VRS Anecdote
I used to instruct new pilots to the Puma in a high altitude night hovering role. As part of the instructional package, the incipient stage of Vortex Ring was induced to:
a. Demonstrate how a relatively benign manoeuvre could rapidly go wrong.
b. Enable the new pilots to recognise when things were starting to go wrong.
c. To demonstrate how to recover from the situation if ever they got into it.
The scenario was a stabilised into-wind hover at night at about 8000ft using a hover-meter. Allow a slight forward drift to develop (1-2 kts G/S). In order to correct the forward drift, the cyclic beep trim would be beeped back once to give a slight nose-up attitude to arrest the forward movement. Then WAIT. Relatively quickly, the forward drift would be arrested but if no further corrective action was taken, the helicopter would begin a slight rearwards drift; and this is where the problem begins. As the rearward movement increases, airspeed reduces and translational lift reduces accordingly inducing a RoD. It doesn't take long, maybe 10 seconds or so, for the RoD to catch up with the downwash and you're looking at the incipient stages of VRS. (ShyTorque; do you recognise this mate?)
The interesting point in my experience, having done this demo dozens of times was that the Puma was very predictable. Allow the rearward drift and RoD to develop without any corrective inputs, watch the VSI (RCDI) increase to 750fpm and then feel the rapid increase in vibration level, note the yaw fluctuations as the T/R bit into the very disturbed VRS airflow, see the aircraft attitude randomly fluctuate without cyclic input and most startlingly, watch the VSI peg off-scale low. To recover, slightly lower the collective and apply forward cyclic. Within a couple of seconds and a few hundred feet (maybe a thousand+ if you are slow to react) the aircraft would happily be flying again, ready to climb up for the student's attempt. Except on one particular occasion........
Same scenario as above. Going through my usual instructional patter and explaining just before we hit 750fpm RoD what is about to happen. Except this night, for a reason I am still unable to explain, something completely unexpected, and scary happens.
Instead of the VSI showing 750fpm and then the usual vibration and undemanded pitching/yawing as the VSI pegs off-scale, this time there is initially no-vibration at all. Infact, the whole thing went eerily smooth; there was no pitching, rolling or yawing for a few confusing seconds. My 'slick' instructional patter dried up! However, the VSI still pegged off-scale low. After what was probably about 3 - 4 second (I wasn't counting!) the vibration I was expecting appeared as did some very harsh pitching/rolling and yawing movements. The engine RPMs (NGs) were fluctuating wildly. Perhaps scariest of all however, was when I tried to recover by taking my usual conditioned actions; the cyclic didn't not want to respond. I had to apply full forward cyclic but the helicopter just didn't want to pitch nose-down to fly away. After a few more tense seconds it slowly began to respond and the ASI reluctantly showed the magic 30 knots whereupon I applied full power and recovered to the climb. My altitude at recovery was 1800ft. We'd lost 6200 ft.
What's the moral of the story. Well, for what it's worth in my opinion. VRS or Settling with Power or Power Settling, whatever you want to call it, is not predictable at all. There are many variables as Nick L so elequently covers on his web-site. It is beyond the vast majority of pilots to compute all these variables and decide where, on any particular day, the boundaries are. So, stick to what you've been taught and ALWAYS keep your wits about you on approach, or indeed, when hovering OGE.
I normally agree with Nr Fairy's posts, however, as for his suggestion about getting an instructor to demonstrate the incipient stages of VRS at low level to make it more realistic, I entirely disagree. If you decide to go down this route - the best of British luck to you, you'll need it one day! The same logic could be applied to a fixed wing stalling / spinning demo - "lets do them on finals because that's when we're most likely to encounter them for real" - I don't think so!
Finally, Nick L, or anybody else care to try to expalin why I experienced a few seconds of super-smooth descending flight before I entered fully developed VRS? J