Won't be differing with any of that!

Will only add that recovery method has to be 'tailored' to height available. At 3000'
that's one thing, at 75' into a confined is something else. Poor speed/power management is the major cause but a downdraft would be very unfortunate at the latter height.

I stick firmly to the 30/30 rule, no more than 30(0) ft per minute if speed is less than 30 kt. You won't fall into any trap then. I have also heard that it cannot develop at approach angles of LESS than 30 degrees but have had no time to go into that.
Maybe someone else has?

Reduce power, forward cyclic; forward cyclic, reduce power. I agree.
It's much like the fellow who showed up to work in a new duty site at 6000' in the summer, having arrived from a long period at sea level.
I said " The book tells you that the 180 auto has several steps. Collective down, pedal adjust, cyclic into the turn, check rate of turn, etc, etc. Not correct. At this elevation it's one step or you won't finish the turn till your on the runway" He didn't believe that. Half way through the turn I caught sight of the man with no forhead that I've seen a few times before. From cheek bones to hairline he was nothing but eyeballs.
To take a position, I do agree that you won't get what you need from forward cyclic till you reduce the collective.
The worst aircraft for VR and S w/P for me has been the CH-47. He was flying, I was digging for a map (not doing my job of paying attention). There were no vibrations to alert me out of the map case. When I looked up we were falling like and anvil out of trim, so smooth. Not what anyone who has flown a few Chinooks would ever expect in terms of vibratory level. He was still operating well within the bliss that ignorance provides.

Lu Z,

As I understand it the BERP blade was designed to overcome problems with compressibility at the high speed end of the flight regime. They are in effect a swept tip but to reduce twisting moments the rear of the blade shape is modified too.

SPS/Randy_G:

I agree with you completely, settling with power is not the correct term for VRS (Power Settling).

A classic example of Settling with power is the young hotdog trying to impress his onlookers and starting a return to target into wind! The resultant course reversal into a downwind return sets you up for a classic settling with power situation. Too heavy for the ambient conditions without enough power to recover.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

[This message has been edited by offshoreigor (edited 23 January 2001).]

[This message has been edited by offshoreigor (edited 23 January 2001).]

When I posted the earlier comments on why a VRS intensifies, I wanted to add the following comments but chose to wait to see if the comments would be anywhere near correct.

A vortex also has the property of being "semi-static". By this I mean the rotational energy and velocity of a vortex causes it to stablize its position in the airmass that it was created in. However it will move with the prevailing winds and air currents that move the airmass it exists in. That's why it's "semi-static", it remains positionally static within the surrounding airmass, but moves when the surrounding airmass moves.

I think a very good example of visualizing a vortex ring occurs when a prop driven aircraft is taking off in high humidity air. In these conditions you can "see" the prop tip vortexes being generated by the prop tip, by seeing the moisture inside those vortexes. I think this happens because the rotational energy of the vortex causes a low pressure area inside the vortex (like in a tornado or hurricane) to develop, and the low pressure area causes the inside air's "dew point" to be reached and the inside air "fogs". You also notice that the visible moisture soon dissipates as the vortexes dissipates.

Being able to "see" these circular "ring" shaped vortexes on the prop tips makes it easier to understand how vortex ring state develops in a helicopter. If you've ever seen this, you know that the visible circular vortexes move towards the rear of the aircraft in a spiral corkscrew shape as the aircraft accelerates down the runway. On occasion you can even see this corkscrew pattern when a prop driven aircraft has its engines under power but is stationary with its brakes applied.

The corkscrew pattern is being developed because the vortexes are stationary within an airmass that is being moved towards the rear of the aircraft by the force of the propellers. Now use your imagination and suppose that a rather hefty tug were put on the nosewheel of the aircraft. Imagine that a pilot in the aircraft pushed the throttles forward on a very humid day (so you could see the vortexes) and the tug operator began to push the aircraft backwards until the speed of the aircraft moving backwards began to equal the speed of the airmass being driven backwards by the propellers. The ring vortexes on the prop tips would no longer be stretched out in a corkscrew pattern, but all of the vortexes being generated would now accumulate in one spot around the arc of the propeller, within the propeller driven airmass that is now stationary with respect to the rearward moving airframe. I hope you can visualize this.

