Vortex ring

A vortex ring is an undesirable aerodynamic condition inherent to rotary-wing aircraft, sometimes induced by rapid altitude changes. In typical flight, the rotor disc directs the airflow downwards against the terrain, creating lift. A vortex ring state though involves a toroid-shaped path of airflow circumscribing the blade disc, as the airflow moves down through the disc, then outward, and then down through the top again. This circulation can negate much of the lifting force and cause a catastrophic loss of altitude.