It has to resonate somewhere!
But it may not necessarily be the engine itself. The engine mount/engine
will have a particular resonant frequency as will anything else hung off it
e.g. the exhaust system.
However, your resonance is so tightly focussed around 3250 as to suggest
that it could be a compounded effect eg a more general resonance that causes
a problem in one of the systems (eg carb vibration and shortlived
overfilling of the float bowl).
I have the same set-up on my Rotax as yours. There is a slight
roughness/unhappiness between 4200 and 4500, which is only detectable when
the prop and everything else is well balanced, but enough to want to avoid
that speed range in cruise.I suspect this particular vibration is a
propeller/gearbox interaction.
Balancing the carbs throughout the speed range is impossible on many
Rotaxes, due to a combination of tolerance differences including the length
and mounting of the throttle arms on the carbs. Best therefore to balance at
cruise engine speeds and not worry about other speeds.
Duncan Mcf
----- Original Message -----
From: "Tony Bale" <tony.bale@virgin.net>
Subject: Europa-List: 912S Resonance / Vibration
>
> Folks,
> We have just completed the first 2 1/2 hrs of ground engine
running (912S Warpdrive @ 21deg XS mono) apart from a couple of starting
problems which we think are fuel related, the only issue which we are
concerned about is a vibration / resonance at 3200 - 3300 RPM.
> Could find nothing loose, re-torqued the blades and had a good poke and
push around with all the bits and pieces, anyone any ideas / experience ? We
have spent considerable effort in reducing throttle cable drag and balancing
the carbs.
> The rev band the resonance occurs at is very specific, and goes as quickly
as it starts, very consistent and repeatable, however the engine is smooth
at all other speeds.
>
> I must say being a spam can pilot I was very impressed (not to mention
releaved) at how controllable the aircraft was on the ground in some cross
winds and up to high taxi speeds on grass
>
> Yours with blurrrrrred vision
>
>
> Tony.
>
>
> PS, I think CKT will be releasing an exhaust mod to prevent the manifold
pipe (left rear) cracking some people have experienced. Also they were
unaware of the tail pipe falling off some owners have experienced, they are
developing a better clamping device.
>
>
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