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Downwind Quickstops

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Downwind Quickstops

Old 2nd Feb 2007, 07:36
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Downwind Quickstops

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
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Old 2nd Feb 2007, 08:07
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DTP,

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

Hovering Downwind


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!
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Old 2nd Feb 2007, 08:19
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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
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.

Last edited by Gaseous; 2nd Feb 2007 at 08:34.
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Old 2nd Feb 2007, 08:30
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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...
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Old 2nd Feb 2007, 08:50
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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.
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Old 2nd Feb 2007, 09:09
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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
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Old 2nd Feb 2007, 09:32
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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.
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Old 2nd Feb 2007, 10:08
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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.
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Old 2nd Feb 2007, 10:53
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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
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Old 2nd Feb 2007, 11:23
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Downwind Quickstops

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.
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Old 2nd Feb 2007, 11:48
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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.
 
Old 2nd Feb 2007, 11:51
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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?
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Old 2nd Feb 2007, 12:11
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Quickstops completed downwind

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.
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Old 2nd Feb 2007, 12:35
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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.
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Old 2nd Feb 2007, 12:47
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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!
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Old 2nd Feb 2007, 14:52
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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!)
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Old 2nd Feb 2007, 15:49
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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.
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Old 2nd Feb 2007, 16:55
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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.
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Old 2nd Feb 2007, 18:57
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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?
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Old 2nd Feb 2007, 19:08
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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. !

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!

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.
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