Vortex Ring / Settling with power (Merged)

[BlenderPilot]

WAIT A MINUTE!

The last time WE discussed this I left the thread with a whole different view of VRS, and from what others have written in previous threads it seems to me they have all gone to the previous paradigm of VRS, or some of us are still confused.

CONTRARY to what some have said . . .

Its EASIER to get into VRS . . . . .

WHEN APPLYING LITTLE POWER, the less power you use, your downwash velocity is less and easier to catch up to, the MORE suceptible to VRS you will be, .

WHEN YOU ARE LIGHT IN WEIGHT, the lighter you are, the lower disc loading, the lower downwash velocity, lower ROD required to get into VRS, EASIER to get into VRS.

WHEN AT LOWER DENSITY ALTITUDES, the lower the DA, the lower the downwash velocity, the rotor doesn't have to move as much air to produce the same amount of lift thus less downwash, harder to get into VRS.

VRS depends largely on Disc Loading and Downwash velocity.

I understand you have to catch up with your own downwash (match downwash velocity with ROD) in order to get into VRS,

Let not again get "VRS/Settling With Power" confused with "Settling With Not Enough Power"

I think I'm right aren't I?

[UNCTUOUS]

The Downwind Aspect

Didn't really see any mention of it above so thought I'd mention that in my view a steep downwind approach is a great way to end up settling into your own downwash.

My first exposure to it was in a UH-1B and I encountered it as I approached with a mild downwind for a low flyby inspection of a prospective LZ at the junction of an L-shaped gorge. It was only the latter perspective on the high terrain that made me aware that something was seriously wrong. Introducing lots of collective just accelerated that downward vector and induced a very distinctive sonorous beat in the Huey's blade-slap. As I'd probably slept through the lectures on retreating blade stall and VRS, I can only conclude that it was a 19 y.o.'s superior reflexes that made me shove the nose down and go for an accelerating disc. Or maybe the recovery was helped by an enforced turn away from the gorge walls.

It was very attention getting and adrenalizing and I've no doubt that if it had been at night or with NVG's, I'd not have picked up the cues early enough to have escaped. As I recall, the a/c was very light-weight (just me and 800lbs of gas, no winch).

[Vortex what...ouch!]

I also had the misfortune of experiencing VRS.

I was practicing transitions, upwind, then turning downwind then upwind and slowing for landing. On this particular day the wind changed without my noticing, not by much but enough to make a difference. I found myself in VRS after making the turn upwind. Slow with a high rate of descent equivalent because of the change of wind direction. Very Scary I can tell you, although it was only afterwards I realised what had happened.

That feeling you get when you drive over a bridge and leave your stomach behind. Well that was it. I hit quite hard but fortunately I was only about 6 feet high straight and level and so no damage was done, except for my nerves. I was very scared for a few minutes afterwards until I realised what had happened.

My experience is limited and so I did not realise entirely at first how I had got into it. I now understand fully after talking to other pilots about what I did wrong.

I was too low to do anything about it, then that is probably always the case, but more importantly too unaware of how easily it can strike.

I am so grateful for people like Nick et al who come on here and take the time to explain to us novices the multitude of ways we can kill ourselves in helicopters if we do not take the time to understand them fully.

I got away with it, but only just. There are so many other ways I could get it wrong, but hopefully will learn on here enough not to do so.

[Nick Lappos]

Chosing words carefully, let me say that the experiences described below were settling, and they did have power, but they were not VRS (or settling with power) aerodynamically, at least not to start with.

The classic downwind turn is a killer because the aircraft is slowed through zero knots and then accelerated backwards. This drives the rotor through the peak power point, where there could very well be a big power shortfall. The aircraft starts dropping, the pilot pulls what power he has, the bottom falls out and he calls it settling with power.

VRS or settling with power requires that you descent to eat your downwash, so the descent speed must already be there to start the VRS. The situations described start from a mild descent, and with the big changes in power required due to the downwind condition, the descent begins because of a power deficit (This is NOT settling with power, as confusing as the words are).

[helmet fire]

Apart from the excellent stuff on Nick's site, there are some intresting discussions of VRS and the issues asked about here on two previous threads:

Black Hawk accident

and

BH Accident not VRS

[offshoreigor]

Hello All, Its been a while.

