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If you are in a vertical descent [no wind] with rotor at max RPM Auto, and you abruptly pull full collective, the momentum of the blades will drive air down creating down wash.

Now obviously this is a totally hypothetical question with no real world use, but, presuming a high enough inertia rotor system, in this very specific scenario is it possible to at least enter the incipient phase of VRS? [before the rotor folded and you fell to your death]

Please no answers just saying you go into auto to get out of VRS, I am looking for a very specific yes or no.

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  • $\begingroup$ Seems to be very well answered already here: aviation.stackexchange.com/questions/43028/…. Essentially, you cannot get VRS in autorotation, because in autorotation you are falling and leaving the downwash behind, above the rotors. $\endgroup$ – Penguin May 29 '18 at 11:34
  • $\begingroup$ Autorrotation and VRS are very distinct and incompatible states of the rotor. The answer is no unless you add enough qualifiers to make one of the two states unrecognisable (and thus no longer aptly named) $\endgroup$ – AEhere supports Monica May 29 '18 at 11:43
  • $\begingroup$ Are you actually trying to ask "Is it possible to enter VRS from autorotation, without engine power?" to which I suspect this comment may be helpful (although I believe it deserves its own answer here by a more knowledgeable person than me). $\endgroup$ – Sanchises May 29 '18 at 12:28
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    $\begingroup$ That's not what he's asking. He's asking if VRS can be induced in the brief interval where you exit autorotation by pulling pitch, as in the post-flare landing phase, in some hypothetical helicopter that had oodles and oodles of rotor inertia. Not a totally loopy question. A UH-1 can lift off the ground, do a 360 degree pedal turn, and set down again on rotor inertia alone if the rotor rpm is at the top of the green arc when it's started. So the question is how much time does it take to get a VRS going in a vertical descent under power (VRS and SWP are two different things). $\endgroup$ – John K May 29 '18 at 13:08
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    $\begingroup$ Possible duplicate of Why is vortex state impossible in autorotation? $\endgroup$ – TomMcW May 29 '18 at 23:27
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To enter VRS you have to have near zero forward airspeed and a descent rate of about 500 fpm or more under power. Note that VRS is not "settling with power" regardless of what the FAA says - they are two different phenomena. So in practice you would never encounter the conditions that could create a VRS in the first place while autorotating because you normally autorotate with substantial forward airspeed if you want to be able to land in one piece.

But say if you were autorotating, then slowed to zero airspeed so you were now autorotating straight down (and the descent rate goes from holy crap to oh my god), and then you pulled pitch to arrest the descent rate using rotor inertia. Could a vortex ring state be induced during that brief interval? Perhaps one could start to form in that brief few seconds of inertia-generated downwash, but by then the rotor would be stalling and you are now an anvil, so the point is moot.

However, for the purposes of our just-for-fun theoretical discussion, I think that if you had enough rotor inertia, it would be possible theoretically I would say, because for the purposes of the time interval involved, it would be the same as having engine power. Maybe if you had a rotor with super duper uranium tip weights that could generate inertia lift for say 10 seconds or more.

Watch this video to see how fast the vortex ring state can form in a vertical descent with power

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