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What are the signs and symptoms a pilot can recognize, that the helicopter is approaching vortex ring state (VRS) and measures should be taken?

A related question discusses how to prevent or recover from VRS, but it does not address the recognition technique. Similar to stalls (which happen unintentionally), being able to identify a worsening situation before the stall fully develops is better than stall recovery.

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  • $\begingroup$ You can see the symptoms quite well in this video. NB, see my comments below as to why this is not settling with power, it is vortex ring state. $\endgroup$
    – Simon
    Sep 28 '16 at 16:59
  • $\begingroup$ @Simon - I would guess that it started at about :40 in the video, and he'd recovered by about :56? $\endgroup$
    – FreeMan
    Sep 28 '16 at 17:39
  • $\begingroup$ @FreeMan Exactly. You see it start to shake then yaw left. After that, it's like a bone shaker until he flies out of it! $\endgroup$
    – Simon
    Sep 28 '16 at 18:47
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Vortex ring state is characterized by rapid descent and reduced effectiveness of control inputs. At the onset, it is characterized by increased vibration and buffet of the airframe, with uncommanded changes in helicopter attitude. Also, any increase in the collective is ineffective and counterproductive.

From skybrary.aero:

  • Incipient vortex ring conditions are typically:

    • increased vibration and buffet,

    • the onset of small amplitude ‘twitches’ in pitch and evidence of longitudinal, lateral and directional instability.

  • Established vortex ring conditions are characterised by:

    • a very rapid increase in rate of descent towards and beyond 3000fpm,

    • reduced effectiveness of cyclic inputs in roll or pitch

    • the application of collective pitch failing to arrest the rate of descent and usually increasing it.

From FAA Helicopter Flying Handbook- Helicopter Emergencies and Hazards:

A fully developed vortex ring state is characterized by an unstable condition in which the helicopter experiences uncommanded pitch and roll oscillations, has little or no collective authority, and achieves a descent rate that may approach 6,000 feet per minute (fpm) if allowed to develop.

The best measure against vortex ring state is quite simple- go forward i.e. increase airspeed. Basically, you enter into forward flight so that the rate of descent doesn't oppose the induced flow. You need 'undisturbed' airflow over the rotor disc. Power is to be applied after gaining sufficient airspeed. From skybrary:

  • Incipient Stage ... Keep the collective position unchanged and apply forward cyclic to achieve an accelerative (nose down) attitude so as to increase forward airspeed quickly. As soon as a steady increase in airspeed is indicated, and above 30 KIAS, more power can be applied if necessary without waiting until the best rate of climb speed is reached.

    If this action does not resolve the situation rapidly then it is best to treat the condition as established and take the actions below.

  • Established Condition. Recovery can only be effected by changing the airflow around the rotor and will inevitably lead to significant loss of height, which makes recovery from a low level occurrence impossible.

    There are two theoretically possible actions: Moving the cyclic forward and lowering the collective. Combining these actions is likely to produce the quickest recovery with the least height loss.

    Application of forward cyclic should increase airspeed but a large input held for several seconds may be required before significant pitch attitude and consequent speed change is achieved, with a significant nose down attitude resulting. Lowering the collective to reduce power towards auto-rotation, so unstalling some of the inboard portion of the blades, may also be effective but forward airspeed must be gained before power is re-applied during recovery.

Autorotation has also been suggested as a measure for overcoming the vortex ring state (by the FAA handbook, for example), but it may lead to further loss of height, which has to be taken into account. Another issue is that there are some variations between the different helicopter types- in case of tilt-rotor aircraft like V-22, the only way is to go into forward flight.

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  • $\begingroup$ This defines what it is, but doesn't address the "measures to be taken", could you elaborate? $\endgroup$
    – Ron Beyer
    Sep 28 '16 at 15:07
  • $\begingroup$ @RonBeyer I've added it. See if its ok. I'd left it out as the OP clearly said the the linked question discussed recovery. $\endgroup$
    – aeroalias
    Sep 28 '16 at 15:34
  • $\begingroup$ I read the question wrong as him asking for the measures as well, but I think its good to include here, thanks! $\endgroup$
    – Ron Beyer
    Sep 28 '16 at 15:57
  • $\begingroup$ I thought this was called power settling. $\endgroup$ Sep 28 '16 at 16:03
  • $\begingroup$ @CarloFelicione Oooh, you've opened Pandoras' box. In most places, vortex ring state and settling with power are used interchangeably, even in helicopter literature. But they are different states. Put simply, settling with power is when you don't have enough power to stop a descent. It is distinct, and aerodynamically different, to vortex ring state. It can occur when changing from downwind to upwind, when heavy, when overestimating performance etc. $\endgroup$
    – Simon
    Sep 28 '16 at 16:39
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The main rotor of a helicopter enters the Vortex Ring State when it enters its own downwash. In order for that to happen, it must be producing downwash, have airspeed less than Effective Translational Lift (ETL), and be descending faster than its downwash.

The pilot can recognize that they are approaching the conditions for VRS whenever the aircraft is in powered flight (not in autorotation), airspeed is slow and descent rate is high.

This typically occurs during steep or vertical approaches; normal approaches with a tailwind; or hovering OGE when altitude is not precisely controlled and a descent rate develops. Accordingly, it is during these maneuvers that the pilot must be most alert to the hazard.

Helicopter pilots are commonly taught that there are three "ingredients" necessary for the main rotor to enter VRS:

  1. Using power
  2. Flying less than 30KIAS AND
  3. Descending faster than 300fpm

Although these are not hard numbers (in reality, they vary depending on aircraft design, disk loading, ambient conditions, etc.), they are useful warning signs to remind the pilot to guard against VRS. If a pilot is flying with two of the ingredients, they must take measures to avoid the third.

E.g. When a rapid descent, but not autorotation, is required, the pilot must maintain airspeed above ETL. When slow flight, less than ETL, is required, the pilot must closely monitor altitude and vertical speed to descend no faster than the downwash (generally around 500-1500fpm, increasing with aircraft gross weight). If descending rapidly, while slowing airspeed, the pilot must ensure that descent rate is decreased before losing ETL.

Different helicopter designs respond differently to VRS, but there are common symptoms. Generally, when the main rotor begins to enter the VRS, the helicopter will vibrate and begin an uncommanded descent, and controls may lose some degree of responsiveness. Although plenty of engine power is available, increasing power--raising collective--actually makes the condition worse (hence the term used by helicopter pilots in the U.S. to describe what is happening, i.e. Settling With Power). At this stage, the appropriate measures are to lower collective and fly into "clean" air, or ETL, in whichever direction is most appropriate.

Note: any rotor system can enter the Vortex Ring State. Tail rotor VRS is one of the conditions that can result in Loss of Tail rotor Effectiveness (LTE). It is generally assumed when you say VRS, that you mean MR VRS, but it might be useful to clarify.

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VRS is settling of a helicopter in its own downwash. Settling rates can be as high as 6000fpm. To develop VRS you must have all three of the following conditions 1. Speed < ETL. 2. Vertical speed > 300fpm 3. Power on between 20-100% If any one of these conditions is missing, NO VRS settling.

The best technique to recover from VRS is the Vuichard Recovery. Which is a power ON Side slip to the Right (Counter rotating rotors) or left for Clockwise rotating rotors.

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