Hard Mount???

[Bob Richards]

Some of you may remember that, years ago, I flew a Cap21 in pattern with a hard mounted 1.20. What can I say, I was an un-initiated HEATHEN!!! :-)  Never had a problem. But, it was a wood fuselage with a (for then) large cross section. IMHO, a wood fuse does a lot to absorb vibration. Fiberglass transmits it, with or without Nomex.
 
It is my opinion that whatever mount is used needs to be "tuned" to the airframe and engine. I believe it is possible to have a soft mount that is harder on the equipment than a solid mount. The goal is to not have a strong resonance in the normal operational range of rpm. It might also be good to have other things in the plane soft mounted as well. Everything on the plane (wing, stab, ailerons, rudder) all have a resonant frequency. Ideally, none will be a very strong resonance, and everything will have different frequencies at which they resonate.
 
All of the soft mounts that I have seen try to force the engine to "shake" around the centerline of the crankshaft. This is OK for a horizontally opposed engine where the engine CG is on the crank centerline, but not ok for a single cylinder engine. With the cylinder head shaking back and forth, this will cause the fuselage to shake in the opposite direction. I believe it would be better to soft mount the engine around its natural CG. I don't think it matters if the prop shaft moves back/forth a little, the only shake that will be transmitted to the fuse will be rotational, not side/side (except for a small amount due to the piston travel).
 
This is one of those experiments that I have wanted to try for years, but never got around to it. :-)
 
Bob R.


"Atwood, Mark" <atwoodm at paragon-inc.com> wrote:

Question for all you vibration guru’s out there.  I’m tired of soft mounts.  Seriously.  

 

Has anyone RECENTLY, tried using a reasonably firm, or hard mount on a pattern plane with a YS???   I know I know
my servo’s will take a beating, blah blah blah.  And I’m sure it’s true to some extent.  But I have several sport planes with YS 1.20’s hard mounted, they run like tops, idle at 1300, 
they literally purrr
  And I’ve been flying them for YEARS
and I mean 8 or 9, with no issues on servos, airframe, etc.   

 

I’m just wondering if we’ve over played the soft game and shredded a zillion headers in the process.    Now I know my sport planes are all wood and therefore handle the vibration better
  But wouldn’t the newer Nomex construction planes handle it fairly well too?

 

Would the current firewalls hold up??   

 

Is there a happy medium?

 

I’m considering mounting a 1.60DZ on a Brio with simple rubber washers for isolation ala a gator style mount.  It’s virtually a hard mount with minor “give”.  

 

Thoughts???  Opinions??  I’m all ears


 

-Mark

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