Wingsail Story
Ridicules debate of thin profile vs thick, heated since Wright brother’s first flight on December 17, 1903, four miles south of Kitty Hawk, North Carolina. Despite scientific discovery by Russian scientist Zhukovsky, who gave us first mathematical description of thick profile, Wright brother’s plane was made with thin (curved plate) wing. And because of that, their flight was very unstable and hard to control. Early theorists in the science of aerodynamics, including Zhukovsky, assumed that the air has no viscosity and that the flow over the wing section is two-dimensional. Therefore they calculated lift without drag, even for thick wing profiles. Early wind tunnel experiments did not show any loss of lift to drag, because at that time, air flow in wind tunnels was slow and turbulent. Air flow than considered frictionless. Until later when technology allowed to build more powerful, fast, turbulence free tunnels, than all kinds of drag were discovered. People’s mind was so fogged with first flight euphoria, that no one even payed attention to many very important issues with thin profile. On the contrary, some people even filed patents for these profiles! Thin profiles were used (and failed) on first planes, flight suites and hang gliders (watch some early history “Birdman, the original dream of flight”). Finally, after many years of R&D and unfortunately many lost lives, modern planes, hang glider and wingsuit pilots use thick airfoil. Generally, thick airfoil proved to be more efficient and easy to control. So why sailboats are still using thin sails these days? Answer: Simply because majority of boat sailing is done when wind speeds are very low. At these low wind speeds (Reynolds number below 40,000) thin profile generate more lift. When it comes to boardsailing (windsurfing), it’s done in much higher winds (Re above 40,000) where thin sail profile drastically gives up. Solution: We simply load our vehicles up to the roof with many different sail sizes, so we are able to cover (not very wide) wind range. Yet, not so great. Sometimes it is almost impossible to predict size of the rig. Some days wind speed shift is so high that even technology like wind meters and digital data are totally useless.
Trial and failure to produce reversible wing in windsurfing industry goes back to early 70s. It seems like everyone follows the same path. First, they put their efforts to develop bidirectional sail. Then, after fast failure, one directional and at last hard surface…. Some of these designers were looking for fast results, others still in their R&D. Bottom line, many tried but it just didn’t happen. Majority of these developers came to foolish conclusion that the wing is just not working at all. At some point, one guy from Sweden even got in to production, but all problems with wing design hasn't been solved and product was simply not fully mature for commercial release. There are 6 major points that need to be addressed:
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Durable (able to withstand crash)
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Easy to rig (practical)
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Easy to water start
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Safe to sail
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Hold its shape under load
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And most of all, deliver 100% better or same performance in all aspects.
If you have ever seen asymmetrical, reversible wing sail in action, consider yourself very lucky, because there is no asymmetrical wing sail on the water these days (except mine). However, benefits of the wing is obvious in theory and now in practice. Let’s acknowledge well-known fact that all sailing speed records on land and water were set with wing. But all these sail machines can go only one direction, because its hard wing cannot be reversed.
My idea of wing design was born a while ago, however practical experiments started only in 2012. At that time, I was thinking that my very simple idea will unfold in just a few month. But small hidden problems started popping up one after another. As soon as the last one was solved, brand new one has surfaced. But thanks to the modern world where technology is available to an average person, asymmetrical reversible soft wing sail became a reality.
What makes thick profile better than thin?
According to R.T. Jones in his book “Modern Subsonic Aerodynamics”, thin curved plate superior to thick airfoil below Reynolds number 40,000. But after we reach Reynolds number 120,000 thick airfoil gives its advantage.
Here is formula to calculate Reynolds number.
Re = V x I / v
Where:
V = Relative speed (m/sec)
I = typical "length" of a solid body (M)
v = kinematic viscosity of the air (sec/m2)
Reynolds number calculator: http://airfoiltools.com/calculator/reynoldsnumber?MReNumForm%5Bvel%5D=7
This is happens because curved ting profile generates turbulence under lower surface when at low angle of attack. This turbulence cause center of lift travel back and forth. Normally Center of lift on thick airfoil located about 25% off the chord from leading edge, while on thin profile it has tendency to travel from approximately 25% to 28%. With hi wind speed this turbulence will cause leading edge stall. Thin airfoil will perform well only at high angle of attack. If this condition is not met, leading edge stall will take effect to the point where sail is impossible to control because leading edge stall and recovery will occur very rapid. In windsurfing we call it overpowering. Cause of the leading edge stall is due to Venturi effect (same effect that creates suction in car carburetors and spray guns). Air, just like any fluid, has viscosity, because of that, stream of fast moving air particles will interfere with surrounding area creating all kinds of turbulence. On traditional thin sail, at small angle of attack, air that passes under camber will capture stationary particles, pulling them out, creating suction and low pressure area. This happens right under the leading edge, where lift is generated. When wind speed increases, air pressure under the leading edge will go lower accordingly, up to the point where lift becomes 0 or even negative. This is how the leading edge stalls. To avoid leading edge stall at hi speeds, sail needs to be always at high angle of attack. This explains why windsurfing speed sailors setting their speed runs steep downwind. Despite the fact that during downwind runs relative wind speed and lift will go down. Because so many conditions need to be met, thin profile is very unstable and sometimes is very dangerous, especially at high speeds.
