What is autorotation in rotary wing flight?

Autorotation in rotary wing flight refers to the phenomenon where a helicopter descends rapidly without engine power. This occurs when a helicopter's rotor system continues to rotate due to the upward flow of air through the rotor blades even after the power is cut off. In this state, the pilot can control the descent rate and direction by adjusting the pitch of the rotor blades, allowing for a controlled landing even when power is lost.

The essential feature of autorotation is that it relies on the aerodynamic principle of generating lift contrary to the gravitational force acting on the helicopter during descent. Pilots train extensively on autorotation procedures to ensure they can manage scenarios where engine power is lost and to facilitate safe landings from substantial altitudes.

The other possible answers do not accurately represent autorotation. Ascending without power suggests a scenario that does not align with the mechanics of flight when power is lost. Hovering at a specific altitude involves maintaining a stable position against gravitational pull using engine power, which contradicts the idea of autorotation. Changing flight direction, while an important aspect of helicopter maneuverability, does not capture the essence of autorotation either, as autorotation specifically describes a state of descent when power is no longer supplied.