For what its worth, Marvin Alvarez recently took his 914 powered tri-gear to
Lockwood due to vibration problems. He was told that his rubber engine
mounts were too hard. They were replaced with softer ones. Sounds contrary
to what was said on the forum.
Bob Berube A166
----- Original Message -----
From: <DJA727@aol.com>
Subject: Re: Europa-List: Night and Day
>
> 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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