TC, I know you are pretty good at what you do, but give Prouty and other aerodynamist credit for their work.
If Vd/vi (thats decent velocity over induced velocity) is above 1.5, they at least consider the vortex gone for good. That's about 2000ft/min in anything I know.
If I say VRS, I mean both, IVRS and FDVRS.
I at least learned something about it during the course of this discussion.
a. Normal recovery or DT is frankly a moot discussion. If I get into really bad shacking and waggling I have messed it up so profoundly in the beginning, that 30 or 50 feet do not make a big difference.
b. From reading a few things, dusting of Prouty's books, and yes, quite a bit googling, I learned that The Beast is elsewhere, not in the descend rate.
“Much of our knowledge of this state comes from flight and wind tunnel tests. Based on this experience, we know that unsteadiness starts at about one-quarter, peaks at three-quarters, and disappears at 1¼ times the hover induced velocity.”
Raymond W. Prouty. “Helicopter Aerodynamics – Vertical Climbs and Descents”
And if Dr. Gordon Leishman's book wasn't so expensive, It would be on the way to my desk by now (if anybody has a copy he does not need anymore? And there is an article about VRS in the AHS Journal, could somebody scan that for me, please with a cherry on top?). But now I know, I can much safer descend straight down, than in a 40° angle and suddenly the Super Puma accident makes sense and a lot more does, too. It think, I am at least on the right track to understand VRS a bit better. I was taught in beginning things that are not true. I truly believed in gyroscopic precession (and I looked at a rotor head and the angle wasn't 90°. That's were I started to get suspicious), in the centrifugal force, density altitude affects turbine engines. You can count down almost all the myths Nick brought up in that famous thread, I would probably been a believer at one time in my life.
If anybody wants to believe the FDVRS with high sink rate, be my guest. You can try to explain to me the VRS diagram above with the high sink rate, I am all game and I will not interrupt and I will admit that I am wrong and I can't read the diagram correctly, but in that case, you might contradict a few giants in helicopter aerodynamics, behind which I hide.