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aviation pros!

New to the aviation/RC world and would like to ask a serious question about using composites for the skin of a UAV I am developing my first time out.

Want to use carbon fiber for a planned UAV approx 6-8 feet in length and weighing in at about 200 lbs powered by electric motors. But, instead of making a carbon fiber skin from a stryofoam (or similar) mold, I am thinking of printing out the skin (using a carbon fiber filament) from a large format 3-D printer as separate panels up to 20" x 20". They would then be attached to the UAV's metal airframe (of a semimonocoque style).

It just seems like a much simpler way than molding the skin from a plug. Yes, I am concerned about the necessary strength and elasticity needed from the skin but I am hoping that a well thoughtout design for the panels (and especially for the nose and empennage) along with a CF/nylon filament will provide that. I have also considered the production time and filament costs associated with this method, but it just seems so darn simple and easy.

But what do the pros think.

Thanks.

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"Carbon fiber filament" for Fused Deposition Modelling (FDM) printers doesn't have the properties you believe it does. While perhaps a little stronger than a plain filament, in comparison to traditional items made of carbon fiber it is actually quite brittle and easily snapped, because the fibers are all chopped up into short pieces and mixed with the plastic resin.

To build something useful for an aircraft, you need fibers of meaningful unbroken length - either large pieces of cloth, or to run a continuous filament back and forth many times.

There have been thoughts of making FDM printers which would place a continuous carbon filament along with a thermoplastic "printer filament" as a binder, but that's not what a roll of "carbon fiber printer filament" gets you. Also note that even a filament-stringing machine would only make structures which were strong in the plane the layers, but not against delamination between layers; to get a strong 3d shape like an aircraft panel, you would need a six axis machine which built in a way sensible for the shape of the component being built.

You could potentially use some ordinary or soluble filament to print a mold for conventional carbon fiber layup.

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  • $\begingroup$ Wonderful answer! Part of what you wrote I was aware of--that aircraft skin requires longer fibers than the particles of fiber in filaments. But I thought that maybe by thicken the panels it might compensate for the weakness inherent in filament fibers. And someone on YouTube had that problem of brittleness when trying to print an AR-15 lower receiver even w/ 85/15 CF to nylon. However, I was toying w/ the idea of using the printed panels assembled together as a complete fuselage to use as a plug for a CF mold. I'm glad to hear that's possible. Any more thoughts on doing that? $\endgroup$ – VSS Defense Jan 3 '19 at 8:41
  • $\begingroup$ @VSSDefense: Short fibers will have random orientation while long, unbroken fibers will be oriented along the direction of maximum load. Much of the chopped-up fibers will not contribute to the desired strength because they have the wrong orientation. $\endgroup$ – Peter Kämpf Apr 12 at 13:32

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