 02:26 PM

Added my comments in between the quotes above.
Brian

In my last comment, I was trying to say that the mast on a mast aft rig must be some distance from the very aft end of the boat for the back stays to have enough leverage to resist the head stay tension, even if the back stay only goes halfway up the mast.

And, if it does go only halfway up the mast, an upper back stay is required, along with a spreader, to hold the top of the mast back.

Once you have all that rigging aft the mast, you can not hang any kind of a sail there. So, to get adequate sail area you have to go higher.

I am not knocking the aft mast rig here. It is one of Phil Bolger's favorite rigs, especially for ocean going boats. All I'm saying is Mr. White's rig allows a mizzen sail hung from the mizzen mast, where the mast aft rig doesn't.

I might add here that stay sail schooners have been around for a long time. This is the first stay sail ketch I have ever heard of. Having the shorter mast aft allows better back stay geometry for both masts.

As for the wing masts, I have no idea what he is trying to do. Perhaps he is trying to cut down mast drag. I think they are a bad idea on a boat that can flip. Just my opinion.

Anything that is intricate and complex is bound to fail in the worst conditions. If it fails to feather in a storm, the boat and crew are in great jeopardy. I would go with a round mast section and a tear drop section mast sock. It's not as good as a real airfoil section, but it's a lot better than a plain round or oval mast section. And it can be pulled down, if it misbehaves.

Paul Scott

11-26-2011, 11:36 AM

Can the masts become assymetrical foils that might develop lift at 0 or negative aoa?

http://www.worldofkrauss.com/foils/784

I'll see if I can find a more specific example, but generally...

Paul

P Flados

11-26-2011, 07:09 PM

Can the masts become asymmetrical foils that might develop lift at 0 or negative aoa?

http://www.worldofkrauss.com/foils/784

I'll see if I can find a more specific example, but generally...

Paul

Some foils do provide lift at negative AOA.

This is not really anything special. Each profile has an angle of zero lift. More AOA gives positive lift. Less AOA gives negative lift.

Your example was an apparent attempt to use a highly asymmetric foil to approximate a symmetrical foil designed to operate with a flap extended well off of centerline.

This points out a fundamental gap where the most available technology on wings does not fit our desired use.

Most airplanes use flaps only to generate extra lift at takeoff / landing where they can tolerate much higher drag. They then reposition the flap to "restore" the design profile - a lower lift but high efficiency profile for cruising.

For boating, wing flaps are being used to address additional needs. One major need is a just the power and efficiency of a cambered wing that can be reversed during tacking. The other is adjustable camber to provide for the different needs of upwind vs downwind sailing. For upwind we want efficiency (max lift vs drag). For downwind, it is more of a need for max lift (similar to an aircraft at take off). The other major consideration is twist. This is where we are dealing with different apparent wind angles from near the surface up to the top.

Given that the vast majority of wing studies and wing publications are for airplane applications, it is not surprising that sailors do not find what we really need.

For the Chris White boat, his web page is a bit confusing but it would seem he is using of a inner (probably round) structural element (mast / spar) with an exterior that provides a wing with flap. The round spar as a main structural element has been used quite a bit and allows for easy construction of a very strong element.

I like the basic concept where you want to skip the traditional main and use large roller furling Jib/Genoa as your soft sail surfaces and wing sheathed mast(s) to provide small but efficient rear driving surface(s). However, I am not sure I like his choices for and/or placement of the components. Then again it is easy to sit back and dream stuff up. He is to be commended for pushing to actually get the concept on the water.

daiquiri

11-27-2011, 07:07 AM

Most airplanes use flaps only to generate extra lift at takeoff / landing where they can tolerate much higher drag.
Just for general info - one of the most important functions of airplane flaps is to allow a steeper (up to 10°-12° more than a clean wing) and slower approach path during landing. That is made possible because of the combined effect of high lift and much lower L/D ratio of the wing with fully extended flaps.
Cheers

brian eiland

11-27-2011, 08:07 AM

In my last comment, I was trying to say that the mast on a mast aft rig must be some distance from the very aft end of the boat for the back stays to have enough leverage to resist the head stay tension, even if the back stay only goes halfway up the mast.

And, if it does go only halfway up the mast, an upper back stay is required, along with a spreader, to hold the top of the mast back.

Once you have all that rigging aft the mast, you can not hang any kind of a sail there. So, to get adequate sail area you have to go higher.
On my aft-mast design I was able to put a mizzen sail on one of my primary backstays (the one that goes down to the base upon which the mast is stepped). That was better in my opinion that having that mizzen sail blanketed by a mast itself.

