Tonight's Dumb Idea...

[vicenterc at comcast.net]

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.
> 
> ----- 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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