Gary Casey wrote:

Here's a (long winded) question regarding

gyroscopic precession moments.

When I was long ago talking to Windmill people

they told me why they used

3-blade turbines.  If you have a two-blade

propeller the precession

moment varies during the rotation.  Say you are

pitching nose-up and the

blades are horizontal.  At that instant the moment

is zero because you

are simply rotating the blades not changing the

plane of rotation.  When

the blades are vertical the moment is at the max

with the curve a sine

wave.  This means that the peak precession moment

is, I think, 1.4 times

the average and the average is what the equation

for precession moment

calculates.  Worse than that, as the propeller

passes through horizontal

(for a pitching movement), the force goes to zero,

the engine mounts

"relax," rotating the engine and making it worse

when the blades move out

of the horizontal.  This effect would tear the

windmill blades apart and

that is why vitually all wind turbines have

3-blades.  The precession

moment for a 3-blade prop is essentially constant

throughout rotation.

That says before you do aerobatics get a 3-blade

prop unless the 3rd

blade adds 50% to the weight.
This all makes sense to me, although I didn't go

through all the math.

Does it to you?

Sounds plausible to me. What is worse is it is
cyclical. Twice per RPM,
which adds to the fatigue on the prop shaft.

Paul Lamar

It sounds wrong to me - but I will have to think about
it - it also would seem to me that the disc angular
change on a windmill would be slow enough to not make
precession a major problem - There are several good
reasons for liking 3 blades, but I doubt that this is
one of them - Vance

 
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