The clutch is engaged any time that the engine is kicking back (including at
shut-down) and the immense forces that are felt to be at play are actually a
manifestation of the interaction of the prop., crank and dog clutch in the
gearbox (the latter in particular as the dogs bounce from face to face). No
doubt the sprag and starter is suffering too, but that would not be
discernable from outside.
My point was that the whole system must have (by law of nature) a primary
resonant frequency somewhere; it's usually engineered to be below cranking
speed, before the point where combustion loads can commence and below the
speed that the engine will normally run. So, as you say, once the engine is
running it is beyond the speed range that can produce kickback Of interest,
some two-stroke set-ups can be seen with a resonance well above cranking
speed to the extent that the engine struggles to power-up through this
resonant speed once the engine is running
I can't think that Rotax would have been so thoughtless as to have provided
too much advance at cranking speed (4 degrees) and others have confirmed
that trying to start on one ignition only (which would crudely "halve" the
rate of advance of the combustion flame-front, effectively retarding the
ignition) makes no difference to the shaking. However, if the cranking speed
is below par for whatever reason and the engine fires whilst at or close to
the resonant frequency, then the combustion loads will massively antagonise
the shaking.
I had the same problem on the BMW (the BIG starting problem on that was
similar to that of the Rotax and was the last straw for this project) which
of course had mapped ignition and a flywheel. What helped resolve the
problem was adjustment of the gearbox preload, which moved the engine-prop
resonant frequency away from cranking-speed frequency. Problem then was that
the preload was then wrong for operation.
Its also worth noting that the BMW starting was eased by leaving the
ignition switched off until the starter had run up to cranking speed.
Maybe this is just another way of looking at the same problem, but I think
it more accurately reflects the dynamics at play.
Duncan McF.
antagonise the whole regime.----- Original Message -----
From: "Fergus Kyle" <VE3LVO@rac.ca>
Subject: Europa-List: Rotax starting - 912S
>
<nigel_graham@btclick.com>
>
> 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>
> To: <europa-list@matronics.com>
> 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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