If our engines are smooth and the entire engine is

well balanced, why is it necessary to isolate the

engine from the engine mount/airframe.

 

‘Smooth’ is a relative term.  The rotary is much smoother than a Lyc; but that doesn’t mean that it doesn’t vibrate.  Add a prop that is trying to transfer all that power to the air, and the resulting turbulence and the various vortices adds more vibration.

 

Aside from contributing to noise and discomfort in the cabin, vibrations cause failures.  Somewhere in your airframe there is a mounting bracket, an electrical connection, or whatever that could have a natural frequency that tunes to a vibration coming from your engine (resonance); and eventually succumb to vibration fatigue failure.

 

The job of the engine mount dampers is to isolate the engine/prop from the rest of the airframe.  You can never get them all, but you can significantly reduce their amplitude generally get most except the lower frequencies.  The objective would be to make the mount as soft as possible consistent with limiting the range of movement and not resulting is sag or failure of the mounts in less than a thousand hours or so.  Mostly in custom engine installations this becomes an educated guess.

 

FWIW,

 

Al