Building with carbon-fiber tube

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Written by Rob Caso Scale
As seen in the Fall 2020 issue of Park Pilot

>> For me, scale modeling has been a somewhat evolutionary “experience is something you get after you need it” process throughout the 2020s. Lately, I have been taking a harder look at using pultruded carbon-fiber tube as part of a model’s basic infrastructure instead of merely as a means of reinforcement. There are two big problems with carbon-fiber tube: It doesn’t bend and it doesn’t allow adhesive to penetrate. On the plus side, it doesn’t bend and it is seriously strong and lightweight. A 1/8-inch section of carbon-fiber tube weighs approximately the same as a piece of somewhat hard, 1/8-inch square balsa. A scale benefit is that carbon-fiber tube can be used to replicate a prototype’s framework. My two latest builds, a 30-inch Free Flight, rubber-powered Fieseler Fi 156 Storch and a 52-inch Focke-Wulf Fw 56 Stosser, use carbon-fiber tube as part of the basic fuselage structure. The 3-ounce Storch’s fuselage is comprised of 1 mm tube. Carbon-fiber tube was also used to replicate the internal trellis of the Stosser, on which stringered bulkheads were hung.

 


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Stringered fuselages are notoriously twisty, so all of them on the Focke-Wulf Fw 56 Stosser have an internal trellis.

After having duplicated these structures with carbon-fiber tube, it’s easy to see why a welded, steel-tube frame was the basis of many aircraft of the day. It just won’t twist or distort, and it is incredibly lightweight, thereby eliminating tail weight—the nemesis of scale models. Many traditional models feature either a light plywood box or large-dimension square balsa framework, the latter of which can still flex, twist, or break and is only marginally lighter than plywood. A similarly configured carbon-fiber tube structure does away with all of these shortcomings. Sizing the tube is still pretty much a guess for me, but it somewhat depends on the overall size of the model and the spans of unsupported sections of tube. As an example, my 1/8-scale Stosser uses .096-inch tube for the longerons and verticals. This scales up to approximately 3/4 inch—which sounds about right to me. Although heavier-diameter tube is generally used for longerons and verticals, the angled, corner-to-corner bracing can be perhaps 75% of the diameter of the mains. When designing such a structure, remember that properly anchored triangles rule, and many full-scale airplanes were built this way for a reason. The general procedure is to construct a pair of identical sides then join them as you would a wood stick fuselage, using crossbracing and corner bracing to make a long, four-sided pyramid. Because you can’t easily adjust a carbon-fiber tube structure, it is imperative that each side be identical and that the sides are held vertically square to apply the crossbracing. I use laser-cut jigs for this, but they really don’t have to be that elaborate. A flat board with the plans over it and sections of square balsa defining where all of the straight pieces go would be fine. Position the sides at a perfect 90° with blocks or machinist squares to do the crossbracing.


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The bulkheads are located at the crossmember stations.

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Aluminum wire was used to join carbon-fiber tubes or assemblies to accommodate a sharp bend.

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Gussets were used on the Storch to increase the glue area and support the tube joints.

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This is one of the Storch’s framing jigs for the cabin bulkheads.


Carbon-fiber tube may be cut with a Dremel and an emery cutoff wheel, but be smart about this. Unreinforced wheels can grab and shatter, and the resulting dust is unhealthy to breathe. Safety glasses and a mask are required. I usually cut sections a little long, rolling the tube around while the wheel does the cutting. Go slowly and don’t force it, then sand the tube to the proper length and miter. It abrades easily. Sand up to the ends with 220-grit sandpaper to roughen it and glue it in with a dab of medium CA adhesive. The tube must be sanded wherever adhesive is to be applied. Although the CA is strong, it cannot penetrate the tube. After the framework has been inspected for trueness and accuracy, I go over all of the joints with a mix of fiberglass strand and 30-minute epoxy. This adds weight, so don’t use much, but you don’t need much anyway to get a secure joint. The fiberglass strand creates an interlocking matrix that acts as a collar to keep all of the tubes together. If you’re neat, it will actually look like weld. With the low vibration levels of electric motors, this will hold up well, although I also use small balsa triangles for 90° corners. For areas where the framework makes an abrupt bend or corner, such as where a straight fuselage section meets the tapered tail, I use the tube to my advantage by inserting small lengths of aluminum wire to negotiate the bend and to keep the assemblies aligned. I use CA on the one side and epoxy on the other, and I nick and/or sand the wire first. Such assemblies might be constructed and wired together flat on the board initially for alignment, and then oriented properly for joining. How it took me so long to try this I will never know, but this technique is now part of my go-to repertoire. Let’s see how lightweight you can go!


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The Storch is ready to go with folding wing halves and working landing gear.

 

By Rob Caso | quack916@aol.com

 

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7 comments

This is great Rob - would be even better with some close ups and video. Especially would like to see the way the fiberglass strand is used to join the parts. Thanks!

Would be good to see exactly what you did in terms of quantity and quality for the fiberglas and epoxy glue joints with some close-up shots, if that's possible somehow, somewhere. Fascinating topic for a guy that grew up building free flight rubber models and small glow powered free flight. Great article. Thank you!

"I go over all of the joints with a mix of fiberglass strand and 30-minute epoxy."
Elaborate or more photos please. Thanks, Jack

Nice Article Rob,I been using C/F rods on a lot of my Gliders for years .

Sal

Without RC control, rubber band planes probably crash often, I THINK. When carbon fiber tub models
Crash do they break at junctions of carbon and other more flexible materials?

It's not enough for aircraft structures to have just enough strength, without being overweight. Stiffness -- resistance to unwanted flexing -- is just as important. Thank you for your article explaining how both strength AND stiffness may be achieved without adding additional weight, by using carbon-fiber tubing. And you've explained well how to work with the material's quirks -- which all materials have. The ONLY thing missing is recognition that such tubing is WAY more expensive than equivalent amounts of even the best balsa -- but I would point out that the cost of materials for our models is always DWARFED by the value of our time building them. It's false economy to evade the expense of carbon-fiber tubing by making an inferior, 'flabby' structure.

Rob, I'm very happy to see this article and the successful use of pulltruded carbon fiber rod/tube as a structural element. This rod is available in sizes as small as .010" diameter and has miraculous qualities! It should not be delegated to simple roles like pushrods and wing joiners. The rigidity of this Storch fuselage must be off the charts! Not to mention, it looks fantastically scale from the inside out.

For my part I have been experimenting with commercial rods and also the indoor RC technique of creating rods from wetted tow (in 1K, 3K, and 12k bundles). With the wetted tow, I use it to "weave" a fuselage outline/truss pattern using steel brads treated with mold release on an MDF board. I will prepare about 20 feet of tow and take several passes over the pattern. This creates a fuselage side, ready to go, and I create a second side in another session. Then I glue them together (CA or epoxy) with commercial rods forming the truss of the top/bottom of the fuselage. This is a bit of a weak point, but the joints can reinforced with kevlar thread. This is still experimental but I have a test fuselage weighing just under a gram which is for a plane with a span about 24". It has some interesting qualities - despite being constructed "too light" (double the carbon would be more reasonable for its size), it is very stiff at light loads which are more likely to experienced in the air. Also, when exceeding the limits, the truss members buckle like little springs. I will toss it on a concrete floor and it bounces without damage.

Lots of good work to be done here!

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