Hi Paul. I am revisiting the long drive shaft for a pusher design.
With the progress made on composite drive shaft, is the option of
long and light prop shaft becoming more of a reality now? I saw a
lot of automotive carbon shafts on the market now for very decent prices.
Could a simpler design with rubber couplings instead of power/slip
clutch be made to work without outlandished development cost and
headache? I would appreciate your thought.
Luke F Wings.


How long?

I would use a minimum of 3.5 inch diameter aluminum tube.
Cut the wall thickness on a lathe to  .062" to save weight.
Then anodize it so epoxy will stick .
diagonally wrap carbon fiber to a total thickness of .125 tp .2 inch.

If not too long get a stock  RX8 carbon drive shaft from a junk yard.
Yes rubber couplings [are] the way to go.

Paul Lamar


Luke,

Please consider using a thin layer of fiberglass as the first layer
next to the aluminum.  Even with a good epoxy, this will ensure there
is no galvanic corrosion between the carbon and the aluminum. A forty
five degree angle to the longitudinal axis will ensure the fiber wrap
is in tension during load.  The large diameter will save weight and
the higher the driveshaft rpm, the more torque that will be transmitted.

Would it be possible to put the PSRU on the opposite end of the drive
shaft from the engine?  Maybe more damping could be incorporated
that way?  In any case, the torsional resonate frequency of the
drive shaft must be much greater (or much lower) than the torsional
vibration of the engine.  Otherwise you end up with a BD5 scenario.
This is difficult throughout all the RPM ranges of the engine.
That is why the BD5 eventually had to go with an overrunning clutch
after many forced landings at the Newton, KS airport where I used to work.

Regards,

Norm Deboer


Good points.

Paul Lamar

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