Understanding Wing Technology

I hope this thread will eventually become a resource for anyone looking for information on the design, construction, control and sailing of solid wings for powering performance sailboats. I hope that anyone with experience or questions in these areas will contribute when they can. I am building a small wing now as an excercise in learning about the technology and will add a wing-my second-to an RC cat I have to learn more about it.
The first link is to a thread in this forum: http://www.boatdesign.net/forums/multihulls/patient-lady-wing-control-system-34619.html

And for practical and theoretical work on wing sails: http://www.tspeer.com/

The LAC thread has links to some good info: http://www.boatdesign.net/forums/multihulls/little-americas-cup-2010-c-class-real-one-32301-9.html

I will add more and more as I have time.......

 

Twist

My biggest hassle right now is trying to understand how the wing is twisted. I continue to read and re-read Steve's PDF on Patient Lady but am still having a hard time getting it. Any help would be appreciated....
-Do the completed elements have a tendency to twist due to wind pressure alone?
-Does the wing design used on current performance boats include a designed -in method of gust response?

 

Twist

Post from SA with more on twist by Steve Clark:

1 element twist:
The main structural element of the wing is a round mast. This is inside the #1 element of the wing forward of t6he quarter chord.
The wing surface is attached to this spar above the hounds such that the spar rotates with the wing surface.
Below the hounds, the wing is located on the spar with saddles, but is free to rotate relative to the spar.
At the bottom of the wing , there is a large boom like structure ( called the Boom Box) which is the foundation of the wing control system. This is attached to the wing surface but can rotate about the spar. At this height the spar has a rotation tiller, similar in all respects to what you see on any beach cat. If the tiller is centered on the boom box, the wing above the hounds lines up with the boom box and there is no twist. If the tiller is eased, the wing above the hounds will be eased by the same amount, thus twisting the #1 element below the hounds.
As the flap angle is controlled by a system that has all of its foundations on the wing, flap angle is unchanged by twist.
So all the twist happens in the bottom 25' or so of the wing, which is where the greatest speed differences in boundary layer occur.

That is at least how all of the Cogito wings operate, and 5 of the 7 wings at Newport will work this way. Patient Lady VI's wing was built before we thought of this and only has the ability to wash out flap angle. Aethon's new wing has an extended play version of the twist control which allows us to twist the entire span of the wing instead of the bottom 25' or so. This comes at the cost of some additional complication that has me channelling Lindsey Cunningham on a nightly basis.
SHC

 

Twist control and soft reefable furlable wingsail

Twist control is very easy and precise if you use a euphroe type junk rig sheeting system, as contrasted with a Blondie Hasler purchase-based junk rig sheeting system.

In the junk rig, the sheets run along the leach rather than along the boom. It is very effective and practical, but for some reason, almost incomprehensible to most sailors. By running the sheets along the leech, the Chinese gave staying to, not the mast, but the sail, and in so doing, they also made it supremely easy to reef.

To me, the Western way of arranging the parts of a rig look like the rig was put together by someone heard the parts of a sailing rig designed, but assembled them incorrectly because he had never seen one. I say this because the Chinese way is so much more sensible - easy to handle, safer to reef and furl, etc.

Handling the Western rig is the real Chinese fire drill. Huge, straining, poorly retained sails billow and flog, complex devices try to cope and occasionally fail, dangerous gybes, you can't free the sheets on the mainsail while running without losing your mast, you struggle to into the wind to furl a mainsail, you crank giant, highly loaded sheets, and every once in a great while someone loses a finger, etc.

This is, of course, by no means an exhaustive list of the the whole Western sailor's fire drill, the vast majority of which could have been avoided by correct assembly of the rig parts.

In contrast, the Chinese rig is balanced such that you have whatever sheet loading you want - exactly like a balanced rudder.

It's no great challenge to turn a junk sail into a wingsail, either. Just make an airfoil batten assembly to hide the mast inside, and hinge the after part against it to create a flap. It is exactly like a cloth and wood spar airplane wing, with the added advantage that you can reef and furl it with astonishing sang-froid. The sheeting system imparts the desired reversing camber by holding the leech to windward of the batten assembly.

