Rotating Wing Mast – theoretical discussion

In the Spirit of Cogito

Thank you Peter for your comment. My English is too poor to be able to put that so clearly in words. But as you have paved the road, I feel more confident to go out of the wood.

In the title, I make a reference to René Descartes and his famous "Cogito Ergo Sum" when revisited by Steve Clark should be read "Cogito Ergo Twist"

In addition to this famous addage Descartes also used to say "When you have a big problem to solve, try to slip it into smaller problems". That is exactly the spirit of your recommendations Peter. I am investigating an alternative rig solution I would like to disclose.

Basically, you blend the Cogito twist concept for the frame: (telegraphic main mast lower part + tapered upper part + 1st element structural trailing edge equivalent + twist lever and houd + boombox), with a cambered inducer-like system.

Then considering the performance of a well designed 2D mast+sail arrangement as proposed by Tom Speer at many occasions., I think there is an intermediate way to be explored between soft sail and 3 elet wingsail:The single element wingsail.

The only ancillary difference with the single sail/teardrop mast concept as proposed by Tom Speer is that you need a bit more thickness everywhere.
1-At the leading edge to store your telegraphic tube (around 5%).
2-At the 50% 60 % point chord where the second vertical spar is located (around 3%).

As far as I know it is just a question of parameter in XFOIL.

You wrap the structure with a 25% to 40% cord mast leading edge made of composite skins, with little morphing possibilities including:
1-Twist
2-Asymetrisation of the leading edge
3-Morphing lips around the sail luff/mast connection (in order to control the bubble ramp). It is not mandatory the sail slot on the mast is only 10mm wide thick if we have a double side thick soft sail at the luff.

The "1st elt trailing edge equivalent" will be basically a vertical spar located somewhere aroud 50 to 60% from the leading edge, inside the double-side sail.

This spar could be use to control an inflexion shape on the leeward side (ala LIEBECK) as designed by Tom Speer on some recent Tear drop mast revisitation (I cannot remenber which thread) for the high L/D and high lift sailing range.

But it will hold the horizontal structure (aft part of the cantilever-batten/camber-inducers)

The sail
would be a double side soft sail with double batten arranged together in order to provide inertia and trailing edge shape control for aft loading possibility (max lift sailing range)

The 2 trailing edges of the 2 sides of the sail would have to skip on each other, to adjust to the new tack-shape, so 2 vertical "batten" trailing edges seem necessary.

My cogitations are pending at this point. And because the structure is the same than those used for a 3 elets wingsail, and could be therefore very easily recycled, I think it worth the experience (for A-Cat or a Moth).

Thanks for readings

Regards to everybody

Erwan

 

Hello Erwan, Is this what you are decribing? http://www.omerwingsail.com I'll add something as well, I'm not an aerodynamicist so may get this wrong but. Solutions lie in the soft sail area (like Omer Sails, double sided soft sails which are generally banned in racing), the rigid sail area (C Class cats) and combinations which have been done in the past. RIGID (target Cl=2.5 to 3.0+) if rigid then we require a slot to create camber. But technically if we are aiming at a multifoil arrangment the elements need to overlap to provide the max lift. If the foils overlap they can't change sides easily due to mechanical difficulties. So they usually are very close so they can swing side to side. This is not aero optimum. The flap is used to close the gap so it behaves like a single element (this is the bit I may have wrong) So using two or three panel rigid wings have compromises. Plus they have many mechanical hurdles to jump. SOFT sails like Omers are single element designs having a Cl target of 2.0 -2.5 and have not really been explored enough. HYBRIDS rigid forward section and soft aft section have been explored but again not enough. So there is much scope to move forward in sail design in general. Plus most sailors are looking for more lift (power) which is good but they should also be looking to reduce DRAG. Any design that reduces drag is good even tidying up the deck is a good thing. This is rarely discussed but needs to be understood better. The drag on a moth for instance is of the same order as its lift so if you halved the drag you would go much faster! Peter

 

Another thing I'd like to mention is that I watched the USA17 and Alinghi races for every second and I was amazed at the hugh camber in the USA17 wing. We will never be able to camber membranes (soft sails) like this so ultimately rigid solutions for racing are the go. The acceleration and ease at which USA17 beat Alingi is the big signpost for racing sails. In an interview with James Spithall at the C Class champion ships (I think from memoery) he stated that once the hull was up they feathered the top 1 or 2 panels to depower. It was then a balancing act to depower more than power up going through the course, its rare a racing sailboat has to depower more than power up!. Peter

 

A while back, I had mentally worked out a mechanism that would generate a reversible cambered wing. It would have have a rigid partial ellipse (or similar shape) for the leading edge beam/spar, a series of hinged internal ribs, control lines to pivot the ribs (similar to C Cat technology) and a "sock" mylar film covering.

