I Didn’t Know That . . .

Q: Is a fan with five blades more effective than one with three blades?

A: The more blades a fan has, the less efficient it becomes because each blade is moving in the turbulent wake of the blade preceding it. But a fan with fewer blades has to turn faster to move the same amount of air, so it will have a higher tip speed and thus be noisier. The same principles apply to airplane and ship propellers. The optimum efficiency is achieved with a single blade attached to a suitable counterweight, and some planes have actually flown with such a design. The primary reason for multiple blades is to reduce the diameter and the noise.