Dave Wrote:
> @@@@@ I don't really thing the mounts effect the resonant frequency
of an
> engine/prop combination. The mounts absorb the resulting vibrational
> energy, but do not cause it. In an extreme case, hard bolting the
engine
to
> the airframe would result in zero energy absorption and all that
energy
> would be taken either in the airframe or in a reaction force in the
engine.
PZ: In your extreme example where the engine is hard-bolted to the
ariframe,
the resonant frequency of the Prop/Engine+Airframe is indeed different
---From the the Prop/Engine alone as the mass of the "Engine" in the
relationship
is effectively quadrupled (assuming infinite stiffness in the airframe) over
infinitely soft mounts.
Dave Wrote:
> @@@@@ With softer mounts, the engine will move more at this
frequency
PZ: And that is the major factor/problem. Once you start letting the
engine
move more, depending on where the engine is in its 'loaded' or 'twisted'
position (as it is moving back and forth - with it's velocity in the form of a
sinewave), it could very well kick back with much more force against the prop
during a power-stroke. These 'opposite' phase impact forces between the prop
and the engine would load the gears up higher than with the harder mounts which
allow less movement. Add to this whole mess the gear-slop in the gearbox, and
the whole thing gets even worse to figure out.
PZ- It could very well be that these effects are only a problem at
start-up
or idle, but I would be very interested in seeing some test data on the
airmaster/914
configs (with a gearbox slop as per specification) with both soft
and hard mounts to see what the problematic RPMs would be.
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