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According to the wikipedia page :

Air vortices can form around the main rotor of a helicopter, causing a dangerous condition known as vortex ring state (VRS) or "settling with power". In this condition, air that moves down through the rotor turns outward, then up, inward, and then down through the rotor again. This re-circulation of flow can negate much of the lifting force and cause a catastrophic loss of altitude. Applying more power (increasing collective pitch) serves to further accelerate the down wash through which the main-rotor is descending, exacerbating the condition.

The vortex ring state has also been listed as a potential hazard in the Wikipedia page for helicopters. What are the various preventive measures used to prevent such a condition?

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  • $\begingroup$ Vortex ring state and settling with power are two different things. For example, one can also settle with power in high density altitudes or in OGE hovering $\endgroup$ – rbp Jun 22 '15 at 13:40
  • $\begingroup$ VRS from the high compound walls is attributed as the cause of the helicopter crash in the Bin Laden operation (en.wikipedia.org/wiki/Death_of_Osama_bin_Laden). Since that was a state of the art U.S. military stealth helicopter, it seems likely that it would've had VRS countermeasures, if any existed. $\endgroup$ – RBarryYoung Jun 22 '15 at 18:53
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There's an interesting article on the hazards at wikipedia.

The avoidance tactic mentioned in that article is to avoid rapid descents at low forward airspeed - this would seem sensible as that's where the rotors will be exposed to their own downwash.

And the preferred reaction also described there is to use the cyclic to fly forwards, to move into less disturbed air.

(I have heard that another cause of the condition is flying into narrow spaces among tall buildings, but unfortunately I haven't been able to find a reference.)

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    $\begingroup$ Flying forward is sometimes correct but there are other recoveries which would be more appropriate in certain circumstances. In essence, either stop the recirculation, e.g. enter auto-rotation, or fly out of the descending air in any direction. $\endgroup$ – Simon Jun 22 '15 at 11:34
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    $\begingroup$ Narrow/Tall buildings causing VRS: The high compound walls were what caused VRS and the helicopter crash in the U.S.'s Bin Laden operation: en.wikipedia.org/wiki/Death_of_Osama_bin_Laden $\endgroup$ – RBarryYoung Jun 22 '15 at 18:50
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    $\begingroup$ this is definitely the wrong answer $\endgroup$ – rbp Nov 21 '15 at 22:24
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Helicopter driver here.

I am not aware of any technical features which can prevent it. Quite simply, the disc must accelerate air downwards to generate lift. If the helicopter descends into that downward accelerating air, it is recycled through the disc resulting in a catastrophic loss of lift.

The preventative measures are:

  1. Be aware of the flight conditions under which vortex ring state may occur.
  2. Do not fly into these conditions.
  3. Be able to recongise the symptoms of vortex ring state.
  4. Understand, and be practised in, the correct recovery technique, which is most often fly in any direction away from the vortex.

I am not an aerodynamics engineer but I doubt that there is a physical solution to this - and if there was, it would likely have been implemented by now.

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    $\begingroup$ I would recommend to land with some forward speed, either provided by wind or by motion of the craft. This might not work in tight conditions, but then the vortex ring will also be weaker. $\endgroup$ – Peter Kämpf Jun 22 '15 at 12:03
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    $\begingroup$ I meant to make sure there is a horizontal flow component when landing, as a precaution. Once you are in a mouse trap, it is hard to get out again. $\endgroup$ – Peter Kämpf Jun 22 '15 at 12:13
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    $\begingroup$ @PeterKämpf ah, I see, you mean avoid it? Yep, on my most common types, avoid <30 kts when >300 fpm. $\endgroup$ – Simon Jun 22 '15 at 12:16
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    $\begingroup$ @rbp Yep, but that as long as you have HOGE power and your ROD is minimised, you still won't get into VRS. If I fly with > 30 kts airspeed, with a tailwind (and ignoring the problems with aft cyclic authority and weathercocking) then I can land with no risk of VRS. I've got lots of other things to worry about and I need a lot of room to skid to a stop but being downwind does not change the incipient entry conditions for VRS. The problem arises because inexperienced pilots keep the same sight picture and apparent ground speed which leads to higher ROD and lower airspeed. $\endgroup$ – Simon Jun 22 '15 at 14:07
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    $\begingroup$ BTW, I'm not saying downwinds are OK, they aren't, they are truly dangerous for all but the pros and highly experienced, just saying that no matter where the wind is, keep airspeed up and ROD under control and you won't get into VRS. $\endgroup$ – Simon Jun 22 '15 at 14:09
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Late reply, but I wanted to bring attention to this very excellent video that shows vortex ring state, using a helicopter with a sprayer, spraying a mist of water to visualize the airflow around the rotors. At about 2:28 into the video, that helicopter enters vortex ring state, and you can see exactly what happens to the airflow over the rotors.

