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The concep described below for animation shown is by no means the only VAWT you can build with flap concept. Possibilities are endless, this is just a thought experiment and I believe that it will work. This idea was floating around such a long time with one major problem, that is how to reduce drag created by the sails moving against the wind. The flap concept overcomes this. Also this size and shape creates so much area that in the storm it is so hard to prevent damage to the turbine, if sails are as shown with or without flaps. All of us see fallen trees after the storms. Such mighty trees coming down, because thousands of small leaves on it creates so much drag. To prevent damage to this conceptual turbine pitching section of the sails horizontally will reduce wind drag to manageable level.
In this Google sketchup 3D animation you see a conceptual large VAWT. The sails of this turbine attached on the moving platforms which are mounted on small railroad carts. The carts do a circular motion on two concentric roller coaster tracks. This design allows one to build very large VAWT economically. Under the platforms very large and heavy gear is attached which transform rotation movement of the turbine to the fixed generators under the tracks. The weight of the carts and the gear keep the turbine in balance and prevent it from tipping during storm. To prevent tipping further, the same principle to prevent roller coaster derailing can be used here. Also pitching mechanism for the sails, horizontal or vertical, will reduce drag substantially and prevent damage to the turbine. In order to have efficient turbine and extract maximum power the width of the sail and distance of the sails from center of the rotation is kept as small a possible.