Vortex ring state

April 17, 2009 by Tim McAdams

One of the more confusing subjects for helicopter students to fully understand is known as the vortex ring state, also (correctly or incorrectly – depends on who you ask) referred to as settling with power or power settling. Much of the confusion comes from the terminology as these terms are used interchangeably in many textbooks. Yet, there are instructors who teach that settling with power is very different from the vortex ring state. This is true when settling with power is defined as simply not having sufficient power to hover, thus causing the helicopter to descend or settle when power required exceeds power available. However, many textbooks do not use that definition – hence the confusion.

Regardless of what labels you use, the important point to know is that there are two very different situations that can affect a helicopter in an out of ground effect hover or a steep approach. One is simply running out of power as described above, the other is the vortex ring state, which is an aerodynamic condition that forms when a helicopter is allowed to descend into its own downwash. It can happen even when a helicopter has more power available than needed.

To understand the vortex ring state imagine a helicopter hovering at 1,000 feet. The rotor system is drawing air from above and accelerating a large amount of air downward in a column underneath the helicopter. If the pilot allows the helicopter to descend vertically at too high a rate of descent into the column of downward moving air, the air that is now above the rotor system will still be moving downward. As the rotor system tries to draw air that is moving downward a re-circulation of air forms, causing the rotor tip vortices to become much bigger. (Normal rotor tip vortices are small and only cause a small loss of rotor efficiency.) Additionally, a secondary vortex ring will form near roots of the rotor blades.

In a well-developed vortex ring state, most of the engine power is consumed by accelerating air in a circular pattern around the rotor system. The vortex ring state causes turbulent rotational flow across the blades with increasing roughness and possible loss of control. The helicopter continues to descend and a natural reaction is to increase power by raising the collective control. This merely increases the strength of the vortex ring and the helicopter will settle even faster.

To recover from this situation, the pilot needs to remove the helicopter from the column of air. This can be done in any direction, however, it is best to accelerate forward and reduce the collective pitch slightly. This will normally result is a minimum altitude loss. In the early stages of development, a large application of power (if available) might be sufficient to overcome the upward moving air and initiate a recovery. Theoretically, entering autorotation would change the airflow and result in a recovery, although it would also produce a large loss of altitude.

The best plan is to understand what type of conditions can cause the vortex ring state and avoid them. The three basic conditions are: a vertical descent rate greater than 300 fpm (the actual descent rate required may be higher depending on density altitude and aircraft weight); the rotor system must be consuming 20 to 100 percent of available power; and airspeed less than about 15 knots.

Some of the maneuvers that are susceptible to encountering the vortex ring state are steep approaches (especially downwind) or hovering out of ground effect at high density altitude. To stay out of trouble, keep approach angles shallower than 30 degrees, and when performing steeper approaches keep your rate of descent no more than 300 fpm and don’t let airspeed get too slow.

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14 Responses to “Vortex ring state”

  1. Ken Cashwell Says:

    Good Article Tim. Being a former Army Rotorwing instructor, I found it to be right on point. I’d like to add one other possible scenario. From the very first flight, students are instructed to land into the wind. But if one flies helicopters for some length of time it is very possible that one finds his/herself making an approach in a tailwind. Making an approach with a light tailwind has a similar effect of either descending in a hover or making an approach at a steep angle. The tailwind has the tendency to blow the downwash back into the approach path of the helicopter, especially if approach is abnormally slow. The perfect situation is making a tailwind approach where there is only one way in and one way out, and the landing spot is small. More often that not, pilots will fly a slower than normal approach speed. This may set up the conditions Tim described in his article. One more thought, with “one way in, one way out” tailwind approaches there aren’t many options for “flying out” of settling with power or vortex ring state “. Here forewarned in forearmed. Regards.

  2. Jim Corey Says:

    Yes, I agree it is a nice brief on vortex ring state. I am left with some comfusion about the comments: “it is best to accelerate forward and reduce the collective pitch slightly. This will normally result is a minimum altitude loss. In the early stages of development, a large application of power (if available) might be sufficient to overcome the upward moving air and initiate a recovery.”
    1) Most helicopters will roll faster than they pitch, suggesting that the sideward movement may result in quicker recovery. This is especially important to keep in mind when forward space is limited as in a one way in one way out landing site.
    2) Aren’t we trying to overcome the “downward” moving air?

    Thanks for the otherwise well written and concise explanation.

    Jim

  3. Jean Schwarzkopf Says:

    Flying forward is not the best direction to fly when trying to arrest a max power sink. Always fly towards your left shoulder. This will reduce the amount of left peddle needed and the power lost to the tail roter.

  4. Duane Zentgraf Says:

    Great topic, most pilots don’t even think about this until they experience it and may be to late. Pilots should always remember to when flying helicopters is to “Stack the Deck in their Favor” for all situations. Always enjoy reading your articles.

  5. Tim McAdams Says:

    Jim, I appreciate the good comments. I probably should not have used the word “best” as I agree that exiting to the side could be a better option. Thanks for pointing that out.

    Jean, would your theory be true in an Astar as well?

