Duncan McFadyean
Joined: 18 Jan 2011
Posts: 220
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 Posted: Fri Sep 04, 2020 7:47 am Post subject: Propeller? Gearbox? "Wow-wow-wow" resonance |
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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
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=496930#496930
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