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{{Short description|Aerodynamic condition related to helicopter flight}}
{{Use dmy dates|date=December 2020}}
{{Use Canadian English|date=December 2020}}
{{Too technical|date=February 2022}}
[[File:Vortex ring state.png|thumb|300px|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.<ref name="Helo">{{cite book |title=Helicopter Flying Handbook, FAA-H-8083-21A |date=2012 |publisher=U.S. Dept. of Transportation, FAA, Flight Standards Service |pages=11-8 - 11-12, 11-17 - 11-20 |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch11.pdf}}</ref>]]
The '''vortex ring state''' (VRS) is a dangerous aerodynamic condition that may arise in [[helicopter]] flight, when a [[vortex ring]] system engulfs the [[helicopter rotor|rotor]], causing severe loss of [[lift (force)|lift]]. Often the term '''settling with power''' is used as a synonym, e.g., in Australia, the UK, and the US,<ref>{{cite web |title=Assessment Requirements for AVIY0029 Operate rotary wing remote pilot aircraft systems |url=https://training.gov.au/TrainingComponentFiles/AVI/AVIY0029_AssessmentRequirements_R1.pdf |publisher=Australian Government |access-date=16 April 2023 |quote=recognition and avoidance of settling with power/vortex ring state}}</ref><ref>{{cite web |title=Private Pilot Licence Examinations – 070 Operational Procedures Aeroplane and Helicopter |url=https://publicapps.caa.co.uk/docs/33/PPL%20070%20Operational%20Procedures%20A%20%20H%20Learning%20Objectives.pdf |publisher=Civil Aviation Authority |access-date=16 April 2023 |quote=Settling with power (vortex ring)}}</ref><ref>{{cite web |title=Helicopter Emergencies and Hazards |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch11.pdf |publisher=Federal Aviation Administration (FAA) |access-date=16 April 2023 |quote=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.}}</ref> but not in Canada, which uses the latter term for a different phenomenon.<ref>"[http://www.tc.gc.ca/eng/civilaviation/publications/tp9982-exercise26-973.htm Helicopter Flight Training Manual (TP 9982) – Exercise 26 – Vortex Ring]" ''[[Transport Canada]]'', 20 May 2010. Accessed: 13 September 2014.</ref>

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 phenomenon (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 [[tiltrotor]]s, and it was responsible for [[2000 Marana V-22 crash|an accident]] involving a [[Bell-Boeing V-22 Osprey|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.<ref>{{cite news |url=https://www.bloomberg.com/news/2011-05-05/commando-black-hawk-downed-by-air-vortex-not-mechanics-in-bin-laden-raid.html|title=Helicopter Carrying SEALs Downed by Vortex, Not Mechanical Flaw or Gunfire|last=Capaccio|first=Tony|publisher=Bloomberg L.P.|date= May 5, 2011}}</ref>

== Description ==
[[File:État d'anneaux tourbillonnaires.png|thumb|300px|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 [[stall (flight)|stall]]ing.{{Citation needed|date=June 2022}}
  
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.{{Citation needed|date=June 2022}}
  
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 (flight)|stall]].{{Citation needed|date=June 2022}}  
  
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.<ref name="Helo" />
  
=== Occurrence ===
A helicopter normally encounters this condition when attempting to hover out-of-[[Ground effect (aerodynamics)|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).<ref>{{Citation |title=Helicopter Flying Handbook |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/ |chapter-url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch11.pdf#page=9 |chapter=11: Helicopter Emergencies and Hazards |page=9 |author= Federal Aviation Administration |date=December 2019 |location = United States |isbn=978-1-61954-992-0}}</ref>

=== Detection and correction ===
The signs of VRS are a vibration in the main rotor system<ref name=wayne>Johnson, Wayne. [https://books.google.com/books?id=SgZheyNeXJIC Helicopter theory] pp99+106, ''Courier Dover Publications'', 1980. Accessed: 25 February 2012. {{ISBN|0-486-68230-7}}</ref> followed by an increasing sink rate and possibly a decrease of [[Helicopter flight controls|cyclic]] authority.<ref name="AC 61-13B">Advisory Circular (AC) 61-13B, ''Basic Helicopter Handbook'', U.S. Department of Transportation, Federal Aviation Administration. 1978</ref>

In single rotor helicopters, the vortex ring state is traditionally corrected by slightly lowering the [[Helicopter flight controls#Collective|collective]] to regain cyclic authority and using the [[Helicopter flight controls#Cyclic|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.<ref name="Helo" />

