Autorotation (fixed-wing aircraft)

For fixed-wing aircraft, autorotation is the tendency of an aircraft in or near a stall to roll spontaneously to the right or left, leading to a spin (a state of continuous 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 and lift coefficient decrease which tend to restore the wing to its original angle of attack. Conversely any decrease in angle of attack causes a decrease in lift coefficient which causes the wing to descend. As the wing descends, the angle of attack and lift coefficient increase which tends to restore the wing to its original angle of attack. For this reason the angle of attack is stable when it is less than the stalling angle. 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, which causes the lift coefficient to decrease and the angle of attack to increase. 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 causes the lift coefficient to increase further towards the maximum lift coefficient. For this reason the angle of attack is unstable when it is greater than the stalling angle. 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 reaches the stalling angle the aircraft is at risk of autorotation. This will eventually develop into a spin if the pilot does not take corrective action.

Autorotation is the basis of a large sector of airborne wind energy (AWE) technology. High altitude wind power research and development centers frequently are dependent on blade autorotation: SkyMill Energy,Joby Energy,Sky Windpower, BaseLoad Energy, Magenn Power, and Makani Power are making and testing airborne wind energy conversion systems (AWECS) that employ autorotation of blades to drive the shafts of generators to make electricity at altitude and send the electricity to earth via conductive tethers.

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Wikipedia