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?
In: 7
The eli5 answer it really it depends on many many factors and the number is selected based on what the design prioritizes and the power transfer required.
It’s a fluid dynamics exercise. For wind turbines (or water, etc) ideally the fluid hitting your blade stops. If it does you get all the momentum available, but if it did it interferes with new moving fluid arriving. That would slow down fluid speed on the blades and take away power. That all changes with fluid viscosity, density, and velocity. So we have to strike a balance for real applications.
We also consider what you prop needs to do, be light, efficient, maximize power transfer, be quiet, etc.
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