Eli5: Why airplanes have shorter propellers, when long helicopter propellers could move more air?

Question

Isn’t forward motion created by moving more air?

In: Engineering

Answer 1

An airplane pushes itself forward by pushing air backward. It can move a little bit of air slowly, or a lot of air fast. It takes less energy to get the same thrust if you move a lot of air slowly, *but* if the air comes into the propeller moving faster than it leaves, then your plane is actually going to be slowing itself down.

So a big propeller moving air slowly is way more efficient, *if* the plane is moving slowly. Helicopters don’t move up or down very fast, so moving a lot of air slowly is ideal. Airplanes move pretty fast, so they *have* to move air fast too, and a small propeller generates enough thrust because the air it moves is fast. A bigger propeller would be less efficient, moving extra air.