Hi Pete, Dont know if this is of any relevance to your issues but I too had
some
vibration issues a few years back (912ULS/ Ground Adjustable Warp Drive).
My symptoms were a horrid, high frequency, grinding vibration after start for
a few seconds which tended to crack the rear exhaust down pipes - I later found
I could usually avoid the vibration by getting the starter motor up to speed
before turning the ignition on. This was not the dreaded kick back - Ive
never had that. Also the same horrid vibration could occur inflight,
specifically
when throttling back for descent from a normal fast cruise, ie when the
prop was changing from loaded to unloaded. Again I found a way to avoid it
by slowing the airspeed right down (by a small climb), then throttling back to
descend, increasing airspeed as required. I (unwisely) lived with it for a
couple of years with no issues, avoiding the vibration as described above.
Eventually I got it sorted. Kevin Dilks (UK Rotax genius and top chap)
immediately
diagnosed it (over the phone) to be worn prop shaft splines, hed come across
it many times. The main cause being low idling RPM causing the wear.
My engine was second hand and had come off a weight shift microlight - he said
this issue can be very common on those as they often use foot operated throttles
which makes the RPM control very imprecise and they can often allow them
to idle very slowly on the ground.
So I took the box off and took it to Kevin who replaced the prop shaft - and
while
he had it fitted a slipper clutch which mine didnt have then. He said that
not having a slipper clutch can accentuate the development of the prop shaft
spline wear. The labour cost was minimal - I think he had the job done within
half an hour while I watched him. The parts were eye-wateringly expensive
though!
The good news is that it has completely cured the problem and touch wood it has
been completely perfect ever since - Thanks Kevin! The difference, even in
normal
running was unbelievable - I would never have believed that an engine could
run so smooth. Whilst very expensive (1,700 GBP for prop shaft & slipper
clutch) - and that was with me minimising the cost by removing and refitting
the gearbox myself and taking it to him - it was the best single financial
investment Ive made on the aircraft since I bought it and the moral of the story
is I should have done it straight away rather than keep putting it off!
Cheers!
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G-TERN
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