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Old 7th April 2009, 10:40 AM   #1
Join Date: Nov 2008
Posts: 27
Default who killed the wing sail?

What happened to double camber wingsail development in windsurfing? Every time I see an independent developer or enthusiast develop a working soft wingsail, they seem to disappear without a trace in a year or two.

Obviously most of these projects are self financed.

But still, were is BIG WINDSURFING when it comes to buying up these good designs?

Camber induced sails have always had very good aerodynamics, but if you're an aero purist then you have to believe in a double surface technology and it's potential to make small powerhouse sails and turn gusty holey winds into plane-able days.

I tried snowkiting this winter for the first time on a lake. (Dedicated ice boarder). The smallest kite they had was a 4m FOIL designed for land only. Everyone was using 7m-8m inflatable water kites. Another guy carved in to the launch area with a foil kite. He had a somewhat tangible fear in his eyes.
As a lightweight, i'd probably be using a 5m Windsurf sail on snow in the 15 knots filtering over the bay, so I said "ok, ill try the 4."
The lesson guy launched the kite for me.
"Woa, it's pulling like a 7m, you'll probably be able to jump with this."
Maybe i''ve bitten off more than I could chew, I thought, but gave it a try anyway.
After a few uncontrolled face plants and body drags downwind and some tedious slow riding upwind I decided I'd mastered the kite's 3rd line release system and exceeded my endurance for another sesh as a kite noob.
The thing was a 4m powerhouse. It wasnt just more powerful than an equivalent ws sail, it was like a force of nature, an example of what a true wing shape can do.

I don't expect that windsurfers would be able to ride 4m on 7m day, but with a true wing who knows what's possible? Who wouldn't want to rig small with a wingsail?
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Old 8th April 2009, 04:51 AM   #2
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It's an interesting topic that's come up a few times over the years. The closest parallel I can see is Hang Gliders. Not dissimilar to two race sails foot to foot. Similar speeds (at least close enough in aerodynamic Reynold numbers terms). Double surface wiped out single surface in competiton overnight. Although single surface remains for lightweight recreation ; but even then there is some DS at the leading edge. Pretty much like a current race sail.

When the wide luff race sails came out some said it was a gimmick, I thought the sock size and shape would mutate into a wing sail. But intensive development but several brands has proven that
1) They do give a (small) performance improvement
2) There is an optimum DS luff size
Exactly why is debatable, but it does seem to be a fact - at least until someone proves otherwise.

Ten years ago a start up in Scotland got as far as taking orders - but didn't get many. Boards magazine did test an early example and said that as a lightwind sail it did have amazing glide thru' the lulls. But commercially a big name brand would have to sell it to convince a sceptical public. Weight and production costs are huge issues.

It was interesting to see a solid wing in the A class cat championships too, against the best in current sail technology. (Their sail weights make formula look lardy). Competitive in it's first version at world level , but didn't make everything else obsolete.

Actually I've just been playing with an 8m Neo foil kite, they're just some much neater than inflatables to me all round. Jumping with these is more an inevitability than a skill ! Will be interesting to see if foil kites find a niche in the new kite course racing. If so we might need some lightweight high twist DS formula sails to avoid being left behind !
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Old 8th April 2009, 05:36 PM   #3
Join Date: Aug 2008
Posts: 34

K.lauman (or anybody else) you don't have a picture of an wing sail? Never seen one. Try to googel it but didn't find any except strange looking sails for boats and catamarans.

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Old 9th April 2009, 05:03 AM   #4
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there is a picture of the powerfoil sail I mentioned on
You have to look carefully to see the battens on both double surfaces.

On the thread there is also some heavweight aerodynamics from the respected Tom Speer. He has run many simulations of various configurations. He says that single surfaces sails work very weel - but only over a narrow apparent wind angle. Writing in 2002 he concluded

"So I think we're seeing a convergence on a sail shape that has a properly formed leading edge, smooth lee side contour, a double luff for 20% to 40% of the chord, and a single surface for the remainder of the mast-sail section. And the purpose of having a double luff is to cut the drag asociated with the windward side separation bubble."

Sounds pretty much like a current race sail no ?

btw Loft sails are pushing an RDM formula setup as a way of extending mast life. I Just wonder if it may also offer an aerodynamic edge having a slimmer leading edge ?

Paul M
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Old 13th April 2009, 09:25 PM   #5
Join Date: Nov 2008
Posts: 27

Thanks for the thoughtful posts. First of all i'd like to qualify that I sail in the Great Lakes and spend most the summer months on cammed 7.5 or 6.2. and 110 freeride board, or 150 GO. Planing power is a big consideration here, the wind is usually gusty, variable and front based. When your slogging between gusts you have plenty of time to critique your sail's shaping.

