Autorotation

Autorotation is a theory that applies to the operation of fixed-wing aircrafts and rotary-wing aircrafts.

during an engine failure, a helicopter may be able to slow its descent before landing and lan in a controlled manner, using the characteristic autorotation.

Fixed-wing aircrafts

When an aircraft is in stall, or approaching the stall, it displays a tendency to roll spontaneously to the right or to the left. A fixed-wing aircraft in a spin, rolls continuously to the right or to the left, which displays the characteristic 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.

Rotor-wing aircrafts

in operation of rotor-wing aircrafts, such as helicopters and autogyros, the generation lift by the main rotor, even though no power is being provided to the rotor by an engine is known as autorotation.

Autogyros main rotor is un-powered, which results the autogyro to completely rely on continuous autorotation to generate the lift.