Aerodynamic condition related to helicopter flight
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Vortex ring state, in which airflow is upward on the inner blade section, producing a secondary vortex in addition to the normal wingtip vortices. Turbulent airflow results in loss of rotor efficiency. If allowed to continue, uncommanded pitch-and-roll oscillations may occur, with a large descent rate.[1]
The vortex ring state (VRS) is a dangerous aerodynamic condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor, causing severe loss of lift. Often the term settling with power is used as a synonym, e.g., in Australia, the UK, and the US,[2][3][4] but not in Canada, which uses the latter term for a different phenomenon.[5]
A vortex ring state sets in when the airflow around a helicopter's main rotor assumes a rotationally symmetrical form over the tips of the blades, supported by a laminar flow over the blade tips, and a countering upflow of air outside and away from the rotor. In this condition, the rotor falls into a new topological state of the surrounding flow field, induced by its own downwash, and suddenly loses lift. Since vortex rings are a surprisingly stable fluid dynamical phenomena (a form of topological soliton), the best way to recover from them is to laterally steer clear of them, in order to re-establish lift, and to break them up using maximum engine power, in order to establish turbulence.
This is also why the condition is often mistaken for "settling with insufficient power": high-powered maneuvers can both induce a vortex ring state in free air, and then at low altitude, during landing conditions, possibly break it. If sufficient power is not available to maintain the airfoil of the rotor at a stalled condition, while generating sufficient lift, the aircraft will not be able to stay aloft before the vortex ring state dissipates, and will crash.
This condition also occurs with tiltrotors, and it was responsible for an accident involving a V-22 Osprey in 2000. Vortex ring state caused the loss of a heavily modified MH-60 helicopter during Operation Neptune Spear, the 2011 raid in which Osama bin Laden was killed.[6]
^Helicopter Flying Handbook, FAA-H-8083-21A(PDF). U.S. Dept. of Transportation, FAA, Flight Standards Service. 2012. pp. 11-8–11-12, 11-17–11-20.
^"Assessment Requirements for AVIY0029 Operate rotary wing remote pilot aircraft systems" (PDF). Australian Government. Retrieved 16 April 2023. recognition and avoidance of settling with power/vortex ring state
^"Private Pilot Licence Examinations – 070 Operational Procedures Aeroplane and Helicopter" (PDF). Civil Aviation Authority. Retrieved 16 April 2023. Settling with power (vortex ring)
^"Helicopter Emergencies and Hazards" (PDF). Federal Aviation Administration (FAA). Retrieved 16 April 2023. Vortex ring state (formerly referenced as settling-with- power) describes an aerodynamic condition in which a helicopter may be in a vertical descent with between 20 and 100 percent power applied, and little or no climb performance. The previously used term settling-with-power came from the fact that the helicopter keeps settling even though full engine power is applied.
^"Helicopter Flight Training Manual (TP 9982) – Exercise 26 – Vortex Ring" Transport Canada, 20 May 2010. Accessed: 13 September 2014.
^Capaccio, Tony (5 May 2011). "Helicopter Carrying SEALs Downed by Vortex, Not Mechanical Flaw or Gunfire". Bloomberg L.P.
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