Vortex ring effect in helicopters
The curved arrows indicate airflow circulation about the rotor disc. The helicopter shown is the RAH-66 Comanche. (Graphic)
In typical flight, the rotor disc directs the airflow downwards, 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.
A helicopter typically induces a vortex ring state by descending into its own downwash. This requires low airspeed and a moderate rate of descent with power applied, and can lead to an undesirable phase of flight known as settling with power. This condition can be corrected by lowering the collective, which controls the pitch angle of the rotor blade, slightly pitching nose down, and establishing forward flight. The aircraft will fly into "clean air", and will be able to regain lift.
And:
Settling with power
From Wikipedia, the free encyclopedia
Settling with power is a hazardous helicopter flight condition in which the aircraft's available power is not sufficient to overcome gravity and inertia, and may not have enough power to stop the descent. Settling with power usually occurs under conditions of high all-up-weight and high altitude.
The effects and of settling with power are similar to vortex ring state. A standard procedure exists to correct both conditions.
Moral of the story:
Don't let airspeed get to 0 at altitude/high DA, esp. when heavy....
The physics which describe ground effect are still very much under debate. A common belief is that ground effect is caused by a "cushion" of compressed air between the wing and the ground. However, wind tunnel testing and experiments have indicated that while a "cushion" effect is present, ground effect is almost solely due to the ground interrupting the formation of wingtip vortices.