When the helicopter descends too quickly with little or no forward airspeed, the downwash curls around and breaks up the airflow on the top of the rotors. The end result is much the same as breaking up the airflow over an aircraft wing, like an aircraft with insufficient forward speed and a high angle of attack... it kills most of the lift. Essentially, the blades stall, and the helicopter drops even faster.

Pulling up on the collective would do the same as pulling back on the stick a stalled aircraft... it would just make a bad situation worse.

The solution is to get out of the turbulent airflow, by either tilting the helicopter forward, or in the video that demonstrates the Vulchard technique, tilting to one side. As you can see in the video, the helicopter recovers fairly quickly, and the mist illustrates how tilting to one side restores smooth airflow on the tops of the rotors, enough to arrest the too fast descent.

Or, as Simon notes... the better solution is to know your aircraft and not allow it to get into VRS in the first place... just as with fixed wing aircraft, you're well advised not to let it get into a stalled condition.

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There are four factors: low airspeed less than 20 knots, partial power, high rate of descent greater than 300-500 feet/minute, and steep angle of descent greater than 30 degrees. Somehow, the aircraft has to descend, either by settling or other means at great than 30 degrees. Remove any of these four items and VRS is not possible.

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    $\begingroup$ The last "factor" is no factor at all. If you are at <20kias and >300fpm descent, you are by definition making a steep approach. $\endgroup$ – rbp Apr 24 '16 at 20:35
  • $\begingroup$ For which model of aircraft are you making this assertion? The disc loading will change the RoD value for incipient VRS depending on a number of factors. $\endgroup$ – KorvinStarmast Apr 9 '18 at 16:05
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enter image description here

The left picture above from J. Gordon Leishman, Principles of Helicopter Aerodynamics, depicts the Vortex Ring State where the wake below the rotor is re-ingested on top of it. A dangerous situation indeed, since adding power may result in higher vortex velocities only.

enter image description here In section 5.3 of the Leishman book a further detailed discussion of the VRS can be found. A quantification is given as induced power loss in VRS: measured data shows a power loss factor of almost 2 in some circumstances, and here lies one of the issues in trying to recover with adding power. The engine is simply not dimensioned to deal with such effective power loss.

Another problem is flow fluctuation, leading to fluctuations in thrust of up to 40% depending on disk loading:

The VRS is accompanied by an extremely unsteady (aperiodic) flow field surrounding the rotor.

All in all a dangerous situation to be in, since the flow fluctuations associate with a loss of predictability. Best avoided, although one of the methods to recover is to reduce rotor Angle of Attack:

Notice that the fluctuations drop off quickly as the disk AoA decreases below 50° and is consistent with piloting experience on helicopters, which shows that a forward speed component causes the rotor to quickly exit the VRS.

After tuning the flight controls of a CH-53 helicopter simulator and going for a test ride, I inadvertently got the sim in a VRS in the thin mountain air surrounding the modelled airbase. The instructor told me to give full left lateral, wait for the helicopter to reach a considerable roll angel, then full right lateral, then mid again; then regain horizontal attitude while pressing cyclic forward. The initial lateral movement indeed changed the disk AoA enough to exit the VRS quite rapidly.

I don't know if this is taught as a recovery practice in the real helicopter though, or if it only works in the (high fidelity military) simulator.

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