Autorotation is a term used in aeronautics to describe the behavior of aircraft while airborne. It is different between fixed-wing and rotary-wing aircraft, with helicopters and autogyros generating lift from the main rotor without engine power. Autorotation is made possible by a freewheel unit in helicopters, making them safer than fixed-wing aircraft.
Autorotation is an aeronautical term used to describe a particular behavior displayed by fixed-wing and rotary-wing aircraft while airborne. However, the actual move, while going by the same name, is actually quite different between these two types. In airplanes and other fixed-wing craft, it is the phenomenon whereby an aircraft tends to roll one way or another, and continues to turn in the same direction, as it approaches a post. Much more complex in a rotary wing aircraft, helicopter or gyroplane in a self-rotating state is generating lift from the main rotor, without the aid of engine power.
Autogyros are unique in that their main rotor is always unpowered, so they uniquely and deliberately generate lift through autorotation. Only the propeller in a gyroplane has power, and it is solely responsible for providing directional thrust, not lift. Helicopters, by comparison, generally only auto-turn when forced to do so in emergency situations after engine failure. However, the physics behind autorotation in helicopters and gyroplanes is the same.
Essentially, a rotor starts spinning automatically as soon as it starts spinning faster than the motor that drives it. Again, in the case of an autogrio, this happens instantly since there is no rotor motor. The math that describes how autorotation works is complex. Generally speaking, however, it is based on a balance of different amounts of torque on the outer and inner portions of the main rotor. Balance must be maintained through constant and painstaking corrections to keep the ship controllable.
As air moves upward through a moving rotor, the shape of its blades causes them to spin naturally. This occurs in the same way that a ceiling fan can spin slowly in a drafty house, even when turned off. In the case of helicopters, this means that the craft can be safely controlled and landed even without engine power. Many helicopter advocates argue for autorotation, and the resulting ability for a pilot to maintain control even when there is no forward motion, makes them much safer than their fixed-wing counterparts.
Autorotation in helicopters is made mechanically possible by a special type of clutch, known as a freewheel unit. Attached to the rotor, this device works in roughly the same way that a car clutch allows a car to continue moving even while depressed. The free-spinning units only work in one direction, making it impossible for the rotors to spin in what could be considered the wrong way, driving a helicopter downward. Free-turning units and automatic rotation are generally so important to rotary-wing flight that all single-engine helicopter designs in the United States and many other countries must demonstrate their capability before being released. considered fit for flight.
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