In a helicopter "settling with power", this is exactly what you have. The rotor is driving down an airmass around the helicopter's airframe, while the airframe itself is falling down at a rate that is at or near the airspeed of the decending air column generated by the rotors. The rotor tip vortexes are no longer being spread out in a corkscrew pattern, but are being allowed to accumulate at the rotor. This causes the effect of "vortex ring state" to intensify as the rotor tip vortexes have nowhere to go, and they just keep adding energy to the vortex ring that already exists.

I agree completely with the methods of escape described in the other posts on this topic. Since the vortex ring is "semi-static" within the descending air column, then the best method of escape is to cut collective and/or power and to push the cyclic forward, leaving the vortex ring behind. Side stepping the vortex ring with side cyclic could also work, which the Chinook pilots seem to find helpful. For a tiltrotor, forward cyclic would seem to work best as this would seem to remove both rotors from the vortex rings at the same time.

Please correct me if any of this sounds wrong.

------------------
Safe flying to you...


[This message has been edited by Flight Safety (edited 25 January 2001).]

Flight Safety - A.W.T. ('Appy With That) (quaint British Army expression denoting satisfaction with a statement, theory or state of affairs)

Agreed. Although 'lever down' is part of the generally accepted recovery , IMO , it is more of an attempt to make clear in the mind-set of the 'victim' that 'normal' rules no longer apply and raising lever in response to increased sink rate is futile. Once fully-developed I have found a very stable elegant aerodynamic condition (v. high R.O.D.) and have still had good cylcic authority (Offshore; not also true in your types? inertesting.) to maintain or 'fly out' of V.R. - min hight loss recovery being Attitude (only) till VR eliminated then Power up and nose up to climb - recoveries which reach more than 30-40 kts are just energy (height)inefficient. IMO.
Auto recovery is nice (but does not minimise height loss).

Offshoreigor: Goldie / s61 / New york, early 90's. I believe!!!!
And they all walked away too....

http://www.gograph.com/Images-7298/A...if/redstar.gif

------------------
Thermal runaway.

TC:

Spot on! And Goldie is still as funny as ever!

Did you know they use his DND video documentary in CRM these days? The way he describes it, it was the perfect setup for the fall, no pun intended.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

Unrecoverable Vortex Ring State
 

After reading the discussions on the Blackhawk accident threads, could it be possible to get into an unrecoverable VRS? Say a helo is in a full blown VRS with a huge ROD of 4000+ fpm. Could it be possible that:

1. The rotor is completely stalled and so cyclic pitch changes will have no effect on disc attitude, or

2. With such a huge ROD, even with the collective on the floor, the ROD airflow is large enough to give a resultant relative airflow and angle of attack that exceeds the stalling angle on the rotor blade?

I have been in many doozies in the training environment and have always effected the recovery before the ROD exceeded 3000 fpm, but I always presumed some parts of the rotor disc were still doing something right. Any ideas or test data to prove or disprove my theory?

(Edited to reflect light disk loading helicopters)

The state where the rotor is in VRS is strictly defined by the ratio of the descent rate to the downwash velocity of the specific helicopter. Only within that boundary is VRS possible. One caviat is that we read RoD with a static instrument, which can go flakey when the wash around the fuselage is skewed beyond the normal certification envelope.

For modern helicopters, VRS is unlikely below vertical speed ratios of .5 and beyond ratios of 1.5. For an R-22. descent rates less than about 700 fpm are below the VRS threshold, and descent rates beyond 2100 fpm are above VRS. For Black Hawk, descent rates of about 1200 FPM are below VRS and greater than 4000 FPM are above VRS. The Vortex in VRS is produced when the downward velocity matches the rotor's downwash, so the exact match causes the air to simply wrap around the rotor tips. Lift is lost quickly, and the bottom falls out. If the power is left up and the condition is stable, the descent rate is constant and very high (about equal to the downwash velocity - maybe 1400 fpm (robbie) to 3000 fpm (Hawk)). The thrust variations might be 20 or 30% thrust (producing low frequency turbulence that is about .3 G's).

