Vortex ring state

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 USA,[2][3][4] but not in Canada, which uses the latter term for a different phenomenon.[5]

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]

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 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 manoeuvres 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 afloat 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]

Description
Airflow in normal flight (A), in rapid descent (B) and VRS (C)

Because the blades are rotating about a central axis, the speed of each airfoil is lowest at the point where it connects to the hub-and-grip assembly. This fundamental physical reality means that the innermost portion of each blade has an inherent vulnerability to stalling.

In forward flight with translational lift, there is no upward flow (upflow) of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins simply because there are no airfoil surfaces in the area of the hub, mast and blade-grip assembly.

Then, as the volume of upflow increases in the central region (i.e. between the hub and the innermost edges of the airfoils), the induced flow (air pulled or "induced" downwards through the rotor system) of the inner blade sections is overcome. This causes the innermost portions of the blades to begin to stall.

As the inner blade sections stall, a second set of vortices, similar to the rotor-tip vortices, begins to form in and around the center of the rotor system. This, combined with the outer set of vortices, results in severe loss of lift. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and catastrophic ground impact.[1]

Occurrence

A helicopter normally encounters this condition when attempting to hover out-of-ground-effect (OGE) without maintaining precise altitude control, and while making downwind or steep, powered approaches when the airspeed is below Effective Translational Lift (ETL).[7]

Detection and correction

The signs of VRS are a vibration in the main rotor system[8] followed by an increasing sink rate and possibly a decrease of cyclic authority.[9]

In single rotor helicopters, the vortex ring state is traditionally corrected by slightly lowering the collective to regain cyclic authority and using the cyclic control to apply lateral motion, often pitching the nose down to establish forward flight. In tandem-rotor helicopters, recovery is accomplished through lateral cyclic or pedal input or both. The aircraft will fly out of the vortex ring into "clean air", and will be able to regain lift.[1]

Another correction now widely known as the Vuichard Recovery Technique after gaining recent popularity, was taught by Claude Vuichard, a Federal Office for Civil Aviation (FOCA) inspector in Switzerland. This technique uses a combination of all three controls together to reduce altitude loss and recover more quickly: apply cyclic in the direction of tail rotor thrust, increase the collective to climb power, and coordinate with the power pedal to maintain heading (cross controls). Recovery is complete when the rotor disc reaches the upwind part of the vortex.[1][10][11][12]

Powering out of vortex ring state

It is possible to power out of vortex ring state, but this requires having about twice the power it takes to hover. Only one full-scale helicopter, the Sikorsky S-64 Skycrane, is documented as being able to do this, when unladen.[13]

Pilot or operator reaction

Helicopter pilots are most commonly taught to avoid VRS by monitoring their rates of descent at lower airspeeds. When encountering VRS, pilots are taught to apply forward cyclic to fly out of the condition and/or lowering collective pitch.[9] While transitioning to forward or lateral flight will alleviate the condition by itself, lowering the collective to reduce the power demand decreases the size of the vortices and reduces the amount of time required to be free of the condition. However, since the condition often occurs near the ground, lowering the collective may not be an option; a loss of altitude will occur proportional to the rate of descent developed before beginning the recovery. In some cases, vortex ring state is encountered and allowed to advance to the point that the pilot may severely lose cyclic authority due to the disrupted airflow. In these cases, the pilot's only recourse may be to enter an autorotation to break the rotor system free of its vortex ring state.[1]

Tandem rotor helicopters

In a tandem rotor helicopter, forward cyclic will not arrest the rate of descent caused by VRS. In such a helicopter, which utilizes differential collective pitch in order to gain airspeed, lateral cyclic inputs must be made accompanied by pedal inputs in order to slide horizontally out of the vortex ring state's disturbed air.

Radio control multirotors

Radio controlled multirotors (common on drones) are subject to normal rotorcraft aerodynamics, including vortex ring state. Frame design, size and power affect the likelihood of entering the state and recovering from it. Multirotors that do not have altitude hold are also more likely to succumb to operator error, where the pilot drops the craft too fast resulting in the upwash at the rotor hubs that can lead to vortex ring state. Those that are equipped with that feature, on the other hand, tend to control their descent automatically and can usually (but not always) escape the dangerous condition.[14]

See also

References
  1. 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.
  2. "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
  3. "Private Pilot Licence Examinations – 070 Operational Procedures Aeroplane and Helicopter" (PDF). Civil Aviation Authority. Retrieved 16 April 2023. Settling with power (vortex ring)
  4. "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 20 percent up to maximum 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.
  5. "Helicopter Flight Training Manual (TP 9982) – Exercise 26 – Vortex Ring" Transport Canada, 20 May 2010. Accessed: 13 September 2014.
  6. Capaccio, Tony (5 May 2011). "Helicopter Carrying SEALs Downed by Vortex, Not Mechanical Flaw or Gunfire". Bloomberg L.P.
  7. Federal Aviation Administration (December 2019), "11: Helicopter Emergencies and Hazards" (PDF), Helicopter Flying Handbook, United States, p. 9, ISBN 978-1-61954-992-0{{citation}}: CS1 maint: location missing publisher (link)
  8. Johnson, Wayne. Helicopter theory pp99+106, Courier Dover Publications, 1980. Accessed: 25 February 2012. ISBN 0-486-68230-7
  9. Advisory Circular (AC) 61-13B, Basic Helicopter Handbook, U.S. Department of Transportation, Federal Aviation Administration. 1978
  10. Tucker, Tim (September 2015). "Flying Through the Vortex". Rotor & Wing. Aviation Today. Archived from the original on 10 January 2022. Retrieved 13 February 2016.
  11. Robinson R22/R44 Flight Training Guide, R22 Maneuver guide, Settling-With-Power/Vortex Ring State, Page 29, Revised: October 2013
  12. "Claude Vuichard & Tim Tucker tell the story behind the Vuichard Technique". Vertical Mag. 29 April 2021. Archived from the original on 29 April 2021.
  13. Dziubinski, Adam; Stalewski, Wienczyslaw (2007). "Vortex Ring State Simulation Using Actuator Disc" (PDF). Instytut Lotnictwa / Institute of Aviation - Computational Fluid Dynamics Department. Archived (PDF) from the original on 18 August 2021. Retrieved 14 March 2022.
  14. "Quadcopter "Wobble of Death": VRS Recovery and Avoidance". YouTube. Archived from the original on 12 December 2021. Retrieved 21 September 2014.
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