VRS Continued
Nick L,
you wrote "The yaw wiggling is the rearward flight, mostly, but as you enter VRS, the yaw is in concert with the torque jumping, as the rotor flow changes rapidly enough to make the engines work up and down to keep up with the varying rotor torque"
Just to clarify; during my demos, we rarely encountered a negative airspeed. These demos were flown at 8000 ft AMSL in the UK where there is usually always a wind blowing. You'd be hovering over a fixed point with usually 15 - 40 kts airspeed. By allowing a rearward (ground) drift to develop with no collective lever input, the loss of ETL causes the helicopter to descend - all the time however, maintaining a positive airspeed. In this regime, I believe the yaw 'wiggling' at the incipient stages of VRS is due to disturbed flow over the T/R. Once fully developed in VRS however, I entirely agree with your torque jumping argument.
As regards your suggested recovery method of applying collective, I have quite serious reservations. I can see that this recovery would work during the incipient stages of VRS when only a proportion of the disc is stalling - provided you have sufficient power available to pull the extra pitch. However, in the fully developed stage (as I obviously encountered as described above) I beleive it would be entirely wrong to pull in the collective to recover. If you follow through the conventional wisdom of the doughnut of airflow on the outer portion of the disc causing increased induced flow and reduced lift. Couple that to the stalling of the inboard portion of the disc due to high alpha caused by the RoD - by raising the lever, you can only exacerbate the situation.
This leaves us with a bit of a problem. Do you raise the collective to recover during the incipient stages and achieve minimum height loss. Or do you use the same recovery action as you would during the fully developed phase - lower the lever slightly and pitch nose down to recover the airflow but lose more height. With my experienced pilot hat on, I reckon I might be able to decide, fairly accurately which method to use and when. However, with my instructors hat on, we are all aware, things need to be suitable for the lowest common denominator ie. the low hour ppl (no offence intended to low hour ppls - i was one once!). If you give the low hour ppl a choice at such a critical time, Sod's Law says he will make the wrong choice and kill himself. If on the other hand you lay down in tablets of stone a set of actions to take in a particular set of circumstances, chances are, if he recognises the problem, he'll fix without conscious thought.
What I'm trying to say is, to recover from VRS, either incipient or fully developed there needs to be a standard recovery action that will work in all events. The conventional recovery of lowering the lever slightly and diving for airspeed works - period. If on the other hand, you raise the collective to recover, it might work, it might not. This depends upon how quickly you recognise the VRS condition and how much power the aircraft has in surplus.
The only problem the conventional recovery gives us is when we encounter the condition at low altitude. In this regime, you may have insufficient height to recover using the conventional method, then again you might! Likewise, by raising the collective to recover, it might work, it might not. This is why, once again set in tablets of stone (well, more like soft jelly (that's jello I think in the US)) we have a region of the RoD and IAS envelope which we avoid at all costs. J
Nr Fairy
I take your point about the 1 hour of VRS in the syllabus but I can't do anything about that.
However, what I can do is try to give people (usually my student's) the benefit of hard earned experience. As far as VRS is concerned, on the Puma at least, I got pretty familiar with it's behaviour. Nonetheless, one day it bit me. Luckily, my 3 other crew members and I are here to tell the tale (we even made the video!). As I said in my original post, we used to recover from the incipient stages in hundreds of feet, and occasionally just over a thousand. However, I always practiced from 8000ft; and it's just as well I did. Much lower and we would all be dead!
I guess, in a Robinson or other light helicopter, you can practice VRS recovery all day long from 800ft and I dare say you'll get away with it 99 times out of 100. It's the time when it takes you by suprise, as it did with me, that you'll have insufficient height to recover and you will die.
If it's any consolation, the VRS demo I had from a Robinson instructor (at a premier (?) UK heli FTO) in an R22 was not VRS at all, not even the incipient stage. The only symptoms we had were a high'ish RoD and low IAS but none of the accompanying vibration, yawing, pitching and rolling. The recovery was the 'conventional method' and the aircraft flew away immediately. I would wager that you could have done that demo quite safely from 800ft as it wasn't really Vortex Ring.
Incidentally, the UK military do not demo VRS to ab initio students. They simply explain the risk areas and encourage students to avoid them. This seems to have worked successfully for years now, so why change? J