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 outlandish 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 to .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

Will anodizing be as good as the process used on grumman GA? THANKS.
L F WINGS.

I don't know what they used on that airplane. You can buy a kit in West Marine to treat aluminum for epoxy bonding.

How long is the prop shaft?

Paul Lamar

Isn't there a vibration issue with anodized parts. There was an award winning Experimental helicopter where the builder had anodized a lot of parts for a blingier look, including I think the drive shaft. After about 60 hours the blingy part failed catastrophically causing a complete wreck of the machine. IIRC the anodizing process introduces microscopic stress risers. Feel free to correct me if I've got this all buggered up.

John Gibbons


The aluminum tube is just a male mold for the carbon fiber.

BTW when you have a high modules of elasticity material (carbon) sharing the load with a low modulus of elasticity material (aluminum) the high modules material takes most of the load.

Paul Lamar

But when you attempt to adhesively bond two materials with widely different Elastic Modulii you are asking for bond line failures - Ask me how I know this Roland Friestad

I agree but not only that. When thermal expansion coefficients differ you can also get bond line failures.

Again we are only using it as a male mold.  The carbon will take all the load.
 It is too hard to remove the plug so it can be left in there. If the bond fails it will act like a torsional damper :)

Paul Lamar



It would be better to use a tube of the correct diameter and slice it accurately longitudinally and use it as an OUTER mould. Use pre-preg carbon and lay it up around a partially inflated tubular balloon. Clamp the outer mould in place and inflate the balloon whilst warming the outer mould with a heat gun. When cured remove outer mould and balloon and you should have a perfect carbon tube.
   Chris in NZ.


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