Rod Buck wrote:

In message , lid writes
You lost me there. I was talking about the use of a tilt rotor (with a
fixed pitch) on a helicopter. Other than losing auto-rotation, what is the
downside? I know losing auto-rotation IS a big deal. I saw a coaxial kit
that uses a tiltrotor (no swashplate, no collective...no auto-rotation) But
they have two engines. Seems like dropping all the extra hardware for pitch
control (they use engine speed to control lift) would be a good thing.

Look, you can alter the lift force of a rotor by two methods. Either you
keep the same rotor rpm, and alter the pitch angle of the blades, OR you
keep the blade angle the same, and alter the rotor RPM.

Or, of course, a combination of the two.

The problem is that, without collective pitch control, you can only vary
the lift force by increasing or decreasing rotor rpm, ie by altering
engine power.

This is very slow to act, due to rotor inertia (flywheel effect) and
means that control is extremely sluggish and imprecise compared to
collective pitch control, where the rotor speed is constant, but the
attack angle of the blades is changed instantly.

You do NOT need a collective-pitch control to change from powered flight
to autorotation - you could just set the blades to autorotation angle to
start with, and then vary power to increase lift - then, if the engine
quit, the freewheel device in the drive chain would let the blades
outspeed the engine, and you'd enter autorotation automatically.

The confusion you are having is, I think, because you (and several
others) think that the blades have to be at positive pitch for powered
flight, and altered to negative pitch (nose-down to the plane of the
rotor disk) for autos. THIS IS INCORRECT.

Blades autorotate perfectly well with a small positive pitch angle -
normally about 1-3 degrees (depends on the airfoil used)

--
Rod Buck


Well...

You also change the flight path. The blades need to see an
updraught to absorb power to spin, to generate lift somehow.

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.

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.

The apparent wind on the blades must be at a + aoa to
autorotate, even if the aoa at the reversed hub wrt the shaft may
be -, 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. Autorotation is a
delicate balance, yes? One must fly the collective carefully to
do autos. 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, 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.

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

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?

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