Excellent topic.

Just one thing I'd like to point out, maybe it's symatics, who knows.

I keep hearing people associate "Settling With Power" with "Vortex Ring State". IMHO, and many others in the Check & Training department, settling with power is not VRS, but a case of the A/C simply not having the power available to arrest the ROD on landing, i.e. "Hot and heavy above RTOW to an offshore platform with little or no wind".

I think where the confusion comes in is related to an old military expression "Power Settling" which is in fact the same as VRS.

What do you think Nick?

Cheers, OffshoreIgor

[Nick Lappos]

Igor,
I wish that were how it is, because that is the best way to describe the two different things. In a perfect helicopter world, Vortex ring is aerodynamic downwash eating, and settling with power is too much weight or too little engine. Unfortunately, the "official" definitions sort of trap us.

SWP and VRS are the same, according to the FAA:

http://www.geocities.com/flyingmouse1/Chapter_9.html

By the way, I put the VRS web site up on a real server, where it won't shut off:

http://www.s-92heliport.com/vrs.htm


note that even the FAA is confusing the two things, as they warn you to avoid, "hover out of ground effect at altitudes above the hovering ceiling of the helicopter" as if that is VRS.

OH well.

The important thing for ppruners is to make sure they have enough power to perform the hover they intend. If they do, then they can make a vertical descent with relatively low ROD with no worries about VRS, until they get to a ROD of about 50% of their downwash speed.

I have been in a real case of VRS during some Cat A vertical rejected takeoffs, where I was at about 8 knots forward speed, and at 25 fps ROD, when the bottom fell out, and I was suddenly doing 50 fps when passing through 50 feet (that means 1 second to crunch time, sports fans!) I pulled an armfull of collective at about 5 feet, hit the top collective stops at 1 foot, and whacked the ground at 13.6 feet per second (twice the design sink speed) bounced up and (to make sure I had learned my lesson) hit again at 8 feet per second. To its credit, the S-76 was a lot stronger than advertized, otherwise someone else would be writing this. We wrinkled one stringer in the tail cone.
Theoretical VRS should have been about 25 feet per second.

[jellycopter]

VRS Anecdote

I used to instruct new pilots to the Puma in a high altitude night hovering role. As part of the instructional package, the incipient stage of Vortex Ring was induced to:
a. Demonstrate how a relatively benign manoeuvre could rapidly go wrong.
b. Enable the new pilots to recognise when things were starting to go wrong.
c. To demonstrate how to recover from the situation if ever they got into it.

The scenario was a stabilised into-wind hover at night at about 8000ft using a hover-meter. Allow a slight forward drift to develop (1-2 kts G/S). In order to correct the forward drift, the cyclic beep trim would be beeped back once to give a slight nose-up attitude to arrest the forward movement. Then WAIT. Relatively quickly, the forward drift would be arrested but if no further corrective action was taken, the helicopter would begin a slight rearwards drift; and this is where the problem begins. As the rearward movement increases, airspeed reduces and translational lift reduces accordingly inducing a RoD. It doesn't take long, maybe 10 seconds or so, for the RoD to catch up with the downwash and you're looking at the incipient stages of VRS. (ShyTorque; do you recognise this mate?)

The interesting point in my experience, having done this demo dozens of times was that the Puma was very predictable. Allow the rearward drift and RoD to develop without any corrective inputs, watch the VSI (RCDI) increase to 750fpm and then feel the rapid increase in vibration level, note the yaw fluctuations as the T/R bit into the very disturbed VRS airflow, see the aircraft attitude randomly fluctuate without cyclic input and most startlingly, watch the VSI peg off-scale low. To recover, slightly lower the collective and apply forward cyclic. Within a couple of seconds and a few hundred feet (maybe a thousand+ if you are slow to react) the aircraft would happily be flying again, ready to climb up for the student's attempt. Except on one particular occasion........

Same scenario as above. Going through my usual instructional patter and explaining just before we hit 750fpm RoD what is about to happen. Except this night, for a reason I am still unable to explain, something completely unexpected, and scary happens.

