Building With Carbon-Fiber Tube

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There’s never been a better time to be a flying-model builder! Yes, this is true. The materials and technology that are readily available today to amateur modelers (like me!) are astounding. Combine these with some hands-on building experience and maybe some dope and balsa and you have the makings for a great model—one that both flies well and stays together. These competing objectives might be brought closer through the use of pultruded carbon-fiber tube in our models.

Designing for Carbon-Fiber Tube Construction

I am primarily a Scale model designer and builder and, although I will use such models as examples herein, the ideas presented can be applied to nearly any aircraft. For example, the bane of any flying model is tail weight. An ounce too much necessitates countering weight in the nose, generally by a factor of three. Do you want to take 4 ounces off your model? Simple. Take an ounce out of the tail. This, and keeping things together, is where carbon fiber shines. A section of 1/8-inch diameter carbon-fiber tube weighs roughly the same as a piece of hard, 1/8-inch square balsa. But guess what? You can use a smaller-size carbon-fiber tube to get the same or greater strength. Although I have thus far used carbon-fiber shapes rather superficially for wing dihedral supports and struts, I have since begun to make practically entire airframes out of them. Having such a rigid primary structure allows me to skimp in other areas to save weight. A WACO biplane requires a lot of stringers aft to replicate its round, fabric-covered fuselage. The stringers add a ton of weight exactly where it’s not wanted. They don’t add much rigidity and because stringered fuselages are notoriously "twisty," and a stout internal framework is needed to counter any torsional movement. A light, rigid carbon-fiber tube internal trellis, not unlike the prototype’s, is the solution. I recently did exactly this on my 52-inch, electric-powered Fw-56 Stösser, which has a prototypic internal carbon-fiber trellis aft that is surrounded by quarter bulkheads and stringers. It came out so rigid that I could have used a smaller-diameter tube, which brings me to my next point.

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Tube size is a giant guess, but it is nevertheless an important strength-to-weight consideration. A good starting point is to determine what the prototype had and simply scale it down to the size of your model. This method will likely get you close to what you need. As an example, my 30-inch Free Flight (FF) Storch is 1/19 scale. If you assume a 3/4-inch tube was used on the fullscale Storch, this scales down almost exactly to the 1.0mm tube that I used on the model. I used the same methodology with my Stösser. In that case, I decided on 3/32-inch tube. While "real world" structural properties generally don’t scale well to our models, in this case, surprisingly, they do—or at least might. As a final comment on design, I would say that using carbon-fiber tube for frameworks lends itself more to electric-powered models because there’s little or no vibration trying to shake everything apart. Nevertheless, if the joints are sound, I see no reason why it could not be adapted for liquid-fueled models.

Working With Carbon-Fiber Tube

There are a couple of tricky things to remember here, the first of which is that carbon fiber does not allow glue to penetrate as balsa does. You’re stuck with a slightly superficial, mechanical bond that does not interlock with organic fibers. To counter this, the tube must be sanded thoroughly where it is to be glued and you must increase the gluing surface as much as possible without adding tons of weight. On the little Storch, I used tiny, 1/64-inch plywood gussets behind each intersection of two or three tubes, keeping these internal so that they wouldn’t be seen under the covering. I used medium CA adhesive for these intersections, and not very much. On larger models, the gusseting should be both behind and between the intersecting tubes. When the framework will not be seen when you’re done, use an epoxy/fiberglass strand "collar" at the intersections. For 90° interior joints, such as with the crossing tubes on the trellis, I glued in triangular pieces of balsa, orienting their grain along the hypotenuse. Glue surface is the key and, as a general rule, never use an unsupported butt joint. The next issue is that carbon-fiber tube doesn’t bend, but don’t forget to take advantage of the fact that you’re using tubes. Bends in a length can be accomplished via the insertion of aluminum wire or rod inside the ends of tubes to be configured into a bend or angle. No, you can’t bend the carbon fiber, but you can bend the embedded aluminum rod at the intersection. Use rods that "just" fit and do so without putting any pressure on the carbon-fiber tube walls. Use medium CA adhesive to bond it. It absolutely, positively, will not come out. Be careful, however, with overstressing the loaded end of a carbon-fiber tube, such as with landing gear. Because the pultruded fibers run parallel with the length of the tube, the tube can split at its ends. Solve this with a wrap of fine Kevlar sewing thread and CA glue.

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Keep in mind that carbon-fiber tube structures are very unforgiving of building errors and take more time to do. If one side is out of alignment even slightly, this error will magnify itself many times in other areas. My Storch was built on rather sophisticated, lasercut jigs with slotted tube-placement tabs all over the place. Even then, I had minor work around some areas because the tube was so small, but, in general, the jigs worked quite well. You must employ some kind of jig that guarantees that the sides will be the same under construction and be positioned precisely upon assembly. If you’re careful, you don’t need a laser. Hard balsa strip pinned or glued to the board in strategic locations would be a good method to establish fidelity to the plans, and simple 90° blocks will help with vertical alignment. What you ultimately decide on will be largely determined by the shape of what you are constructing. I will short-circuit the next question because I know you are going to ask: "Did I ‘fish-mouth’ the tube?" I didn’t do this on the Storch, which would have been nearly impossible, but I did do some on the larger Stösser, but only for the angled, corner-to-corner bracing. Don’t forget, the Stösser has quarter formers around the perimeter of the framework for the round fuselage, so these tend to function as more bracing and glue surface. Between that and gusseting, I didn’t feel that it was necessary. With larger models, I feel that fish-mouthing would probably be necessary.

Ongoing Projects

A major project of mine is the design of a 1/6-scale, 93-inch wingspan Storch with a carbon-fiber tube framework and electric power. This was the ultimate goal of making the small model, using it as a study to determine whether carbon-fiber tube could work. My calculations indicate that 11 pounds or less is doable and this represents a 3.5-pound difference compared with my existing 1/6-scale Svenson Storch.

Covering a Tube Framework

Under the I-never-did-that-before category, my little Storch required me to attach the covering directly to the tube framework. Full-scale pilots do this all of the time. I had some NLA CoverLite laying around, which is lightweight and durable, but it has no adhesive. I therefore "schmutzed" both the framework and the back of the CoverLite with reduced Sig Stix-It. Applying the adhesive to the covering was key, and this facilitated a good bond to the tube at the expense of some weight. A Hangar 9 trim iron is a must-have for covering small models.

Safety

Be careful when cutting carbon fiber. I use an unreinforced emery wheel in a Dremel to precisely cut the tube, and these can shatter without warning. This is especially true when cutting larger-diameter tubing that has more opportunity for the wheel to grab. Roll the tube as you cut it and let the wheel do the cutting. The wheels will still break, so don’t position yourself in the line of fire. Cut it long and sand it with a block to the correct length because it abrades easily. A final comment is that carbon dust is not good to breathe and safety glasses are necessary. Part of the fun of this hobby for me is pushing myself and learning new techniques and ways to solve problems. I find that the amazing technology and materials we have today actually enhance the foundational skills and techniques that we, as modelers, continue to develop by doing and trying.

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