Downwind Quickstops

[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.