Downwind Quickstops

[Dump the pole]

Hi All

Why are downwind quickstops dangerous? I know not to do them because of Vortex / SWP but when I try to use my rusty Principles-of-Flight to work out why I'm getting confused.

When INTO wind: I've got the lever down and I'm flaring. As the machine slows I begin to pull in power.
At this point I've got Power Applied with Low Airspeed and because the disc is tilted backward (but the machine is still moving forward) I have a rate of descent ?

If I'm DOWNWIND, because the disc is tilted backwards the wind behind me would help lift ???

What have I got wrong ?


While I'm here why does it take more power to hover downwind ? Must be some interaction with the tail ?

I can't find these answer in any of my (old and dusty) books.

Thanks, DtP

[Bravo73]

DTP,

To answer the second half of your query, have a look at this thread here:

Hovering Downwind (http://www.pprune.org/forums/showthread.php?t=157371)

Essentially, it doesn't actually require more power to hover downwind. It's just that slightly poor handling on the part of the pilot can tend to consume more power.


I'll let the assorted aerodynamics experts answer your first query.


HTH,

B73


Oh, even if your textbooks don't have the answer, the subject might well have been discussed on here already. Remember, the search function is your friend! ;)

[Gaseous]

They are not dangerous if you know what you are doing. They are a bit harder though. The main thing that springs to mind is when you stop with a tailwind you are flying backwards. You may find you are caught out by weathercocking and end up inadvertantly rotating:eek:
You only have a rate of descent when descending and you should not do that until the quickstop is finished.
edit: Oh yes, I THINK the tailrotor consumes more power holding the tail into wind but I have never noticed it as a practical effect. I spend more time looking outside when hovering downwind than examining MAP gauge for small changes.

edit again. I just read one of Nicks post and he says it doesnt.

[teeteringhead]

You only have a rate of descent when descending ... seems obvious but ain't necessarily so when talking quickstops and vortex ring...

..... the problem of "rate of descent" when approaching vortex ring is in fact flow upwards through the disc opposing induced flow (hence "power on" being a prerequisite for vortex ring)...

... in a level flare with forward motion (aka a quickstop) you have precisely those conditions. And of course for all the downwind hover and rearward airspeed reasons you would have much more power applied if you tried to quickstop downwind.

Not sure why anyone would want to do a quickstop downwind, when a "flare and turn" of some sort can almost always be accomplished.

Did have a stude once try and quickstop (a heavy Wessex) downwind when invited to "do a downwind quickstop of your choice"....

... he was subsequently chopped! But not just for that...

[Gaseous]

TH, Ah semantics. Dont ya just love it. I would suggest a better phrase to describe what goes on in the flare as a change in the direction of relative airflow. Descent as a word is strongly indicative of going downwards.
I suggest you read nicks post (3) in this thread re: power requirements in the hover
http://www.pprune.org/forums/showthread.php?t=157371


By the way Its a gorgeous day. I'm going flying now.

[Dump the pole]

Thank so far,

I don't WANT to do downwind quickstops but I just want to understand what happens.

TH: You have grasped the point that I was trying to make. In the flare the disc is tilited backwards. This surely has the same effect as a descent when you're moving forwards.

Now, with power applied, low air speed and this rate of descent I don't see why I wouldn't get into SWP / VR when INTO wind.

DtP

[Arm out the window]

Is this the same fella who was asking about the backwards autos?!

Just quickly, if you fly a quickstop into wind, you start at high speed, reduce the power and flare. With wind on the nose and a low power setting, you're not likely to be descending (I use the term referring to the relative airflow) into your own downwash, and therefore are unlikely to get vortex ring. When at the desired speed and approach angle, you pull power and lower the nose to smoothly join the normal approach profile, again not a vortex ring risk.

However, comma, if you decided to do this downwind, you may find that when pulling power in during the latter stages of the manoeuvre, the tailwind sets up a condition where you're effectively settling into your own downwash, and you're in vortex ring close to the ground and in a bad way. I believe this was cited as the cause of a UK military Puma crash where they picked the wind wrong and ended up quickstopping downwind with a resultant prang.

[NickLappos]

There is simply no way that performing a quick stop will result in vortex ring state, because the rates of flow through the rotor are nowhere near the velocities required to cause vortex ring state. On the other hand, it is certainly possible to get yourself in trouble with the downwind quick stop if you don't carefully fly the maneuver. Why?

Because, when you bring the aircraft to a quick stop down wind, you first have to decelerate through 0 knots of air speed. And of course your power required its highest at 0 knots of airspeed, which you pass through somewhere short of the stopping point. This means the aircraft will sink a bunch at that 0 knot point, and ground contact is possible. Additionally as you pass through the zero knot airspeed, you actually begin accelerating rearward through the air so that the aircraft will start to try to point into the wind, becoming very unstable in yaw. This instability occurs right where the collective is being moved a bit due to the power required needs. As a result, you find yourself passing through the peak power point and dancing on the pedals at the same time. The chances of making a mistake are substantially higher than otherwise.

Typical quick stop problems include ground contact (especially the tail, because you are in a flare) and also poor yaw control. Overspeeds are more likely (big power changes) and sometimes there is a tendency for the helo to swap ends.

The old wives tale about vortex ring state being encountered while maneuvering near the ground simply will not go away. Let me specifically state that it is impossible to get into vortex ring state while maneuvering around the hover and performing maneuvers like a quick stop. Why? Because vortex ring state requires rates of descent of approximately 800 feet per minute to 2000 feet per minute, depending on the type of helicopter (actually depending upon the helicopter's disc loading.) if you achieve those descent rates near the ground, you have bigger problems than vortex ring state. The books that say that vortex ring state can be encountered in a 300 foot per minute descent are simply not correct.

[Dump the pole]

Thanks Nick - That's what I was looking for.

Good explanation without mentioning "descending on a pocket of air" - and no that wasn't me

As a student we're told not to do these things and when out of the training school sometimes it can be embarassing to ask.

Its important to ask if in doubt.

Cheers
DtP

[Teefor Gage]

When I trained on the Bell 47, many moons ago, Downwind Quickstops did not involve stopping whilst still facing downwind but were, as previously suggested by teeteringhead, a low level flare and turn into wind. Quite exciting but safe if performed accurately.

[TheFlyingSquirrel]

I was always told that the biggest element of the manouvere that made it unsafe, was the tail stabilizor surfaces being pushed into the ground by the wind as your ground speed reduces. Try a 50 knot flare and turn, and you'll feel the downwind portion just doesn't feel quite right through the controls.

