Al Gietzen wrote:

Looks sort of like my mount.

Al

Yes it appears similar but this mount violates a few rules of the beam
mount which leads to indeterminant (not easily calculated short of a
finite
element analysis) local bending loads in the tubes. Tubes A, B, C, D

and

E
all need to be in the same plain. The joint where tubes A, B, C and E
come together
should be moved forward closer to the front motor mount beam. Not
entirely
necessary as there is very little load on the front motor mount as the
CG of the engine/PSRU/prop is much closer to the rear mount.
Cantilevering
tube E forward to take these loads as done here is workable. Also tube

D

is in the
wrong position leading to unnecessary bending loads in tubes E and C.
Also this mount needs to be diagonally braced laterally under the

engine

or over
to the spar from one or more sides. Currently tube bending stiffness

is

relied on to do this job.

I would monitor this mount on a regular basis for cracks.

Paul Lamar
-----------------
Good points, and I'd only disagree on the CG being "much" closer to

the

rear mount.  Based on how the full-up engine/redrive balances on a

chain

on the hoist, I estimate the CG within an inch back of  being centered
between the mount bushings.  This is without the wood prop and prop
extension.
Also I'm not sure about the need for diagonal bracing; the way in

which

the tubing is 'out-of-plain', and with the added gusset make it strong
sideways, and torsionally quite rigid.
But I have no detailed stress analysis on this mount and will monitor

it

closely.  A broken engine mount could really ruin your day.  It works
fine for the 220hp Franklin for which it was designed.

Al

Have you given it the six G hard landing static test? If your engine
weight
is 400 pounds six G's would be 2400 pounds. Assuming for the moment
the engine/psru/prop CG is in the center of all four mounts that
is 600 pounds down force on each rubber bushing. Not counting torque
reaction. I know you know this but we have a lot of new guys on here.

Possible it will be OK. Some of those old mount designs violate all the
rules. Due to years of experience and AD's & repairs along the way with
various
doublers and gussets they eventually become very reliable. My C 182
mount
is a good example. Plenty of short sections of doubled up tubing and
gussets.
These eventually add the required local bending strength to delay the
onset of
fatigue cracks indefinitely.

One can remove a lot of uncertainty by never putting a tube in bending.
In other words by designing the motor mount as a pure pin jointed space
frame.

I find that CG position hard to understand as the PSRU is 45 pounds
behind
the rear mount and the bare 20B is 260 pounds. Also your front mount
appears to
be several inches ahead of the front cover. Do you have a cast iron
water pump
housing on there? :) Well I guess the alternator could tip the balance
forward.

The CG of my 13B with alternator, starter and Tracy PSRU is very near
the back (as in the car) end housing. The prop will put it right
at the end housing.

Do you have the starter on there Al. Most wood props are about 30
pounds.

If the PSRU is 15 inches long and the CG is in the about middle of the
engine
(12") the moment due to the prop alone would be about 700 to 800 inch
pounds.

Paul Lamar

No; I haven't done the 6G load test.  Do you recommend I try that before
I fly it?

I don't think you will be doing aerobatics in your type of airplane.
As long as you don't drop it in from ten feet it looks like it will hold up :)
PL

No doubt about it; you're right in suggesting the best way to keep the
stresses low is to keep the tubes in compression or tension.  If I had
stared from scratch on the mount I would have done it differently.  The
Franklin mount was included in the Velocity kit, so modifying it seemed
to make sense.

One reason the CG is as far forward (toward the firewall, in my case) as
it is that very heavy intermediate housing in the 3-rotor is toward that
end.  Also there is the alternator, pullys, water pump, coils, oil sump
that I have kept toward the one end.  The starter is on.  I don't recall
what the 3-blade Catto weighs, but I'm sure it is less than 30#, maybe
20.  However, the 6" prop extension weighs a few pounds, and puts the
prop further back, so when I hang that on it will move the CG back a
couple of inches.

Al

 
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