Maybe you have thought why wheels sometimes appear to spin backward. This is so called the stroboscopic effect.

Of course, human’s eyes are not capable of catching all of the motion, especially very fast and continuous ones. In addition, some of cameras have limitation to record every single frame of the motion. Therefore, sometimes wheels appear to spin backward, or appeared to switch the directions of the spin.

Now, here is a very special case. (Well, maybe not.) The blades of a helicopter don’t look like spinning, but the helicopter is being levitated very slowly.

The frequencies of the spinning blades and camera frames look like synchronized.