On Wednesday, December 18, 2013 10:33:51 PM UTC-5, Morgans wrote:
I'll bet any money the problem will turn out to be torsional harmonics.

This is way over-simplified, but useful to think about.
And I am absolutely not expert in this area! But...
You can think of the assembly as a set of springs:
- the drive belt
- the structure between the engine and prop hub
- the prop itself

When the engine fires, it:
- stretches the drive belt on one side, loosens on the other
- flexes the support structure
- bends the prop (in both torsion and load directions)

These produce fatigue loads on all components involved.
All of which have conspired to produce numerous failures
in many different makes and models over the years,
on all of the affected parts.

Hope that helps!
Be careful out there, and ALWAYS have a plan for a failure
at all points during motor operation,
Best Regards, Dave "YO electric"