Finn Lassen wrote:
I visited Tracy today (Such a nice day that I had to fly somewhere).
The subject of the modifications to his mount plate came up. He's very worried about the
suggested modifications (extending the plate to sides for engine mount points) and even more so
about the milling of pockets in the plate. It's not a question whether the modified plate can
carry the static and G loads, but if it will be able to handle the torsional (gyroscopic) loads.
Please consider the loads during pitch, yaw and bank changes. You have big spinning disk out
front (how much depends on prop weight and RPM) - a gyroscope that's trying to twist the mount
plate.
When I asked Tracy why he hasn't been more verbal on this list regarding this, he said he has
already posted his warning here. I think it would be wise for those of you considering these
modifications to realize the severity of Tracy statement of "you're on your own" if you modify
his design and that's he's not kidding.
Maybe these modification will hold up on a low-G non-aerobatic gently flown airplane with a light
low RPM prop. I don't know.
If someone here has the knowledge and tools to do an actual analysis of the gyroscopic loads, it
sure would be nice to have that done before someone actually flies one of these and kills
himself.
Finn
Anybody care to do an FEA on this?
With a 40 pound metal prop at a pitch rate of one radian per second (typical) the torque
on the plate is 610 foot pounds. Page 437 Sky Ranch Engineering Manual.
That is equivalent of having the airplane nose up at 30 degrees and dropping the nose
down through zero to 30 degrees nose down in one second. BTW that is enough to
bend the prop flange on a Lycoming engine.
See also page 217 & 242 formula 5... Formulas for Stress and Strain R.J. Roark.
Chapter 10. "Flat Plates". A classic work in the field.
For the stock plate.
Assume a round PSRU plate with radius Ro of 6 inches.
Assume Tracy's PSRU is six inches in diameter (Ri = 3) where
it bolts on to the center of the round plate.
Assume worst case unsupported edges.
Max angle in radians is M/a X E X T^3
M is in inch pounds 600 foot pounds 7200 inch pounds.
E is 10,000,000 for aluminum
T^3 is .125 inches = .5 X .5 X .5
"a" is 6 for unsupported edges and Ri/Ro.
See page 242.
7200/7500000 = .00096 radian.
57.295 degrees per radian or .055 degree.
That is half of one tenth of one degree. I would be surprised if
you were able to measure that directly.
Check my references and my arithmetic. Please!
The current twelve inch wide half inch thick plate is way way overkill
as are a lot of other parts in Tracy's design.
You can measure the deflection with a dial indicator if you wish
by hanging a 600 pound load on the end of Tracy's PSRU one foot out from
the plate. Make sure you also measure the vertical deflection of the
spacers when you do this as any shear deflection in the spacers
relative to the engine will erroneously appears to be an S shaped
deflection in the plate.
I am not worried at all and I am really sorry that Tracy's feels like
that. I don't think he made any calculations on its stiffness or strength.
I am doing this on my airplane mainly for cosmetic reasons as I think
the current plate looks crude and unprofessional.
The weight savings is minimal at less than three pounds.
The pockets represent a missing metal disk with a six inch hole in it 12 inches
in diameter and 3/8ths of an inch thick. It would have an area of 85 square
inches, a volume of 32 cubic inches and at .1 pounds per cubic inch for
aluminum would weigh about three pounds. Actual weight savings would be
less than say 2.5 pounds.
Paul Lamar
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