Have you ever wondered what would happen if a helicopter engine just stopped running? It would drop like a rock, right? Well not quite, actually not at all. Provided the blades are still intact, the pilot can employ a technique called autorotation to safely bring the chopper back to the ground. Autorotation works as air below the descending helicopter moves up through the rotor causing it to continue to turn thanks to a freewheeling unit. Using this technique pilots can safely bring a helicopter to rest if there would happen to be an engine failure.
The freewheeling unit in a helicopter consists of a clutch that disengages whenever the rotational speed of the engine drops below the speed of the rotor. This allows the upward flow of air to drive the rotor
There are several factors that will lead to the successful landing under autorotation. To start, the collective pitch must be lowered to reduce drag and lift which will begin descent.