SNIP<

There is about 5" distance between gear mount bulkhead and firewall. The
plans mounting hard points can support O-360 with no problems. Do you

think

we have to be concerned with a smoother 13B?
Bulent

The steel tube is supposed to take up that space.
The further apart the rubber bushings the softer they can be.
I would like Cadilac smoothness. :-)

Paul Lamar

Paul, educate me on this motor mount concept.  I still don't understand why
the wider separation results in a smoother airplane for the pilot.

In my simple mental model of the engine/airplane system, all of the
vibrational torque from the engine ends up acting on the entire airframe.
This being the case, does it matter whether you transmit a smaller torque
through a long lever or a higher torque through a shorter lever?  Seems like
it would come out the same when it reached the pilot.

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

Some of the torque pulse is converted to heat in the motor mounts
just like the rubber bushings installed in your torque isolation
coupling. The longer the travel the better the isolation and the
higher the damping factor. Big mushy tires on your car ride better
than smaller less mushy tires.

Also the spring rate of a rubber bushings rises as load increases.
Unfortunately Barry shows only the linear range and not the overload
conditions. Obviously with enough force the rubber becomes nearly
solid or is squished out between the bracket and the washer.

What you want is a rubber bushing that won't bottom out on that
800 foot pound torque pulse. Since the rubber bushings are the same
size it helps to use a softer rubber bushing and apply less of the
torque reaction by using a longer torque arm. The lower load version of 
the larger 22003-11 would be better than say a 22002-13. Unfortunately that 
is the lowest load 22003 we can get. 

The static deflection will be higher with the softer rubber bushing
as static loads don't change with the length of the lever arm but 
the torque pulse will be one third as much. 

Paul Lamar
 
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