There is some great info out there, but the data is for engineers, so it might be way too pointy-headed for us drivers. I can try to translate it into english, however. The points the below is intended to show is
1) That true VRS does not occur at 300 fpm, it takes a lot more descent rate.2) that VRS is not a destruction of rotor lift as we know it (like "crossing the streams" in Ghostbusters!) but rather a place where the thrust is unsteady and the power required goes up a bunch.
The paper that says it all is here, done by Dr. Gordon Leishman, who is a real pro at the U of Maryland:
http://www.enae.umd.edu/AGRC/Aero/AH...hman.pdf<br />
Vortex ring is a state where the recirculation makes the rotor less efficient, so it chews up more power making the lift.
Rate of descent for vrs:
Here is a rotor in a steady hover. See the ring of tighter vortexes forming at the bottom, which seems to form "just because", and is probably the reason why we feel the puffs of downwash from a rotor, instead of a steady stream as it flys by:
Here it is as the rotor is descending at 0.15 times the downwash speed (225 fpm for a R-22). Note the downwash has been stopped and a ring forms below the rotor, at the place where the downwash (which is slowing down as it disburses) matches the upward wind:
Here is a sequence as we descend ever faster, all the way to incipient vrs:
at 675 fpm, the rings are less steady, and closer to the rotor:
at 900 fpm, the rings are more compact but still push away from the rotor:
at 1050 fpm, 0.7 times the downwash speed, the rings start to get closer to the rotor, and sometimes puff through it. This 3 shot sequence takes place in about a second (six rotor revolutions!) so these rings are fast. The pilot would feel a 1 per second torque variability, cyclic and collective control problems, and lots of yaw, due to the torque spikes. This is incipient vrs, for the first time in this sequence:
For such descents, the power rise is shown below, with the curve illustrating the amount of increase in induced power needed to trim at flight thrust (full lift). Induced power might be 30% of the total power, so you can estimate the total power increase as maybe 1/3 of the amount shown. Note that for a descent at half the downwash speed (that is the -0.5 on the horizontal scale), the induced power would increase by about 40%, so the total power might fluctuate at about 15% above normal. The curve is experimental data, cited in Dr. Leishman's paper.
Here is an animated file of a rotor seen from the side at Dr. Leishman's web site. The animation takes a rotor from steady hover, then has it go faster and faster downward until it gets into VRS and then past VRS into autorotation (windmill brake state). Each blade's tip vortex is plotted as a different color green, red, orange, yellow. As the descent rate goes from steady hover to VRS and then into autorotation, see how the ring forms below the rotor then is pushed into the rotor as the descent rate increases, then finally the ring drifts away upward in smooth autorotational flow:
If the animation does not work, here is the link so you can just go directly to the site:
http://www.enae.umd.edu/AGRC/Aero/images/wake1.gif
Also, the thrust of the rotor does not change and make the rotor stop producing lift. here is a plot of the thrust for the experimental rotors as they are forced to go through descent, VRs and finally autorotation:
One place where I must admit I was wrong, even leishman says that some inboard sections of the rotor experience stall in vrs. As I have always said, we all learn on pprune!
The detailed discussion from Dr. Leishman, and dozens of animated gif files are shown at his fantastic web site:
http://www.enae.umd.edu/AGRC/Aero/vring.html#Case%201st
I should point out for pprune's benifit, Dr. Leishman is a transplanted Brit!