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Autorotation is an aeronautical term used to describe particular behavior displayed by both fixed wing and rotary wing aircraft, while aloft. The actual movement, though given the same name, is actually quite different between these two types, however. In airplanes and other fixed-wing craft, it is the phenomenon whereby a plane tends to roll one way or the other, and continue spinning in the same direction, when approaching a stall. Much more complex in rotary wing aircraft, a helicopter or autogyro in a state of autorotation is generating lift from the main rotor, without the assistance of power from the engine.
Autogyros are unique in that their main rotor is always unpowered, so they generate lift solely and purposely through autorotation. Only the propeller in an autogyro is powered, and it is tasked solely with providing directional thrust, not lift. Helicopters, by comparison, typically only autorotate when forced to in emergency situations following engine failure. The physics behind autorotation in helicopters and autogyros is the same, however.
Essentially, a rotor begins to autorotate as soon as it starts to spin faster than the engine powering it. Again, in the case of an autogryo, this happens instantly as there is no rotor engine. The mathematics that describe how autorotation works is complex. In general terms, however, it relies on a balancing of differing amounts of torque on the outer and inner portions of the main rotor. Equilibrium must be maintained by constant, minute corrections to keep the craft controllable.
As air moves up through a moving rotor, the shape of its blades causes them to spin naturally. This occurs in much the same way as a ceiling fan may spin slowly in a drafty house, even when turned off. In the case of helicopters, this means the craft can be controlled and safely landed even with no engine power at all. Many helicopter proponents argue autorotation, and the resulting ability of a pilot to maintain control even when there is no forward motion, makes them far safer than their fixed-wing counterparts.
Autorotation in helicopters is made mechanically possible by a special type of clutch, known as a freewheeling unit. Attached to the rotor, this device works in roughly the same way that an automobile clutch allows a car to continue moving even while it is depressed. Freewheeling units only work in one direction, so it is impossible for the rotors to ever spin in what could be considered the wrong way, and drive a helicopter down. So critical are freewheeling units and autorotation, generally, to rotary wing flight, that all one-engine helicopter designs in the United States and numerous other countries must prove they are capable of it before being deemed airworthy.