I am not knocking the aft mast rig here. It is one of Phil Bolger's favorite rigs, especially for ocean going boats. All I'm saying is Mr. White's rig allows a mizzen sail hung from the mizzen mast, where the mast aft rig doesn't.
I don't think Chris has any intentions of placing a mizzen sail on that aft mast of his.

I might add here that stay sail schooners have been around for a long time. This is the first stay sail ketch I have ever heard of. Having the shorter mast aft allows better back stay geometry for both masts.
One of my original inspirations for the aft-mast design of mine was an experiment carried out on a Morgan Out Island ketch, where the traditional mainsail was removed and a staysail (might be termed a mizzen staysail) substituted in that area
http://www.runningtideyachts.com/sail/
"I once had a copy of a test on a Morgan 41' Out Island ketch , where upon removing the mainsail, the boat lost only 1/2 knot of speed, but cut its leeway in half (from 11 to 6 degrees). A staysail was then rigged between the masts in place of the mainsail, and the boat regained 1 knot of speed while retaining its decreased leeway."

I think there are a number of other examples here:
http://www.boatdesign.net/forums/multihulls/main-less-rig-21274.html

As for the wing masts, I have no idea what he is trying to do. Perhaps he is trying to cut down mast drag. I think they are a bad idea on a boat that can flip. Just my opinion.
He is looking for more efficiency from a lower aspect sail plane, along with total feathering of those mast shapes without 'reefing' them.

Anything that is intricate and complex is bound to fail in the worst conditions. If it fails to feather in a storm, the boat and crew are in great jeopardy. I would go with a round mast section and a tear drop section mast sock. It's not as good as a real airfoil section, but it's a lot better than a plain round or oval mast section. And it can be pulled down, if it misbehaves.
I believe he is utilizing round section mast shapes hidden inside the foil shapes, and those outer foils are free to rotate 360 degrees so they are not venerable to worst conditions.
For the Chris White boat, his web page is a bit confusing but it would seem he is using of a inner (probably round) structural element (mast / spar) with an exterior that provides a wing with flap. The round spar as a main structural element has been used quite a bit and allows for easy construction of a very strong element.

Paul Scott

11-27-2011, 10:36 PM

My post about -aoa giving lift was intended to beg the question about coming away from a dock with an on dock breeze.

Also, would a -aoa + lift mast section develop enough circulation to be of interest to a jib set in front of it? Especially if there was less drag than a mast with mainsail?

Paul

tspeer

11-28-2011, 12:30 AM

...And I purely can't fathom that Chris White penned the optimistic lines that would have the cat sailing sideways off the dock with an on-dock breeze, by setting each mastfoil to lift to windward.

You're not the only one.

The wings can be useful for springing off the dock in a crosswind, producing forward or aft thrust to push against a spring line to rotate the boat away from the dock. However, in this case the force pushing the boat into the wind comes from it pressing against the dock. But there's no way the wings can pull the boat into the wind without any other outside interaction.

tspeer

11-28-2011, 12:46 AM

...Also, would a -aoa + lift mast section develop enough circulation to be of interest to a jib set in front of it? Especially if there was less drag than a mast with mainsail?

I'm not sure what you mean by "-aoa + lift mast section", but the influence of the wing on the jib will be similar to a mainsail producing the same spanwise lift distribution. The main difference is the lift on the wing is produced further forward and may make the jib act as though it has more overlap than it actually does.

There's a widespread misconception that jibs are more effective per square meter than mainsails because they are staysails and not mast-mounted. This leads to the conclusion that one should remove the mainsail and concentrate the area in the jib.

Instead, it's the case that the jib benefits from favorable interference from the mainsail, and the mainsail sacrifices lift because of interference from the jib. But the combination of the two is better than either one alone. If you remove the mainsail, the superiority of the jib disappears. If you remove the jib, the mainsail looks better. This is why development classes that only limit sail area and make no distinction as to how the area is used (A & C class catamarans, for example) evolve to cat rigs, not mast-aft rigs.

Paul Scott

11-28-2011, 08:27 AM

Tom, I meant this by -aoa +lift (just as an example, first one I found, no editing, I'm sure there are better ones, don't know if it would generate enough lift to get off a dock when pinned):

http://www.worldofkrauss.com/foils/784

Sorry for confusion. Low battery. Race against current, as it were.