[gedney]

I'm afraid that I can't fully agree with Nick L when he says:
"let me specfically state that it is impossible to get into vortex ring state while maneuvering around the hover and performing maneuvers like a quick stop. Why? Because vortex ring state requires rates of descent of approximately 800 feet per minute to 2000 feet per minute".
On my type we fly quickstops nb 50 ft agl, and of course you should never see a RoD anything close to the 800 to 2000 fpm figures to which Nick refers, even if the quickstop is grossly mis-mandled and a RoD is allowed to build up during the manouevre.
However, as I see it it is not just the physical RoD as you sink that should concern us. As you flare the aircraft into the quickstop the angle at which the forward airspeed passes over (and through) the disc now changes. Consider it as a velocity, and take its 2 components (horizontal and vertical relative to the disc) and you will see that the disc is now exposed to an apparent RoD airflow that can be very significant. This apparent RoD airflow, allied to a sink during a mis-handled quickstop, could indeed lead to problems.
(I have drawn a lovely picture but I can't work out how to import it!)
It would be an extreme case that approached vortex ring parameters, but the difficulty that Nick described with the zero airspeed condition when operating downwind would only push you closer to this extreme condition.
Does that make sense?

[jeepys]

Maybe I am off track here but I always understood a downwind quick stop to be of the flare and turn/flare or turn and flare variety. Yes you enter the manouvre downwind but terminate into wind. It reads to me that you are talking about doing a complete quickstop from entry to completion downwind.
If this is the case then I am all ears but would not do one myself, then again as I said I may be well of the beaten track.
I shall wait to be enlightened.

[NickLappos]

gedney,
The VRS regime is where the rotor is fed a stream of air at a precise angle where the velocity of that air is 75% of the downwash velocity of the rotor. For a Robbie, this is about 800 fpm, and it must be aligned precisely at about 85 degrees angle of attack to the disk. ANY other angles or velocity and it simply is not VRS. If you achieve those speeds and angles while doing quick stops (85 degrees of disk angle to the air) you are really stopping quickly!

What most pilots call VRS is actually "too little power" and very nicely called "over pitching" in British parlance. A rapid power demand, faster than the throttle increase or engine accel, and the helo falls through, and the investigator pulls out his rubber stamp that says "VRS" and we have even more proof.

In reality, any discussion of VRS is a discussion of a rotor very close to autorotation (since windmill brake state is the next phase, and occurs about 120% of the downwash speed.) True VRS is extremely rare.

[FH1100 Pilot]

Couple of things...

1) VRS is not always about absolute rate-of-descent of the airframe. Remember the V-22 crash in Marana, Arizona that killed all those Marines? It wasn't that the ship was descending vertically so fast. In fact, the pilot still had "forward" airspeed. But this is deceptive. He also had the nacelles tilted backward (which I believe NATOPS now prohibit) as he was trying to decel/descend and maintain position on Lead, who'd screwed up the approach (and subsequently hard-landed his machine, a fact that was overshadowed by the crash of #2). It was this "apparent" RoD that the proprotors were seeing that caused them to start to go into VRS, evidenced by the roll instability the pilot experienced just before one of the proprotors "let go" into full VRS, putting the aircraft into unrecoverable A-VRS at which point it rolled pretty much inverted and crashed.

2) We need to be careful when we speak in absolutes. Although I generally do not like to disagree with Nick, when it comes to the absolute RoD's needed to induce VRS, one image sticks in my mind: That amateur video of the Canadian Sea King hovering around at the airshow in upstate New York. One minute he's hovering there, pretty as you please, camera steady on him. Next thing you know, it's falling like a wet turkey, blades coned up like a ballerina, tips nearly touching. (I wish I could find the full video that was shown on the local news the day it happened. The truncated clip that we see in the archival footage just doesn't give the complete story.)

I think that maybe nature doesn't always cooperate with our pat theories and "rules." I think that maybe a sudden gust...an updraft through the rotor can make it "think" that the whole aircraft is in a rate of descent. When that happens, if all the other stars are lined up correctly, boom-crash!

Downwind quick-stops to termination? Yikes, there's a scary thought!

[NickLappos]

FH1100,
That video is a screamer, I have found it a dozen times, but it is NOT VRS. The touchdown velocity of the helo precludes VRS. It is "over pitching" as can be seen by the slow rotor rpm among other things.

VRS is extremely rare.

I will post (again) the plots to show where and how it occurs, and let the debate run rampant (because it is a good thing!)

[i4iq]

Nick

Can you clarify...

When we're practicing "settling with power", climbing to 2,000 Ft, slowing to zero airspeed (sometimes needing to induce a descent), raising collective and watching the VSi swing to indicate a descent of 1,200 fpm or more - are you saying that the initial indications are not VRS? From a safety point of view, isn't this a moot point, as we're trained to recognise the onset? Or is "real" VRS more perilous than what we actually practice?

I have heard that in some types - such as the schweizer, you're not really in VRS until the tail "drops away" behind you.

[FH1100 Pilot]

I can take my little ol' FH1100, go up to altitude, bring 'er to a downwind hover, get a decent descent goin', the pull up on the pole and watch it come down faster while not even getting near a power limit. Done it, in fact. Done it in a 206, too. Maybe VRS is a myth, or maybe it's "extremely rare" but I can experience "settling with power" any time I want to.

In fact, I used to demonstrate this to young aspiring commercial pilots. Photographer in the back, wants to shoot something on the ground from a certain angle - which puts you in the position of being in a downwind OGE hover and (of course) sideways to the object. You sit there, looking out the side window (not really at the instruments), maneuvering around at "the photographer's" command, "up a little...now back...down a bit...hold it there...down a little more..." Suddenly - whoa! - you find yourself in a big descent and pulling pitch doesn't stop it. It's a fine, easy, fun demonstration up at 3,000 feet, where the ground is nice and far away. But, I ask them, what would happen if you were only at 500 feet? It's an eye-opener, baby.

And yes, I have done such ball-buster photo flights. He says, "No, we can't be moving - I just HAAAAAAAVE to get this shot from this specific angle." And you go, "Yeah, but the wind is...I mean the ship doesn't...<sigh> oh okay, we'll give it a try." Hey, we were all young and stupid once. (Okay, some of us were.) Now I know better. Back then I didn't. Nobody took me up and showed me some of the wacky situations I'd find myself in as a commercial pilot, oh no!

So I dunno...maybe VRS, maybe not. Or maybe there's something else going on in the rotor system - weird flow transients or patterns that are hard to quantify and equally hard to understand. And come to think about it, a momentary RoD of 800 fpm in a hovering or descending Robbie might not be all that hard to produce. I mean, think about it. Think about being up at 400 feet and needed a whole 30 seconds to get straight down to the ground. Not a very fast descent, wouldn't you say? Doesn't seem all that amusement park-ish to me. Maybe VRS sometimes only needs a half-second of that to get excited.

Rotors are weird.

[JimEli]

Thanks Nick, I’ve been hounding this “urban legend” for years. Let me take a stab.

Vortex Ring State (VRS) is a condition where the rotor ingests it's own vortices. This of course, is an over simplification. But what makes matters worse, VRS is a relatively unstudied phenomenon--just ask anyone in the V-22 program office.

During powered flight the airflow moves through the rotor system from above to below. When airflow is moving from below to above the rotor, we call this condition autorotation. VRS is entered when the flow is roughly between these two situations.

