The Texas Turnstile WindCatcher uses a rigid upper sail to create a low pressure area that draws up a flexible lower sail to it as the wind passes over both sails on the return side of a vertical axis windmill. This aerodynamic action reduces the wind resistance of the sails on their return into the wind. This same action acts as a self-damper in high winds because the spinning of the rotor causes the sails to close and resist opening until the spin slows.
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