I was so shocked to see this on Finn's airplane I forgot
to take a picture. So I did this 3D. His rubber motor mount
bushings had melted and allowed the engine to sag down about an
inch or so. This means the engine is flopping
around as he is doing aerobatics. It is also twisting slightly
in an unrestrained manner as he opens and closes the throttle.
This over and under motor mount scheme has other serious problems
as well. The bolt is subjected to super high stresses and Finn's
may be cracked as we speak. I will get into those problems shortly.
Seems like there is little understanding out there for the principles
of space frames and structures. The Egyptians figured all this
out 5000 years ago or they would not have been able to build the
pyramids. I have signed up to do a forum at next years Sun & Fun
on how to design motor mounts.
Finn is a strange guy. He ignores peoples suggested improvements
on his engine installation until something serious happens. I warned
him about the tension bolts and he failed to take them out one
at a time and coat them with RTV. Sure enough he had a tension bolt
fail. Ed Anderson said he would pay for the fuel lines if
Finn replaced the cheap clear plastic lines he is now using with
AN fuel lines. Finn declined.
I also warned him about having the rubber motor mounts
in an aluminum plate sandwiched between engine and pan as that
would conduct too much heat into the rubber motor mounts. Rubber
motor mounts are limited to about 180 degrees max. Sure enough
his rubber motor mounts melted.
Come on Finn. We don't want to lose you. Who would we have
to humiliate if you were gone? :-) Fix the fuel lines and the
motor mount. You don't want to have your engine fall out of the
cowl. Use a Schertz beam motor mount and get rid of that
silly dumb idea sandwich plate.
Paul Lamar
Finn Lassen wrote:
Paul, you didn't look closely enough. The rubber bushings are NOT melted or damaged in any way.
The conical "cups" I had JB-welded to the mount simply broke loose and broke up (thin split alu
tubes filled with JB-weld). Didn't hold up to repeated aerobatics. I'll be replacing them with the
McMaster-Carr mounts you saw. The better solution might be to have someone turn me 8 conical cups
in alu and have them tig welded onto the mount. As for the stress on the bolts. 3/8" bolts! What's
the rated rated shear and tension stresses of such a bolt? What kind of stresses are you imagining
these bolts seeing?
If I were to heed all the "advices" I've been given through the years, I'd still not be flying,
and surely not behind a rotary.
300 hours and counting. How many hours on your KIS cruiser? Oh, that's right, it's still not
flying.
Finn
If you ask me Finn you are lucky to still be alive.
I did look closely and it looked melted to me. The lower rubber part was
down to about 1/4 inch thick and there was about a 1/2 inch gap between the
aluminum plate and the upper rubber bushing. Did you take it apart
to inspect it after you left Shady Bend and flew home to Tampa this
past weekend?
Natural rubber, which the conical mounts you showed
me is made from, is limited to 180F. Barry Catalog page D19.
Since that highly conductive aluminum plate is subject to hot oil
at up to 250 F an inch or two away from the
rubber this configuration is a really bad idea. The rotary, unlike
a Lycoming uses the oil to provide 1/3 of the cooling.
Furthermore the Lycoming mounts are further away from
the hot oil.
The shear strength of the 3/8 bolt is totally irrelevant in
this application. The bolt in your motor mount and other
poorly designed motor mounts like it is in severe bending as shown
in the rendering. It could break at any moment.
The bolt is seeing bending stresses similar to hanging a 900 pound
weight at the end of a two foot pipe while attempting to
bend the bolt. I would not be a bit surprised if it were not bent
already. Also the welds around the bosses the bolt passes through
are also seeing severe stresses. The mount should be removed
and magnafluxed.
Please don't fly the airplane again until you fix this mount.
Paul Lamar
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