If you lower the collective in VRS (and you have lots and lots of altitude) you will simply get into windmill brake state and autorotate at about 5 to 6000 feet per min.

For data and photos, see my web site at:

http://mywebpages.comcast.net/llappos/

During VRS, typical modern helicopters retain some cyclic control. The rate of descent will make the horizontal tail try to pitch the nose down, which will help you recover. Reduced collective and nose down will produce a fast exit from VRS. Increased collective will only help if the aircraft has a great deal of excess power, not at all likely in anything but an empty machine with powerful turbine engines.

Note from the data on the web site that VRS is unlikely in a purely vertical descent, some forward speed is needed, maybe 8 knots. In practice, it is difficult to attain and hold VRS, as the condition is unsteady and tends to break of its own if any disturbence is induced. That does not mean it can't do harm, since the first 1000 feet of drop might be several hundred feet too many!

So that's a "no", then ? :D

Great thread,

Nick, thanks for the charts and especially the picture of the little midget helo on your site.

Imagining myself as very small and inside the little helo in the picture I was wondering about the recovery at that point.

The state is fully developed,

Cyclic not making desired effect,

Collective has become excercise machine for left arm,

Disc not happy, pilot not happy.

But the tailrotor can still provide translating force to the fuselage. Tractoring it (and the disc) away from the bad ju-ju happening at the time.

I'd think that a fair amount of pedal input (not enough for a spin) should slowly edge your disc out of it's nasty state to the point where the other controls start to take effect again.

Holes in the idea?

Obviously pedal input topping the engines would be a concern.

tgrendl, for most modern aircraft, cyclic is still quite effective, because you do not need to be developing thrust to develop control. The exception is a teetering rotor, which needs thust to develop lateral or longitudinal control power.

In any case,reduce collective, push the nose forward, and fly out. It would be the rare case that everything fails to respond. If you get stuck, be sure the collective is fully down, and you should slide down into autorotation.

Here is a very good web site that has flow depiction of the rotor flow as a descent progresses from gentle descent to VRS and then to autorotation, it was put up by Dr. Gordon Leishman of University of Maryland (and a Brit, I believe, formerly of Westland). Scroll down to the single rotor cases which are well labeled, and run the movies. Note that he shows how little forward component is needed before the VRS is broken. For those on dialups, it might take a while to download each. I would be glad to post some stills from them if anyone needs that done.

http://www.enae.umd.edu/AGRC/Aero/vring.html

Recovery from High state VRS
 

Last year 2001, I was with a very experienced Ex mil Cfi who showed me what can happen in a very fast descentVRS(Heli was R22) we went very high(5000ft) and he showed me a rate of decent to 3500 fpm, recovery was achieved by simply completely closing the collective, after recovery to height again it was my turn, and the same sort of recovery was made, It was very fast, very intimidating but after the event I feel able to cope with this problem( if it arises) in the future, brilliant for the old ticker!!:eek:
Regards Peter RB

A simple question perhaps:

Why do we need a small amount of forward speed to induce VRS, why doesn't a purely vertical descent induce it. I can see from the data that it doesn't just wondered why ?

cheers all
:D :D :D

I recently did an Enstrom Type conversion and the instructor could not get any sort of VRS, even when lifting the lever from the bottom in low speed auto to 38 inches MP. The aircraft just stopped decending and started to climb. No fuss. He had several goes all with the same result. This was NOT the case last time I did it in a R44. Anyone got any ideas why the difference? Articulated vs teetering? Rotor RPM? Conditions on the day?
I have no idea.

(Edited to accurately reflect Robinson and Black Hawk downwash velocities, based on the coyote's calculations)

VRS is only encountered when the downward speed of the helicopter matches the downwash velocity from the rotor. In the cases mentioned by vfrpilotpb it is likely that the VRS was passed through, and then a zero knot autorotation was flown, thus the 3500 ft/min descent rate.

