Tonight's Dumb Idea...

[Amir Neshati]

Good points Jeff and Jim.....Jeff, I believe the load carrying structure is
actually lighter
yet. About 4~5 lbs. for these 2 meter birds....may be even less.

Amir



> 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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> >
>
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