Soon I will be installing a dubro vibration reducing mount on a 90 sized plane with a cowl. Being that the entire purpose of the mount is to absorb shock I assume that it will flex a little bit as the engine runs. HOw much movement can I expect and do I need to cut the cowl any different around the carb and prop to all ow for side to side movement?

What about forward and backward movement? Will I need a little extra gap between the spinner and cowl to allow for slight compression of the mount?

The DuBro soft mounts are fairly stiff so the engine will not flex much. I usually make all cuts in the cowl so that I have about 2/32" margin around all parts of the engine that protrude through the cowl. That is usually enough.

There is no compression of the mount because the rubber grommets are mounted between two aluminum bushings that keep the distance to a fixed value.

/Red B.

Ditto what Red B. wrote. I've never had problems with 1/16" clearance.

Vibration mount is to absorb vibration and NOT to allow the engine to move.
IF the engine is moving, then there is something wrong.

Red and mirwin, you suggested leaving about 2/32 or 1/16 clearance. I was thinking more along the lines of 4/64.

Bravo Zulu, Gringo....!

Gringo, I don't know about 4/64 how about increasing to .0625

Guys,

I belive the measurement you are seeking is a "poofteenth" which is slightly smaller than a "micky hair".

Now that is out of the way i have never had a problem with the motor moving in vibration adsorbing mounts. However rubber grommets or other measures such as fuel tubing over the fasteners really help with the longevity/crack resistance of the cowl mounting

Cheers,

Colin

I strongly recommend against that, Gringo. The coeffishal nalamyute of 4/64 creates harmonious imbalance between the propeller and the tail wheel. As a consequence, the plane will fly upside down and backwards. If you really want to try it, mount your landing gear above the fuselage intead of below it, and point the wheels backwards.


Mike