Highly Assymetrical Wing Sail

The idea is to mount a wing on a Vee-mast that can be pivoted from side to side. When changing tack in this way, effectively the wing is flipped top to bottom, rather than side to side. Therefore it no longer has to be symmetrical and can instead adopt a more efficient profile such as those used on sailplanes. To eliminate problems controlling alpha, the wing has a small controlling foil mounted on a stub, operating like the tailplane of an aircraft, so alpha control is automatic.

It would be a practical sail for a small boat such as a kayak, because the thrust vector always points towards the pivot which is mounted low, reducing heeling. Same advantage as a kitesail but no launching problems. Obviously, multiple wings can be used, biplane, triplane, ad nauseum.

I plan to call it the SwingWing, assuming it works.

 

I see no reason why it wouldn't work.

Difficulties might include: high stresses on the pivot fitting, leeway control (since it will lift the boat upward and laterally downwind, as well as forward) and you might need some kind of shock damping system for tacking- if that swings over too quickly, that's a fair bit of rotational inertia going in the wrong direction on a tack.

But I think those problems should be easy to overcome with a little ingenuity.

Doesn't that "Sailrocket" speed-record candidate have an angled wingsail that looks a little like this, but only one-directional (can't tack)?

Looking forward to a prototype!

 

I didn't ghave time to get too much into details of your idea, but at first glance I don't see why it shouldn't work. You'll have to resolve two problems, especialy if you plan to use it with a kayak hull:
1) the center of gravity of the rig is placed on the lee side, creating a heeling moment which will need to be countered. Maybe two small floats at the wing tips would help.
2) you need to give the skipper a mean to luff up the whole thing and stop the boat, even under moderately strong winds.
If anything else comes up to my mind, I'll get back.

 

Marshmat: good point about the shock damping even on a small rig; the best time to flip the rig is during the turn. Also maybe a multiple pivoted lever arrangement of the wing supports so the CoG doesn't get so high when it is flipped, but it might be too complicated. I have a lot of respect for the kind of forces that even a small sail can generate!

I recall several advanced speed sailing designs in a magazine I read a few years back, some looked liked airplanes tethered to a float with a foil and one in particular was probably the "Sailrocket" that you mentioned; the vectors of the wing lift and foil reaction looked to be on the same axis. With the SwingWing it would be too complicated to arrange for that but it gets close. I thought an offset inclined leeboard or Bruce foil would react the remained heeling moment. If course if I wnated to get fancy I could sing the hydrofoil under the hull but ...

Daiquiri: I hadn't thought about the CoG offset, it depends on the weight of the rig which I haven't projected yet. This one is a bit further down the pipeline than current projects. This style of wing is easy to put into neutral, I can either change the control foil alpha to feather the wing or set the wing horizontal, or perhaps both. Feathering the wing would reduce thrust by the lift/drag ratio of the profile, if horizontal the center of thrust of that drag would be higher unless the wing support had the kind of multiple lever system mentioned above. Whatever I have, I would want to be able to dismantle it from the cockpit and store it on deck or toss it overboard in seriously bad weather conditions, for safety. Early days yet.

 

A-K,

Look at photo 2.29 in C.A. Marchaj's book, "Aero-Hydrodynamics of Sailing", page 386 in the first edition. It is a picture of a catamaran called Tackwind that competed for the World Sailing Speed Record in 1974. The wing looks kind of like a surfboard. The lower end is rigged to one ama, and the midpoint of the wing is attached to a pivot on the top of the single mast. To tack, the upper end of the wing is pivoted down to the other ama, bringing the lower end up to the upper position. Overall, the wing is situated more upright than in your diagram. The text in the book uses this photo and idea in its discussion of 3-D flow over rigs.

Eric

 

Pivoting assymetrical Wing: Past experimentation

Hi everybody,

20 years ago I met a young Phd in fluid mecanics. He got an old Tornado hulls/ platform, with a short mast, and put on it a Deltaplane wing.

He experimented it on flat water low wind, and concluded it was very good.

Then, with another bunch of Phd, they build a C Class catamaran, with a pivoting wing, very asymetrical and slots.

When they go to Australia in the ?? early 90's I guess. The boat was able to point higher windward than the US or Aussie boat, but unfortunatly at half the speed.

For downind it was much worse, they had to sail close to the wind and again speed was half classic C-cat., They capsized, in light condition despite hight platform stability and low wind, due to not friendly-user rig manipulation, and the wing was destroyed immediatly.