Here is the most practical explanation of Venturi effect I found on YouTube: https://www.youtube.com/watch?v=Na9ORhYjvJU. If you watched the video, you can clearly see how air particles affect surrounding area. On conventional windsurfing sail, this effect is very obvious. This parasite effect is impossible to fight. No matter how well the sail is designed, or how good your tune up is, it will cripple sail performance to the point where it becomes totally useless. We all experience this on hard wind days.
Fig.2 shows how air that passes under lower surface creates negative pressure affecting lift force. Both sail profiles, on the sketch, is at 5 degree angle of attack. Due to viscosity, air that passes underneath of concave shape, catches air particles (pulling out) creating low pressure area. Because of this effect center of lift will travel back, this is why windsurfers experience increased back hand and back leg pressure. To counter this (to some extend) , windsurfers move harness lines back on the boom. But this is not the only defect of thin curved profile. According to the theory of aerodynamics, due to collision of air particles, lower surface generates about 30% of the lift and another 70% are generated from upper surface. As you can see on figure 1, angle of incidence of air particles on lower surface of thin profile is 11 degrees more than on wing, according to this vector of lift generated by lower surface will be 4.5 degrees more back than wing. With leading edge stalling in progress this effect will only become worse making thin profile totally useless.
Mechanical properties of the wing.
Because our Wing has two surfaces spread apart and 3 stretch points where they meet, it makes sail structurally very strong. Basically, we weld and stitch many triangular panels together. Upper panels of wing are mostly welded. We eliminated most of the stitching (perforated lines), so it makes wing surfaces much stronger, compare to conventional sail. We created very durable wind machine. More so, all battens and stitches are hidden inside under sail surface. And that helped to achieve smooth (laminar) flow. This is the only laminar flow windsurfing sail in the world as of today.
Water start and beach start.
Sailing with wing is not much different than sailing with conventional sail. I mean in terms of how conventional sail feels at its best. All people who tried wing for the first time, were able to take it on the plane right from the start. A little challenging in the beginning is a beach and water start. It takes people by surprise with its power, because wing generates more lift than conventional sail. But you get used to it in about 2-3 times of trying. It is much easier to water start compare to....everything. Our sail built with inflatable cells inside, 3 to be exact. Wing never sinks, never takes water inside and it has triple protection from failure. When starting in deep water, to make wing fly (lift from water surface) is enough to pull it slightly forward holding it by the boom or simply just holding boom. Wing wants to get in the air by itself, especially in high wind days. So I suggest never let it go, cause it will take off on its own. If you fall in the deep water with sail in wrong direction (upwind) - no problem. Wing floats on water surface better than board and it will position itself downwind. All you need to do is just hold the board and let the wing drift around into position. With wing, it is more operating, directing it, than lifting and forcing into position as it is with conventional sail.
Handling the wing.
People think that the wing is heavy equipment, until they see my 116lb wife caring her wing trough the parking lot with no effort. Wing is just 1kg heavier than slalom sail of equal area. But this is on the parking lot where there is no wind. When wing is exposed to moving air, it becomes weightless. People tend to grab sail by the mast, as they do with regular sail. With wing it is uncomfortable, because wing has large round leading edge. So what’s the right way to hold wing? By the boom, with one hand and the other hand on the top of leading. Holding leading edge down, prevent wing to go up when sail is not hooked to the board.
In general, sailing with wing is no different than with conventional sail. There is no adaptation period or change in sailing technique. Wing sail delivers better performance in all aspects. All lessons learned with regular sail will be greatly enhanced using wing. Wing doesn't have fatigue as traditional sail and it's very predictable in all conditions. Because wing has fixed center of lift, ride is much more stable and much easier. There is no back hand and back leg pressure, no leading edge stall, no sudden loss or gain in power. In fact it is so easy to handle that you will never shred skin on your hands. Wing easily outperforms any traditional sail of equal size, especially cross and upwind. Wing has near unlimited wind range. I mean, if you take wing on 45kt wind, you probably experience more problems with aerodynamics of your board and your own body than with wing.
We all know that jibe is very difficult and advanced move and probably we all experienced the fact that in different wind condition jibe technique is different. This happens because traditional sail has different behavior on different conditions. Regular sail needs to be tuned up according to wind and water conditions, and with different tune it has different characteristics. Wing has only one setting for all conditions. Wing delivers that same performance and behavior in all winds. It does exactly what it is designed to do, pull forward. Compare to conventional sail, with wing it much easier to master perfect jibe as well as other sailing techniques.
Is wing faster?
Strait answer YES. Wing easily outperforms traditional sail of equal size with same sailor, board, mast, boom, and fin. But, if you expect that sailing the wing will instantly make you fastest guy on the bay, probably not. There will be a guy who is heavier, has bigger sail, smaller board, better skills, smaller fin, etc. and he will be faster than you sometimes. But you will go faster with wing than with your regular sail of equal size. Also because lift component is directed more forward you can use smaller fin. Coefficient of lift that wing generates is just 0.2 better than traditional sail generates at its best. Also wing has no fatigue. If you can utilize and use properly all advantages of the wing, than you will be faster than others.
Who should consider wing sailing?
Sailors who consider getting wing, should more take in account such values as stability, reliable ride in all winds, better performance, freedom of sailing, and perfecting sailing techniques. When sailing on a good windy day, you will see people go over 2- 3 conventional sail sizes, while you just having more fun with more wind on your wing. With wing you can go safe as fast as you can and as slow as you want. Wing is an engine, not a crazy machine.