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Propeller? Gearbox? "Wow-wow-wow" resonance

 
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Duncan McFadyean



Joined: 18 Jan 2011
Posts: 218

PostPosted: Fri Sep 04, 2020 7:47 am    Post subject: Propeller? Gearbox? "Wow-wow-wow" resonance Reply with quote

Can also be a worn propshaft bearing(s), where the shaft is rotating in the bearing (as normal) and simultaneously orbiting around the centre of that rotation (allowed by radial play).

Duncan McF.
Quote:
On 19 June 2020 at 14:58 clivesutton <clive.maf(at)googlemail.com> wrote:




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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