Editing Vortex ring state (section)

== 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>