CJ Eliassen:
So, its conservativeism. Taken a little to far? Maybe, but its much safer than telling a student the exact numbers because in flight they will never know the exact numbers from the instruments at hand.
I agree with Nick that helicopters will never see VRS at 35 knots. By definition, to get into VRS the rotor must be back below ETL. There is no vortex ring during translated flight. I've never understood why the books use such hard (and obviously wrong) numbers for defining VRS, which is why so many pilots seem to not understand it. I agree with you CJ, that the instruments we use in the cockpit are very imprecise. In my opinion, what the texts really ought to be saying is:
a) Speed below ETL
b) Some RoD
c) Some power applied (not autorotative descent).
The trouble with the above is that there is no Translational Lift Indicator- nothing to look at in the cockpit that would definitely tell us whether we are in translated flight or not. Ergo, the conservative
"Look out below 40 knots!"
There have been many times in my career whilst making landings to offshore oil platforms in grossed-out BO105s on hot, calm days (or when the light wind was variable) when I've wondered if the shaking, shuddering and rivet-loosening was the normal BO translational event or the onset of VRS? Either it never was or I just got lucky.
Finally CJ, I got quite a chuckle out of trying to imagine an CH-53 in the flight conditions you described. A RoD of 4000+fpm with a forward speed of only 38 knots? While that might work in theory, I would imagine that such a manuevre would be impossible to perform with any sort of power applied (have to defer to real '53 pilots there though). But it sure would be exciting to watch, eh!