See: http://bigcatcatamarans.com for a soft, reefable, furlable wingsail that uses such a system on a wing sail.

 

Why are the wing elements usually separated by a gap in between the fore and aft element. I always thought that a configuration like this was only used to allow air flow from the windward side through to the leeward side, helping to prevent separation, enabling higher lift coefficients to be obtained, though this comes at the cost of an increase in drag.

For a wing sail isn't the goal more about the efficiency? and so therefore wouldn't it be better to have a foil shape that has a flap, such that it is just the aft section of the foil that rotates, with no gap, similar to a trim tab on a keel?

So my question is therefore why is the mulit-element wing sail more efficient than a comparable wing section with a flap.

Cheers

 

======================
This is an answer from Tom Speer that I saved-don't know where it was originally posted. Its possible he may post here with further info: (unfortunately the illustration with it didn't transfer)

The sections of the individual elements are symmetrical, but when the flap is deflected it forms a cambered section that looks something like this: the illustration on the left below goes here-click on the image


The hinge line is somewhat forward of the forward element trailing edge. This opens up a slot between the flap and forward element, forming a slotted flap.

This is what the pressure distributions look like as a function of angle of attack for the configuration above: the illustration on the right below goes here-click on the image


The flap has the effect of making the forward element act as though it were cambered, adding a lot of aft loading to it. The boundary layer on the forward element doesn't have to slow all the way down to freestream, but comes off the trailing edge at a higher velocity that matches that of the flap leading edge. This means the forward element boundary layer does not have to have as strong an adverse pressure gradient, and is less susceptible to separation.

It also lets the forward element carry lift without developing a leading edge suction peak. If you look at the pressure distribution for -6 deg angle of attack, the lee side pressures on the forward element form a smooth curve that is almost flat, with a peak velocity ratio that is only around 1.6. Since skin friction is proportional to velocity squared, there is a big friction drag penalty associated with pressure peaks, which can account for the majority of the profile drag. This is why its better to camber up and sheet out than to operate the wing fairly flat.

The flap's job is to complete the slowing of the flow to near the freestream velocity at the flap trailing edge. This is where the slot helps, because the flap gets to start with a fresh boundary layer that hasn't been tired out by passing over the forward element. So the flap can tolerate a steeper adverse pressure gradient without separating. It's like a relay race, with the forward element handing off to the flap to bring the flow home.

15 - 25 degrees of flap deflection is typical. When you get to 30 degrees or more, the drag goes up and there's not that much lift benefit. Zero or small flap deflection is used when you're basically feathering the section. In general, it's better to operate with a fair amount of flap and rotate the whole wing to a lower angle of attack if you need less lift.

The C-class typically have an additional element that is a plain flap on the trailing edge of the first element. Once the desired width of the slot is obtained by deflecting the #3 element (slotted flap), a bracket deflects the #2 element to follow the flap and maintain the same slot. Slot geometry is important to the C-class cats because they have a fixed wing area, and have to go to high lift to get the most out of that area downwind.

High lift was less important for USA 17 because it could have any amount of area in the wing, and could add area in the form of headsails for downwind. Lift/drag ratio was more the driving factor. But light weight and reliability were very important, so they went with the simpler configuration of just two elements.

The section design contributed to the performance of USA 17, but profile drag is a small component of the overall drag. People tend to look at the boat and then overlook the most obvious and most important feature of the wing. It was tall. Lift-induced drag is the biggest component of wing drag, and induced drag goes down with the square of the span. The wing was the fourth rig for USA 17, as they made two masts of different lengths when the boat was first built, and then built a third mast taller than the other two that was intended to be the race mast. And after the wing was tested in San Diego, another 6m were added to the top in Valencia. As the rig got taller, the boat got faster. There were lots of other major changes to the boat, too, but rig height was important.
__________________
Tom Speer

 

thanks very much for the reply, this was very informative.

