Why do modern windmills have such thin sails?
Modern windmill rotor blades are like the wings of a plane. They have one curved surface and one flat one and this is what gives them the turning power. They don’t have to have a large surface area to do this. Just like a plane, more surface area facing the wind just adds more drag. In the case of the windmill, this would put more force on the tower, so it would have to be stronger and more expensive.