Later when reviewing forums discussing C Class wings, I noted that they have figured out that the slot between elements is a big part of the very high lift performance that is so important for downwind runs.

Now just imagine combining both of the above. The attached file combines 2 elements that are each very close to the Clark Y in profile.

Even though C Class wings can generate high lift, I am certain that use of reversible & variable cambered airfoils for a 2 element wing could probably enhance the performance. The front element of the C Class wings have a flap to try to get close to a cambered performance. However flaps just are not as good as a wing section with the right amount of camber for the desired conditions.

The effort for the above is not anything I would ever anticipate being able to justify for any project I would be involved with. However, if a big dollar AC effort really wanted to try for an edge, this would be an interesting thing to explore.

 

Don't designers, sailors and sailmakers talk drag reduction all the time? For example, the Moth designers have a very, very strong idea of the importance of drag reduction, which is partly why the boats and sails are designed the way they are - ask Thorpey about the time he spent assessing freeboard in reality and on MBD's computer. AMAC is a world-class designer of windsurfing speedsails, which is an area where low drag is vital.

Similarly, even back in 1972/73 IOR designers were doing things like hiding spinnaker poles under the streamlined blister decks to reduce windage (see Ydra and Frigate). Couldn't it be that they completely understood the idea, but that it proved to make no significant difference and it was faster to put the poles back on deck? The 55 footer Quailo III from the UK Admiral's Cup team didn't go to a three=spreader mast because they were concerned about the "excess" windage of the third spreader. The Lexcen boats of the time were flush decked partly to reduce drag. All those concepts were dropped and replaced by higher-drag options that worked better; arguably, drag was not under-rated but over-rated. Tidy decks can be slow.

As a side note, coming from boards (which have few, if any, of the rule-based restrictions you mention) you can also see that those restrictions can also lead to a design that is more user-friendly in many ways. There have always been classes that don't have restrictive rules, but the free choice of sailors normally makes them less popular. The reason we see so many boats and events to restrictive rules is surely because those restrictions make a boat or event that appeals to more sailors.

Is depowering that rare? In boards, cats and skiffs there's a tremendous amount of depowering going on. The top Taipan sailors, for example, are probably effectively depowering from about 8 knots of wind, at a guess.

BTW do you know what's happening with the Omer sail? About three years ago we were told that the test Elan 37 was going to start trialling against a stock sistership to prove the superiority of the Omer sail. No reports of those tests seem to be available.

 

I don't agree on drag. For conventional boats like the IOR designs drag reduction is not overrated at all. You have to look at the total drag and see what parts of the whole are the largest contributors. You must also consider that total avail able power is limited by RM. You can't "add lift" past a certain point, the boat tips over. For this limited power the only way to go faster is to reduce drag.

Once "hull speed" is reached the dominate drag producer is wave formation drag. Second is probably induced drag from the rig having to operate at relatively large CL and short rig height to accommodate the RM limit. The keel has to lift equal to the lateral force of the rig so there is induced drag there also. The windage of deck gear is a very small percentage of the total. The difference a "tidy deck" makes might be very hard to measure. It can be measured on multi's that sail faster relative to true wind speed. Shuttleworth claims that once the rig and foils are right, the next thing to focus on is reducing drag of the hulls and deck. This is "free" VMG upwind.

The taller rigs and deeper keels we see on modern keel boats attack one of the major drag producers (induced drag) directly ... simply increasing the span pays off in reduced drag.

I do agree that high performance boats are looking to depower much of the time. This means flatter sails and twisting off the top of the rig. This changes the spanwise lift distribution and has the effect of reducing induced drag. In effect the twisted off portion of the sail acts much like a winglet. As long as it is not flogging, the twisted off main is faster upwind than reefing.

