"Greg Johnson" wrote in message
m...
It it possible to get into vortex ring state (or settling with power)
during an autorotation?

No.

A friend at my radio control helicopter field claimed that you shouldn't
do a vertical autorotation, because you might enter vortex ring state
if you do.

He's talking ********. To reach "that" stage, the air must be travelling
DOWNWARDS through the rotors. In autorotaion, the air is travelling UPWARDS
and turning the blade. (Well the AIRFLOW is upwards due to the downwards
trajectory of the heli)

This doesn't seem right to me based on my limited knowledge of the
aerodynamics of helicopters.

You're right, its not right.

Clearly, during an autorotation the main blades are developing lift, just
like a glider's wings are generating lift when it descends at a constant
velocity.

Exactamundo (almost:-)

And so, presumably there is a vortex at the tips of the blades. But in an
auto, it seems like you would be descending out of this rotor tip vortex;
the wind is driving the blades, rather than the blades driving the wind.

Correct.

Furthermore, the inner counter-rotating vortex that develops during
vortex ring state would seem not to be possible during an auto, because
there is no down-wash over the intermediate part of the blades.


Or over any part of the whole machine.

The air
is going up through the rotor disk the whole way out from blade roots
to tips.

Indeed it is, and one day I'll read the whole bleeding post before I start
typing :-))) save myself a lot of work if I did:-)


(This is all just seat-of-pants intuition; I hope someone with aerodynamic
knowledge can say if my intuition is right, and what the aerodynamics
of an auto are, and why in that regime settling with power can't happen.)

The seat of your pants is working well from where it's sitting:-)

The one place I can (just barely) imagine it might be possible is during
the brief moment at the bottom of an auto when you crank up the collective
to exchange rotational inertia for lift to cushion your landing.

Not going to happen there either. You need to have POWER into the blades
(and lot of it) plus a descent rate exceeding 300fpm.

At that
point it seems like you are adding energy to the rotor head other than
that which is coming from the descent through the air.

Actually, you're USING the energy already stored, so you're losing energy
not adding to it.

My supposition
is that you can turn the blades using the engine, or you can turn
the blades using the stored rotational inertia in the blades, and in
either case you might be able to induce vortex ring state. Is this true?

No. More than one criteria must be met to induce VRS, and one is a LOT of
power going into the blades (not being used as it is when pitch is pulled,
but DRIVING the blades when pitch is pulled) and the other is that rapid
descent rate (300fpm typically). If both aren't present, you're not going to
find yourself in the ****.


--
Beav


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