1. A vertical or nearly vertical descent of at least 300 feet per minute. (Actual critical rate depends on the gross weight, r.p.m., density altitude, and other pertinent factors.) 2. The rotor system must be using some of the avail- able engine power (from 20 to 100 percent). 3. The horizontal velocity must be slower than effective translational lift. entering the rotor from the top. This phenomenon is common to all airfoils and is known as tip vortices. Tip vortices consume engine power but produce no useful lift. As long as the tip vortices are small, their only effect is a small loss in rotor efficiency. However, when the helicopter begins to descend vertically, it settles into its own downwash, which greatly enlarges the tip vortices. In this vortex ring state, most of the power developed by the engine is wasted in accelerating the air in a doughnut pattern around the rotor.  In addition, the helicopter may descend at a rate that exceeds the normal downward induced-flow rate of the inner blade sections. As a result, the airflow of the inner blade sections is upward relative to the disc. This pro- duces a secondary vortex ring in addition to the normal tip-vortices. The secondary vortex ring is generated about the point on the blade where the airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the disc. Rotor efficiency is lost even though power is still being supplied from the engine. [Figure 11-4] A fully developed vortex ring state is characterized by an unstable condition where the helicopter experiences uncommanded pitch and roll oscillations, has little or no cyclic authority, and achieves a descent rate, which, if allowed to develop, may approach 6,000 feet per minute. It is accompanied by increased levels of  vibration.  A vortex ring state may be entered during any maneu- ver that places the main rotor in a condition of high upflow and low forward airspeed. This condition is  sometimes seen during quick-stop type maneuvers or during recoveries from autorotations. The following combination of conditions are likely to cause settling in a vortex ring state: In forward flight, the relative airflow through the main rotor disc is different on the advancing and retreating side. The relative airflow over the advanc- ing side is higher due to the forward speed of the  Recovery should be initiated at the first sign of vor- tex ring state by applying forward cyclic to increase airspeed and simultaneously reducing collective. The recovery is complete when the aircraft passes through effective translational lift and a normal climb is established. To enter the maneuver, reduce power below hover power. Hold altitude with aft cyclic until the  airspeed approaches 20 knots. Then allow the sink rate to increase to 300 feet per minute or more as the attitude is adjusted to obtain an airspeed of less than 10 knots. When the aircraft begins to shudder, the application of additional up collective increases the vibration and sink rate. For settling with power demonstrations and training in recognition of vortex ring state conditions, all maneu- vers should be performed at an elevation of at least 1,500 feet AGL. When recovering from a settling with power condition, the tendency on the part of the pilot is to first try to stop the descent by increasing collective pitch. However, this only results in increasing the stalled area of the rotor, thus increasing the rate of descent. Since inboard portions of the blades are stalled, cyclic control is  limited. Recovery is accomplished by increasing  forward speed, and/or partially lowering collective pitch. In a fully developed vortex ring state, the only recovery may be to enter autorotation to break the  vortex ring state. When cyclic authority is regained, you can then increase forward airspeed. Some of the situations that are conducive to a settling with power condition are: attempting to hover out of ground effect at altitudes above the hovering ceiling of the helicopter; attempting to hover out of ground effect without maintaining precise altitude control; or down- wind and steep power approaches in which airspeed is permitted to drop to nearly zero. Figure 11-4. Vortex ring state. 11-6 RETREATING BLADE STALL