So it follows, to enter VRS, the airflow through the rotor needs to approach that of the rotor downwash. The rate of downwash is related to disc loading—but how fast are you entering this quick stop? More importantly, the movement of air needs to be nearly vertical through the rotor system. How far up is your nose during the quick stop? 85 degrees? Because 45 probably doesn’t cut it.

Now, it is typically understood that a helicopter is vulnerable to "Settling with Power" (I like the "over-pitching" term too) when the following 3 conditions are present:
(1) 20 to 100% power applied.
(2) Zero, or near zero airspeed (not ground speed).
(3) Rates of decent of 300 fpm or greater.
Isn’t this what you’re talking about (especially if your rotorcraft is underpowered and/or heavily loaded)?

VRS and Settling With Power are NOT the same thing, but are typically confused with each other because most pilots learn of them coincidentally. Doubt this? Think of it this way; if you agree about the 3 conditions for SWP above, then relate them to the conditions during a normal approach. Is a normal approach SWP? Is a normal approach VRS?

[EX-PJ]

Wow!

It's awesome how much knowlage and information can be gained from this site.

My two cents with the down wind Quick Stop! Engine failure!


We MUST always be prepaired......

As you excelerate through transational lift, down wind and BANG!

Now you are down wind, low, with a ground speed that is not advised for touch down, do your best not to nose over on contact, maybe contact with slight nose up attitude, or any number of things that are waiting to bite us, and blade to tail contact is made. Reguardless, let's just say something get's bent. :mad: !

Mr. FAA inspector arrives and with his 15 minutes of helicopter experiance he determins that the action is unsafe because it was the take off phase and it was down wind! Then it's all bad!:ouch:

Most auto's with down wind touch down don't go well.

I recomend into the wind quick stop then hover drills back to the starting point. 20 years teaching folks to fly these things, ain't bent one yet (knoock on wood)!

Nick, will see you in Orlando! You continue to amaze me with your wealth of knowlage. I'm honored to know you.:D

[Arm out the window]

A few factors could combine in a downwind quickstop to set up the relative airflow into the disc so that it would be coming in from underneath, perhaps leading to the conditions for VRS, I think.
As gedney said, the components of flow relative to the disc would be the important thing, wouldn't they?
Using Nick's figures of needing somewhere between 800 and 2000 fpm coming into the disc from 85 degrees 'below', I wonder if you could get that in a downwind quickstop?
Just crunching a couple of numbers in a scenario, say you were carrying out a quickstop into a pad with 10 kts downwind, got a bit steep (as you probably would) and allowed a rate of descent to develop, how about this - slowing down with about 20 kts groundspeed, subtract the 10 kt downwind leaving you with 10 kts through the air horizontally = 1000 fpm, approx. Add the vector of your descent, say 600 fpm, which gives a relative airflow from about 30 degrees below the horizontal. Also, you're flaring, maybe 40 degrees nose up (?) giving you about 1160 fpm of relative airflow coming in at 70 degrees from below the disc. Slowing down would continue to increase that angle, if you kept the rate of descent on.
As you pulled in power to try and save a very messy situation, you'd then have all the conditions for VRS, wouldn't you? Just a thought.

[nigelh]

All i know is that the last checkride i did with a very experienced pilot, we went up to 3000 ft twice slowed down to zero , got the rate of descent up , pulled power .......and she started going up again !! Both times she did this so we got bored and did some other tricks. I think it is quite a difficult condition to bring on and more often the accident is too little too late ending up with over pitching.

[i4iq]

works better with a tail wind if you can find one. I've also noticed a difference in older Robbies vs. newer ones...

[woccer woccer]

how does the disk know its going backwards

[dammyneckhurts]

I am a crusty ole long line pilot here in Canada and have been looking straight down out of all kinds of different machines over the last 10,000 hrs or so.

Since production longlining is pretty much all I do in all kinds of terrain I am often dancing on the edge of conditions conducive to vortex ring or just simply mushing due to the aircraft simply not having enough power to do what I have asked of it.

To me the defintion of vortex ring is when I pull collective and my sink rate INCREASES. Doesn't matter much what my vertical speed is because its more about relative airflow. If I pull power and my ass gets LIGHTER in the seat its time for immediate corrective action. Some may call this "incipient vortex ring" vrs full blown VR. Labels dont seem to matter much when it happens at a few hundred feet.


If I pull power and my ass gets slightly heavier in the seat, but I am still decending faster or more than I want too, then to me that is settling with power. This could of course get ugly and turn into overpitching if you keep pulling collective. It could also turn into VR if your sink rate increases.

DMNH

[i4iq]

woccer woccer

"how does the disk know its going backwards"

it would be upside down in your case...

[MightyGem]

Dump the Pole. Most other posters seem to have become embroiled in a favourite Rotorheads topic of Vortex Ring.

Just to re-iterate what a couple have said: a downwind quickstop does not end with you hovering downwind. Imagine it as the helicopter equivalent of a handbrake turn in a car. You end up pointing in the direction that you have come from, ie into wind.

There are two types: flare and turn, and turn and flare. Heights and speeds will vary according to type. For a flare and turn, I would be flying downwind at 90kts and 50'. Initiate with flare, maintaing height and direction, and then start the turn into wind to end up pointing into wind with about 30-40kts still at 50'. Come to the hover, then forward and down.

For the turn and flare, same height and speed, but initiate with a turn and then flare off the speed during the turn still looking to be 30-40kts and 50'.

The difference between the two is the flare and turn gives you a (comparatively) long narrow flight path, whereas the turn and falre gives a short wide flightpath.

I'm sure others will have different techniques.

[topendtorque]

"There are two types: flare and turn, and turn and flare. Heights and speeds will vary according to type. For a flare and turn, I would be flying downwind at 90kts and 50'. Initiate with flare, maintaing height and direction, and then start the turn into wind to end up pointing into wind with about 30-40kts still at 50'. Come to the hover, then forward and down."



One word of warning, this is what most people might think that they are doing and come unstuck.

They start to flare, I.E. decrease airspeed, then sink, (as they wish to descend to the target) then pedal turn instead of cyclic turn, then they’re right where old mates DMNH and AOTW describes.

The only safe way to do it is turn with cyclic (steep co-ordinated turn) first half of the turn, and then second half of the turn as you are coming into wind commence the quick stop. Note how little power is reqd.

Now do it the other way that DtP first asked about (if you are stupid enough), note the power required, safety margins, and where oh where is that safe EOL picture?

AOTW describes the downwind VRS situation well and I suggest that there is no clear cut definition as Nick has isolated. That entry and many variants of it are just as good to get you sailing downawards and light on the seat. Why not look at a recent video of a seaking crash onto the back of a frigate or destroyer or some such. Note that he is sailing along in almost the same vortices configuration thanks to the ships superstructures, as AOTW describes, note the M/R coning flex abruptly two or three times as it unloads in incipient VRS.

As an analogy to the possible combinations reqd for LL VRS one might refer to a recent media report where some of the world best mathematicians finally gave up on trying to formulise the surf, breaking-wave actions.

