A bed mount with the rubber bushings under the
engine CG is a much lighter way to go.
Paul Lamar
'splain... what is reasoning? (not a challenge, just want more info)
We are looking at all the possibilities ...Chrissi
Very hard to design a front cover that will take the bending stresses
of a Lycoming type cantilevered design.
Two pieces of 3/4 OD .049 tube 25 inches long weigh only
.86 pounds each. Four pieces of 5/8 OD .049 tubing 25 inches long
weigh .72 pounds for a total motor mount tubing weight of about 4.6
pounds. Add another three pounds for a larger motor mount/PSRU plate
and you have a total motor mount weight of 7.4 pounds. The loads then
are straight down on the apex of the tripod through the two rubber
bushings in pure compression during a six G pull up. This leaves the
back side of the engine (front in your case) totally unobstructed for
accessory drives and water pump plumbing.
In the case of the cantilever mount you are looking at
a moment around the rubber mounts of about 300 pound times
6 G times the length to the CG of the engine or about 12 inches.
That moment is 1800 foot pounds. If your four rubber bushings
are spaced one foot apart the upper two will see a tensile load
of 900 pounds each and the lower two a compressive load of 900
pounds each. This means any bracket or plate bolted to the
engine back there will see this bending moment.
Assume for the moment the plate or bracket is made from
1/2 thick aluminum 14 inches by 14 inches it will weigh 9.8 pounds
alone. I and Mecanair both believe 1/2 inch plate is marginal for
this application judging by the looks of the beefy casting of what
they are using. If you cut any lightening holes in it it will becomes
less strong and stiff.
This does not include the
weight of a Lycoming type steel tube mount between this
plate/bracket and the firewall. I estimate that will weigh
additionally at least three pounds and maybe five pounds.
the total mount weight for this approach will then
be about 13 to 15 pounds roughly. You then still must get
all the accessory drives dealt with on the back side (or front
side in your case).
The loads on the firewall are the same for both type mounts.
See the ACRE NL web site below for more info.
Paul Lamar
Paul and Chrissi,
I just have to jump in here. My reason for building a cantilever mount was to
simplify radiator and exhaust system design. A RV-4 is only 24.5" wide so width is
at a premium. The cantilever mount was definitely a lot more difficult to build.
Based on the advice of Paul, Bill Freeman, Monty Roberts and Mike Wills I tied my
lower mount directly into the engine block as well as the front cover to keep from
overstressing the '83 aluminum front cover.
Ken Powell
What does this weigh again Ken? I think you told us but I forgot already.
Also you are in a unique position as you are using a peripheral
intake port and that gives you a few more critical bolt holes to work with
on the front end housing.
Paul Lamar
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