I've read through this exchange with interest and I am, of course,
obliged to say that the Chapter on Vortex Ring State in my book, Fatal
Traps For Helicopter Pilots addresses this quite extensively and gives
some Accident Report excerpts in Case Studies (see
www.fataltraps.com).

But, in response to the very first message posted, I would have to say
that in any helicopter I would expect VRS to be demonstrated at
nothing less than 4000' ASL and a full briefing to be given before the
event - aside from recovery the most important thing to learn from the
demonstration is recognition of the early signs of onset - it should
be stressed that these can vary greatly depending on the make & model
of helicopter. I doubt that entry to VRS was an intentional one on the
part of the Instructor (who was, after all, the Pilot in Command)...
if it was intended, it was poor practise indeed.

I note with interest the mention of the fact that the rotor thrust
must equal the weight of the helicopter. I was fascinated with a
display with a model helicopter I once saw on a Science program on TV.
They weighed the model on a scale and then hovered it over the same
scale (which had a large plate placed on it and reset to zero). Sure
enough, the weight of the downwash onto the plate was exactly that of
the model. With enough decimal points you could actually see the
figure steadily decreasing with fuel burn-off. I'm not saying I ever
doubted the principle, but practical demonstrations (safe ones) are
always interesting and valuable.

As a newcomer to this site, thanks all for your input, makes for
interesting reading.

Greg Whyte