The flow around the rotor is established by the balance between the rotor's push on the air, and the upwind matching that push. Tales of VRS sort of locking the rotor up aerodynamically, and causing tremendous fall rates are simply not true. Those descents are experienced, we can be shown them, but they are not VRS. The reason for VRS is that the rotor downwash is pushed upwards by the free stream and then recirculated back down through the rotor. If the ROD is very much higher than the downwash velocity, the free air just passes through the rotor, and you have to raise the collective to keep from overspeeding the rotor.

For Gaseous, the autorotation first could be a difficult way to experience VRS. If you are into auto, the rotor must be transitioned to powered state, and you must try to trim the descent rate at somewhere between the range of 50% to 150% of the downwash speed. The rate of descent varies a lot depending on how heavy the disk loading is. For a Robbie, the VRS range is 700 fpm to 2200 fpm, for a Black Hawk, it is 1400 fpm to 4000 fpm.

One of the difficulties comparing what your instructor shows you with the actual VRS is that there is no telling what your instructor knows about actual VRS, and there is not standardization of techniques for the demo. The texts are poor, and much pilot lore surrounds the maneuver. I am sure many well intentioned instructors show a descent, some vibration and then an awesome vertical autorotation, and call the whole thing VRS. Why not, mine did in flight school back in 1968.

For VRS to be established in a rotor, the rotor must be lowered to about half the downwash speed under powered conditions. By 75% of the downwash speed (R-22 = 1100 fpm, H-60 = 2000 fpm) the VRS will show its head, the thrust will oscillate (you will feel low frequency vibration like turbulence, with maybe 3/10 of a G of magnitude, really big) and the aircraft will pitch and roll somewhat, the cyclic will be sloppy, and raising the collective will not necessarily produce a reduction in ROD. If the descent is increased to about 150% of the downwash speed, VRS is gone, clean air passes through the rotor, and you are in a vertical descent.

Any nose down (or even lateral tilt to slide out sideways) will help break up the VRS, and a climb will probably start (or at least the rate of descent will reduce somewhat. If you are falling at 3000 or 6000 feet per minute, you are not in VRS, you have slid through it and are now into a vertical descent or autorotation.

Nick, based upon your formula to calculate the downwash velocity and the following data:
R22, at MAUW=1370lbs, rotor diam=7.7m/25.26ft, I calculate the downwash velocity to be 1440 fpm. Am I right? Is the 0.002378 number in your formula, is that specific to a particular rotor system or a constant for all helos?

When I used to demonstrate VRS to students, I would get it into the hover (obviously with at least 3000 feet to spare) and in the R22 this would usually require pretty much max power. I would then very slowly lower the collective. As the ROD got to around 500-900 fpm or so, (and sometimes you would have to gently search for the downwash with cyclic), bingo you would be into it and most times so quickly you would lose your stomach. All the normal control sloppyness, pitch and roll and vibrations just prior to it. On a windy day often you couldn't get into VRS no matter how hard you tried for obvious reasons. From your formula it seems like a higher ROD is necessary but experience tells me otherwise, maybe I am under-estimating the lag in the VSI? Or does the downwash velocity vary considerably depending upon the power being applied at the time?

Thanks for your input and a Disclaimer for students: DONT do the above solo.

the coyote,
You are exactly on. The disk loading of the R-22 is very light, so it does not need to push the air very hard to get its lift. The downwash velocity of 1440 is correct. The .002378 is the density of air at sea level. The density is important because the formula simply matches the downward air momentum change needed to make the upward lift for the rotor. With denser air, less velocity is needed, and vice versa.

OK, so we estimate that you get into the beginning of VRS at 50% of the downwash speed. That means by about 700 fpm ROD you should start to get VRS effects. You have observed that at 62% of that speed, 900 fpm, you are fully into the effect (that is the orange area on the plot on my website.) It all checks very nicely.

I think it all fits. The misleading thing about my post is that I have assumed downwash velocities more in the range where turbine helicopters are built, note the use of a Jetranger for the website chart. I will edit the post right now to add Robbie numbers! I will also put a Robbie scale on the chart right now.

