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