I think that would be very simple compared to our rotor blades actions and reactions, not to mention the unseen winds vortices and currents afore you even got there.

At the end of the day when close to the ground, Downwind, Descending and Decreasing A/S, in the same handful is the same as Drinking, Driving and Destruction.

WW. No the disc has no idea which way it goes and it is also totally irrelevant as to which way the aircraft is pointing, the only relevance is the actual movement of the disc relative to the surrounding air and whether you might also be encountering or projecting vortices in front of it to then fly into.

There was a super good article in the Rotor and Wing Mag. (The Subtle Hazards of Light Winds) sometime in 1984, I use it as a bible and the numbers are nowhere near Nick’s, mind you it referred to light helicopters.

Nigelh, I agree at times esp. in the R22 in can be difficult to simulate and the running oneself out of puff and then overpitching, which is another story, is something that will get everyone sooner or later if they live close to the ground.
tet

[Arm out the window]

I'm sure I remember being told on a flying safety course that the UK Puma accident mentioned on the previous page was vortex-ring related - obviously there could be a number of explanations depending on how much information the crash investigators had to go on, but it sounded plausible.
As Mighty Gem points out, the quickstop manoeuvre we want to fly starts out downwind and finishes into it, but the story with those guys was apparently late identification of the pad, picking the wind direction wrong leading to a quickstop that terminated effectively downwind, winding up with them falling through and hitting the ground in a scenario fairly close to the conditions I went through above, ie relative airflow into the disc at the right angle and rate to set up VRS.

[Head Turner]

Hi Dump the Pole... there is ample info here about Quick Stops but what I am amazed at is the primary factor that you have not been taught properly. I suggest that you bring this up with your instructor and if he/she has difficulty teaching quick stops in the correct manner and technique that you find an instructor that knows his/her trade. Really I'm appalled that the basic techniques are poorly taught.

[rudestuff]

Downwind Quickstop. Hmmm.
Well of course, you can do the handbreak turn thing - all banked over at low level - lots of fun - but you can also deccelerate straight downwind. It all depends on why you're stopping, and if it really needs to be a quickstop - lots of people simple incorporate the maneuvre into a rushed landing (fast approach > airtaxi > quickstop > land) I think most if not all of us have done that - after all, these things cost money and flying a 10 degree textbook approach into wind after a full pattern takes time.
As far as the aerodynamics goes - the Helicopter has no idea if its going downwind, upwind, crabbing or not - it simply flies relative to the body of air.
The only difference is when and how much power needs to be applied:
When you do a Q stop upwind, you never really get to zero A/S, so all you have to do is lower, then raise as you slow down - obviously your G/S will be lower.
When you do a Q/S downwind, you have to flare all the way down to Zero A/S (requiring more power) THEN you have to start accelerating BACKWARDS to get to Zero G/S.
This is obviously harder because it requires higher power, and downwind your G/S will be quicker, so everything happens much faster.
The key to downwind operations is to fully understand how much power you need at each phase, and to know your that airspeed can already be zero - even though you're nose-up and hurtling towards the pad at 20 kts!
The reason most people 'fall through' and crash is that they flare all the way to zero, then don't recognise it - and keep flaring, nose up, power down, then BANG.
Same reasoning goes for downwind takeoffs - anticipate that your power requirement will go up first, before it goes down, and be smooth...

[Dump the pole]

Head Turner: I believe I was taught correctly (I don't do them !)

If you check my original question you'll see that I just wanted to understand WHY I shouldn't do a downwind quickstop.

Thank you everyone who has actually answered me and for the descriptions of correct technique and how to avoid having to do it !!

DtP

[NickLappos]

The amazing thing about pprune is the wide difference between all the separate helicopter worlds. This thread illustrates it quite well.

For small pistons, underpowered and flown by new guys, downwind quick stops are to be flown carefully or avoided.

For scout/attack helicopters, maneuvering low to the ground among trees and obstacles with some disregard for the wind is actually safer. When flying helos with good power and good controls, pilots who know how to use them consider that the norm.

The Real Lesson:
Please don't take the admonishment of good instructors talking to Robbie pilots as "natural" helicopter limits! And please obey those instructors until a Black Hawk or Apache comes your way!

[topendtorque]

DtP
Yours is a very valid q. and even though I refrained from talking about it before I also agree with everything that Rudestuff says. Except of course if the noise stops then lookout.

The following quote of his qualifies his story.

"The only difference is when and how much power needs to be applied"

May I attempt to explain that.

If you are doing a qs and slowing down in the downwind diretion you will get to the point where you are at zero A/S where you understand that you will need more power and further you are trying for a negative forward A/S, then that is where things go wrong for the unwary.

This is where the rudestuff quote comes in. Imagine the air that you are flying on as smooth above your rotor and very tubulent below it. Say that boundary between the two projects in a straight line in front of the disk. (If you are bloody lucky in the real world I say)

If you then apply enough power to stay above it you will be OK. Drop into it and you will be looking for all sorts of trouble, simple.

The limiting thing apart from all of the other eroded safety issues, is how much power do you have?

The same goes for doing pull ups into torque turns. When doing from the downwind direction to turn into wind it is real easy, as there is plenty of fresh air coming at your disk from behind you if you are stationary relative to the ground at the top of the turn.

When you do them the the other way from the into wind to the downwind recovery, always allow enough of a power margin to pull a bit of collective at the top so that the disk will be in fresh air. In other words plan to be travelling backwards a bit relative to the ground to give zero A/S.

It's can be kinda spooky if you don't. Remember what I said about being above eighty feet if you really stuff it up.

Doing anti-torque turns in the same manner becomes just a bit more complicated.

[aclark79]

Nick: I'd like to see those charts, especially for the R22 as thats what I'm instructing in. If your descrption is correct, (and I'm not doubting you) I guess I've never been in actual VRS. I have never allowed a demonstration of VRS/SWP to develop past a 500 fpm decent rate which would mean that I wasn't in VRS by your definitions.

I find that when I induce SWP with a tailwind and I attempt to make it worse by pulling in power the helicopter (R22) has a reduction of the climb rate and an eventual climb. Certainly not true VRS if an increase in power is supposed to result in an increase in rate of decent.

So what am I really in? SWP but not VRS? I certainly am settling, have not reached a 800fpm decent rate, have power, and am yawing and shuddering until I recover.

I still teach that downwind quickstops are dangerous, maybe I'll just be changing the explanation of why from the danger of VRS and other factors to just the other factors. Simply put I wouldn't want any of my students making a downwind quickstop, regardless of VRS or not due to the other dangers already mentioned.

[helimutt]

aclark79, I very much doubt you've actually been in VRS. What you seem to be describing is incipient stage, which is about as much as you need to demonstrate to a student. What you need them to appreciate are the dangers and symptoms. 500'min isn't much. Take it a bit further to see it develop. 2000'/min+! UK and USA call it two different things but it pretty means the same thing VRS or SWP.
Anyway, can someone answer this question:- Why do a downwind quickstop or any quickstop for that matter?
I always thought it was purely a dual co-ordination exercise whilst training.
As for those people who seem to blame every R22 crash on VRS, I say to them, it's probably because they don't understand many of the other possible causes of crashes close to the ground, but it's easy to just say 'oh yeh it was VRS'.