 

To add onto what BigCat was suggesting about an airfoil-batten assembly, and to the diagrams which Tom Speer posted via Doug Lord, here is an easily-made soft wing sail controlled by multiple sheets to the leech:

http://wharrambuilders.ning.com/for...2195841:Comment:15950&x=1#2195841Comment15950

 

junk rig biplane catamaran wingsail stuff

You will find links to Bertrand Fercot's sites (he owns "Pha", shown above,) on my website at http://bigcatcatamarans.com .

The above left photo is an experiment of Bertrand's. His boat actually has a foil shaped forward part rather than shown above left.

Bertrand started out with a 'Swingwing' rig designed by Robin Blane. The Swingwing rig has a sleeve forward, but it is not a foil shape.

Another very good and very recent article is by David Tyler, who is cruising in his yacht "Tystie." He has posted a comprehensive article on his junk - wingsail hybrid update at:

http://groups.yahoo.com/group/junkrig/files/

The file is titled:

Tystie's rig update, Dec 2009 to jan 2010 rev1.pdf

All three of us post about rig design at http://groups.yahoo.com/group/

The concept of a junk wingsail hybrid has been independently invented a number of times, as it is a logical development of combining the ease of handling of the junk rig with the aerodynamic advantages of the foil.

Neither David's nor Bertrand's rigs started with a foil section forward. You can actually watch me groping towards the idea, and perfecting it, with helpful feedback from the Junk Rig group at Yahoo, by reading old posts there. It is a very strong group, with over 17,000 posts.

I just looked over old posts on the subject, and I see that I was posting about what I called the "Snowshoe" rig in 2002, and mention that I had been thinking about it for years.

 

Doug, you wouldnt happen to have some info hidden somewhere on the sections that are being used in these wings? I see there are some on the plot above... do you have any more?

 

Wing Tech

This is another post from Tom Speer. I assume you've checked his site? You might pm him or e-mail him from his site.

This depends on whether you are considering plain flaps (no gap at the hinge line) or slotted flaps formed by two or more symmetrical airfoils in tandem. A 30% chord flap will raise the mzximum lift coefficient of the FX 71-150 to 1.8 at a Reynolds number of 1 million.

I know of no published data besides that on my website for symmetrical sections with slotted flaps designed to be used as rigid wing sails. The methods used to calculate these data are VERY crude andthe maximum lift coefficients are HIGHLY questionable. I was also interesed in fairly low Reynolds numbers. At the higher Reynolds numbers of the data quoted above, the maximum lift coefficients would be substantially greater.

One of the best combinations from landsailing competition has proven to be two NACA 0012 sections, with the forward element 60% of the chord and the flap 40% of the total chord. The hinge line is normally located at about 90% of the forward element chord, and the links extending from the flap leading edge (through slots in the forward element trailing edge) sized so the flap just clears the trailing edge of the forward element. A flap deflection of 20 degrees will typically provide best performance. Higher flap deflections may increase CLmax, but the drag also increases rapidly from, say, 30 degrees to 40 degrees flap deflection.

In general, one can use a thick section for the forward element, but the flap should be a thin section. The thin flap section would normally produce sharp pressure peak at the leading edge that would lead to leading edge stall. However, when used as a flap, this pressure peak can be suppressed by moving the flap closer to the trailing edge of the forward element, making the gap smaller. There is an advantage to having the flap and forward element overlap somewhat when the flap is deflected. However, this adds complexity to the mechanism to keep the two elements from interfering when tacking.

Maximum lift coefficients for multi-element rigid wing sections are probably in the range of 2.5 - 3.0+. One must do considerable tuning and adjustment of the slots and flap deflections to optimize the performance.

