Lu Zuckerman asked:
How fast is fast?
In a conversation with Boeing Engineers I was told that the ROD in autorotation would be between 4000-6000 FPM. Granted the accident did not occur during autorotation but it is something to be considered in the design.
Lu, it doesn't matter. To excite VRS, the rotorcraft has to be descending pretty much vertically, and its rotor has to be generating some downward thrust which is not being dispersed by wind and/or relative speed, therefore the need for "power applied" as opposed to autorotation in which the flow through the rotor would be "upward."
You are correct that an autorotating V-22 would be falling out of the sky like a greased lorry...and that would be in an auto
with some forward airspeed. The RoD in a vertical auto would likely be heart-stopping. All the more reason to believe that V-22 pilots will never be making "blottle-on-the-bottom" approaches.
Further, I would go out on a limb and state that you
cannot get into VRS of any sort with forward, translated speed...any speed above ETL in other words.
As CAC Runaway points out, VRS is unpredictable. The margins needed to avoid Asymm-VRS in the tiltrotor might be so large as to effectively limit the aircraft's usefulness - or at least guarantee that
every future accident will be blamed on "pilot error"...
"They violated NATOPS!" Asymm-VRS will
always be a "problem" plaguing tiltrotor aircraft. Just how much importance we give or don't give this niggling, trifling little...little...flight characteristic
peculiarity depends I guess on how much you believe in the inherent "goodness" of the tiltrotor design. Sorry, but I'm not sold.