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20 degrees AOA in aviation terms – “nothing less than extreme".

Even canard wing aircrafts, which resistant to stall, advised AOA during landing , do not exceed more than 15 degrees. Generally low AOA means better efficiency and less drag. Just real life example: when sailing big cambered sails during low wind conditions, it is very difficult to go upwind or even maintain upwind position. Those who didn't rig right or have less skills have to walk upwind, ironically called "walk of shame". At low wind, we have to maintain hi angle of attack to get maximum lift in expense of maximum drag. But as soon as wind speed increases we are able to sail with much less AOA and going upwind is not an issue anymore. Sail efficiency could be defined by ability to maintain lift at low AOA.

At 20 degrees AOA and 35kt apparent wind, traditional sail (curved plate profile) will generate 101650Pa Hi and 101050Pa low air pressure. The difference is 600Pa. And this is just an overall picture. Let’s take a closer look at smaller details. Notice, right behind the mast, green line goes dark, almost blue. This is negative pressure build up due to venturi effect, causing leading edge stall and working against lifting force. This is like natural built-in speed limiter. I just want to point out that our model is racing sail profile with pocket behind the mast. In regular sail this effect will be magnified.

Image below shows wing sail and it has exactly the same profile as sail on upper image with one difference, the lower surface is absolutely flat.

Notice that lower surface Hi pressure area didn't change its pressure redistribution from previous AOA at all! Also because thickness of profile displaces more air, max air velocity magnitude is 6ms higher. This cause better lower pressure generation on outer surface which equals more lift. Total difference in static air pressure is 1800Pa. THIS IS 3 TIMES MORE THAN REGULAR SAIL! Not to mention that low pressure area will bet against ambient pressure 101325Pa just adding to that 3 times number. With new generation of sailing - foiling and rapidly growing ice sailing, low AOA is crucial. Sailmakers are already making sails dedicated to foiling. They have less camber and tight leech so they can operate at low AOA. But still these sails are very much prone to leading edge stall and also very low lift due to shallow profile. Compare to wing, traditional curved plate sail, more than 3 times deficient and ineffective.

Next time we will take a look at 15 and then 10 degrees AOA before we can summarize all pros and cons on both profiles.

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