  6. Jean Schwarzkopf Says:

    Tim,

    Of course not. Always slide towards the side of the retreating blade. If you’re holding right peddle in a hover, slide to the right. The Navy teaches this trick in case you fly off the flight deck in an overloaded condition. Once clear of the deck you will sink towads the water, 70 feet below.

    Jean

  7. Tim McAdams Says:

    Jean,

    Great idea, thanks for sharing it.

  8. Jerry Robinson Says:

    I think part of the confusion re: terminology comes about because the terminology has changed. When I was a young Army helicopter pilot in the ’60′s, the exact verbiage you use to describe “vortex ring state” was described to us in official Army documentation, as “settling with power.” But then “vortex ring state” sounds so much better … it’s more” technical” … and does allow us to couch the obvious in terms of the obscure.

  9. Jake Says:

    Great article and I am always learning something new.

    Maybe other topics could be dissymmetry of lift, dynamic roll overs, or ground resonance?

    Just some ideas :)

  10. Christian Tillmanns Says:

    In the very well know forum rotorheads in pprune, we discussed this matter back and forth. Nick Lappos, Sean Coyle and other well know test pilots and engineers contributed with very interesting inputs. In the end most agreed, that there is a lot of confusion about the vortex ring state. There is no way, that you can get VRS with a descend rate of 300 ft/min. VRS begins to develop when the descend rate is about equal to the downwash velocity. That is about 700 ft/min in a R22.
    For a more information have a look here:
    http://www.pprune.org/rotorheads/116124-vortex-ring-settling-power-merged.html
    You may be required to get a login, but it is worth it. The forum is very well moderated.
    Cheers
    Christian
    @ Jake: All those subjects you mention are also discussed

  11. Chris Says:

    Does anyone know where to find some good graphics to help explain this concept? THANKS!

  12. Ehud Gavron Says:

    Edit: s/this will result is a /this will result in a/

    Well-written article. I am pleasantly surprised to see AOPA putting rotary-wing material up, and your writing is excellent with good scientific basis for your comments!!

    Ehud

  13. Robert Says:

    Good discussion, fellas. Three items regarding above comments.
    1). less than T-lift is the 3rd Rotorcraft Flying Handbook criterion for VRS. Everyone should understand T-lift is not always 15 kias – rather, it calculated to be Vi/101.4. (Where Vi is induced velocity in a steady state hover) Numbers become greater when you see heavier gross weight helos.

    2). Above comment states R22 Vi is 700 fpm. I don’t know what study the FAA used to validate the 300 fpm warning, but I don’t doubt it’s veracity. That is because Vi is velocity in a STEAD-STATE hover. If you increase collective, that number goes up, and vice versa, when you reduce collective, that number decreases. The demo prescribed in the Rotorcraft Flying handbook precisely describes entry into VRS at 300fpm or greater, when the aircraft may begin to shudder. I find it easy to envision a minimum pitch decel, with a slight descent, that would encounter vortices as you add power.

    3. Any approach angle is safe from VRS when sufficient wind exists to remain in T-lift. ie. I promise, you will not encounter VRS while you have T-lift, even descending vertically. Conversely, with a tail wind, approach angles less then 30 degrees could be very problematic. I’ve flown thousands of steep approaches targeting 45 degrees. Proper energy mngmt. is the key. (continuous descent/decel) More than likely, severe turbulence from VRS would occur at descent angles greater than 50 degrees. I personally would rarely target a 300 fpm rate of descent during an approach, let alone less than 300fpm. That advice might be more helicopter specific, more pertinent to very small light helicopters.

    You may calculate Vi for any helicopter on any given day, and Vi will change. But once again, that will give you numbers for vortices in a steady state hover. That bubble is hanging out underneath you, probably .66Vi to 1.22Vi, but think of it as an inverted-plastic-ball-on-an-air-column demo,and you are manipulating the ball up and down with collective inputs. Good luck.

  14. David R Says:

    Nicholas Schmidle’s thoroughgoing article entitled Getting bin Laden in the Aug. 8 New Yorker describes the mishap at the bin Laden compound when the Blackhawk inserting SEALs within the walls via ropes instead landed hard and sustained damage sufficient not to fly out. There’s bound to be more discussion about “settling with power” among rotary wing pilots as a result.
    Apparently, conditions were calm, appreciable DA and weight, obstructions, a closely bounded air column below due to high walls, and hover out of ground effect. The pilots reported loss of cyclic response and presumed vortex ring state, but had no way to fly out of the problem, despite using the space available. Practice runs on a mock-up compound in the U.S. a few weeks previous had succeeded, but possibly the walled space below permitted quick and severe vertical rotation to develop, versus open wire fence in the practice sessions, a contributor remarked.
    Fortunately, preparations included two pairs of Chinooks staged to help if needed. One flight advanced to the compound and the other refueled the remaining Blackhawk on the way out. It was frustrating to read of the mishap, knowing that the mission was well planned and involved expert aviation input and participation. Apparently, circumstances came together in the wrong way and weren’t emphasized enough in the planning nor given priority in the cockpit until the aircraft slowed out of translational lift and entered a hover. Altitude loss commenced at once, apparently. SEALs never managed to get the ropes out, but felt the attitude change and drop and were able to avoid injury and deploy into the compound.
    The article is a good read overall, but will cause discomfort to helo drivers everywhere, especially military.

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