Making small part from carbon

[Andy W]

I want to make a small yoke for folding prop blades on a small sailplane - the standard Graupner plastic part is not going to work. I was thinking of just making a small mold from wood pieces that break apart, coating them with paint, waxing, then stuffing the assembled parts with chopped up carbon tow saturated with epoxy. Is that all there is to it?

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[soholingo]

Andy,

I would think building the yoke would have the same caveats as building a prop. Somewhere you would want to put in some Kevlar tow to reinforce the area the props attach.

Also if the the graupner part is the right size but not strong enough I would use that as the mold, and then use a pourable silicon to make the mold. www.smoothon.com has a lot of info on molding these sort of odd shaped objects. look up oomoo-30.

As far as people who would know how to do this try David Fee and Wolf F.

J

[Andy W]

Thx, jay..

[Andy W]

Ack. Looked at the prices of that stuff and decided to come up with another solution. Used some pre-fab carbon plate I had to make up what I needed. Took less than 30 minutes, plus time for epoxy to cure. 9g for the yoke & blades, 25.8g total for what you see..
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[sarmoby]

Just an FYI-

On a highly loaded bearing application like this it may make sense to pot a metal bushing for the screw to go through. This will spread the load of the screw out into the plate material without causing as much of a stress riser. Any cavitation/vibration in the blade will be transmitted back to the plate and can cause a stress cracking action when superimposed on the high centrifugal forces. Last thing you want is a blade to come loose at speed.
On helicopter parts the bushing end is often made larger and is wrapped with continuous tow to carry the loads back to the hub.

Hope this helps
Scott

[soholingo]

Ack. Looked at the prices of that stuff and decided to come up with another solution. Used some pre-fab carbon plate I had to make up what I needed. Took less than 30 minutes, plus time for epoxy to cure. 9g for the yoke & blades, 25.8g total for what you see..
..a

Man that looks good. Have you tested it to see how it holds up?

Keep us posted.

[Andy W]

Thx. I have run it up and it's fine, but I'm going to wrap the center section with tow to hold everything together. Again, this is a pretty low power application - 36W peak..
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