Turbines have higher disk loading bcause they have more power, so they can afford to spend more making higher downwash velocity. I will eventually add that to the web site, too.

Not tryingto be cheeky by asking again but why is it that we need as Nick said around 8 knots of forward speed for VRS why is it less likely in a vertical descent ?

3D,
I didn't ignore your original question, I just can't answer it. I do note that the data on Dr. Leishman's site shows no such forward speed bias, but his data is rotor only.

I have a gut feel as to the answer. The steep descent makes the fuselage and tail want to tuck nose down, with the forward moment provided by the horizontal tail. To stay level, some aft cyclic is necessary to balance the moment. This aft flapping could result in the rotor wash having a forward bias, requiring some slight forward speed to keep it reingesting.

That is pure speculation, but it has a certain ring to it, I think.

Nick

Thanks Nick Thats a very good explanation I think.

reply to gaseous
 

in attempting to minimise the airspeed ie:transverse flow across the disc, we tend only to think of IAS.

it is possible that in your example a floww across the disc existed (eg: left,right or even tail wind) hence no VRS.

i have no time in the enstrom but the bell47 'sometimes' displayed similar inconsistancies.

The only way I have been able to repeatedly demonstrate the onset of vortex ring state (and I don't want to get to the fully developed version, thank you very much), is to decelerate downwind (at a sufficient height above the ground to permit a timely recovery)
Find out where the wind is at altitude (watch the clouds drifting by, or know it from some other source), but downwind is the only way I can get this to work.
Start at 60 KIAS in level flight - reduce the power to something slightly below the power for level flight at that airspeed and maintain altitude.
As the airspeed passes through about 20 KIAS, there will be increased airframe buffeting, unlike any you get anywhere else. The helicopter may have uncommanded pitch, roll and yaw (i.e. you're not moving the stick and it's dancing around on you). The rate of descent will be 500-800 fpm, maybe less.
Increasing the collective might increase the rate of descent, but this is not very repeatable.
WHen you've had enough, smoothly, but positively lower the nose to 20-30 degrees below the horizon and fly out of it.
Doing it into wind won't work.

Nick,
when you say "That is pure speculation, but it has a certain ring to it, I think.", I assume that was a totally unintentional pun! :D


Shawn,
I'm a little confused by you talking about being upwind or downwind when trying to enter VRS. I mean if you're starting off at 60 knots IAS what difference does it make what the air is doing relative to the ground? I mean, you're still flying relative to the airmass. I'm not trying to pick holes, I just don't get it! Maybe I'm a bit slow today. :(

Irlandés

P.S. I really enjoyed your book! :)

Irlandes,
The pun was intentional, I settled upon it easily.

Shawn,

The maneuver that you describe sounds like a way to force the rotor into VRS with a combination of desent and deceleration, all quite par, for it forces the downwash/upwash fight that creates VRS.

Coyle mentions this as one of the scenarios that the V-22 can possibly find, where little ROD is needed.

I, too, find the need to be downwind interesting, but I assume that you have really decelerated to effectively zero velocity, and perhaps even a bit rearward. This could also explain the wobbling, as you pass through ETL and then into rearward flight. Where is that omni-directional airspeed system to help settle this powerful discussion?

Nick

Nick,
I had a look at your website (very good!) and have a couple of questions. In your formula for downwash speed, you have the three variables, weight, density and disc area. I would have imagined that angle of attack and possibly rotor rpm would have to be in this formula somewhere. If I'm on the ground at 104% (in the Robby say), with no pitch, then my downwash velocity is zero (correct me if I'm wrong). As I increase pitch and subsequently AOA, my downwash speed increases until such point that I can take off. So I see a downwash speed varying with AOA. By inference I assume my downwash speed in a descent will be lower than in straight and level flight everthing else being equal, thus permitting me to descend. So is your formula valid for a concrete flight situation or is it valid for all flight situations? My worry is that I could calculate my 700fpm (50%) and 2100fpm (150%) as being the avoid descent rates for a given all up weight and air density, only to find that these can dramatically lower, with decreasing downwash speeds due to lowered collective.

