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Downwind Quickstops

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Downwind Quickstops

Old 7th Feb 2007, 23:35
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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.
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Old 8th Feb 2007, 01:12
  #42 (permalink)  
 
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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
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Old 8th Feb 2007, 02:20
  #43 (permalink)  
 
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Hiro Protagonist, The term 'incipient' means 'beginning to exist or appear'.
Uhhhhh, really??? Thanks!...

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.
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Old 8th Feb 2007, 02:42
  #44 (permalink)  
 
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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.
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Old 8th Feb 2007, 08:05
  #45 (permalink)  
 
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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.
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Old 8th Feb 2007, 20:17
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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.
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Old 8th Feb 2007, 22:47
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Shytorque.

They are part of the FAA Private checkride, into the wind of course, if you want to pass!!
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Old 9th Feb 2007, 11:08
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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.
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Old 9th Feb 2007, 11:28
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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.
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Old 9th Feb 2007, 11:49
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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?

Last edited by rudestuff; 9th Feb 2007 at 11:50. Reason: spelling
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Old 9th Feb 2007, 12:46
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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
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Old 9th Feb 2007, 15:16
  #52 (permalink)  
 
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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.
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Old 9th Feb 2007, 15:54
  #53 (permalink)  

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

If the aircraft is flown by the pilot, rather than the pilot being flown by the aircraft, that just cannot happen.
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Old 9th Feb 2007, 16:38
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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!
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Old 9th Feb 2007, 17:54
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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.
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Old 9th Feb 2007, 22:04
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PPL and Quickstops...

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?).
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Old 10th Feb 2007, 00:56
  #57 (permalink)  
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...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.
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Old 10th Feb 2007, 07:37
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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".
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Old 10th Feb 2007, 12:14
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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!
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Old 10th Feb 2007, 14:30
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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
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