Keep Calm and Autorotate T-shirt

Keep Calm and Autorotate T-shirt.

Autorotation is a state of flight in which the main rotor system of a helicopter or similar aircraft turns by the action of air moving up through the rotor, as with an autogyro, rather than engine power driving the rotor. The term autorotation dates to a period of early helicopter development between 1915 and 1920, and refers to the rotors turning without the engine.

In normal powered helicopter flight, air is drawn into the main rotor system from above and exhausted downward, but during autorotation, air moves up into the rotor system from below as the helicopter descends. Autorotation is permitted mechanically because of both a freewheeling unit, which allows the main rotor to continue turning even if the engine is not running, as well as aerodynamic forces of relative wind maintaining rotor speed. It is the means by which a helicopter can land safely in the event of complete engine failure. Consequently, all single-engine helicopters must demonstrate this capability to obtain a type certificate.

The longest autorotation in history was performed by Jean Boulet in 1972 when he reached a record altitude of 12,440 m (40,814 ft) in an Aérospatiale Lama. Because of a −63 °C temperature at that altitude, as soon as he reduced power the engine flamed out and could not be restarted. By using autorotation he was able to land the aircraft safely.