I may of course be talking utter non-sense. :confused: If you could point me on the straight and narrow, I would be very grateful!

Irlandés :rolleyes:

Irlandes,

The formula is a simple translation of the speed change that the rotor must make on a column of air in order to make the hover thrust. It is only an approximation (a good one) but it is restricted to the case of hover. The factors that you describe are of course necessary, but they tend to describe the details that operate the wind machine. In the end, small angle of attack on a high speed rotor, or large angle of attack on a slow one all must move the air, and it is the net momentum change on the air that determines the thrust of the rotor.

In a climb or descent, the downwash speed is the same, as long as the ROD or ROC is steady state. Recall that lift equals weight as long as the flight is unaccelerated. It takes a bit more thrust (downwash) to initiate the climb, and a bit less to start the descent, of course.

I will make this change to the web site to clarify the limits of that formula.

Thanks!

Nick

Thanks Nick for that reply. I've been mulling over what you said and sorry to whip a dead horse but I have one more question if you feel up to it.

Is rotor downwash speed measured relative to the plane (cone?) of the rotating blades or relative to the undisturbed air mass surrounding the helicopter?

I mean, if a helicopter with a theoretical downwash speed (in an OGE hover) of say 1400 fpm is descending vertically at say 400 fpm. Is the downwash velocity such that the downwash is approaching the ground at 1800 fpm (the vector sum of the two) or is is that the downwash velocity relative to the blades is actually 1000 fpm which added to the 400 fpm descent rate gives a total downwash velocity (again relative to the approaching ground) of 1400 fpm??

I feel this second solution would tie in nicely with both your explanation and my possibly misguided idea of downwash velocity (relative to the blades) reducing with reduced collective.

Thanks again!

Irlandés

More on being downwind to demonstrate Vortex Ring State.

I'm not sure where or when I picked up on having to be downwind to get this to work. All I know is that having done it many times starting downwind and having it work reasonably well, and tried it several times into wind and having no results, is that downwind works well for the demonstration.
The aim of the demo is to educate the student on what the symptoms of VRS are, not explore the depths of the monster.
Avoidance should be the key!!
I've investigated at least one accident where VRS was the culprit and the board of inquiry didn't even consider it - shows how little the matter is understood.
This thread is actually pretty awesome in what it is explaining, and I'd like to thank Nick Lappos especially.

VRS
 

Nick and Shawn - I have read your posts and have found them very interesting and enlightening. As a 7000 hour pilot and examiner, I thought I had this Vortex Ring thing sussed. It turns out that I didn't quite know as much as I do now and, as with just about everything else in aviation, there is much more going on than Lofty Marshall explained to me at CFS.

The main point that I note from your discussion is that the larger helicopters with more power and higher disc loadings seem to have significantly different entry parameters to VRS. As a simple operator, instructor, then instructors' instructor now examiner, I have been lulled into the general train of thought that rates of descent more than 300 fpm when below 30 knots are not acceptable. Whilst this is a good policy to adopt and has always kept me well clear of the offending phenomenon, I now realise that there have been situations where I possibly could have achieved something from which I had chickened out.

I will continue to emphasize the 300fpm and 30kt alarm bell as I still feel this is a point beyond which, very careful consideration is required.

Shawn, having taught (to aspiring instructors) and demonstrated
to others how to get into and out of the danger zones, I generally used the downwind approach or downwind hover (at recoverable altitudes), to show how easily things can go wrong. I think the visual cues from downwind help to mask the onset and make the demonstration more effective. In addition, helicopters do not generally "like" being downwind at slow speed and this increases the pilot workload, control inputs and the likelyhood of a rate of descent inadvertantly building.

You readers and writers of Greek flute music have given me something else to think about and I am grateful, however, I think we all agree that exploring the envelope in that direction should be left to those paid and taught to do so under controlled circumstances and for the rest of us, no rates of descent in excess of 300fpm when below 30 kts will keep us in a nice cosy corner of the envelope.

