Shake ... Rattle ... and ... Roll

[Dean Pappas]

Hi Jim and Earl,
This is a real nice thread, going here.
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.
 
The fact that our planes are not cinder-blocks, but have relatively low inertias, raises the resonant frequency. (you both know that)
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.
 
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. 
 
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.
 
Regards,
    Dean P.

-----Original Message-----
From: JOddino [mailto:JOddino at socal.rr.com]
Sent: Thursday, February 19, 2004 11:35 AM
To: discussion at nsrca.org
Subject: Re: Tonight's Dumb Idea...


Hi Earl,
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.
Interesting stuff but it will probably all go away when we switch to electrics.
Regards, Jim

----- Original Message ----- 
From: EHaury at aol.com 
To: discussion at nsrca.org 
Sent: Thursday, February 19, 2004 4:50 AM
Subject: Re: Tonight's Dumb Idea...

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. 
 
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!
 
Earl

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