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Old 24th April 2009, 04:01 AM   #11
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Andretsin,

Thanks for the summary of the theory, which I'm sure is sound. I think the best L/D would give the best angle upwind ?

I also find it hard to explain several practical competition results, such as why C class catamaran have proven fastest with solid wing sails - despite their weight. Also why double surface hang glidiers wiped out single surface in competition , see
http://www.british-hang-gliding-muse...ration_64.html

I suspect response to varying angles of attack is as important as coefficent of lift and L/D. Actually our current 'flexy' sails seem more tuned for wind range than max lift coeffiecnt ? Could a wing sail be so user friendly ?

I agree downwind looks like an area where experimentation might still produce new concepts. Back on the phantom 380 thread I posted

"No arguing that drag will increase with slots, which is why I've only really imagined them in the downwind dash scenario. There we are operating at as high as angle of attack as we can , proabaly well past most efficient. I wonder whether by running deeper, if a little slower, with slots (ie closer to the spin equipped boats) it would be possible to make more VMG. For general aviation take landing drag is probably pretty irrelevant to the designer. However if the goal is different then the placement and sizing of the slots would differ to meet the changed target. The only windtunnel work I know of in this area has been done for paragliders, and is shown on
http://www.skywalk.info/Content/103/?mnid=328
According to independant tests (in the table 2/3 the way down) they got virtually 20% decrease in stall speed (ie high angle of attack) for 2% increase in drag at minimum angle of attack (equivalent to upwind). Certainly those things sell well enough. I've spoken to owners and they say the only disadvantage is a narrow 'poor performance window' at medium angle of attack - might be a problem for a sail on beam reach, but not up/downwind. Just makes me wonder - but we don't see so much experimentation nowdays with rigs, that all seems to be on the kites."
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Old 27th April 2009, 05:18 PM   #12
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I think the reason that C Class cats use wing masts but other boats don't has been explained by top aerodynamicist Mark Drela, whose foil designs hold things like the human-powered flight and boat records.

Mark notes that with a board or boat, the issue is not the l/D of the rig in isolation BUT THE LIFT DRAG OF THE RIG AND PLATFORM (ie the rig and the hull or hulls). Looked at it that way, everything comes together.

A glider consists of a huge wingspan with a tiny bit of parasitic drag caused by the fuselage, so going for the lowest-drag wing is vital. In contrast, a boat or board consists of a smaller (proportionately) "wingspan" and a massive amount of parasitic drag caused by the hulls. No matter how efficient the L/D of the rig, the L/D of the whole boat is dominated by the huge drag of the hull. Therefore a lot of the time, it's more important to maximise the lift of the rig, even at the expense of maximising the drag of the rig, because the rig drag is a small fraction of the total drag of the entire boat or board.

For example, a C Class cat is an inherently low-drag beast for its size and sail power. Therefore the low drag of the wing sail is very important (and it helps that the boat is so big that it rarely crashes and blows the wing!). So wings work in Cs.

At the other extreme is a cruising yacht; it's got an inherently high-drag hull and therefore maximising lift is vital, and since rig drag is a small amount of overall drag, an inferior L/D in the rig is no problem if L increases.

In boards, we go from longboards in light winds (high drag compared to windspeed) to speed boards in strong winds (low drag compared to windspeed) and the fastest sails follow the same rules; increase lift when your hull drag is high even at the expense of higher aero drag, and vice versa in other conditions.

I'm no expert in this, just passing on theory that matches practice 100%.
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Old 28th April 2009, 06:11 AM   #13
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On a yacht, where rig size is limited (either by exterior dimensions or via the handicap measurement) what you say is correct. C class cats, for example, have very specific limits.

On a windsurfer, however, you can use whatever rig you want, so it would follow that the best possible L/D would produce the best result. IMO, the 'gray area' of wing viability is the boundary conditions. Windsurfers have slightly more than 1:1 speed to wind speed ratio (for comparison, Yellow Pages just went over 50 kts in less than 25 kts of wind which is in excess of 2:1). So to go 50 knots on a windsurfer, you are looking at nearly 50 knots of wind speed. The impact on the water surface, gust dynamics etc is extreme, and so far it has seemed that the advantages that result from the ability of a properly designed conventional rig to 'autotune' outweigh the disadvantage of the intrinsically lower L/D.

My 2 cents, for what they are worth!
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Old 28th April 2009, 09:46 PM   #14
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Good poin!

By the way, even very open-minded boat classes with no rig limits have found that wing masts, double luff sails etc are often less efficient than just making a taller, lighter, simpler rig that cuts down on induced drag by its longer span.

There's pics of double-surface windsurfer sails in use (slowly) at speed trials as early as about '82. They still seem to have an attraction because people believe that gliders are the best model for sails, and while I'm no aero expert in any way it's interesting to see that guys like Drela seems to believe that "normal" sails are better in many ways, particularly (as you pointed out) their adjustability; I can recall a post to a glider forum where he seemed very impressed by the way teh America's Cup rigs worked.

Tom Speer (a Boeing aerodynamicist who posts on Boat Design Forum) has noted when writing about solid wing sails that "the notion that because aircraft wings are very efficient and have thick sections, while sails have thin sections and generally lower lift/drag ratios, and therefore a thick sectioned sail will aerodynamically superior to a sail rig with a thin section simply because it is thick, is a mistaken idea. Airplanes have thick sections because they are structurally stronger and because they have to operate efficiently at low lift coefficients in cruise. This is generally not the case for most sailing craft, except for very high-speed craft like landyachts and iceboats."
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Old 6th November 2012, 08:36 AM   #15
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Quote:
Originally Posted by k.lauman View Post
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?

........
it is more than alive!!!
www.herusails.it
check on youtube and facebook for video and pics

rider Marco Begalli (Starboard) and Roberto Dacosta

giulio iviani
Sails Designer at Herusails
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