Thank you all for a very interesting thread.

hh

Shawn,
Good evening,

The Ex Raf, and now Cfi/examiner who spent quite a lot of time explaining and actually demonstarting this VRS state to me on one of my annual's headed the Heli into wind, brought the heli to a high hover and when there slowly started to shed power by pushing the leaver down very slowly, we felt the entry into VRS by the vibrations and uncontrolled yaws, at this stage the power was slowly reintroduced and we could see the huge/fast loss of altitude, it was recovered as you have stated by pushing the nose down at the same time as pushing down on the collective and after the airspeed built up to around 15 Knts the power was reapplied and we flew out of this VRS state, but to reiterate we started by flying and hovering into wind, frighteningly it seemed to me the only ref to loss of many hundereds of feet could really only be judged by the instruments, until we were flying again.

downcast by the downwind
 

The downwind part is easily apparent to me, because that was not only my first frightening encounter with VR but also the probable cause thereof.

I was doing a low speed recce of a potential confined area at the base of a deep gorge, in the bend of a river and it was obvious that it would have only limited approach directions, and they would be relatively steep. There were also some power-lines around (and a flying fox wire-borne cross-gorge funicular close-by). Because I wasn't planning to terminate in an OGE hover but just do a slow flyby for my crewman to take some photos, I disregarded the wind - which wasn't really significant anyway, although it was certainly about a 5 to 9 kt downwind component....and the air was quite smooth. It was only as we came down to the level of the rim of the gorge that I started paying attention to the rate of descent, because it seemed excessive. I pulled in some collective (UH-1B) and the rate of descent and blade-slap increased quite noticeably (being below the level of the surrounding high terrain). Half my mind was concentrating on the constraints of the flight path for a good photographic pass, but the other half was becoming increasingly concerned with the plummeting aspect. Because we were quite light I wasn't really concerned at this stage, more perplexed really. At about the stage where I was beginning to get ground rush I had pulled it in to max torque but hadn't done anything really about the flight-path, still fixated on the photography aspect and still inexplicably intent on maintaining the angle so as to bypass the pad at a good ground-surface recce height. We may have actually been arcing over somewhat because of that downwind approach effect.

This all happened over the space of no more than 35 to 50 seconds and you have to remember that I'd slept through the VR and retreating blade stall lectures and still had only about 50 hours on choppers. Dumber than dog-droppings really. At the point where I threw it all away and looked desperately for survival solutions I wasn't thinking engine failure or anything machine related, because everything sounded right and felt right, it was just that we were in a hell-bound and cable-cut freight elevator. To this day I think that my instinct of "overshoot" was a valid solution (i.e. I didn't consciously lower the nose with the cyclic as an answer to VR - because I hadn't realised that that was what it was). At about the same time, around 400ft AGL the river, we entered the narrow bend of the gorge and the air changed direction and speed with a bit of turbulence - and it was as if we'd jettisoned a very heavy load. I felt a real surge of lift upon the rotors. Might have been the venturi effect of the gorge narrowing or just the change in local wind velocity but it was something akin to the Hand of God as far as I was concerned. I was quite shaken by it, later discussed it with a few people smarter than I and it all then became clear.

The real clue to the significance of the approach being downwind is that I was flying in relation to the ground and so enhancing the probability that I'd be staying in my own downwash. Highly probable that it was an arcing over approach because of the overshoot trend effect of the downwind component. The very same consideration applies to what happened to the MV-22 at Marana, except that he arranged for the downwash to be on his flight-path by fully utilising that 95 degree nacelle-tilt capability (to kill his wingman overshoot). Think about just what that extra 5 degrees does. Great for slowing down, but it also puts your downwash right in place, beneath and ahead, ready for recirculation.

So I think that "downwind" is very relevant to your chances of encountering vortex ring. Been there, done that, still got the stained undergarment.

Those unfamiliar with the night-crash of the Puma VH-WOF on oil-rig approach to a tanker's helideck off the Coast of Western Australia might care to download the 170 kb file from

this url. It has some good lessons to be learned, first person commentary in there (as I recall). It may have been that a moving vessel's deck has the same effect as a downwind component? Too late to think that one through. Anyway, it sucked in two experienced rotary drivers, so it's worth reviewing.

