Eli5: Why is it when you see a propeller (like from a turbine airplane) starting up it seems to switch directions as it spins faster and faster ?

Question

Eli5: Why is it when you see a propeller (like from a turbine airplane) starting up it seems to switch directions as it spins faster and faster ?

In: Physics

Answer 1

You typically don’t see this with your eyes, but do with a video camera. It has to do with aliasing, which means you aren’t taking pictures fast enough. If your camera is doing 30 fps, it takes one picture every 1/30th of a second. If the propeller is spinning at 30 rotations per second, it will appear to be still, because it will be in the same spot every time the camera takes a picture. If it’s going a little bit less than 30 rotations per second, it will actually appear to move backwards because it rotated *almost* all the way around by the time the next picture is taken.