[Hiro Protagonist]

aclark79 said...I have never allowed a demonstration of VRS/SWP to develop past a 500 fpm decent rate which would mean that I wasn't in VRS by your definitions.

Out of curiosity when 500 fpm was indicated how long were you stabilized? If your rate of descent was increasing rapidly, and you initiated a go-around at 500 fpm indicated your actual ROD was higher because of the lag in the VSI (unless you have an IVSI).


and now...

Nick swears that VRS can't be established as easily as most of us were taught, and I'm not going to argue with Nick, because he's almost certainly right, and I'm just repeating what I learned from books/instructors which could have been misinformed. It seems that there is an intermediate phenomenon which most of us call VRS or SWP, but some call incipient VRS which exibits the signs and symptoms taught to me as VRS, but without the actual vortex ring. Some have called this overpitching, but I was taught that term referred to a condition where the high pitch / AOA was creating more drag than the engine could overcome leading to a decrease in Nr etc...

Is it possible that while descending at very low, or no airspeed, the fact that we are descending into our own downwash causes such a reduction in angle of attack that we loose a significant amount of lift, even without the development of a vortex ring? (I mean more than just the loss of ETL, but an additional loss of AOA due to descending into air already accelerated by the rotor.) Additionally, in this state, would the addition of some pitch not give us the increase in AOA/lift that we expect, and therefore our rate of descent keeps increasing, and we feel that we are "settling with power"?

I ask because there is definitely a phenomenon when descending at at airspeed less than ETL whereby increased collective pitch seems to have no "bite", no increased lift... and it's quite disconcerting when near the ground.

[ShyTorque]

Helimutt, Downwind quickstops (flare and turn / flare and turn as req'd) are taught to military pilots as a good method of arriving in a high threat area to a landing from a low level transit.

[Whirlygig]

Yeah but no but yeah but no but ... Is there really a need for quickstops to be taught as part of the PPL syllabus, other than being bloody good fun!! :ok: One appreciates it is because the syllabub was derived from that used in military training but can't see it being much use unless I want to impress the blokes! :}

Cheers

Whirls

[helimutt]

Whirlygig, you don't need to do quickstops to impress the blokes.

But I agree. Military may use it yes, but not many ppl (h) holders find themselves in a position which requires use of said quickstop. Never arrived into a hot LZ in a robbo myself. Although anywhere near Liverpool could be called that.:E

Hiro Protagonist, The term 'incipient' means 'beginning to exist or appear'. This means you are demonstrating the symptoms just as it starts but not letting it fully develop. That would be bad. Show a student what to expect and how not to be there in the first place. I guess after the incipient stage, there is only Vortex Ring State (Settling with power). It's not as easy to get into as you may think. But fully developed? :ooh:

I feel this is being done to death and there will always be people on the fence about it. A lot of instructors maybe just don't understand what they're teaching and make too big a deal out of it all??

I agree with Nick though.

[Two's in]

Learning to handle any helicopter smoothly and safely while close to the ground and in varying wind conditions can only be a good thing, not just a military requirement, but don't become the self-taught statistic. Knowledge, good handling skills, anticipation, and mechanical sympathy will often keep you out of the more exciting corners of the flight envelope.

[eagle 86]

Ever since I started in helos - some considerable time ago - the use of terms such as Vortex Ring - settling with power and overpitching have been mixed and confused. My understanding is:
Vortex Ring - minimum collective pitch (read minimum power), ROD greater than 200ft/min, IAS less than 20 Kts THEN increasing power but nowhere near full power, just like the final couple hundred feet of a sight picture approach (SPA) - this is the incipient stage. What's happening on the rotor blade - outer section is reaching stalling angle of attack due recirculation of tip vortices effecting relative airflow - inboard section reaching stalling angle of attack due to increasing collective pitch exacerbated by washout - middle section is only bit producing lift - as VR develops (more collectve pitch to try in vain to slow increasing ROD - note engine[s] may not be anywhere near full power and RRPM will still be in the green) the section of blade producing lift reduces even further. Recovery - collective down - reduce angle of attack away from stalling angle - cyclic forward - carry vortices away from disc so that recirculation is minimised.
ROD in fully developed VR well in excess of 5000 ft/min! Obviously takes a bit of height for recovery!
Settling with power/overpitching - sight picture approach/hot/high/heavy - lot of collective - engine(s) topped out - drag being produced by rotor system can no longer be overcome by engine power - blades slow down - cone up - heavy landing at least!
Moral - know your helo's capability backwards - don't venture too far into the VR regime and make the final bit of all SPA's slow and steady.
GAGS
E86

[Hiro Protagonist]

Hiro Protagonist, The term 'incipient' means 'beginning to exist or appear'. Uhhhhh, really??? Thanks!...:hmm:

I was using the term "incipient VRS" as used in this and other threads to describe a state different than VRS.

Those who use the term incipient VRS say that few of us have ever experienced "real" vortex ring state, and they have and it was a whole 'nother beast than the mamby-pamby little demo's we do in flight training.

[aclark79]

There is just no way I will ever enter a 2000 foot per minute decent rate in an R22 on purpose! I understand what you are saying though, 500 ft and 2000 ft are two diffrent animals altogether.

Out of curiosity when 500 fpm was indicated how long were you stabilized? If your rate of descent was increasing rapidly, and you initiated a go-around at 500 fpm indicated your actual ROD was higher because of the lag in the VSI (unless you have an IVSI)

I am/was probably stabalized at 500 fpm for no more than 3 to 5 seconds. It feels like forever up there though... I don't wait for the rate to hit 500 before initiating the recovery, I teach a recovery as soon as the onset is recognized. That said, I do hit a real decent when demonstrating it.

I feel this is being done to death and there will always be people on the fence about it. A lot of instructors maybe just don't understand what they're teaching and make too big a deal out of it all??

I don't really think we could do this one to death, I had great instructors but none of them could give me the technical instruction thats in this post, I for one am enjoying it. Never to late to learn right? I'm not sure how this will affect what I teach regarding VRS in the PPL, I still want the student to have a healthy respect for those 3 SWP criteria and avoid them at all costs.

Regarding doing quickstops in the PPL at all, I can't say I ever had a reason to do a real life quickstop where the choices were stop NOW, or hit something, but that doesn't mean it won't happen.

[Arm out the window]

A lot of Aussie CPLs would be using it often for mustering - charging along at low level at a decent speed and propping quickly to chase an uncooperative beast - lots of quickstops in their line of work.

[ShyTorque]

Quickstops are definitely not just a military manoeuvre. I'm quite surprised if they aren't in the PPL syllabus; I would see them as a normal way to fly a helicopter, especially a small one.