As a practical matter, I belive it is possible to design a wingmast & sail that is competitive with a rigid wing under most conditions. The A-class catamarans seem to have settled on soft sails while the C-class cats have settled on rigid wings. From what I've seen, a single slotted flap is probably comparable in maximum lift and a double slotted flap will probably have a higher maximum lift than a wingmast & sail. However, the wingmast may have less drag at high lift and will most likely be lighter than the rigid wing and better able to accommodate a wide range of conditions. In landyachts, the best of the rigid wing rigs and the best of the wingmast rigs are neck-and-neck with each other.

As important as the section design is, once one has a reasonably well performing section the more important thing is to get the distribution of lift correct along the span of the rig. A major factor in Cogito's success in winning the C-class Challenge Trophy ("Little America's Cup") was the ability to twist the wing to optimize the spanload distribution. This reduces the induced drag and also allows the different sections from top to bottom to perform near their best angles of attack. This is more important than the differences between one section and another.

 

Understanding Wing Tech

Tom Speer from SA:

Twist control is comparatively slow compared to sheeting the whole wing, so sheeting ("traveler" in USA 17 parlance) would be the way you'd react to fast changes.

Twist changes the basic lift distribution. But the change in moment due to a change in wind speed or angle of attack is not affected by twist (the additional lift distribution). It is possible to generate negative lift at the head and lower the center of effort. However, when the gust hits, it acts through the aerodynamic center, not the twist-lowered center of effort. The hydrostatic analogy might be center of buoyancy and center of flotation. The total buoyancy acts through the center of buoyancy but any change acts at the center of flotation. So if you're using twist to lower the center of effort and balance the boat, beware! Because the gust is going to hammer you at the higher aerodynamic center.

The good news is changes due to sheeting also act at the aerodynamic center instead of the center of effort. So the reaction will probably be to sheet first, and adjust twist or camber second.

The other problem with depowering downwind is the rotation of the wing is limited by the shrouds. It is more effective to sheet in on the main element and let the flaps out to leeward until the trailing edge reaches the shroud than it is to blade the whole wing out with the flaps flat until it's against the shroud. But that's an unnatural act, and it depends on how quickly the flaps can be deflected from their normal position to dump lift. Dumping the flaps through a given angle while holding the main wing fixed is approximately 80% as effective as dumping the whole wing through the same angle with no change in camber. And the flaps can be dumped to leeward through a greater angle than the whole wing, thus ending up in the configuration with inverted camber mentioned above. So we may see the development of control systems that allow the flaps to be dumped rapidly, even if they have to be pumped back up more gradually afterward.

The wing is twice as high as the length of the boat. I think there will be danger of pitchpoling even with control of the wing above the hounds.

 

Understanding Wing Tech

More from Tom Speer from SA:

I'm using the term "aerodynamic center" in the context of the vertical plane as well as the horizontal section plane, because I don't think there's any other accepted term in yacht design for the distinction between it and the center of effort.

The center of effort is defined to be the heeling moment (or pitching moment, depending on the context) divided by the lift. This is the same as the concept of center of pressure for an airfoil. If the moment went to zero at the same time the lift went to zero, then this would be the same as the aerodynamic center. If the wing had no effective twist (zero lift lines of all sections were parallel), then this would be the case. However, things are different with a twisted wing.

If you sheeted out a twisted wing until there was no net lift, then there would still be a heeling moment because the head would be pushing one way and the foot would be pushing the opposite way. When you calculate the center of effort, it is at infinity because you have the heeling moment due to twist divided by zero lift! Suddenly the concept of center of effort isn't making much sense.

When the wing is operating below stall (linear lift range), then the change in the lift distribution will be pretty much the same for a change in the sheeting of the wing (angle of attack), whether the wing is twisted or not. It's just that the change at a given section will have a different starting point when wing is twisted. So it makes sense to consider the height at which the change in lift is centered in order to give the same change in the heeling moment. This is the aerodynamic center in the context of heeling moment, just as it is when considering pitching moment in the context of section aerodynamics. So now we have the lift acting at the aerodynamic center producing a heeling moment and we have an additional heeling moment that is due to twist. If you look at it this way, the effects of angle of attack and twist are decoupled. The local lift at the head can go negative if the twist is large enough or the angle of attack is low enough, but that doesn't change anything - it's only a matter of degree.

