Hi Jonathan, what you describe is 'Heterodyning' or 'Beat-frequency' created by
the difference between two noise or vibration frequencies very close together.
Do you have a recording on e.g. a mobile phone that i could listen to - along
with
the engine speeds in play at the time? And what about:-
1) How the frequency of 'wow-wow' change if you speed up or slow down the engine
a bit?
2) How does the 'wow-wow' effect change with Airspeed?
3) the CS mode idea is viable too - does it occur when that is turned off/the
prop
is not trying to hunt for optimum speed vs pitch?
It's impossible to say from just the description given, but assuming the above
items make a significant difference, my initial suspicions would be one prop
blade
with a slightly different prop pitch to the others and/or blade tip tracking
differences between blades. Both of those parameters should be quite easy
to check on the ground (no engine running needed) and reporting any variation
you find?
Clive
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