Rotorcraft Flying Menu >Aerodynamics of Flight  >Autorotation

Autorotation is the state of flight where the main rotor system is being turned by the action of relative wind rather than engine power. It is the means by which a helicopter can be landed safely in the event of an engine failure. In this case, you are using altitude as potential energy and converting it to kinetic energy dur-ing the descent and touchdown. All helicopters must have this capability in order to be certified. Autorotation is permitted mechanically because of a freewheeling unit, which allows the main rotor to con-tinue turning even if the engine is not running. In nor-mal powered flight, air is drawn into the main rotor sys-tem from above and exhausted downward. During autorotation, airflow enters the rotor disc from below as the helicopter descends. [Figure 3-20]

Figure 3-20. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. In effect, the blades are “gliding” in their rotational plane.