The test I'm talking about uses transducers to vibrate the prop through
a range of freqs that the prop will see in use, similar to the way an
airframe is ground tested for flutter freqs. About 25 years ago Thorp
T-18 & Mustang II flyers were cutting down a common fixed pitch metal
prop & repitching it for faster cruise speeds.  Several of the cut-down
props shed a blade at almost exactly the same distance from the hub.
Vibration testing using the same model cut-down prop showed a ring node
at that point on the blade.  This problem is far less likely with 'glass
blades, but it's cheap insurance to do it anyway.

Charlie England

Finn Lassen wrote:

Transducers?
You mean "shakers"? Basically loudspeakers without the membrane and a mount plate fixed to the
moving coil. You of course also need a pickup (accelerometer) to first calibrate the shaker and
then measure the spectrum with the prop tip attached to the shaker.
I used to work at a company (Bruel & Kjaer) that specialized in such equipment.
These day it should be easy to do a cheap set-up, using a computer to generate the sweep frequency
and record the output from the pickup. An old 100W speaker and amp would probably be enough to
exercise the prop enough to discover the resonance frequencies. I'm thinking one could glue a
piece of tubing to the front of the coil and use that to connect to the prop tip.

Just an idea, and no, I'm not volunteering to do it.

Finn

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