If the head is twisted off, then the heeling moment due to twist will be a restoring moment, and will oppose the heeling moment due to angle of attack. At any given moment, it will be as though the center of effort is lowered. But the center of effort will be going up and down as the lift changes, which again kind of defeats the whole purpose of being able to think of the lift as acting at one fixed point. So it may be more useful to think of the lift as acting dynamically at the aerodynamic center, and being controlled by sheeting the wing in and out. Then use the twist as a means of heel trim, because it will be slower to effect the twist than to sheet the wing.

When a gust hits, it will have two effects (assuming the shear is similar in the gust). There will be an increase in apparent wind speed, which will affect both the heeling moment due to twist and the heeling moment due to angle of attack, and the apparent wind angle will increase, which increases the angle of attack. If there's a lot of twist, the heeling moment will be greater than what one might expect because the change in relative wind angle will be acting through the aerodynamic center and no getting a corresponding offset from the twist.

 

Uwt

This from an unknown source-sorry:

Well the wing we use on the C-class in not one element, its 2 or three depending on how you look at it.

I just call them front and back, the front one having a flap on it that is 15% of the chord of the front part and deflects up to about 20 degrees.

Normally sailing upwind you have about 20 degrees of "camber" bewteen the front and back sections, and the little flap(s) is not off center line at all.

Down hill you have a maximum of 40 degrees of camber between front and back and the number 2 flap kicks in about 15-20 degrees held there by the idlers, which are little fingers that hold the trailing edge of the front section.

So uphill, on the leeward side what you see is a nice fair curve with a little slot half way along the wing. Thats the important bit is that the leeward side is nice and fair with a little slot in it.

downhill, it looks the same, just deeper fatter more powerful and draggy section.

So essentially when you tack or gybe the thing flips inside out and goes the other way.

The point here being that by combining two symetric sections, you get a more powerful assymetric section with the option to power up or de-power. so as for donwhill, you have a big fat deep section that is way more powered up than uphill, so thats what makes it work down hill.

As for apparent wind, it should still be forward of the beam to be useful, any further aft and you're sailing like a retard with the wing, she just does not work well when too deep unless its 4 knots of breeze and then you just drift straight down hill with the barn door up there nudging her along (Not as fast as the wind for those on that other thread)

illustration that goes with it:

 

uwt

From Steve Clark-10/1/09-

I'm going to sa some stuff that is going to shock some people.
I think it is easy to overthink some of this stuff, The small detail of section shaping and selection probably maters less than getting the weight down and making the controls simple and reliable.
I mean you are never going to be able to tell the difference between a Eppler or a NACA section if the flap sags and the wing is inside out half way up it's span.
As a result, I encourage you to make nice conservative choices for your sections and then spend most of your effort on the structural and mechanical engineering side of the problem.
A leading edge element that looks a lot like a NACA 63-12 ( with a 20%flap) and a trailing edge that looks a lot like a NACA 0009 is a pretty safe bet. Make it light and it will be really good. Make it lighter it will be better.
Make it so you can tack and gybe without pulling a bunch of strings, you will be happy, make it so you can sail it day after day without rebuilding it, you will be facking brilliant.
As regards the Yellow Pages wings, these were very high lift configurations, they paid for that lift with complexity and weight. In order to get various elements to set, there were miles of very light wire that had to pay out and take up proportionally. This resulted in a a system that was really not very reliable because any hang up would either result in a loss of controll over one of the elements or a failure of the wing control system in total. It may have been possible to add twist to these wings, but I really don't know haow it could have been accomplished within the constraints of the flap control system.
The Dave Hubbard system, which has been the basis of the Patient Lady, Stars and Stripes and Cogito wings, while not as ideal, has the advantage of simplicity and reliability.
In short, I think there are many right answers to the aerodynamic questions and we can spend the rest of our lives trying too determine which is really better. There is more risk in the controls and the structures.
SHC