In a message dated 2/14/2003 8:23:58 AM Pacific Standard Time,
peterz@zutrasoft.com writes:
> I think Bill Lockwood may have a point in that softer mounts would surely
> change the harmonics of the engine/prop combination. But I wonder if his
> blanket statement is valid for all prop/engine combos?
> @@@@@ 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.
>
> I wonder what rpm ranges are the trouble points ("torsional peaks") for the
> redrive with the airmaster prop, on the 914 with the softer mounts? with
> the harder mounts?
>
@@@@(what is redrive??? Is that a British term?) I have seen vibration
increases in the 2600 - 2700 range and at the 5,000 range. Initial
observations of about 30 minutes of running with these new mounts show the
same vibrational resonant frequencies, just a lower perceived noise and
vibration level in the cockpit.
> I wonder if anyone has done a tortional resonance/vibration analysis on the
> combination... likely not I suspect. Is there a shop in the states capable
> of doing such testing (like the Bob Harrison had done accross the pond on
> the Jabiru 3300/MT combo)?
>
> Cheers &full of questions (what else is new?)
@@@@@@@ I may be speaking from a point of ignorance, but again I still
think the rotating parts are what cause the vibrational resonance. Since the
mounts are fixed, they will not contribute to the vibration. For example,
when the eingine/prop is at the 2700 RPm range and hitting a peak, the harder
mounts will allow this vibration to transfer into the airframe more than the
softer mounts. With softer mounts, the engine will move more at this
frequency, but of the prop is balanced and the engine is running smoothly, I
wouldn't think it would be a problem. I would hope that that this engine is
not so sensitive that it cannot be mounted via aircraft quality engine mounts
without having a problem.
My "gut" reaction to this engine operation with the kit provided mounts,
after thousands of hours siting behind lycoming engines, is that it felt like
it was bolted to the airplane. It did not feel right. Now, with these new
mounts it does feel "right". For 24 hours or so, I was feeling way better
about this engine installation, of course now there is water dashed upon
that. I too am full of questions!!!!
By the way, I don't know how old the mounts are I have with the kit. They do
stiffen with age. These continental mounts have a manufacture date for that
very reason.
Cheers!
Dave A227
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