Continuing through my misconceptions dispelled since I started studying aviation… A helicopter is doomed if its engine fails. After all, what does it have left to work with?
Well, it still has its main rotor. In an emergency maneuver called autorotation, the main rotor is allowed to freely windmill as the helicopter drops. This does two things: (1) it exerts a drag force so that the aircraft has a reasonably low terminal velocity, and (2) it builds up the angular momentum of the main rotor. As the helicopter nears the ground, the rotor blade pitch is changed, causing the stored angular momentum to be converted to lift. The lifting force is enough to slow the descent to near zero and gently set the helicopter on the ground.
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