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
Here is the earlier post on my weights"
The original conical mount from Van's weighed 11 lb 6 oz. The
new mount
weighs 13 lb 3 oz, or 1 lb 12 oz more. Lot's of extra .75" tubing
there!
The engine adapter mounts weighed a total of 5 lb 12 oz. The
lower mount
weighed 2 lb 12 oz, the right upper weighed 1 lb 11 oz and the
upper left mount weighed 1 lb 12 oz. The three adapter mounts
were weighed together as well as individually to eliminate
rounding errors. Note that the upper adapter mounts (on both
sides) tie the front and rear of the engine together to eliminate
extra loads on the tension bolts.
Ken Powell
13.2 + 1.75 + 5.75 = 20.7 pounds. Did I add it up right?
Of course that includes the landing gear socket tubes for the RV4. My
guess of about 15 pounds for a cantilevered mount would be about right
if it were not for those socket tubes.
If anybody wants to calculate the weight of a motor
mount design 0.049 wall 3/4 dia. steel tube weighs about
0.035 pounds per inch. 0.049 wall 5/8 dia. steel tube weighs
about .029 pounds per inch. Steel weighs .3 pound per cubic inch
and aluminum weighs .1 pound per cubic inch.
I suggest you first make a 1/4 scale balsa model using 3/16 inch
square balsa sticks and then add up all the lengths of tubing you will
need.
Paul Lamar
Paul,
Total weight of both the engine adapters and the traditional motor mount was 18 lb
14 oz. Note that the original Van's mount weighed 11 lb 6 oz, so my mount weighs
7 lb 8 oz more than Van's. All those little pieces of 4130 add up!
Ken Powell
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