For example, they are useful for safely "expediting" an exit from a runway when being chased down the approach by a fixed wing aircraft. Also any situation where it's useful to operate close to the ground, such as a rapid move from one side of the airfield to the other (obviously, not in the avoid curve), rather than flying a full circuit.

[IntheTin]

Shytorque.

They are part of the FAA Private checkride, into the wind of course, if you want to pass!! :}

[topendtorque]

Yes the QS is the #1 manoeuvre for mustering as AOTW describes. I should imagine that AG drivers might find it handy at times as might any other driver who may be faced with a once in a lifetime - do something now - situation.

At the immediate conclusion of a QS when the A/C is still with the disk in the handbrake position, (I.E. tilted against the direction of travel and power coming on) then coming to the hover is a secondary manoeuvre, instead you may wish to beat the hell out of there sideways, backwards, forewords or go up even.

The Rotor and Wing article I referred to above, was written by a Dan Manningham, January 1984. Don’t know if he is still around. One of his choice words was,‘re-ingest’.

He described flying into your own turbulence say on a day with light wind with a fairly steep approach, flare a bit quickly then continue on down into that disturbed air which ‘re-ingests’ itself and you have the perfect makings for VRS.

The numbers he used were the same as I was taught, AT OR BELOW ETL and a ROD of 300 to 600 FTM.

Well, at the ab-initio stage I was never taught the procedure just told the numbers, and told lookout if the machine drops then put the collective down and cyclic forward.

I did however get demonstrated some fairly high sink rates which in hindsight were basically well developed SWP regimes, I.E. the M/R had NOT picked up any recirculating vortices, but was still functioning in clean air. Those discussions are best left to the mountain instructors / operators.

Of course overpitching, which is a separate article but can be a resultant of each phenomenon and its recovery procedures are also best left to other discussions.

To the VRS exercise;


Of the two most conducive elements, the most pertinent by far is the first, AT OR LESS THAN ETL. Put very simply when the disc has translated it has then progressed into CLEAN AIR, the vortices having been left behind. This is why it is recommended; forward cyclic. In reality it can be cyclic any which way, just to get the disc breaking through into clean air.

When I first started doing them big time ~ looking for ways to prepare pilots to not then overpitch ~ I found that the A/C would fall straight, if it’s CofG was close to the centre. Then I noticed that if I loaded, my Bell 47, well to the left of CofG then EVERY time when the A/C fell in VRS it would naturally start falling left toward the weight and HEY PRESTO, the disc would translate by itself. The A/C was always stable, there was never any flapping of the blades or disc and it could be easily steered to any attitude.

I guess that is because it is still working under the 1G premise and the M/R was not stalled in that recirculating air. Obviously, as the M/R is imparting thrust to the air to keeps it recirculating. Duhhh.



The second bit is about the ROD to get into it. I found the numbers to be fair, but the difficulty when at altitude was to gain a direct appreciation of the actual wind strength and direction so as to position the A/C each time in my down flow. However when held in it is difficult to get the A/C to stay in VRS for any more than say 120, maybe 200 feet. Then as it progresses into clean air, collective should be lowered to stop overpitching. As A/S builds up so does the lift and power can finally be re-used to arrest the ROD.
Let me now introduce another phenomenon. Firstly, you should easily imagine that when the A/C is in well developed VRS that in effect it is flying inside its own balloon of air, totally separate from the surrounding clean air. OK?

If the disc tilts it slips out of the balloon and translates, however if you keep it level it doesn’t.

I mentioned 1G above, that is the maximum acceleration that can be achieved when the A/C is isolated from the surrounding air OK?

All bodies falling in Free State will suffer from atmospheric air friction as a slowing effect on that 1G acceleration, even a rotorcraft with a stalled rotor system. However if we drop a heavily weighted balloon then its skin friction from the fast increasing relative velocity ~from the surrounding and relative up-coming air ~ on the big balloon will be much more than on the smaller surface area of the hapless stalled rotorcraft.

The balloon ~of air ~ will ONLY stay there until it reaches a point where the ‘limiting coefficient of the static friction of the surrounding air’ becomes too much for it to be retained as re-ingesting air and it is simply ‘blown away.’ OK?

This is not rocket science, simply the same principles as used in establishing the speeds at witch a car say, may lose traction when its direction is changed,
due to the limiting coefficient of static friction on the road surface.

That is why you can only maintain VRS properly for about a couple of hundred feet, the rest is recovering from an induced high ROD. That fact which is verified by your VSI, a lagging instrument, is still winding up AFTER you feel that you are slowing down.

Once I got to understand that, then I realized that it wasn’t a phobia to be scared of, like say being in a lift the cable of which was cut at the top of the empire state building might be.

I learnt that if pilots were properly demonstrated the VRS and how to recognise the incipient stage of it even if it goes straight into a gut wrenching drop and if their reactions were conditioned to be sharpish in recovery, then we found that they usually would not drop more than eighty feet.

Let us go back to your downwind QS. Usually to stay above that disturbed air you will adopt a slight climbing attitude to terminate at a hover at say forty feet if you entered at thirty feet from say 70 knots. Not good for EOL’s from that point, you would agree.

Now if you didn’t climb enough and ended up in the disturbed air at say thirty feet which re-ingested and you start dropping like a stone you will now see why it is that I say, never-stuff-it-up-below-eighty-feet as it would only ever be the last fifty feet of your eighty foot fall that will pop the wax outa yer ears.

The reactions are always, cyclic forward first, collective down a bit (if you can at low level) then sort it.

There is another technique which relates back to the comment that Nick made about having heaps of power. If you are developing vortices and starting to feel that terrible sinking feeling, you can give the collective a quick heave and utilise all of that power to;
a) Keep the rotors turning within operational limits, and
b) Utilise the as yet clean areas of the blade to effect enough lift to stop it. (jump it out so to speak)

If you are quick enough.

If in that circumstance and you are in a Bell47 or R22 you will most likely then enter an over pitched state, which can be most unpleasant if it’s over the tops of dense foliage you are at the time.

[NickLappos]

topend,
Your post makes perfect sense. let me add:

VRS is not a state where the rotor is dead, and where up collective makes you go down faster - more mythology.

It is a place where the rotor gives fits and jerks, and where it needs much power to develop the lift you need. In full blown VRS, the actual power to keep you from falling can be considerable, perhaps 20 to 50% more than it was a few seconds earlier. Pilots then translate this into "pulling power makes the helo go faster downward." If you have enough power, there is not state where you cannot pull collective and climb out. Most turbine helos with no payload are at that condition.

Why does it feel better to lower the lever and go down? Because VRS is almost at windmill brake state, so a bit more descent and you are a nice clean autorotation, where the lurches are gone, and a fly-away feels so nice.

[rudestuff]

Seeing as everyone's talking about VRS & SWP (and i've got nothing to do right now) - I thought i'd chip in...

