Autorotation

In aviation, the word autorotation is applied to operation of fixed-wing aircraft and rotary-wing aircraft. The word has significantly different meanings in each of these two applications.

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.^[1][2]

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, a helicopter may be able to slow its descent before landing and land in a controlled manner, using autorotation.

Autorotation in fixed-wing aircraft

A typical graph of angle of attack versus lift coefficient and drag coefficient. At any angle of attack greater than the stalling angle an increase in angle of attack causes a reduction in lift coefficient, and a decrease in angle of attack causes an increase in lift coefficient.

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, which tends 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 increases, 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.^[1][3] The aircraft displays damping in roll.^[4]

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. ^[1][3]

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 in helicopters

Autorotation is the phenomenon which results in the rotation of and lift generation by a rotorcraft's primary rotor through purely aerodynamic forces under certain conditions. Autorotation is employed in the normal operation of an autogyro as the primary lifting mechanism whereas it is used in helicopters only in an emergency mode after failure of the helicopter's powerplant or transmission.

Blade regions in vertical autorotation descent.

Autorotation is a 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.^[5] 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.

Autorotation of the main rotor in helicopters allows a controlled descent to an emergency landing in case of powerplant failure. Proper design of the helicopter assures that autorotation can be 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.

Autorotation in kites and gliders

1. Magnus effect rotating kites that have the rotation axis bluntly normal to the stream direction use autorotation; a net lift is possible that lifts the kite and payload to altitude. The Rotoplane, the UFO rotating kite, and the Skybow rotating ribbon arch kite use the Magnus effect resulting from the autorating wing with rotation axis normal to the stream.^[6]
2. Some kites are equipped with autorotation wings.^[6]
3. Again, a third kind of autorotation occurs in self-rotating bols, rotating parachutes, or rotating helical objects sometimes used as kite tails or kite-line laundry. This kind of autorotation drives wind and water propeller-type turbines, sometimes used to generate electricity.^[7][8]
4. Unlocked engine-off aircraft propellers may autorotate. Such autorotation is being explored for generating electricity to recharge flight-driving batteries.^[9]

See also

• Autorotation (helicopter)
• Kite types
• Samara (fruit), a natural structure evolved to autorotate

References

• Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London. ISBN 0 273 01120 0
• "Autorotation and spin entry". http://www.av8n.com/how/htm/spins.html#sec-Spin-Entry. Retrieved 2009-02-24. 
• Stinton, Darryl (1996), Flying Qualities and Flight Testing of The Aeroplane, Blackwell Science Ltd, Oxford UK. ISBN 0-632-02121-7

Notes