Instead of the VSI showing 750fpm and then the usual vibration and undemanded pitching/yawing as the VSI pegs off-scale, this time there is initially no-vibration at all. Infact, the whole thing went eerily smooth; there was no pitching, rolling or yawing for a few confusing seconds. My 'slick' instructional patter dried up! However, the VSI still pegged off-scale low. After what was probably about 3 - 4 second (I wasn't counting!) the vibration I was expecting appeared as did some very harsh pitching/rolling and yawing movements. The engine RPMs (NGs) were fluctuating wildly. Perhaps scariest of all however, was when I tried to recover by taking my usual conditioned actions; the cyclic didn't not want to respond. I had to apply full forward cyclic but the helicopter just didn't want to pitch nose-down to fly away. After a few more tense seconds it slowly began to respond and the ASI reluctantly showed the magic 30 knots whereupon I applied full power and recovered to the climb. My altitude at recovery was 1800ft. We'd lost 6200 ft.

What's the moral of the story. Well, for what it's worth in my opinion. VRS or Settling with Power or Power Settling, whatever you want to call it, is not predictable at all. There are many variables as Nick L so elequently covers on his web-site. It is beyond the vast majority of pilots to compute all these variables and decide where, on any particular day, the boundaries are. So, stick to what you've been taught and ALWAYS keep your wits about you on approach, or indeed, when hovering OGE.

I normally agree with Nr Fairy's posts, however, as for his suggestion about getting an instructor to demonstrate the incipient stages of VRS at low level to make it more realistic, I entirely disagree. If you decide to go down this route - the best of British luck to you, you'll need it one day! The same logic could be applied to a fixed wing stalling / spinning demo - "lets do them on finals because that's when we're most likely to encounter them for real" - I don't think so!

Finally, Nick L, or anybody else care to try to expalin why I experienced a few seconds of super-smooth descending flight before I entered fully developed VRS? J

[Nick Lappos]

the new VRS web site:

http://www.s-92heliport.com/vrs.htm


jellycopter,

Thanks for the great post, it is this stuff that we need more of, where actual events are described in enough detail to help distill the real story. Let me try to describe what was probably happening aerodynamically to your helicopter, which I believe is entirely consistent with the data on the web site:

As you describe, the demo you normally give was really not full VRS, mostly, but nibbling (even biting) at its edges. The downwash velocity for a 13,000 lb Puma (Is this an OK value?) is about 32 ft/sec (1950 ft/min). If we follow the rule of thumb for VRS, we can see that we should start getting worried about VRS at about 50% of the downwash speed, and by 75%, katie bar the door.

With a torque fall=off of 20% in the demo you gave, you might get from 750 to 1000 ft/min descent, for most helos (sort of exactly the opposite as a forward climb, where the ETL gives you that climb rate.) So in most helos, if you shed ETL by decelerating, and make no collective/power adjustment, you will get 800 ft/min descent. For a Puma that is probably 40 to 50% of the downwash velocity, right at the edge of our calculated VRS speed. Note that one of the first effects of VRS nibbles is to increase the power required (see the cartoon I drew of the power required curve, where there is a hump at 7 knots as you descent at the .5 Vdown speed). This means that at this descent rate, the bottom begins to fall out, and you get increasingly more power shortfall, and therefore more descent rate (if you don't raise the collective). You ended the demos in the past with an acceleration forward, and all was fine.

The yaw wiggling is the rearward flight, mostly, but as you enter vrs, the yaw is in concert with the torque jumping, as the rotor flow changes rapidly enough to make the engines work up and down to keep up with the varying rotor torque. Those torque jumps are a fine indicator of VRS. Note the load factor trace on the V-22 as it gets into VRS, where the smooth slow oscillation (probably the product of a poor autopilot which is not optimized at that regime) becomes a choppy, random one. The load factor gets screwy as the rotor develops randomly oscillating lift and therefore oscillating torque.

In your one massive descent case, the initial descent was a bit more, and the conditions were perfect for the death spiral of descent/power required rise/descent to push you right into full VRS where the simple recovery you used was not enough. Your cyclic effectiveness loss was probably due to the rearward flight effects, where the horizontal tail might have stalled backwards, and the rear part of the disk also lost more lift that the front, leading to the inability to get the nose down. The Puma is a relatively low control power machine, anyway, and you were certainly outside of its normal envelope. Were it daytime, you could have pedaled it around so the nose pointed into the wind, which would have helped things (but at night, the disorientation would be a real problem!)

