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.
I've been thinking (sorry).
We used to take precautions when starting big radials in the following
manner:
While cranking up the engine, we pumped the fuel into the intake manifold
by which time the engine was at proper rev, so turned on the ignition and
she fired. There was no premature popping to damage starter mechanism,
and there was insufficient time for fuel to pool in the intake.
Why does there have to be sparking before its time?
I'm no 912S specialist, but couldn't that preclude some of the problem?
Ferg
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