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<DIV><SPAN class=402322821-19022004>Hi Jim and Earl,</SPAN></DIV>
<DIV><SPAN class=402322821-19022004>This is a real nice thread, going
here.</SPAN></DIV>
<DIV><SPAN class=402322821-19022004>I am not surprised at the 100:1
isolation: if the typical "good" mount resonates at about 1,000 RPM
(actually the best are at about 1,500, by my testing) and you are running W.O.T.
at 10,000 RPM, then the 10:1 RPM ratio gives a 100:1 isolation, when you
consider the second-order nature of the spring-mass system.</SPAN></DIV>
<DIV><SPAN class=402322821-19022004></SPAN> </DIV>
<DIV><SPAN class=402322821-19022004>The fact that our planes are not
cinder-blocks, but have relatively low inertias, raises the
resonant frequency. (you both know that)</SPAN></DIV>
<DIV><SPAN class=402322821-19022004>My purely experimental observation about the
effect of finite airframe mass is that, at worst, light and rigid
airframes roughly halve the effective engine inertia, because the typical
resonant frequency rises about one-half. For a while there, I tached every
plane with Hyde mount I could find, after searching for the resonance. The
reference was bolting a Hyde-mount to a cinder block ... yes I did this, back in
'92 or '93! The engine was an AC.</SPAN></DIV>
<DIV><SPAN class=402322821-19022004></SPAN> </DIV>
<DIV><SPAN class=402322821-19022004>The exceptions (when disaster struck)
was when the airframe was floppy, and resonates (it looked like
side-to-side!) at about the same 1500 RPM. (25 Hz) That wasn't lightness, it was
flimsiness. I used to set the needles on my 33% laserwith the soft-mounted 3W
60cc by removing the wings ... the fuse wobbled, and the engine stayed still!
That made it easy to put a screwdriver on the needles. </SPAN></DIV>
<DIV><SPAN class=402322821-19022004></SPAN> </DIV>
<DIV><SPAN class=402322821-19022004>I forget who posted the observation about
hard mounting on wooden airplanes, but the radio still appreciates the soft
mount with a wooden fuse. Really.</SPAN></DIV>
<DIV><SPAN class=402322821-19022004></SPAN> </DIV>
<DIV><SPAN class=402322821-19022004>Regards,</SPAN></DIV>
<DIV><SPAN class=402322821-19022004> Dean P.</SPAN></DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV class=OutlookMessageHeader dir=ltr align=left><FONT
face=Tahoma>-----Original Message-----<BR><B>From:</B> JOddino
[mailto:JOddino@socal.rr.com]<BR><B>Sent:</B> Thursday, February 19, 2004
11:35 AM<BR><B>To:</B> discussion@nsrca.org<BR><B>Subject:</B> Re: Tonight's
Dumb Idea...<BR><BR></FONT></DIV>
<DIV>Hi Earl,</DIV>
<DIV>Can you give us some idea of the frequencies where you measured the peak
amplitudes? I assume you saw some narrow band peaks where various things
went into resonance. I was talking to Jerry Budd and he figures the
resonant frequency of the modern light weight airframe is down around 1.5
Hz. There is probably a way to "ring" the airframe and measure the
resonances if you've still got the measuring equipment available.</DIV>
<DIV>Interesting stuff but it will probably all go away when we switch to
electrics.</DIV>
<DIV>Regards, Jim</DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=EHaury@aol.com href="mailto:EHaury@aol.com">EHaury@aol.com</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=discussion@nsrca.org
href="mailto:discussion@nsrca.org">discussion@nsrca.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, February 19, 2004 4:50
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: Tonight's Dumb
Idea...</DIV>
<DIV><BR></DIV>
<DIV>When the suggestion that soft mounting provided more advantages than
not was proposed, most of us subscribed to the theory that power loss from
anything less than solid was unacceptable. I set up a test using a fuselage
that could be equipped with either a hard (cast aluminum) mount or soft
(same mount with radial Lord mounts and a Lord mounted nose ring). A
miniature accelerometer was mounted to the inside of the firewall by the
lower mount bolt and another to the mount at the same location. (Stud for
solid, Lord studs for soft.) The accelerometers were connected to a dual
channel spectrum analyzer to display amplitude g's as millivolts on a
vertical scale vs. frequency on the horizontal. Engine was a piped OS 61
with the MK prop of the time. </DIV>
<DIV> </DIV>
<DIV>After gathering full throttle data several times, and reversing the
accelerometers to ensure similar output, the results dramatically told
the story. The solid mount produced a 600mv amplitude signal and the soft a
6mv signal. A 100 / 1 difference. BTW, no measurable difference in engine
rpm. Switched to soft mounts and never looked back!</DIV>
<DIV> </DIV>
<DIV>Earl</DIV></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>