vortex ring state

The operating state of a helicopter main rotor in which the axial flow through the rotor is opposite to the axial flow outside the rotor disc area and to the rotor thrust. It is an unstable condition of powered flight in which the helicopter settles in its own rotor downwash. It is a flight condition in which if a pilot tries to hover the helicopter out of a ground effect at an altitude above its hovering ceiling, the helicopter descends in the turbulent air that has been accelerated downward. In this state, the helicopter experiences very rough flight and the pilot may lose control of the helicopter. High rates of descent develop that cannot be stopped with a full-up collective-pitch stick and full power. In fact, these control positions aggravate the situation and even higher rates of descent may develop. In a normal hover, all the airflow through the rotor is downward. When the helicopter is settling with power, some air flows upward through the rotor, and this upward flow causes two sets of vortices that destroy the lift produced by the rotor, resulting in descent. Also called settling with power.

Aviation dictionary. 2014.

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