The clutch is only vulnerable when it is engaged and the engine kicks
back (i.e. during starting). Once the engine is running, the clutch will
be disengaged and the low RPM torsional resonance can't affect it.
The real damage seems to occur if the engine fires at just below the
required cranking speed and kicks the crank back the wrong way. As Paddy
points out, the forces are immense. It looks just as bad from outside
the aircraft as it feels from within.
While the more powerful starter may well alleviate the problem in the
short term by increasing cranking speed, I feel that the problem will
not be truly fixed until Rotax develop (or get Ducati to) an ignition
system with a mapped "starting" ignition retarded to TDC or even
slightly later. This soft-start would get the engine running without
damage, at which point the normal ignition mapping would take over.
This concept is not new. It is employed successfully on the Europa
"primary trainer" that I fly (DH82a) and was developed over seventy
years ago to protect the starting system - your fingers!
Nigel
----- Original Message -----
From: "Ami McFadyean" <ami@mcfadyean.freeserve.co.uk>
Subject: Re: Europa-List: Rotax starting - 912S
<ami@mcfadyean.freeserve.co.uk>
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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