Tonight's Dumb Idea...

[Lance Van Nostrand]

Earl,
I really enjoy the scientific approach you take to things, and the level of effort to get objective results.  Thanks.

I wanted to respond to the people that are looking for that elusive resonant frequency that, if found, could be designed out of the mount.  It ain't possible.  The resonance of a plane is not the result of its composition, or its mount, or the isolation, or its stiffness or weight.  It is all of these and more.  In fact it is possible to change these frequencies by installing a servo rail at the right location, or by going to a harder or softer mount.

I had the opportunity to test a high resolution laser system that I designed in the Kodak imaging labs on a vibration table.  This thing was cool.  After putting accelerometers at key locations the very expensive table began to vibrate.  The frequence and amplitude was computer controlled to cover the range of operating requirements and after running for an hour or so would print out graphs and charts of all the data.  

I learned that the difference by changing the type and thickness of rubber isolators was amazing.  An unquantified "Soft mount" is so imprecise it's laughable.  Each mount has its own range of effectiveness and range of ineffectiveness.  Besides changing shock absorption we also moved parts closer to the vibration source or farther away, depending on their mass, to find the right balance.  I was not the expert in this area but learned that our practice of slapping rubber on our mount (be it Hyde style, or lord mouns, or Gator grommets)  is crude, but effective enough.  Just don't expect it to be equally effective at all RPMs or comparable from plane to plane.

--Lance

  ----- Original Message ----- 
  From: JOddino 
  To: discussion at nsrca.org 
  Sent: Thursday, February 19, 2004 10:34 AM
  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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