Paul, this is an old one I just ran across, some interesting stuff on resonant failures.
This would be a good subject for onboard PC's gathering data. How about some home brew transducers.. modified phono cartriges?
...Chrissi

What Chrissi? :-)>

Paul Lamar

DOH..oops! (couldn't for the life of me figure out "what's not to understand?")

http://cozy.canard.com/newsletters/news_47.html#prop_hub_break

Quote:
FAILURE OF PROP HUB EXTENSIONS

 There was a recent reported failure of a Brock Mfg. 6" prop hub extension on an
 E-Racer with an 0-360 Lycoming after 70 flight hours. The propellor flange
 separated from the extension, and the flange and the propellor bolted to it
 departed the airplane. The propellor and flange were not recovered, but
 examination of the remaining portion of the extension (by others) indicated that
 it was a fatigue failure. Since this was the only known failure of a Brock extension,
 and similar extensions on 0-360 powered pusher aircraft have accumulated
 hundreds of hours of satisfactory service, there has been considerable speculation
 as to what could have been the cause of failure in this instance. A fatigue failure in
 metal can occur if a sufficiently great load is applied repeatedly over a large
 number of cycles - usually in the millions. If there were one (or two) impulses per
 propellor revolution, 70 hours at 2700 rpm (or greater) would be over 11,000,000
 cycles. This would be enough cycles. The next question is, what kind of a load
 would it take.
End of quote.


Chrissi, O-360s have horrific torque pulses. This is the driving force
that will excite a part into its natural resonant frequency.
Here is a chart of same. Nothing to do with pushers or 
Canards per se. You get two of these monsters per engine
rev. We don't recommend our prop for the O-360. A rotary
with or without a gear box is much much smoother.
Particularly a three rotor.

In this case it sounds like a flaw in the material.

Paul Lamar
 
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