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Vortex ring state
The vortex ring state, also known as settling with power, is a dangerous condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor causing severe loss of lift. The FAA sees these terms as the same thing, whereas Transport Canada sees them as two different phenomena.〔"(Helicopter Flight Training Manual (TP 9982) - EXERCISE 26 - VORTEX RING )" ''Transport Canada'', 20 May 2010. Accessed: 13 September 2014. Quote: There are some uninformed pilots who use “settling with power” to describe vortex ring, in fact some publications use the terms interchangeably. Confusion results when symptoms are related that do not describe true vortex ring but rather describe “settling with insufficient power”.〕
Essentially, the helicopter descends into its own downwash. When the condition arises, increasing the rotor power merely feeds the vortex motion without generating additional lift.〔(Main page ). ''(Chapter 11: Helicopter Emergencies and Hazards )'' ''(Rotorcraft Flying Handbook, FAA Manual H-8083-21. Complete manual, 84 MB )'', Washington, DC: Flight Standards Service, Federal Aviation Administration, U.S. Dept. of Transportation, 2001. ISBN 1-56027-404-2, page 11-5.〕〔Advisory Circular (AC) 61-13B, ''Basic Helicopter Handbook'', U.S. Department of Transportation, Federal Aviation Administration. 1978〕 This condition also occurs with tiltrotors, and was responsible for an accident involving a V-22 Osprey. Vortex ring state caused the loss of a heavily modified MH-60 helicopter during Operation Neptune Spear, the raid that killed Osama bin Laden.
== Description ==
In forward flight, there is no upward flow (''upflow'') of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins because there are no airfoil surfaces in the mast and blade grip area. As volume of upflow increases, the induced flow (air pulled or "induced" down through the rotor system) of the inner blade sections is overcome and the blades begin to stall near the hub. As the inner blade sections stall, a second set of vortices, similar to the rotor tip vortices, form in the center of the rotor system. The inner set of vortices decreases the amount of lift being produced and causes an increase in sink rate. In an accelerated condition, the inner and outer vortices begin to feed each other to the point where any increase in rotor blade pitch angle increases the interaction between the vortices and increases the rate of descent. In this state, the helicopter is operating in its own downwash, descending through descending air. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and impact with terrain.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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