|6th November 2012 09:36 AM|
check on youtube and facebook for video and pics
rider Marco Begalli (Starboard) and Roberto Dacosta
Sails Designer at Herusails
|28th April 2009 10:46 PM|
|C two four nine||
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."
|28th April 2009 07:11 AM|
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!
|27th April 2009 06:18 PM|
|C two four nine||
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%.
|24th April 2009 05:01 AM|
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
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
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."
|22nd April 2009 10:49 AM|
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.
|21st April 2009 08:21 PM|
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!
|21st April 2009 06:05 PM|
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.´
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.
|16th April 2009 06:26 AM|
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.
|14th April 2009 05:48 PM|
"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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