Cammed sails are great, and I have one, but I can't help but wonder if everything above the cams is just stalled out and not driving. I just wonder if there's a "softer solution" with fewer mechanical parts and lower rigging tension.

Haven't tried a DS leading edge cammed sail, but forgive me if i'm wrong, most sails like the Pryde RS are high wind blasting sails with low-down shaping, not especially good for improving your planing threshold.

I've been reading about the windward separation bubble/vortex. A big cause of drag i'm assuming. Leeward separation bubbles caused by the mast are worse and can trigger a full on stall.

Really what i'm hoping for in the future is a wing sail with more power for it's size, less mass, adjustable camber, and less torquing effect on the sailor.

I've found some pics of the POWERFOIL which I think was the scottish design from 2001. I'll include them later. Very low aspect with little twist or cord taper towards the top.
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Old 14th April 2009, 04:48 PM   #6
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"I can't help but wonder if everything above the cams is just stalled out and not driving"

Well, put tell-tales on your sails and answer your own question.
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Old 16th April 2009, 05:26 AM   #7
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Well there's been a raft of sails designed to lower the planning threshold without adding weight, Plasma et al. Choose the one from your favourite brand.
The Formula sails are more orientated to wide usable wind range (and downwind use), so are more flexible.
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Old 21st April 2009, 05:05 PM   #8
Join Date: Oct 2008
Location: Spain
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I would like to tell you some of the theoric characteristhics of profiles:
1-Concave profile: This is the common one of sails, the single skin profile. Is the one which gets the biggest force for a given intesity of wind. But it is also the one which has the worst Lift/Drag ratio.
2-Symetric profile: The one of fins. (It's most used property is that it can work in both sides, but we are now not speaking about this). Is the one which produces the less force for a given wind, but best L/D ratio.
3-Flat-Convex profile: Flat in one side, cambered in the oher. It has medium characteristhics between 1 and 2, of course.
4-You can also find profiles which are concave-concave, but doubble skin, or convex-convex, but not symetrics. You would get intermedium propertis with them.ยด

For high speed vehicles flying at 800km/h (turbine airplanes), you use asymetric convex-convex profile. For medium speed vehicles flying at 100-200km/h (sportive airplanes), you use flat-convex profiles. For very low speed vehicles, sailing at 20-70km/h (windsurfing boards), you use concave shapes to produce enough power in small sail. Otherwhise you would need bigger sail to compensate the lost in power.
We can consider then that a wide luff sail is an intermediate concept between concave sail and flat-convex. Acording to all i said, now you understand why this design is only used in huge sails. And like this you can get better upwind angles.
A thing I would like to add is that if you want to take more power from a given size of sail, what you can do is to divide it in two sails. So that you have a rig looking like the rig of a boat. Consisting in two sails. Like this you would get more power but worst L/D ratio. Good idea for downwinds. What about putting a zip from up to down in the sail to open it in downwinds and close it in upwinds?

Here i put the link of an interseting web page where you can see wing sails.

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Old 21st April 2009, 07:21 PM   #9
Join Date: Jun 2008
Location: Switzerland
Posts: 196


that is a very nice link...but there are no details on all the concepts...
It is very interesting, just curious why is not being used anymore...I know that the AA record sail was also a prototype but not sure if it was a wing sail or some asymmetric profile.

I understand though that the development is not that cheap and that getting the money back is nearly impossible.

Post more...it is an inrteresting reading!
ciao Michal.
PA 86 & eVo 100 (with S-1); iS 101 & 122 & 144 (with Overdrive)
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Old 22nd April 2009, 09:49 AM   #10
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Unregistered: I'll try that as soon as the water temps around here are over 5 degrees celcius, till then i'll have to speculate.

andretsin: very interesting about the concave foil's max force yet poor lift to drag. So going to a wing sail wouldn't help downwind speed in this case and you couldn't substitute a smaller wing sail in place of a larger concave sail?

A thought on ergonomics. I'm sure the lrage amount of "heeling force" that the concave foil makes at a high angle of attack is taken into account in windsurfing design and allows windsurfers to hang body weight off the rig, (counterbalancing heeling force) and water start at high angles of attack ( ie sheeted in).

Maybe a wing foil would force the rider into a very upright and on-top-of-the board position that would be very unwindsurf like? ( Though they're always talking about this kind of stance in freestyle equipment design).

It's interesting that you say a concave foil inherently generates more power than a convex/ flat foil, because in my little anecdote a stationary (no apparent wind, true wind only) 4m kite with aforementioned foil shape seemed to have much more static power than a stationary 5.0 windsurf sail. This might have been due to wind gradient factors, ect ect, but at zero degrees angle of attack it still pulled hard in 15 knots of winter wind.

It would make more sense though that this convex/flat foil kite would be optimized to fly in 80 km/hr and above airspeeds, as a kite can reach these speeds power diving even if the sailor is only traveling at 40 km/hr.
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