That being so, the 'shaking and banging' is likely caused by the low
frequency primary resonance (torsional and/or blade-flap) that is inevitably
present below normal cranking speeds. In the absence of any damping, the
forces produced by resonance will build-up towards infinity. The crank and
gearbox get hit with similar forces too; its just that the sprag is the weak
link in the chain (and, if it's any consolation, cheaper than a new crank!).
However, damping of resonance is provided by the friction clutch in the
gearbox; hence Rotax's insistence that this be maintained at the upper
limit. Also anything that accelerates the cranking engine quickly through
and clear of the resonant band should help (i.e. powerful battery, good
contacts, high torque motor). We have to hope that it is the 'shaking and
banging' ONLY that causes the problem, not the loads of normal starting.
Last Summer, there was a certain 912S CT that was regularly shaking the
carburettors out of the mounting sockets on start up, possibly exacerbated
by the big floppy 2-blade prop it had.
Duncan McF.
----- Original Message -----
From: "P.A.D.Clarke" <paddyclarke@lineone.net>
Subject: Re: Europa-List: Rotax starting - 912S
<paddyclarke@lineone.net>
>
> Hi All,
> First a word of caution. I'm not an engineer and don't
have
> any specialist knowledge. What I have to say may well be ( and probably
is )
> a load of old rubbish. Having said that, I'll press on anyway.
> The problem seems to arise when the engine kicks back on
> start up. The poor little starter motor turning one way meets the engine
> violently kicking back the other. The location for this titanic conflict
is
> the sprag clutch - no wonder it can't cope. The higher output starter
spins
> the engine so fast that there is plenty of inertia to carry it over the
> compression, whenever ignition takes place. This is how it feels anyway.
> I agree that the higher power starter will put greater
> torque into the clutch, but I suspect the increased forces are relatively
> small compared to those involved in meeting a backfiring engine. I shudder
> to think what it would feel like to have a kick back against the high
power
> starter.
> Cheers, Paddy Clarke
> ps. I hope this message doesn't appear twice - we had a power cut just as
it
> was being sent!
> ----- Original Message -----
> From: "Ami McFadyean" <ami@mcfadyean.freeserve.co.uk>
> To: <europa-list@matronics.com>
> Subject: Re: Europa-List: Rotax starting - 912S
>
>
> <ami@mcfadyean.freeserve.co.uk>
> >
> > How is it that the higher power starter motor does not put a higher load
> on
> > the sprag clutch, which would lead to the earlier demise of the clutch?
> This
> > assumes that the clutch is the victim rather than the cause, which seems
> to
> > be the case.Or is it that the violent starting (and the 'standard' but
> less
> > violent stopping) is the sole cause?
>
>
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