Prouty and 40/800
Good post, Matari. It brings up other writings by Ray Prouty:
In the R/W article ( could not get to the associated figures, unfortunately ), Mr. prouty concentrates on the V-22 aerodynamics, whereas in the 1986 ( 1990 revision ) text, his section on VRS has to do with single and tandem rotor machines.
In the R/W article, the main discussion centers on the 40 kt VRS data for the V-22, whereas in his 1990 text ( writing about a smaller tandem helicopter, not the V-22 ), he writes: " The results showed that for the test helicopter, forward speeds above 10 knots were sufficient to avoid vortex ring vibration at all rates of descent." (The cited reference was a 1958 paper and how they could attest to a 10 knot speed in those flying conditions is not stated ).
The other issue I had raised, and which the Prouty article skirts around is one of controllability. I mean controllability in the technical sense: does the pilot have the rotor control moment capability to offset a disturbance ( or "roll-off" to use the term favored by some in the V-22 community ). It is clear from personal experience that the single rotor machines I have flown have no issue there, but the Prouty article leaves the V-22 situation in doubt. Since a Chinook is in one sense a sideways V-22, rotor-wise, isn't an apt analogy a SAS-OFF Chinook hovering in gusty weather, nose into the wind? Is the hands off dynamic stability aperiodic/unstable? You bet. Does the pilot have the control power ( controllability ) to manage the disturbances? Certainly does. All I am suggesting is that the Prouty article does not go very far in explaining the details behind the Marana accident. Doubly so given the experience of the crew.
Oh, I did want to respond to one thing Lonewolf wrote:
"I note that when checking this out, John took the bird up to 6-8000 feet."
Two reasons. First is that it isn't easy to find a fully developed VRS situation, and the second is that once found, a respectable length of recorded data is appreciated by all except the flight crew being bounced around.
Thanks,
John Dixson