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
 
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