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Though carbon fiber is more expensive than aluminum as a base material, one of its biggest costs is creating the final composite parts made of carbon fiber (molding, layering, resin, curing, baking, etc). It is very time consuming and demanding.

As they do with aluminum tubes, why don't aircraft manufacturers simply buy prefabricated carbon fiber tubes (built in large quantities and at a cheaper price), cut them, use joints/unions, and use them in aircraft, especially those with low weight limits like ultralights, Light Sport Aircraft, etc?

Aluminum tube frame

join/union of carbon fiber tubes

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4 Answers 4

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If you're going to build from carbon fibre, you generally aren't going to bother with a very "old fashioned" structural method like tube and fabric; you're going to go with the natural advantages of composite construction by building a monocoque structure with carbon cloth and roving instead of fibreglass, and if you're really smart, Graphlite pulltruded carbon rod for key structural elements like spar caps.

There's also a certain amount of hassle in that you need to create joints that have the same strength as the tube, which requires fittings that are a bit more complex than the molded plastic fittings pictured. Bicycle frames do it using layups of carbon tape around joints, but they don't have to cope with the sort of multi-tube joints you have with a 3 dimensional warren truss fuselage. A bit of trouble to design and control, quality wise.

In any case, it's tube and fabric, not considered optimal for modern designs. It's pretty easy to cut and fit steel tubes to come together in a cluster of 5 or 6 tubes all meeting at a single point, and welding the whole mess together. That kind of intersection in CF tube would require an elaborate moulded or machined joint fitting.

You're basically building an 80 year old design, just replacing the steel tubing with carbon. More trouble than it's worth when you can make something more efficiently in a female mold using standard composite methods and have a ready to use part when it cures.

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They sometimes are. The Carbon Corsair is an ultralight built with a fabric-covered frame of CFRP tubes for the fuselage.

enter image description here

This is an all-new design. Using CFRP tubes allowed it to have a +6/-4 g limit while fitting into ultralight weight limits. There's even an electric version, which still fits.

As to why CFRP spaceframe construction is rare, compared to semi-monocoque, is that the latter is even better. There, CFRP also adds high stiffness for a load-bearing skin, and the ability to make very complex shapes (at a price).

Early carbon bicycles (and very few modern ones) also used lugged construction. It has been replaced by monocoque frames since. And that's bikes, which only need the frame, not the skin. For aircraft, CFRP semi-monocoque allows for a lot more design flexibility - below is one example.

enter image description here

Other than not being as good as CFRP semi-monocoque, or as cheap as all-metal, using carbon tubes is a perfectly valid technique. Just perhaps rarely optimal.

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In addition to the other great answers, consider some drawbacks of carbon. E.g. Fibreglass will be more wear resistant than carbon fibre, which is why you prefer it on leading edges, etc. (see first picture)

Also the joints you mention are not so trivial, and are likely the weakest parts of the structure. The loads your plane experiences in usage will be complex, so you can better accommodate your needs with a tailor-made structure, with varying thickness and material choice (including the fibre orientation, which is VERY important, and mostly chosen for you in a tube - see image 3).

Last but not least, your new (but inferior) structure needs to be tested again before the plane can go into service.

There are developments where carbon stringers are used, which is a kinda-yes-but-not-what-you-thought to your question :) (see image 2) Airframe materials distributions and percentages for the Boeing 787

carbon stringers on the inside of a fuselage section

Strength vs fiber orientation for different materials

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CIvil airliners are already using "Composites" in construction. Despite it's benefits carbon fiber has a notorious expense to maintain.

  • Carbon fiber is considerably more expensive than aluminum.
  • Carbon fiber has lots of rigidity, however it is a brittle substance. YES you can break it. enter image description here Since it is a baked fiber product, repairing it is basically a rebuild.
  • Aircraft need a certain degree of flexibility or creep reflex. Smaller planes Cessna, 3-10 passenger size props alrady have extensive use of CF. they're more rigid. Lots of planes like 787 Dreamliner, A380 are already moving to new composites.
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  • $\begingroup$ You can repair carbon fiber, and you probably would have to, if something were to strike your plane. (although it is very labour intensive) Interesting side note. It is sometimes VERY difficult to gauge the damage to a carbon fiber part from the surface, which is why we test with ultrasound for delamination, as the damage will usually be more extensive on the back-side from the impact, where you wouldn't be able to easily asses it. $\endgroup$
    – Marco
    Feb 8 at 9:04

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