the optimal number of blades on a windmill is 3. Airplane props, helicopter blades, and ceiling fans can all have any number of blades. Why?
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Every blade tip allows air from the high pressure surface to move around to the low pressure side. This is a little simplified but it’s an OK way to think of it. The result is a tip vortex – a swirling flow that trails behind the blade. This is bad for lift generation and it causes drag. So, bad for helicopters, props, and wind turbines. The goal, then, is to minimize the number of blade tips subject to other considerations.
One of the early helicopters had one blade. It was built by a guy named Montgomery Knight. The aerospace engineering building at Georgia Tech is named for him. When I was graduate student there in about 1994, I worked in the Harper wind tunnel. We had to move everything out for a building renovation. Under the wind tunnel, we found that rotor blade and immediately all recognized from the pictures. It is now on display in the Knight Building entrance. Not what you asked, but a good story anyway.
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