Tonight's Dumb Idea...

[Bob Pastorello]

Just to clarify what I'll be testing - I'm going to be mounting the throttle
servo to the beams of a vibration-dampening motor mount - NOT to the
firewall.  ON this airframe, the motor WILL be 'soft-mounted'....
    And I agree with both Jim O and Vicente - having done the test that
Vicente outlines before, I know that there are LOTS of 'resonance' points
even on a SOFT mounted engine.  The idea is to avoid ANY resonances.

Bob Pastorello, Oklahoma
NSRCA 199, AMA 46373
rcaerobob at cox.net
www.rcaerobats.net


----- Original Message ----- 
From: <vicenterc at comcast.net>
To: <discussion at nsrca.org>
Cc: "Jeff Hughes" <jghughes at insightbb.com>; <discussion at nsrca.org>
Sent: Wednesday, February 18, 2004 10:26 PM
Subject: Re: Tonight's Dumb Idea...


> My experience is the oposite.  I will never go back to hard mount.  I
agree with Jim.  If I remember right from ME vibration courses, the
resonance frequency (RF) is a complex function of the square root of mass
divided by stiffness SQRT(K/M)of the mechanical device.  In our 2 meter
planes the mass is low since they are light and they are not as stiff
compared with smaller planes.  That means that the RF of our 2M pattern
planes is a lot lower than the old smaller designs with 0.60 engines.  At
the same time, we are using engines that are very powerful with lower RPM to
reduce noise.  That means that the vibration generating device (motor)
frequency gets very close to the airframe RF and that is the problem.  The
only way to avoid passing the resonant frequency to the airframe is to
isolate the vibration generating device from the airframe.  That is the soft
mount.  This also reduce the noise from the airframe (drum effect) and
planes with soft mounts are less noisy than same planes with hard engine
mounts.
>
> Please don't ask me to prove the theory.  In the practical side is very
easy and I will tell you my experience using my Focus.  The Focus has
removable cowl and is easy to run the engine with the cowl removed.  Make
sure that you hold the plane and a friend help you.  I ran the engine at
different RPMs and put my finger in the engine beam and felt the vibration
level  (if you keep it there too long you will feel vibration pain).  Then
put the same finger in the engine box or firewall.  You will feel how much
the soft mount is isolating the vibration from the engine.  Please be very
careful and don't get close to the prop.
>
> Vicente Bortone
> > The trouble we have is we have a 2 pound motor in an 7lb air frame.
Which
> > means it's virtualy impossible to isolate the engine at all rpms. It's
just
> > too heavy in relation to the plane. That's why sports car designers talk
> > about unsprung weight.  I've had a couple planes with a soft mount and
have
> > gone back to hard mounting. The all wood planes do very well attenuating
the
> > vibration.
> >
> >

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