Magic Helicopter

If the frequency at which the camera takes photographs is equal to the frequency of the blades rotating, then between two successive photographs there will be a time difference of one period of the blades rotating. Therefore, the blades will be at the same spot in each successive photograph.

The camera trick involved here is the synchronism of the shutter speed of the camera with the speed of rotating blades.

Shutter speed of the camera determines the amount of time a camera takes to gather light every time it takes a picture. Less the shutter speed more will be the motion-blur in case of a motion picture.

Let us assume that the camera is filming at 30 Frames Per Second(fps). Also let the speed of the rotor be 1 rev per sec. It means that the camera will take 30 pictures per second so that there would be 30 distinct pictures of the rotor each with a difference of 12 degrees rotation angle with the previous picture. These picture when played at 30 fps ( each picture visible to the eye for 1/30th of a second) allows us to perceive a motion picture. Now, if the rotor of the helicopter rotates at 30 rev per sec and we are still filming at 30 fps, each full revolution will be represented by only one frame. The blades will arrive back in their starting position each time the camera captures a frame, so they’ll look like they’re stagnant.

So there you have it, it's actually moving, but you just can't see it.