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Old 10th December 2008, 06:33 PM   #9
andretsin
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Join Date: Oct 2008
Location: Spain
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Hi,
there are two radical different concepts mixed here. The twist in the upper part of the sail has a bigger purpose than that one of giving control in gusts. That part of the sail work like the winglets of airplane wings. It reduces induced drag. Because having nerarly zero attack angle it makes that the diference of pressure between one side and the other of the sail is reducing until it's nearly the same at the top so that vortex almost don't appear.
It's true that in gusts it also helps when is bended to leeward. And this is what NP is pretending. I always think we have to look to nature. How are the wings of birds? Do they bend in the leach? They do in the top! So, I'm not against this this new design. We will have to wait for it's results.
An other thing wich was not correct at all is to think that is not good to loose surface below the boom. Yo must know there is a big gradient of wind in first 10m from the surface up. It means, in the surface the wind is almost zero because of friction with the ground. It increases it's speed until 30m. After that you can consider that is almost uniform. But in first 10m is where you find the biggest change. That is why an official measure of the wind must be taken at 10m of the ground. With all this i mean that is not a problem to loos surface of the sail in the lowest part. Even more, it would be interesting to study if this lower part is contribuiting to propulsion or it's friction is bigger than the propulsion it mades.
Ah! one more thing. Battens are strong enought to hold this part of the sail. Do you thing the sail makes lot of force? Just divide the force you make with your body in the boom (30kg aprox.) by all the 10sq meters. Is 30g every 10x10cm. I think is not so much. Battens will withstand it.
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