This is a scale rendering of Finn's motor mount.
the first is no load and the second is a 900 pound download
per side assuming the engine, gear box, starter and prop weigh
300 pounds and there is a 6 G pull up or a 6 G hard landing.

The prop torque is shared by four rubber bushing (if they
are not melted :-)) so that would be about 200 pounds per motor
mount. This also ignores gyro loads from the prop when the
aircraft pitches or yaws.

The assumed droop in the engine is about 1/2 inch and the angle change
in the motor mount tubes is about 2 degrees. At this point I have no
idea what the bolt stress is but I guarantee it is high.

The bolts shown are 1/2 inch but my guess is Finn is using much
smaller bolts. Perhaps 3/8.

You do not have to take my word for this. The EAA publishes a book
called Aircraft Detail Design Manual 3rd edition.  ISBN 0-87784-011-5
Aviation Publications. The tubing members of all space frames
SHOULD intersect at a point. This mount design violates that
principle in spades.


Finn, if you don't fix this mount design PLEASE magnaflux these bolts
on a regular basis.  You don't want your engine falling off when
you are doing a loop.

Paul Lamar
 
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