An alternative, the Vuichard Recovery Technique, reduces altitude loss and recovers more quickly.  Developed by Claude Vuichard, a [[Federal Office of Civil Aviation]] inspector in Switzerland, this recently popular technique uses thrust from the unaffected tail rotor<ref name=Vertical2021/> to [[Sideslip#Sideslip|sideslip]] (move sideways without rotating) the helicopter by at least one rotor diameter.  It can be thought of as maximizing sideways thrust from the tail rotor and balancing with the cyclic and collective to avoid rotation, but because the main rotor responds more slowly to the controls, it is actually performed in the opposite order: increase the collective to climb power, and apply cyclic in the direction of tail rotor thrust (as if turning opposite main rotor rotation) to a 15–20° bank angle, all while using the pedals to maintain heading (cross controls). Recovery is complete when the rotor disc reaches the upwind part of the vortex.<ref name="Helo"/><ref>{{cite web|last1=Tucker |first1=Tim |title=Flying Through the Vortex |url=https://www.rotorandwing.com/2015/09/01/flying-through-the-vortex/ |website=Rotor & Wing |date=September 2015 |publisher=Aviation Today |access-date=13 February 2016 |archive-url= https://archive.today/20220110230301/https://www.rotorandwing.com/2015/09/01/flying-through-the-vortex/ |archive-date= 10 January 2022 |url-status=live}}</ref><ref>Robinson R22/R44 Flight Training Guide, R22 Maneuver guide, Settling-With-Power/Vortex Ring State, Page 29, Revised: October 2013</ref><ref name=Vertical2021>{{cite web |title=Claude Vuichard & Tim Tucker tell the story behind the Vuichard Technique |url=https://verticalmag.com/features/claude-vuichard-tim-tucker-tell-the-story-behind-the-vuichard-technique/ |website=Vertical Mag |archive-url=https://web.archive.org/web/20210429170043/https://verticalmag.com/features/claude-vuichard-tim-tucker-tell-the-story-behind-the-vuichard-technique/ |archive-date=29 April 2021 |date=29 April 2021 |url-status=live}}</ref>

=== 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.<ref>{{cite web|url= http://scs-europe.net/conf/ecms2007/ecms2007-cd/ecms2007/ecms2007%20pdf/ese_0121.pdf|title= Vortex Ring State Simulation Using Actuator Disc|access-date= 14 March 2022|last1= Dziubinski|first1= Adam|last2= Stalewski|first2= Wienczyslaw|work= Instytut Lotnictwa / Institute of Aviation - Computational Fluid Dynamics Department|year= 2007|archive-url= https://web.archive.org/web/20210818191550/http://scs-europe.net/conf/ecms2007/ecms2007-cd/ecms2007/ecms2007%20pdf/ese_0121.pdf|archive-date= 18 August 2021|url-status= live}}</ref>

== 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]].<ref name="AC 61-13B"/> 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 (helicopter)|autorotation]] to break the rotor system free of its vortex ring state.<ref name="Helo" />

=== 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.{{Citation needed|date=June 2021}}

=== Radio control multirotors ===
[[Radio control]]led [[multirotor]]s (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.<ref>{{cite web |url=https://www.youtube.com/watch?v=LCret4rv0HE  |archive-url=https://ghostarchive.org/varchive/youtube/20211212/LCret4rv0HE| archive-date=2021-12-12 |url-status=live|title=Quadcopter "Wobble of Death": VRS Recovery and Avoidance |work=YouTube |date=August 2014 |access-date=21 September 2014}}{{cbignore}}</ref>

== See also ==
* {{annotated link|Aerospace engineering}}
* {{annotated link|Ground effect (aerodynamics)}}
* {{annotated link|Helicopter flight controls}}

== References ==
{{Reflist}}

== External links ==
* [https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch11.pdf#page=9 Vortex ring state] FAA [https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/ Helicopter Flying Handbook]
* [https://web.archive.org/web/20140915030343/http://terpconnect.umd.edu/~leishman/Aero/vring.html Free-Vortex Wake Calculations of Helicopter Rotors and Tilt-Rotors Operating-In and Transitioning Through the Vortex Ring State]
* [http://www.military.com/NewContent/0,13190,NI_Myth_0904,00.html Dispelling the Myth of the MV-22] [https://web.archive.org/web/20141109155726/http://www.military.com/NewContent/0,13190,NI_Myth_0904,00.html Archive]
* [http://www.skybrary.aero/index.php/Vortex_Ring Vortex Ring] on [[SKYbrary]] <!--userdriven content, good for an external link but not as a reference-->
* [https://www.youtube.com/watch?v=HjeRSDsy-nE Vuichard Recovery Technique - How to escape a Vortex Ring State] - Video using a water spray to show airflow entering, during, and recovering from a vortex ring state.

{{Helicopters and rotorcraft}}

[[Category:Helicopter aerodynamics]]
[[Category:Aviation risks]]
[[Category:Vortices]]