I believe the old pilot lore of lowering the collective is actually wrong, and more likely to harm you during recovery with greater altitude loss. If you have adequate power to climb vertically at your weight, raise the collective during the recovery, which increases your downwash speed and helps break the VRS. The old experienced Sikorsky test pilots who taught me said that, and it always worked for them. In the old days with piston powered helos, this was never the case, and increasing the collective could cause rotor droop, which is probably the last thing you need!

In a high powered helo, I would increase the collective to max power, but not droop, lower the nose and get out of dodge that way.

This set of events seems to show that the old rule of avoiding the 50% downwash region is a swell idea.

[peter manktelow]

...seem to remember when doing my basic with the RAAF on UH-1...many moons ago

"Below 200 feet then max RoD 200 fpm when below 20 knots"

That seemed to be an easy set of numbers to remember and was aimed at avoiding vortex ring....must have been easy to remember as that was in 1970.......bloody hell...time do fly when you are having fun

[Hingeless Rotor]

Nick,

Just confirm, the lowering of the collective is still a positive way to exit VRS, even in the example above. Why would it not be the most efficient means?

I must admit, it never occurred to me that in some machines it is possible to pull lever to exit. Great food for thought.

Cheers.

[Garry M]

Nick

I carry out mainly longline vertical reference work where we seem to flirt with VRS on a fairly regular basis. I have always found that increasing power and applying right cylic and right pedal the most "minimum altitude loss method" of VRS recovery although it has always seemed slightly at odds with conventional wisdom. Your explanation above has helped explain why. In your opinion is this as good a method of recovery as is available or should we looking for other flight profiles. I have played with many, but the above procedure seems to work the best for Anti-clockwise turning rotor-systems.

[The Nr Fairy]

jelly :

I'll have a word with the instructor I flew with on the day - he's someone I count as a true professional, and I don't think he'd put me (or him) in a dodgy position. Once I have his reasoning I'll re-post.

As for the event itself, it was the end of an hour of VRS. Why the CAA insists on an hour of VRS demos as part of the modular course, I don't know - do it three or four times, it takes all of 30 minutes. The "low-level" was at least 800', if not higher, started into wind, over the airfield, and the surprise was the nose-down attitude I used to recover as promptly as possible. If I ever get into VRS low-level, I'd like to think my recovery actions would be instinctive if I let it get that far !

One last thing - I'm chuffed we seem to agree on most things. I reckon that's pure luck - at my experience level I'm only too willing to let other people tell me how not to kill myself.

[jellycopter]

VRS Continued

Nick L,

you wrote "The yaw wiggling is the rearward flight, mostly, but as you enter VRS, the yaw is in concert with the torque jumping, as the rotor flow changes rapidly enough to make the engines work up and down to keep up with the varying rotor torque"

Just to clarify; during my demos, we rarely encountered a negative airspeed. These demos were flown at 8000 ft AMSL in the UK where there is usually always a wind blowing. You'd be hovering over a fixed point with usually 15 - 40 kts airspeed. By allowing a rearward (ground) drift to develop with no collective lever input, the loss of ETL causes the helicopter to descend - all the time however, maintaining a positive airspeed. In this regime, I believe the yaw 'wiggling' at the incipient stages of VRS is due to disturbed flow over the T/R. Once fully developed in VRS however, I entirely agree with your torque jumping argument.

As regards your suggested recovery method of applying collective, I have quite serious reservations. I can see that this recovery would work during the incipient stages of VRS when only a proportion of the disc is stalling - provided you have sufficient power available to pull the extra pitch. However, in the fully developed stage (as I obviously encountered as described above) I beleive it would be entirely wrong to pull in the collective to recover. If you follow through the conventional wisdom of the doughnut of airflow on the outer portion of the disc causing increased induced flow and reduced lift. Couple that to the stalling of the inboard portion of the disc due to high alpha caused by the RoD - by raising the lever, you can only exacerbate the situation.