Many people use the phrases "Settling with power" and "Vortex ring state" interchangably - personally I think that they are two seperate but linked phenomena:

Imagine you come to a hover at 100ft above a point, you notice you are using 80% torque. To initiate a descent you have to reduce a bit so you go down to 78% and get a nice 100fpm descent rate. Easy.

Now imagine you approach the same hover but you can only pull 78% max - not enough to hover, but enough to give a stable 100fpm descent. As you slow down you hit the power limit and start to descend - "settle" - with power, but not out of control, you just dont have enough power to stop the descent.

This is what I would call "Settling with power" To stop it you still have to 'fly away' and get ETL to recover, but it is not uncontrolled or accellerating - indeed you'll almost certainly stop when you get ground effect again. Put simply, SWP is only having enough power to descend.
Anyone who's flown powerlines in a 300 will be familiar with what i'm talking about - all you need is an extra 1/2 inch to stay up, but you just don't have it...

Of course, should you allow yourself to settle at too high a rate - usually through bad power management (don't USE enough power) or exceeding the A/C's capabilities (don't HAVE enough power) then you'll experience VRS and the ATW (Associated Trouser-Washing) that follows...

Do you think i'm on to something, or talking bollocks?

[Graviman]

Not wishing to distract a very good thread, but several questions:

Did VRS a while back with a talented R22 instructor, and noted -2000fpm. Also noted the spring to allow collective over-travel, on recovery.

From the numbers given i assume this was only insipient VRS, but full VRS would have been -5000fpm? How does full VRS develope, and would recovery be as quick?

Also, i assume a QS is the helicopter equivalent of a fixed wing Chandelle? So it is a coordinated turn in a very steep bank (i used to get several in when poss ;) )? How does the turn and flare differ from flare and turn?

Mart

[rotorfossil]

I've seen the VSI against the stop at -4000 fpm in a heavy aircraft at altitude above 8000 ft and we lost about another 1000 ft on the recovery. One of the dangers on the recovery is that unless the collective is handled carefully, the rotor rpm go sky high with such rates of descent. It's all rather academic. People don't get vortex ring with lots of air underneath them; it usually happens when doing something daft, like an approach with an unrealised downwind component. At this sort of height, anything beyond recovery at the earliest incipient stage will not be successful. One of the values of having downwind approaches demonstrated is how differently it has to be flown to keep out of the VRS area.
Re downwind quickstops. You can do a turn and flare at any sensible speed, but for a flare and turn, the entry speed needs to be above about 60 kt or you get very slow around the second half with the prospect of ending up going backwards over the ground in strong winds. It's not a chandelle as the aim is to keep a constant height. The need for a flare and turn only really becomes apparent in helicopters with speed potential well beyond 100 kt that would do a very wide turn or put themselves into retreating blade stall territory by trying to use too much bank to reduce the radius of turn.

[ShyTorque]

I don't understand the comment about going backwards during a flare and turn. :confused:

If the aircraft is flown by the pilot, rather than the pilot being flown by the aircraft, that just cannot happen.

[rotorfossil]

Trust me, it happens. From the handling point of view it is not dangerous if you have forwards airspeed. The danger is in backing into the obstacle that you are trying to avoid. Students will always find a way of doing it wrong; thats what makes the business interesting!

[ShyTorque]

Students, OK, point taken - but isn't that why we have instructors? Any manoeuvre can be co*ked up in training. If taught correctly to any decent level of competence they are no more dangerous than any other manoeuvre. :)

One of the best pieces of instruction I was ever given was: "If in doubt, go forward and down". It certainly applies to this scenario.

[perfrej]

When I did my type rating on the MD500, most of the time was spent doing two manouvers: autorotaion and quickstops. The latter is, in my humble opinion, the one manouvre that "has it all" in terms of coordination and complexity. It's putting it all together, and if you master the quickstop you definately know the machine in question.

One interesting thing about down-wind ones... My instructor on the 330 taught me to either quickstop with a turn into the wind at the right moment, or to do it with a slide to the side to avoid getting into disturbed air. The latter one is pretty neat. Just flare, and as the flare gets the speed down slide sideways with the same nose direction. You end up in a downwind hover with no disturbances, and of course you have to whatch out for available power.

When I want to "get up to speed" after a while out of the cockpit, I do a few quickstops to get re-acqainted with the helicopter.

And yes, I do beleive that the quickstop is an essential manouvre and is in the PPL(H) syllabus (or is it?).

[Two's in]

...or to do it with a slide to the side to avoid getting into disturbed air...

Stating the obvious, but being low-level at any significant speed with the skids/wheels pointing anything other than in the direction of travel definitely leaves you poorly placed if it all goes quiet back there. Army pilots are taught to recognize the increased risks that are inherent when you do it, which can be a lot of the time.

[rotorfossil]

Shytorque.
Doesn't only happen with students. Not unknown in recurrency checks with low hours pilots. Likewise turning downwind at low level, looking outside (simulating orbiting friends house), speed falls, descent builds up, collective raised - another classic vortex ring "gotcha".

[topendtorque]

Rudestuff
“Of course, should you allow yourself to settle at too high a rate - usually through bad power management (don't USE enough power) or exceeding the A/C's capabilities (don't HAVE enough power) then you'll experience VRS and the ATW (Associated Trouser-Washing) that follows...”

Not bollocks at all, you nailed the SWP examples and I really appreciated the bit about the extra ½ inch. I think in your above scenario I’d be much more worried about blade stall, esp. in an R22. Not VRS

Graviman
“Also, I assume a QS is the helicopter equivalent of a fixed wing Chandelle? So it is a coordinated turn in a very steep bank”

Just to confirm the others, definitely not. In the syllabus the QS should be done at the same level and terminating to the hover – exactly- or do it again.

As a matter of a manipulative exercise it is a good manoeuvre, to learn just the beginnings of the big world of a very manipulatory beast and it may even save your life one day.

You may have seen Dennis Kenyon? (Right spelling??) doing a similar manoeuvre – ad nauseum – to your chandelle in his routine, which is similar to the Torque turn that I talked about in that it is a 180 turn.

The difference with the torque turn when done correctly it is done in a straight line ~ feet on the floor, pull-up from cruise speed, at the top of climb, give it just a slight lead with cyclic, the A/C becomes stationary for a half second as it then rotates from its own torque and go back down the same line.

Antitorque, is same same except you roll the throttle off at the top (Not too much) to rotate the other way. The trick is to never allow the A/C to go back down in its own disturbed air.

We use it mustering, we get to the end of our line (say 6k long) we torque turn back the other way. There are two reasons for the zoom climb;
1) If you are working with another machine your mate with you will be conditioned to look your way at that time and visually check that you are in the right place according to him and of course - all OK. Esp. with RS radios.
2) you have a good look around at the big picture afore you go back down to the recalcitrants

A good thing about the torque turn or any pull up is that it is a good idea to leave your disc pointed at the ground if you can –in any quadrant, but best toward the wind. That gives you a far greater ‘airspeed potential’ if the noise stops. Just follow the disc down.

Shy, the blokes that go backwards, are as you suggest, a bit mixed up with command and control.