Paul

brian eiland

04-03-2012, 01:26 PM

...this could upset the apple cart in understanding aerodynamics of flight...or at least show there are some 'unexplained phenomena' with dealing with these subjects

Cheaper, quieter, and fuel-efficient biplanes could put supersonic travel on the horizon, according to MIT engineers.
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By Jennifer Chu, MIT

For 27 years, the Concorde provided its passengers with a rare luxury: time saved. For a pricey fare, the sleek supersonic jet ferried its ticketholders from New York to Paris in a mere three-and-a-half hours – just enough time for a nap and an aperitif. Over the years, expensive tickets, high fuel costs, limited seating and noise disruption from the jet’s sonic boom slowed interest and ticket sales. On Nov. 26, 2003, the Concorde – and commercial supersonic travel – retired from service.

Since then, a number of groups have been working on designs for the next generation of supersonic jets. Now an MIT researcher has come up with a concept that may solve many of the problems that grounded the Concorde. Qiqi Wang, an assistant professor of aeronautics and astronautics, says the solution, in principle, is simple: Instead of flying with one wing to a side, why not two?

Wang and his colleagues Rui Hu, a postdoc in the Department of Aeronautics and Astronautics, and Antony Jameson, a professor of engineering at Stanford University, have shown through a computer model that a modified biplane can, in fact, produce significantly less drag than a conventional single-wing aircraft at supersonic cruise speeds. The group will publish their results in the Journal of Aircraft.

This decreased drag, according to Wang, means the plane would require less fuel to fly. It also means the plane would produce less of a sonic boom.

“The sonic boom is really the shock waves created by the supersonic airplanes, propagated to the ground,” Wang says. “It’s like hearing gunfire. It’s so annoying that supersonic jets were not allowed to fly over land.”

Double the wings, double the fun
With Wang’s design, a jet with two wings – one positioned above the other – would cancel out the shock waves produced from either wing alone. Wang credits German engineer Adolf Busemann for the original concept. In the 1950s, Busemann came up with a biplane design that essentially eliminates shock waves at supersonic speeds.

Normally, as a conventional jet nears the speed of sound, air starts to compress at the front and back of the jet. As the plane reaches and surpasses the speed of sound, or Mach 1, the sudden increase in air pressure creates two huge shock waves that radiate out at both ends of the plane, producing a sonic boom.

Through calculations, Busemann found that a biplane design could essentially do away with shock waves. Each wing of the design, when seen from the side, is shaped like a flattened triangle, with the top and bottom wings pointing toward each other. The configuration, according to his calculations, cancels out shock waves produced by each wing alone.

However, the design lacks lift: The two wings create a very narrow channel through which only a limited amount of air can flow. When transitioning to supersonic speeds, the channel, Wang says, could essentially “choke,” creating incredible drag. While the design could work beautifully at supersonic speeds, it can’t overcome the drag to reach those speeds.

Giving lift to a grounded theory
To address the drag issue, Wang, Hu, and Jameson designed a computer model to simulate the performance of Busemann’s biplane at various speeds. At a given speed, the model determined the optimal wing shape to minimize drag. The researchers then aggregated the results from a dozen different speeds and 700 wing configurations to come up with an optimal shape for each wing.

They found that smoothing out the inner surface of each wing slightly created a wider channel through which air could flow. The researchers also found that by bumping out the top edge of the higher wing, and the bottom edge of the lower wing, the conceptual plane was able to fly at supersonic speeds, with half the drag of conventional supersonic jets such as the Concorde. Wang says this kind of performance could potentially cut the amount of fuel required to fly the plane by more than half.

“If you think about it, when you take off, not only do you have to carry the passengers, but also the fuel, and if you can reduce the fuel burn, you can reduce how much fuel you need to carry, which in turn reduces the size of the structure you need to carry the fuel,” Wang says. “It’s kind of a chain reaction.”

The team’s next step is to design a three-dimensional model to account for other factors affecting flight. While the MIT researchers are looking for a single optimal design for supersonic flight, Wang points out that a group in Japan has made progress in designing a Busemann-like biplane with moving parts: The wings would essentially change shape in mid-flight to attain supersonic speeds.

“Now people are having more ideas on how to improve [Busemann’s] design,” Wang says. “This may lead to a dramatic improvement, and there may be a boom in the field in the coming years.”

“There are many challenges in designing realistic supersonic aircraft, such as high drag, efficient engines and low sonic-boom signature,” says Karthik Duraisamy, assistant professor of aeronautics and astronautics at Stanford University, who was not involved in the research. “Dr. Wang’s paper presents an important first step towards reducing drag, and there is also potential to address structural issues.”

Published March 2012