World's longest autorotation by keeping the heli in a continuous, theoretically indefinite state of autorotation. The simplest (but incorrect) definition of an autorotation is the landing of a helicopter without a powered rotor. Actually, it is a flight condition -- a well-defined one, to be more exact. During an auto the heli descends with a constant, defined rotor rpm and at a constant, controlled rate; engine not powering the rotor of course.
Theoretically it should be feasible to maintain such a state of autorotation for an infinite time by simply moving an unpowered helicopter through the air fast enough - e.g. by towing it with a car. Problem are factors such as wind and turbulence behind the car. We nonetheless got it to work after an initial crash.
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