But they bloody do at times, one of the traps that we check pilots in our game always set, is to make them deliberately over-run their target when they have done a quick stop into wind and further distract them hoping they then turn to follow it downwind , *@*$%# F”N so and so etc etc. Now Einstein lets do the right procedure which is, you overshot - forget the target - translate into wind – get a safe A/S – then turn back and start again.

It’s also a favourite mistake of the lesser experienced shooting pilots. Following a moving target, especially when they’re looking for an extra 1/2".

It's the main reason why shooting clients look down their snooter at any pilot with less than 1500 mustering.

Also it undoubtedly is the one greatest single cause of low time sad times, when some kid has a pax with a camera. And in the scenario that RF paints, around the mate’s house, ugh!

[Graviman]

Rotorfossil, TopEndTorque, Shytorque, et al,

I am in the situation where, once again, i am feeling a deep seated respect for the airmanship of helicopter pilots! Thanks for taking the time to explain this to me - it sounds like a great deal of fun to get right. ;)

Mart

[MightyGem]

Is there really a need for quickstops to be taught as part of the PPL syllabus
As perfrej says, it's an excellent exercise for co-ordination and planning the actual flight path to be able to end up over the ground where you want to be.

[dammyneckhurts]

I am curious to know how many pilots have experienced some form UNINTENTIONAL incipient vortex ring close to the ground at some point in their careers. What I mean by that is that when you pulled in some power at 300 feet or less your sink rate increased, not to be confused with the helicopter not having enough power to do what you have asked of it and it continues to mush towards the ground so to speak.

VR seems to get covered in basic flight training, and for many it gets reviewed in recurrent company training, so most pilots are exposed to the theory and possibly practice at high altitude, but what percentage have actually unintentionally experienced it close to the ground?

This seems like a good opportunity to share so that others may learn!

[FH1100 Pilot]

Okay, I'll bite. I used to fly a "field ship" in the GOM. Ten platforms total. I was bouncing around the field one day, headed for "Charlie" platform but there was another ship already there, picking up or dropping off someone. He wasn't going to be there long, so I pulled into a 700' hover just downwind of the platform.

Taking a rare opportunity to "play" a little, I started sliding left and right, fore and aft. On one of my slides to the rear, I must have let a little RoD build up (not paying attention?). Next thing I know, the ship is shuddering and sinking...fast backward. Oops. Okay, correct it. Pulling collective did nothing but increase the RoD. Yikes! Water coming up fast...like an idiot I dropped the nose. Wrong corrective action, RoD increased more, dropping vertically now. Heart firmly in throat, I'm waiting, waiting, waiting for ETL. Finally got it and was able to fly away, arresting the RoD at 100' *indicated* (we know how altimeters lag). Got a rate of climb established and started breathing again. It was close.

Glad I hadn't decided to hover around at 500 feet or I would have pranged a perfectly good 206B into the water with no good explanation why. At the time I had about 7 or 8,000 hours. And we wonder sometimes how so-called "experienced" pilots can crash doing extremely dumb things? I do not even question it anymore - I'm living proof that it can happen (the dumb mistake part).

[ShyTorque]

Here's a link to an earlier discussion on vortex ring, which might be of interest.

http://www.pprune.org/forums/showthread.php?t=116124&highlight=vortex+ring

[Graviman]

FH1100 pilot, thanks for sharing. Certainly a lesson that VRS is not just an exercise and really can catch the unwary. Would any instrumentation help to alert incipient VRS? Maybe something linked to collective and VSI - i'm thinking something along the lines of a fixed wing AOA stall alarm...

Mart

[ShyTorque]

Avoidance of the conditions for possible vortex ring is a very basic concept. The best piece of safety equipment in a helicopter is a well trained pilot :)

I concur with FH100 that "backwards and down" is a good way to find oneself losing control, but normally speaking, this is not a usual configuration to be in!

Once seen, never forgotten, especially when the aircraft tends to weathercock into the rearwards airflow and pitch nose down due to "upwards" airflow over the horizontal stabiliser. These factors tend to remove the VR conditions but do nothing to help the piot's adrenaline level. :O .

[rotorfossil]

Can't think of any instrumentation that could easily be made to work cheaply. Presumably something that integrated rate of descent/TAS/engine power information electronically could be made to work but would have to be individually calibrated to a particular type and I'm not sure whether weight might have to be considered as well.
I do remember a presentation a long while ago on a pitot system where the intake aligned itself with the relative airflow and presumably could indicate when the inflow was reaching a critical angle.
Personal experience: Helicopter camourflage trials in the desert, landing with different backgrounds and on specified headings unrelated to wind direction while the fighter recce guy rushed past taking photos. Got bored after about the 10th and didn't pay attention to the downwind component. Classic VR, R of D increasing, raise lever, ground coming up, oh s..t, no airspeed. Fortunately ground fell away to the right into a wadi, so was able to stuff the nose down, roll right, gain some airspeed and just graze the brown bits. Comment from the jet jock, "can we do that one again, it looked interesting". Get stuffed!

[NickLappos]

Believe it or not, downwind is a good way to AVOID VRS because the rotor must have a slight FORWARD velocity to develop true VRS (between 7 and 12 knots forward, and ROD of about 800 to 1500 fpm, depending on the helo's disk loading). However, downwind quickstops make power margin problems even more severe because when downwind, the aircraft must be taken to zero velocity while nearer OGE, thus requiring more power at that zero knot point than will be needed later when the aircraft is in an IGE hover.

When I get some time, I will build a web site that discusses this in detail and lets the pprune crowd tear it apart.

[dammyneckhurts]

FH1100,

Sounds like a real eye opener, thanks for sharing.

There must be a few other guys that have experienced some form of unintentional incipient VR ...with all the very vocal people that have posted about it over the years...anyone?

Many seem to have extensive knowledge in the area, are we all just quoting theories and high altitude practice senarios, or are there people that have actually experienced it?

[ShyTorque]

dammyneckhurts,

The link I posted half a dozen or so before yours will lead you to previous discussions on the subject. :)

[topendtorque]

"Believe it or not, downwind is a good way to AVOID VRS because the rotor must have a slight FORWARD velocity to develop true VRS (between 7 and 12 knots forward, and ROD of about 800 to 1500 fpm,"

Before everyone gets too complicated with something simple, it may be best to repair to the whiteboard before the flight, which is what i do when FIRST discussing DOWNWIND THEORY with so called qualified pilots.

As Nick says 7 to 12 knots.
Same as the books say at or below ETL

Same as it happens, see below,
Helicopter is proceeding in the same direction as the wind.
Say the wind is 20 knots.

Say the helicopter has entered the procedure at 15 knots IAS.
it now slows to zero knots Groundspeed.

After it has gone past the zero knots IAS it then proceeds to a positve disk airspeed of twenty knots (from behind:ugh: ).

Big mobs of room in that equation to find VRS at +7 to +12 knots. All is needed is to add descent at the appropriate rate.

In essence that is downwind theory.