George,
I have attached jpg files from the FEA test. I did do several versions of
this design to come up with one that had minimum stress as well as low
weight. Incorporating the front spacer of Tracy's drive ended up being key
to making this part really strong. It made the outer stiffening ribs much
deeper and they transmit the loads to the outer area of the bell very well.
This design ended with far less deflection than the original design.
The loads applied are 600 foot pounds of positive G load (6G's on a 100 lb
prop, or 12G's on a 50 lb prop), 250 pounds of thrust and 20 pounds of
asymmetric thrust (decending blade theory). The thrust load has nearly zero
impact on the stress loads. The factor of safety on the material is at 1.9
and then only in the upper corner fillets of the stiffening ribs. The total
deflection at the prop hub at this load is 0.011 inches. Stresses are low
with virtually no additional stress from the cutout (see stress distribution
plot).

And have a very Merry Christmas down under!
Larry

Larry, I think you need to rerun those FEA's. When the airplane pitches
in a stall the 600 foot pounds of bending loads due to gyroscopic reaction
are 90 degrees the vertical plane. In other words side to side.
You will probably need a larger radius around the big hole and reinforcement
around the edges.

Paul Lamar ...No rotor no motor.

Paul,
I would support that statement for the only reason that all openings should
have reinforced (thickened) at the edges. I notice this is done on all
commercial housings and is something an old engineer drummed into me.
George (down under)

Paul,
Ran the FEA with the 600 ft lbs at 90 deg to simulate a full throttle
accelerated stall. The safety margins were down near 1, so I have added a
stiffening rib around the opening and increased the inside radius.
Information sharing like this is why this forum is so great from the
engineering side. I'll send you results as soon as I am happy with them.
Larry

This problem came up awhile back. Here is one suggestion.
Ed Anderson has been flying something like this done to
a Ross bell housings casting.

Paul Lamar ...No rotor no motor.

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