In the operation of fixed-wing aircraft, autorotation is the name given to the manner in which an aircraft in a stall, or approaching the stall, displays a tendency to roll spontaneously to the right or left. A fixed-wing aircraft in a spin rolls continuously to the right or left, displaying the characteristic known as autorotation.
In the operation of helicopters and autogyros, autorotation is the name given to the generation of lift by the main rotor even though no power is being provided to the rotor by an engine. Autogyros have an un-powered main rotor so they rely continuously on autorotation as their source of lift. Following an engine failure, helicopters are able to glide safely to the ground using autorotation.
When the angle of attack is less than the stalling angle, any increase in angle of attack causes an increase in lift coefficient that causes the wing to rise. As the wing rises the angle of attack decreases, and that tends to restore the wing to its original angle of attack. Conversely, any decrease in angle of attack causes a decrease in lift coefficient that causes the wing to descend. As the wing descends the angle of attack increases, and that tends to restore the wing to its original angle of attack. For this reason, when the angle of attack is less than the stalling angle, the angle of attack is stable. The aircraft displays damping in roll.
When the wing is stalled and the angle of attack is greater than the stalling angle, any increase in angle of attack causes a decrease in lift coefficient that causes the wing to descend. As the wing descends the angle of attack increases, and that causes the lift coefficient to decrease further and the angle of attack to increase even further. Conversely, any decrease in angle of attack causes an increase in lift coefficient that causes the wing to rise. As the wing rises the angle of attack decreases, and that causes the lift coefficient to increase further towards the maximum lift coefficient. For this reason, when the angle of attack is greater than the stalling angle, the angle of attack is unstable. Any disturbance of the angle of attack on one wing will cause the whole wing to roll spontaneously and continuously.
When the angle of attack on the wing of an aircraft, or on one wing, reaches the stalling angle, the aircraft is at risk of autorotation that will eventually develop into a spin if the pilot does not take corrective action.
Autorotation is a somewhat complex phenomenon involving the balance of opposing aerodynamic forces along the rotor's blades. Because of the rapidly varying airspeeds, Mach numbers, and angles of attack encountered by the blades as they traverse a full rotation, analysis of the aerodynamics of the rotor presents a difficult problem in fluid dynamics. Generally, however, it is the portion of the blade nearer the hub that provides the aerodynamic force (torque) tending to increase rotational speed, and the portion toward the blade tip that provides the preponderance of the lifting force. The aerodynamic drag produced by this outer blade region opposes the torque from the inner region, and thus the rotor's speed arrives at an equilibrium point at which these torques balance each other. The equilibrium speed depends on a number of factors including the speed of the rotor through the air, the angle between the plane in which the blades move and the incoming air, and the collective pitch setting of the rotor.
In helicopters, autorotation of the main rotor allows a controlled descent to an emergency landing in case of powerplant failure. Proper design of the helicopter is necessary to assure that autorotation can be usefully employed, and skilled changes to the collective and cyclic pitch are necessary during the maneuver to manage the energy of the rotor and the airspeed of the craft. Autorotation depends on the maintenance of air velocity through the rotor, and during an emergency autorotation maneuver this airspeed is provided by the helicopter's descent. In autogyros, airspeed is provided by power from an airplane-type propeller which propels the aircraft horizontally; autorotation is the sole source of rotational power for the main rotor.