Tonight's Dumb Idea...

[Jeff Hughes]

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.

----- Original Message ----- 
From: "JOddino" <JOddino at socal.rr.com>
To: <discussion at nsrca.org>
Sent: Tuesday, February 17, 2004 9:59 PM
Subject: Re: Tonight's Dumb Idea...


> Everything has a resonant frequency including a block of aluminum. You
never
> want a critical component to go into resonance.  You want to keep the
> resonant frequency of all the critical components high and keep them from
> being excited by the engine vibration.  The idea of engine soft mounting
is
> to build a low pass filter that attenuates the higher frequencies so they
> don't excite the resonanat frequency of the servos, etc.  If there is no
> isolation, there is no attenuation and you stand a better chance of
shaking
> your servo apart.
> That's about all I remember from my course in vibration umpty years ago.
> Notice I didn't say umpteen.
> Jim
>
> ----- Original Message ----- 
> From: "Bob Pastorello" <rcaerobob at cox.net>
> To: <discussion at nsrca.org>
> Sent: Tuesday, February 17, 2004 5:11 PM
> Subject: Re: Tonight's Dumb Idea...
>
>
> > Actually, Jim, the vibration frequency of a typical YS or large 2 stroke
> is
> > QUITE a bit lower.  Simply, the 049 piston/counterweight is slapping
> around
> > at 22000 or more...the pattern motor somewhere around 8K.  That's an
order
> > of magnitude less.
> >     Where the concern may be is the IMPULSE strength...the "thump" of
that
> > counterweight.  Kinda like an oil rig pump....HUGE counterweight, and
the
> > earth shakes with every thump....smaller pumps, pumping same rate but
with
> > different MASS counterweight - way less thump.
> >
> > Then there's the actual impact....If the servo, attached to the motor
> mass,
> > is moving at exactly the same rate and intensity as the motor itself,
I'd
> > challenge that the RELATIVE force transmitted to the servo is LESS than
> that
> > of a servo constrained someplace in the airframe.  My theory is that the
> > constraining force, distance, and rigiditiy of mounting actually
modifies
> > the resonance or thump that hits the servo.  AND - it's connected full
> time
> > via the pushrod to the motor that is thumping along at it's natural
rate.
> >
> > So we'll try it, and see what happens!!
> >
> > Bob Pastorello, Oklahoma
> > NSRCA 199, AMA 46373
> > rcaerobob at cox.net
> > www.rcaerobats.net
> >
> >
> > ----- Original Message ----- 
> > From: "jim ivey" <jivey61 at msn.com>
> > To: <discussion at nsrca.org>
> > Sent: Tuesday, February 17, 2004 6:59 PM
> > Subject: Re: Tonight's Dumb Idea...
> >
> >
> > > Ok enough. Have you ever held a .049 in you hand,between your fingers
> > while
> > > it was running.Multiply this vibration by 280 times and that is what
the
> > > servo feels that is mounted on the mount of a YS 140.
> > > I'm dodging now.
> > >
> > > Jim Ivey
> > >
> > >
> > >
> > > jivey61 at msn.com
> >
> > =====================================
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> > #
> >
>
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