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post #5 of Old 02-10-2002
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Ayrodynamics of Sailing

Ahoy, Zach.I will answer you query in two parts. First part: The effect you are talking about is called the Bernoulli effect after Jakob Bernoulli, a Swiss mathmetician and physicist who illuminated it. It says, in simple terms, that the faster a fluid flows past a sidewise surface, the less pressure it exerts. Pressure decreases as velocity increases. The air doesn''t slow as it flows over the inside as you said. The air speeds up as it flows over the outside. Sail makers take advantage of the Bernoulli effect by a simple mechanical means. Imagine an airplane wing. The bottom surface is a straight line from leading edge to trailing edge. The top surface is curved making the line from leading edge to trailing edge longer. Now, the wind must travel farther over the top than over the bottom. In order for air split at the leading edge to arrive at the trailing edge at the same time and equalize as it must, the air on top must speed up. The faster air exerts less sidewise pressure. There is no vacuum. There is pressure on both surfaces, but the upper surface has slightly less pressure. A fluid flows from greater to lesser pressure, so the air underneath tries to flow toward the air on the upper surface to equalize the pressure. But, the wing is in the way so it gets pushed in the direction of the upper surface. That gives the wing lift. Turn that wing vertical and you have a sail. Sails and wings work on pressure differential. End part 1.

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