Wiktionary
n. The rotation of the blades of a helicopter driven solely by the action of ascending air
Wikipedia
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).
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 (−81.4 °F) 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.
Usage examples of "autorotation".
He knew there was no room or time at this altitude for a safe autorotation should the engine fail.
Instead of crashing, the helicopter steadied, caught the last bit of available air in the autorotation mode and sank with a loud clanking sound onto the apron.
There I would attempt to hover the machine, talk to the tower, and be ready for a hovering autorotation at any moment.
As we lumbered along, I realized we were so heavily loaded that an autorotation would be a lot rougher than anyone would like when carrying more than a ton of high explosives.
He pushed the collective down like he was making an autorotation and we crashed before I could stop it.
I thought that actually skidding across the ground, finishing the autorotation, was a key experience, so I let each student do it.
When during takeoff a helicopter pilot detects or senses that something is not going exactly as it should be going, he can raise the forward end of the rotor cone, which reduces or eliminates forward speed, and he can thereafter make a powered or unpowered descent an autorotation to the ground immediately under him.
She was an expert pilot and had practiced autorotation hundreds of times.
In addition, the anonymous pirate was to inform Smoke that Cat was busy practicing autorotations and getting his check ride and would meet them at Tangier Island after the race so he had time to do a high recon of the area and set up their new headquarters.
Andy practiced autorotations regularly and not only was good at them, but he loved the excitement of landing a four-ton machine without the help of its engines.