Comments on: Vortex ring state

By: David R

David R — Thu, 18 Aug 2011 17:53:11 +0000

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.

By: Robert

Robert — Sat, 04 Sep 2010 17:20:30 +0000

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.

By: Ehud Gavron

Ehud Gavron — Sat, 06 Jun 2009 17:38:05 +0000

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

By: Chris

Chris — Fri, 29 May 2009 10:42:24 +0000

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

By: Christian Tillmanns

Christian Tillmanns — Fri, 08 May 2009 14:46:08 +0000

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

By: Jake

Jake — Fri, 01 May 2009 13:32:45 +0000

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

By: Jerry Robinson

Jerry Robinson — Sat, 25 Apr 2009 02:28:48 +0000

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.

By: Tim McAdams

Tim McAdams — Sat, 25 Apr 2009 00:05:42 +0000

Jean,

Great idea, thanks for sharing it.

By: Jean Schwarzkopf

Jean Schwarzkopf — Fri, 24 Apr 2009 23:25:18 +0000

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

By: Tim McAdams

Tim McAdams — Fri, 24 Apr 2009 21:49:12 +0000

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?