Figure 11-3.  Vortex ring state.  called the windmill brake state, the point where the airflow  is completely up through the rotor, the only recovery may  be an autorotation. Tandem rotor helicopters should maneuver laterally to  achieve clean air in both rotors at the same time.  For vortex ring state demonstrations and training in  recognition and recovery should be performed from a safe  altitude to allow recovery no less than 1000 feet AGL or the  manufacturer’s recommended altitude, whichever is higher.  To enter the maneuver, come to an OGE hover, maintaining  little or no airspeed (any direction), decrease collective  to begin a vertical descent, and as the turbulence begins,  increase collective. Then allow the sink rate to increase to 300  fpm or more as the attitude is adjusted to obtain airspeed of  less than 10 knots. When the aircraft begins to shudder, the  application of additional up collective increases the vibration  and sink rate. As the power is increased, the rate of sink of  the aircraft in the column of air will increase. that are below translational lift airspeeds are within this  region of susceptibility to vortex ring state aerodynamics.  This condition is sometimes seen during quick-stop type  maneuvers or during recovery from autorotation.  The following combination of conditions is likely to cause  settling in a vortex ring state in any helicopter: If altitude is sufficient, some time can be spent in the  vortices, to enable the pilot to develop a healthy knowledge  of the maneuver. However, helicopter pilots would normally  initiate recovery at the first indication of vortex ring state.  Recovery should be initiated at the first sign of vortex ring  state by applying forward cyclic to increase airspeed and/ or  simultaneously reducing collective. The recovery is complete  when the aircraft passes through effective translational lift  and a normal climb is established. Situations that are conducive to a vortex ring state condition  are attempting to hover OGE without maintaining precise  altitude control, and approaches, especially steep approaches,  with a tailwind component. When recovering from a vortex ring state condition, the pilot  tends first to try to stop the descent by increasing collective  pitch. However, this only results in increasing the stalled  area of the rotor, thereby increasing the rate of descent. Since  inboard portions of the blades are stalled, cyclic control  may be limited. The traditional recovery is accomplished  by increasing airspeed, and/or partially lowering collective  to exit the vortex. In most helicopters, lateral cyclic thrust  combined with an increase in power and lateral antitorque  thrust will produce the quickest exit from the hazard. This  technique, known as the Vuichard Recovery (named after the  Swiss examiner from the Federal Office of Civil Aviation  who developed it) recovers by eliminating the descent rate as  opposed to exiting the vortex. If the vortex ring state and the  corresponding descent rate is allowed to progress to what is  In forward flight, the relative airflow through the main rotor  disk is different on the advancing and retreating side. The  relative airflow over the advancing side is higher due to the  forward speed of the helicopter, while the relative airflow on  the retreating side is lower. This dissymmetry of lift increases  as forward speed increases. To generate the same amount of lift across the rotor disk,  the advancing blade flaps up while the retreating blade flaps  down. This causes the AOA to decrease on the advancing  11-10 Retreating Blade Stall Common Errors—Vuichard Recovery Common Errors—Traditional Recovery 1.  A vertical or nearly vertical descent of at least 300  fpm. (Actual critical rate depends on the gross weight,  rpm, density altitude, and other pertinent factors.) 2.  The rotor disk must be using some of the available  engine power (20–100 percent). 3. The horizontal velocity must be slower than effective  translational lift. 1.  Too much lateral speed for entry into vortex ring state. 2.  Excessive decrease of collective. 1.  Excessive lateral cyclic 2.  Failure to maintain heading