R

 

Actually, high CL increases the profile drag, but doesn't increase the induced drag if the height of the rig is kept the same.

If there's no limit to sail area, the optimum area will be the size that results in the lift coefficient for the best 2D L/D.

The height of the rig then becomes primarily a tradeoff between induced drag and heeling moment. A taller rig has less drag, but the lift has to be restricted to match the heeling moment to the righting moment. The keel comes into it, too, because more lift also loads up the keel and affects the L/D of the hull+keel.

 

I'm struggling with this concept (not news, I know).

Say my section has best L/D at CL = 1.1 and + 6.5 deg AoA (+10 deg from zero lift)
The available RM is 5000 pound / feet
How do I get to optimum area from there?

Don't I have to know what the sailing angle, boat speed, and wind speed are to set the area?

R

 

Maybe I should have been more specific in the part of the post I was replying to.

The points I was trying to make were (1) as we both agree, designers spend a LOT of time working out how to reduce induced drag, wetted surface drag, etc. The reduction of such drag is not "rarely discussed", as was the claim. It was the claim that designers "rarely discussed" drag that I was addressing.

(2) Rather than being "rarely discussed", drag has been an obsession of designers (even of conventional boats) for eons. I was using the windage of deck gear as an example of that obsession. However, practical experience indicates that (as you said) the windage of deck gear isn't that important in "conventional" boats.

Experience in small cats and boards indicates that extra RM from a taller crew is probably more important than the lower drag of a small crew, I s'pose.

BTW, high performance craft will often reduce sail before the gear is flogging, for instance by changing down to smaller rigs in boards and Skiffs. The bigger rigs do feel to have more drag. I know that if the span is longer the induced drag must drop. However, if the upper section of the rig isn't creating lift because it is dead flat and twisted, isn't it therefore irrelevant when calculating the span? You can't just increase span by adding an outboard section that is creating no lift, can you? Wouldn't that go against the ideal of elliptical planform loading.

The windage, in boards anyway (it's been too long since I sailed a Skiff under the small rig for me to recall) certainly feels dramatically higher with a flattened rig rig, even when the head is completely flat and the leach has negative tension.

The practical experience is that the twisted-off section isn't worth it, as demonstrated by the fact that board sails for high-wind sailing are lower in aspect than they could be, given structural and RM constraints. Handling issues also come into play, of course - the R Class guys had even higher-aspect rigs in the past but they were so high aspect that they were too twitchy, so they altered on of their very few rules and increased SA and lowered the rigs for superior speed and handling. Handling is, it seems, one of those things largely ignored by many theoreticians, but vital to high-speed sailing in many conditions.

 

Hi folks,
I don't get the time I would like on this forum at the moment, so I apologise if this has been brought up before (and if it is a stupid idea!).
Is there any merit to having the top of the sail (or top element of a wing) set on the other tack to the lower parts. I know this would have horrific drag penalties, but it would potentially produce a zero heeling moment sail, so the sail could be as big as New York. Never mind efficiency, sometimes there is no substitute for size!

 

Tom went through it some time ago in a thread with Wardii, I think, and IIRC showed that the drag increases higher than the increase in RM.

 

Think its more complicated than that: cue Mr Speer. But I think that a dead flat twisted off section is by no means irrelevant to induced drag. Consider: induced drag is all about the tip generated vortices, and having a big lump of twisted off sail is sure going to affect them and the effective span. I'm not enough of a mathemetician to make sense of the formulae, but as I understand it in the sums for induced drag calculation you have both the span and an efficiency rating for the planform...

I think I'm right in saying too that as you reef a sail - or at least a substantially rectangular sail, then because the chord doesn't change the actual induced drag remains much the same as the area reduces. So if you increase the span and the area increases with it then the induced drag doesn't drop. Interestingly for the cruising folk, now I think of it, that means that in mast reefing should be way superior to in boom reefing in terms of induced drag.

