Carbon rod airframes

Martin Newell

August 2004
Updated November 2005

While balsa is the traditional material for building model airframes, carbon rod offers the advantage of great strength and flexibility. However, it presents some challenges in attaching pieces together. The methods presented here are intended primarily for 0.020" (0.5mm) carbon rods, though they will work for other sizes too.

Joints

The task is to join pieces of carbon rod where they meet at an angle. Three types of joint are described: corner, T junction, and crossing

I have found two methods to work well. The first, and most obvious, is to bind the pieces with thread and apply glue. Thin multi-strand nylon thread works well. CA works well for joining carbon rods. However, beware of using CA accelerator because the heat generated can weaken the carbon rod, sometimes causing an instant failure if the rod is being bent into a curve. Binding with thread works best for crossing rods. It can work for T junctions by first making a crossing joint, then snipping off the unwanted part with a pair of small wire cutters followed by filing. Corner joints can be done similarly by clipping off the two unwanted pieces, but this does not give a very good joint.

A more secure method is to use small lengths of shrink tubing or plastic tubing from wire insulation to reinforce the joints. The plastic insulation from 32 awg wire works well for 0.010" rod. For joining 0.02" rods I use 3/64" shrink tubing.

The simplest is the corner joint. First shrink a short length (say 1") of shrink tube onto a 0.02" piece of wire, using a small flame. Pull the wire out of the shrunken tubing until the shrunken tubing extends past the end of the wire by about 1/8". Insert a second piece of 0.02" wire into the tubing. Using the two pieces of wire, bend the shrunken tubing to the desired angle. Gently re-heat the angle with a small flame, and hold the angle for a few seconds until the tube re-hardens. Remove both wires and trim off the unwanted part of the shrunken tubing, which can be used for more joints. You can now push two pieces of 0.02" carbon rod into this corner piece. You may not want to glue the joint yet if you are just doing a trial assembly. Gluing can be done either by putting a drop of thick CA on the opening and then quickly pushing in the carbon rod, or by using thin CA which can wick into the joint.

If using plastic insulation tubing don't use any heat - just bend the tubing after inserting the carbon rods.

T junctions can be made by starting with a corner piece as above, then using a pin inserted into one opening, push through a hole. Remove the pin and replace it with the carbon rod that forms the top of the T. The other piece of carbon rod goes into the other opening to form the leg of the T.

Cross junctions can be done similarly by pushing a pin through both openings of a corner piece, but they have to be offset. Heating the corner piece will assist the distortion needed to produce the offset.

Other types of joints can be made with combinations of the above joints.

Wings and other curved shapes

Another challenge presented in using carbon rods is forming curved shapes, like wings. If you look on the plans to Martin's Mono or Bipe you will see a line on each side called "Shape Restrainers". These are pieces of nylon tied between the leading and trailing edges. They are located 0.85*C from the wing tips, where C is the chord dimension. This location holds the rod in a curve where the leading and trailing edges would be parallel even with no other constraints.

This still leaves the question of how to put it all together without 4 hands. I have found the following method effective on a variety of wing sizes. Bend the rod into a circle and join it with a piece of plastic or shrink tubing that is 10 times as long as the diameter of the rod. Squash the circle and stick it to the wing plan using 4 or 6 pieces of masking tape. Don't put tape where the nylon shape restrainers go. Tie on the two shape restrainers and attach them with dots of glue. That's it.

The reason for making the joining tube 10 times the diameter of the rod is that we are trying to get the stiffness of the joined rod to be about the same as the stiffness of a continuous piece of rod, so that when we bend the wing to form a dihedral the wing will not be too twisted, and 10x usually works. It never works exactly, sometimes it comes out a bit too stiff and sometimes too flexible. This is a good thing. After covering the wing and bending it to dihedral, getting it to be symmetrical, and shrinking the covering, examine the wing. One of the two edges will almost certainly be bent more than the other. Make this the trailing edge. This gives washout, which is good for stability in turns.