Peter Zutrauen wrote:
> Another question from the uninitiated:
> I wonder, are the "crankshaft oscillation" and resonant RPM's less critical
> on a geared engine like the rotax vs. a direct-drive Jabiru? Are they
> significant at all? I would imagine that the gear-train filters much of it
> out? Or does the gear backlash etc. set the system up for some worse
> resonance's? I assume the former from Tony's subsequent reply?
> Cheers & and Holiday Wishes,
> Pete
Hi Pete,
I believe that resonance in an engine drive train is affected by the inertia
and rigidity or otherwise of all the components, extending from the flywheel if
any, the crankshaft, couplings, gear train clearances and gear teeth stiffness,
and the propeller. Resonances at particular critical speeds can overstress and
break almost all these components if the conditions are right. This is why it
is always best to stay with tried and proven combinations of engine, gearbox
and propeller. Was this the reason that the BMW engine idea some time ago had
so much trouble ? Each design is unique and usually has its own set of
problems.
If you visualise the torsional impact of rear cylinder power strokes on the
crankshaft of a 27 litre Merlin V12 in a Spitfire for example where each
cylinder is ~ 2 1/4 litres volume, you can perhaps appreciate that the
crankshaft winds up like a piece of rubber and the tuning out of resonances is
quite difficult. I know that on that particular engine, the gearbox design was
crucial for successful operation.
Cheers, John
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