Jonathan, a further investigation suggestion that you can do from the comfort of
your home:
1) Download this free sound tool onto your PC:
https://www.nch.com.au/tonegen/index.html
2) Create two sine tones and adjust one to be 1hz higher than the first (i used
440Hz + 441Hz). Play it and you might get the effect you are hearing in the
aircraft. The Wow-Wow you hear is effectively the difference between the two
frequencies so a 'Wow' repeats every 1 second in this case. Play with different
frequencies to get the effect you experience . . . [Caveat - audio speakers
and headphones are not that good at reproducing low frequencies - so the quality
of what you hear may be affected by that factor]
3) For an engine running at 4700 RPM and a gearbox reduction of 2.43, the
gearbox
shaft is turning at 32.23 rotations per second at that RPM. That shaft has
three prop blades bolted to it - so the frequency of pulsation from any one
blade
is 32.23 Hz and the frequency of pulsation from all three blades passing
in front of you at 4700 RPM is 96.69 Hz.
4) based on this logic, if any one of those pulsations is higher or lower than
the other 2 (as a result of e.g. tracking or different pitch setting of one
blade)
i suspect that might be your Wow-Wow source.
I'm not sure this rules out your gearbox - but it's something to check
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=496969#496969
|