This leaves us with a bit of a problem. Do you raise the collective to recover during the incipient stages and achieve minimum height loss. Or do you use the same recovery action as you would during the fully developed phase - lower the lever slightly and pitch nose down to recover the airflow but lose more height. With my experienced pilot hat on, I reckon I might be able to decide, fairly accurately which method to use and when. However, with my instructors hat on, we are all aware, things need to be suitable for the lowest common denominator ie. the low hour ppl (no offence intended to low hour ppls - i was one once!). If you give the low hour ppl a choice at such a critical time, Sod's Law says he will make the wrong choice and kill himself. If on the other hand you lay down in tablets of stone a set of actions to take in a particular set of circumstances, chances are, if he recognises the problem, he'll fix without conscious thought.

What I'm trying to say is, to recover from VRS, either incipient or fully developed there needs to be a standard recovery action that will work in all events. The conventional recovery of lowering the lever slightly and diving for airspeed works - period. If on the other hand, you raise the collective to recover, it might work, it might not. This depends upon how quickly you recognise the VRS condition and how much power the aircraft has in surplus.

The only problem the conventional recovery gives us is when we encounter the condition at low altitude. In this regime, you may have insufficient height to recover using the conventional method, then again you might! Likewise, by raising the collective to recover, it might work, it might not. This is why, once again set in tablets of stone (well, more like soft jelly (that's jello I think in the US)) we have a region of the RoD and IAS envelope which we avoid at all costs. J

Nr Fairy

I take your point about the 1 hour of VRS in the syllabus but I can't do anything about that.

However, what I can do is try to give people (usually my student's) the benefit of hard earned experience. As far as VRS is concerned, on the Puma at least, I got pretty familiar with it's behaviour. Nonetheless, one day it bit me. Luckily, my 3 other crew members and I are here to tell the tale (we even made the video!). As I said in my original post, we used to recover from the incipient stages in hundreds of feet, and occasionally just over a thousand. However, I always practiced from 8000ft; and it's just as well I did. Much lower and we would all be dead!

I guess, in a Robinson or other light helicopter, you can practice VRS recovery all day long from 800ft and I dare say you'll get away with it 99 times out of 100. It's the time when it takes you by suprise, as it did with me, that you'll have insufficient height to recover and you will die.

If it's any consolation, the VRS demo I had from a Robinson instructor (at a premier (?) UK heli FTO) in an R22 was not VRS at all, not even the incipient stage. The only symptoms we had were a high'ish RoD and low IAS but none of the accompanying vibration, yawing, pitching and rolling. The recovery was the 'conventional method' and the aircraft flew away immediately. I would wager that you could have done that demo quite safely from 800ft as it wasn't really Vortex Ring.

Incidentally, the UK military do not demo VRS to ab initio students. They simply explain the risk areas and encourage students to avoid them. This seems to have worked successfully for years now, so why change? J

[Jcooper]

SWP or VRS

Do you mush Settling with power and vortex ring state together or do you keep them seperately? Just wondering as I was always taught that settling with power is the same thing as VRS and then they had another term settling through power which is when you have the power there but your descent rate was to great to be stopped? Just wondering...

[Shawn Coyle]

I don't know of a good, clear definition for 'settling with power' that would make it easy for the pilot in the cockpit to say that he was experiencing that or whether he was experiencing vortex ring state.
Most people say settling with power is when power available exceeds power required, but that can happen without the symptoms of vortex ring state.
Unfortunately the terms get mixed up all the time, adding to the confusion.
I avoid the use of the term settling with power as it doesn't add any clarity to the discussion. But that's my personal opinion.
If someone can come up with a clear distinction between the two, I'd be happy to include it in the next edition of the book.

[IHL]

Practical examples of settling with power:
1) Aircraft is able to Hover in ground effect (just) at SL. It departs and then attempts to land at 8000 feet, maximum power is applied but it is insufficient to arrest the rate of decent.

2) The aircraft at gross weight attempts to land with a 15 knot tail wind. Maximum power is applied but insufficient to arrest rate of decent.

3) If you have ever come into a landing spot and had to over torque to get stopped at the bottom, that is settling with power.

[MightyGem]

JC, this subject has been discussed at some length on previous occassions. A search on "vortex ring" will bring up many results. A good starting point would be here

[Heliport]

Various VRS/SWP threads merged.

[Skycop]

Glad to see this old thread of mine is still going, chaps!