I got my notice from the EAA to sign up for a forum at OSH this
year. In addition to my usual forum on the Aerodynamics of Liquid Cooling
I am thinking of presenting one on motor mount design.

The Mazda wankel rotary does not come equipped with aircraft motor mount
brackets so Cessna and Piper motor mounts cannot be copied.
It is very important that the rotary engine motor mount
be properly designed.

I see some extremely dangerous designs out there. Here is one
example. The load is applied to the middle of a tube without
visible means of support.

It is imperative that all loads are applied at the juncture
of three or more tubes. Any introductory freshman course on space 
frames will emphasize that over and over again right from day one. 
If this is not done you are sure to get cracks sooner or later.
Worst case your engine could rip it self right out of the cowl with
catastrophic effects on the CG and the stability of the airplane.

If you are mathematically challenged and do not want to
pick up a text book on the subject I highly recommend
a publication by the EAA called Aircraft Detail Design
Manual. ISBN 0-87794-011-5. There in you will find many
correct examples of welded tubing joints including
one or two on motor mounts. No math is required
all you need do is copy those examples.

I have helped with Jerry Hey's design and they are safe.
Unfortunately there are some others out there that I can
not say the same thing about. Buyer beware.

If your motor mount looks like this I recommend not
flying it until it is fixed. We don't need any rotary
powered airplanes falling out of the sky in two major
parts. Engine and the rest of it.

It cost no more to do it right.

Paul Lamar
 
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