Irlandes,

The downwash velocity is measured relative to the free stream ie, the air mass that the helo is operating in. The air motion is that which the rotor induces on the air in order to generate the lift. If the aircraft is rising or descending, the downwash velocity is a constant relative to the air around. That is why the VRS state can occur, the downwash velocity matches the descent rate to start the VRS process.

Imagine a swimmer in a body of water, stroking with his arms. As he strokes, he is thrusting packages of water backwards to pull himself forward. We can estimate the drag of the swimmer if we could measure the backward flowing stream of water he leaves behind. The momentum change on the water is required to create the forward propulsive force.

Nick

Nicks website
 

Hi Nick,

It seems the link to your vortex ring page is not working. Any chance of it being reinstated?

http://mywebpages.comcast.net/llappos/

I may be taking things a little off-course here, but can anyone enlighten me on the subject of vortex ring? I have flown helicopters at two difference schools (I now have the grand total of 11-ish hours rotary time).

At one place, the first thing they said to me, while we're preparing the coffee, not even seen the a/c, is AVOID THE VORTEX RING STATE. Below thirty knots, DO NOT let the ROD get beyond 500fpm. I later calculated this equates to an angle of six degrees.

At the second place, it did not have the same emphasis. Bailey volume 1 describes it as though you do all the wrong things and then recover, like incipient spin recovery in a fixed-wing. I've also seen a description of VR as airflow attaching and disappearing in a flickering manner the entire length of the blade (wind-tunnel tests thankfully), which sounds like something one would want to avoid.

Can anyone wise me up? Moderator, don't hesitate to squash this if you think it's inappropriate.

Hilico :

Read all about it in any decent helicopter aerodynamics book, then get it demonstrated - if you're in the UK it needs to be done as part of the PPL syllabus, and is quite fun. If your instructor is game, see it from lowish level - 800' - 1000' with a smartish recovery to see what it looks like when you're most likely to encounter it.

And as for six degrees - angle, shmangle ! You can't see what 6 degrees is like from inside, but you can see IAS and ROD - use them.

Not inappropriate at all Hilico - and welcome to the forum.

Vortex Ring occurs when you encounter your own downwash, and can happen even if your rate of descent is not high.
It’s most likely to occur if you have (1) a low or zero forward speed when (2) descending at a medium rate and (3) a high power setting eg a steep approach where the column of air remains underneath your helicopter. With a rate of descent matching the speed of the downwash, there is no angle of attack, the blade root stalls and thus you have no lift.

To get out of it, you reduce power and go into forward flight or autorotation, but you’ll will lose a lot of height anyway.
You avoid vortex ring by keeping forward speed while you descend, or by descending more gently.

The symptoms sound terrible (vibration, buffeting, pitching, yawing, rolling, accelerated rate of decent and temporary loss of cyclic control) but you’ll learn in training how to avoid it, how to recognise it at the incipient stage - and how to recover safely if it does happen.
(It's more like a stall in a fixed-wing, than a spin.)

[Edit]
As for the instructor who started your introduction to flying helicopters with a briefing on the dangers of vortex ring state - find a better instructor or school. :rolleyes:

Hilico,

You need three things for vortex ring:

1) Low or zero airspeed
2) Power applied
3) a high rate of descent; books vary, but probably more than 400 ft/min

Note you need ALL THREE of these. So if you get rid of one, you solve the problem. So all you normally have to do is move the cyclic forward and get some airspeed.

They say it's most likely to happen in a steep approach, probably to a confined area. However, most people are ready for it then, and IMHO it's most likely when you're concentrating on something else, eg at low speed or a high hover while a passenger tries to take photos or similar. But as The Nr Fairy says, you'll do it in training, probably quite a lot, so you'll get to know what incipient vortex ring feels like.

As for what happens...well, the root of the blade stalls, and the tip has no lift due to increased vortices...read the books if you really want to know.