On Mon, 04 Aug 2003 02:36:31 GMT, Terry Spragg
wrote:

Well...

G'day Terry,

Did the page I pointed you to help at all?

You also change the flight path.

Yeah.

The blades need to see an updraught

Yep.

to absorb power to spin, to generate lift somehow.

Not sure what this means?

The change in direction of the airflow between a nose down
attitude under power in a helo and a nose up, direction slightly
down for autorotation makes the mechanical aoa at the hub
different from the aoa of the airflow on the rotor.

Not sure what you mean by mechanical AoA at the hub, but the AoA of the
blades increases (more +ve), for the same pitch angle, with an upward
flowing airstream.

I cannot claim any authority for this beyond personal
experimentation, and my conjecture, apparantly widely debated if
not misunderstood, seems in at least a common sense way to be
true.

A small toy illustrates a point which becomes intuitive. A stick
twirled between the palms with rotor blades attached ascends
until the energy stored on the rotor is consumed. The toy begins
to descend rapidly. The rotor reverses and spins up. The descent
slows dramatically.

ISTR from childhood playing with this type of device, but I don't
remember them reversing direction of rotation during flight. They would
descend as rpm decreased. Are you just doing a thought experiment and
extending the flight?

The apparent wind on the blades must be at a + aoa to
autorotate,

Yeah.

even if the aoa at the reversed hub wrt the shaft may
be -,

Do you mean the *pitch* is -ve because the rotor is now turning in
reverse?

and the airflow presents a greater + to the reversed blade
with as much - mechanical aoa as it was + rotating in the
ascending phase, spinning it up by energising it, and with
increase in apparent airspeed of the rotor the aoa becomes less +
in regard to the flightpath of the blade arounds the hub, while
being - to the airflow around the craft.

No. I lost it completely there. Are you saying that although the toy,
now spinning in reverse, seems to have a -ve pitch, it actually has a
+ve AoA because it is descending?

Autorotation is a delicate balance, yes?
One must fly the collective carefully to do autos.

As I understand it (and I never got that far in my (limited) experience
in helicopters), it's a matter of engine failure...collective to minimum
ASAP...leave it there until the correct height above the ground...pull
collective to flare. It's not a matter of adjusting the collective
during the auto. I stand ready to be corrected by those in the know.

As for gyroplanes, they will happily autorotate all day without any
means to vary the pitch collectively at all.

It is a skill I cannot claim.

The airflow changes from downward to upward on the rotor blades
of the toy. If the hub rotation does not reverse, the +
mechanical aoa of a powered nose up gyro becomes - wrt the
airflow in the retreating blade,

No. And this is where you are striking trouble, I suspect. The AoA on
the retreating blade does not go -ve. In fact, if anything, the
retreating blade has a higher angle of attack than the advancing blade
(but that is another discussion). the rotor disk is tilted back, yes,
but the blade is doing several hundred miles per hour. The apparent wind
is several hundred miles per hour *in the direction of travel* (plus a
downward component). The direction of the craft has an influence, but
not as significant as I think you think.

the flight path becomes
downward, wrt apparent airflow to the craft now nose down wrt
horizon, but still + in one sense, and - in another. In
autorotation, part of the rotor's flightpath must be - aoa wrt
the aparrent airflow on the blade.

I don't see what you mean. Unless you are thinking that the airflow
caused by craft movement is the apparent airflow. In an aeroplane this
would be the case, but an aeroplane's wings are not doing several
hundred miles an hour and creating their own apparent airflow.

Watch the toy fly again. Meditate some more. Grok the universe.
Be one with gravity.

Ooom...OOooommm...

Is it possible to explain this in some other way? I can't seem to
express it well. Perhaps a cartoon video is available somewhere?

This is possibly a flawed intuit, and I cannot yet grasp it all.
I am not a rotorhead, but have watched many autorotations from
the tower at a helo flight school, complete with commentary from
those "3 dimensional thinkers" that direct the traffic. As a
control tower radio tech, I have had some "unofficial" flight
training, er, perhaps I should say unofficial experience in fixed
wing and rotor, and understand aerodynamics to an extent in model
design and flight.

If I claim to be a little perlexed, at least you must respect my
honesty. In autorotation flight, part of the rotor disk must be
in - air aoa, yes or no?

My vote? No.

Cheers,

Phil
--
Definitions of a pilot - No. 1

The average pilot, despite the sometimes swaggering exterior,
is very much capable of such feelings as love, affection,
intimacy and caring.
These feelings just don't involve anybody else.
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