Tonight's Dumb Idea...

[JOddino]

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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