Hope it can help

Regards

EK

 

Eric: it's a relief that it's been done before, it evidently works, no theoretical reasons why not, just practical challenges.

Erwan: do you know the cause of the capsize? I can speculate a couple of reasons:-

If they were using an asymmetrical wing with slots, assuming it had a fixed profile, then it must pivot laterally to change tack, same as I am contemplating. If control of the wing were lost during this operation, as Marshmat warned, it might well generate enough momentum to cause a capsize, although rig failure would be more likely.

There is an alternative possibility that sailors used to conventional sails might not expect, which has come out of my own theorizing. While beating, during a change of tack while the boat is headed directly upwind the real wind and forward speed produce a strong apparent wind; with a wing that could produce enough lift to cause a capsize with a near-vertical wing. Currently my plan is to set the wind in the midway horizontal position while turning into the wind, or feather the wing. Together with the a Bruce foil to offset heeling moment, I hope that will avoid a capsize.

For sailing downwind, I understand it is faster to tack in order to make use of the lateral component of the apparent wind.

 

Interestingly, in the sailing craft which can cant its rig to windward most easily (the windsurfer) the development has moved the other way - towards an upright rig.

The original Windsurfer did (and still does) sail fastest in a breeze with the rig inclined steeply. However more modern boards keep the rig upright (in the sideways direction) and go extremely fast.

See top Formula racer Sean O'Brien's post about the techniques of the top sailors and how they try to combine an upright rig with the heeled body needed to generate power at

HTML:

http://www.carbonsugar.com/racing/power-to-weight-your-stance-vs-antoines/#more-24

 

All small boat sailors hike out to use body weight to balance heeling force; windsurfers cant the rig windward to a degree to hike out further, but try to keep an upright rig for efficiency. If windsurfers had longer arms they wouldn't have to do that. Not sure why original Windsurfer is faster with rig inclined steeply unless the lift reduces hull drag.

Hiking on of a kayak is virtually impossible of course, at least for me - neither athletic or young. Part of reason for proposed concept; also hopefully can be smaller but still able to delivery performance of large sail.

 

How about small combined ama-winglets in both wingtips?

And now I noticed Daiquiri allready thought about similar thinghy..

 

Good point. there were several good ideas in Daquiri's post and I I didn't give that one its due. If they were light it might help keep a level keel, although a beam sea could push the kayak around a bit. Depends how far from the centerline they are, which in turn depends on wing area and aspect ratio, wing construction, "mast" weight, offset angle , all still up in the air (pun unintentional). My guess for rig moment is around 15 lb-ft; the kayak I plan to use is quite stiff by kayak standards but some of my other boats are tippy, that is about 60% of the available righting moment at the flood point. So something has to be done! On the other hand I may need a Bruce foil or two to offset residual heeling moment from the wing, in which case i can incorporate a float atop of the foil.

 

Yes, that's true. The point is that arguably, on the evidence of some of the most popular classes that have ever been sailed, significant windward cant (even when it increases RM) is a negative.

Frank Bethwaite found one problem - unless there are nice steady winds of reasonable strength, under the force of gravity an inclined sail inverts.

Dunno if the assymetry (in camber) is all that important. Fully battened sails often possess camber at all times however they are not always faster at low speeds (typical of a kayak) or shifty conditions.

As you can tell, I have reservations about highly-canted rigs.

 

Canting the rig is likely to reduce efficiency, I think that's a given. That may offset other benefits.

Based on aerodynamic data provided in another thread, assymmetry improves lift/drag ratio. Profiles with undercamber are commonly found on low speed aircaft where /drag ratio is important such as gliders and performance model aircraft. However, the profile shape can be quite critical and perhaps a battened sail does not always reproduce the optimum profile. Hopefully, I will be able to try this out in the summer as I will be building a sail rig for the kayak soon, and if I can figure out the mechanics I will make an alternative wing sail for it. The proof of the pudding as they say, is in the eating. Or as an American might say, I'm from Missouri.

 

Couple of things...

What holds the wing at the desired angle?

What happens to the control foil as the wing is swung over the top to the other side? You show it near the top of the sail in your diagram.

Chris

 

The control foil acts like the tailplane of an aircraft, its angle with regard to the wing determines the wing's alpha; the wing is free to pivot about its average center of lift. In this case it will experience off-axis wind as in an aircraft that is yawing, but I expect it will still work, it will appear to be half of a swept wing. As the wing is swung over the top it will still behave in the same way, except in the top position the lift will be purely vertical.