So if you stump a rig down without changing the chord then I reckon:-

induced drag stays much the same if the planform stays much the same
form drag reduces
total lift reduces
drag as a percentage of total lift increases

However if you just blade off the top of the rig - thus changing the planform
induced drag increases (worse planform, but maybe not too much)
form drag reduces a little
total lift reduces
drag as a percentage of total lift increases

But, and its a soddin' great big but, we have to figure in the dynamic behaviour of the rig and wind changes. The bladed out rig unblades out, even to a good extent automatically if the wind drops, and the total lift increases a great deal.

So what I'm guessing is that unless the wind minimums are such that the reefed rig is going to be fully powered up/overpowered, the bladed rig is going to be faster. Or, as Uffa Fox said 80 years ago or something, in a racing boat you should reef for the lulls...

 

I don't think we are at odds here.

TSpeer's research shows that when moment is constrained (heeling moment for sailboats) high L/D planforms can have "negative lift" at the top/tip and positive lift at the bottom/root. Soft sails cannot do this, they flog and the drag goes up before the zero lift angle.

Also agree that theory and practice sometimes appear to show that the theory is wrong. Your comment about handling and the reduced AR rigs for the R class are a great example of what works in a wind tunnel may not work in "real life". This is not so much that the theory is wrong but that "real life" wind and wind tunnel wind are not at all alike. Very high AR wings can indeed be "twitchy" this is true for high AR foils used for daggerboards, rudders and keels also. What the theorists don't seem to get in many cases is that out of design range conditions exist often and a lower performance (in theory) rig that works over a wide range of conditions can be faster on the water. They are more "forgiving". Bethwaite's study of wind was an eye opener for me, the smoothest air is much more turbulent than I would have thought. +/- 5 deg angle and +/- 10-20% velocity plays hell with theory. Rather than a design point, you have a design area, if your design requires adjustment within the normal range of wind conditions in "steady" breeze it becomes labour intensive to sail. Rigs that react and are self trimming should be faster even though the theoretical performance is lower.

My reefing comment does not really apply to rig changes to reduce area. They don't have to deal with the drag of a bare mast above the reefed sail. The twisted off rig should have less drag than the reefed rig when the drag of the mast is figured in.

The use of shorter rigs for fast sailing also fits quite nicely with TSpeer's idea that you set the area so that the rig is loaded at the best L/D of the section. A taller rig with the same area has to have lower average CL to keep the same heeling force. Reducing the height/span of the rig so that the section is working at it's lowest L/D should be faster, this seems proved by what we see working in practice.

On the drag reduction topic, how many sailors change rigs and sails looking for better performance and don't spend equal time on the hull surface, fin and rudder profiles? The time and money spent on the keel and rudder of boat is a semi-permanent drag reduction since those surfaces don't flog, don't degrade with UV, and don't stretch and lose their shape over time.

R

 

I think Day first showed negative lift at the head was fast:
Day, A. H., "Sail Optimisation for High Speed Craft", RINA 1990

I think one reason for this is twisting off the head lowers the center of effort, but it does not lower the aerodynamic center. Also, a high AR rig is trying to get performance through finesse - lowering the drag faster than the lift drops due to restricted heeling moment.

Dave Hubbard had a great spreadsheet concerning the sizing of the wing for USA17. He looked at a range of rig heights and sail areas for different wind speeds. At a given wind speed there would be an optimum combination of rig height and area that produced the most drive for the heeling moment. When compared across different wind speeds, the aspect ratio tended to be quite similar, with the optimum rig getting smaller as the wind increased. Very much like what happens when one reefs a soft sail.

 

The bottom line is that we have a load matching problem on a sail boat. The load is applied by the wind which is a function of its Velocity squared. The Righting Moment is basically linear in response and has been designed for low to medium air conditions so the Rm max is at the top of the medium range. Adjusting SA very fast is the only way forward to cope with the big increase in heeling forces on the aero side of the equation. All the aero other strategies are cumbersome. We need a Rm method that increases in tune with the Vsquared part. Has anyone seen hydrofoils that are controlled to force down in response to heeling? This is the only way I can see us matching the gust response (or steady higher wind vel) (Vsquared) as then the hydrodynamics can be more effective in resisting due to its greater density? This would be illegal in most classes (limit on foils) and difficult in the classes that would allow it but seems to be logical. We use a wand to keep Moths up why not use a wand to keep boats flat? I think Happy Feet in Thailand (a cat) is doing this? Peter S