Covering carbon airframes

Martin Newell

August 2004

Here are some techniques useful in covering airframes made of carbon rods, with particular attention to the wings. First the covering material. I have had good results using Reynolds Wrap food covering film. It comes in a variety of colors, though they are not very intense. The material weighs about 16 g/sq.m, so it is not as light as some mylar covering materials that can be as low as 2.2 g/sq.m.

Now the adhesive. Some people recommend using a glue stick, but I don’t get on well with that. I use a product called Dave’s Flexament, which is intended for sealing the heads of fly-fishing flies. It is a thinned version of a popular household glue called Goop. I’m sure other types of glue would work as well, as long as they can be applied thinly with a brush, and dry to a slightly tacky surface. Contact cement would probably work, but may need to be thinned.

The technique. Cut a piece of covering material an inch or two larger than the frame to be covered. For a wing be sure to orient the chord of the wing across the roll, i.e. the span of the wing should be oriented along the long dimension of the roll. The material has a grain and it is important to get this right for proper shrinking.

Attach the covering material to a springy surface, like a soft blanket or a sheet of foam, using masking tape at the corners. It need not be stretched, but should be smooth. I have used a frame with a 1/2" sheet of foam inside. The edges of the frame are handy for attaching the masking tape.

Paint the boundary of the frame with a thin coat of adhesive and leave it several minutes to dry. If there is any wet glue it will crinkle the covering material. If in doubt, leave it a little longer. When dry, drop the frame onto the covering material. Gently push down on the frame all around to ensure good contact. The springy surface helps here.

Remove the frame with attached covering from the springy surface and turn it over onto a firm surface. Using a razor or very sharp modeling knife, cut away the covering material around the outside of the frame, very close to the frame. Do this by running the knife against the frame, angled out a little from the vertical, while pulling the excess film away from the frame. Run your finger around the outside of the frame to ensure the edge is stuck down. The covering material does not wrap all the way around to stick to itself. I have only had the covering break loose once, and that was the leading edge of my IFO after about 50 crashes.

For flat surfaces like the tail section you are now done, unless you have some minor wrinkles. For these I use a small electric fan-driven room heater on low to apply a little heat which shrinks the material. I find hair driers much too powerful.

For wings we need to apply dihedral and shrink the material to create an aerofoil. I like to add a rib to each side of the wing. This gives a better aerofoil out towards the tips, and provides small posts around which the rigging can be wrapped. The rigging is set up as a single loop of monofilament nylon with a slip knot. Monofilament nylon is available as “leader material” from fly fishing stores. I use size “6X” which is 0.005” diameter. Flex the wing to a dihedral shape and mount the nylon loop as a figure 8 over the four posts at the wing ribs. The slip knot should be tight enough not to slip by itself. Tighten or loosen the loop to give the required dihedral, and remove any twist from the wing. Put a small drop of CA on the slip knot to lock it, but don't glue it to the frame yet.

Using a suitable heat source heat the material. All wrinkles should disappear, and a cambered aerofoil should develop. If it doesn’t then you probably have the direction of the grain in the film running the wrong way, so you will have to strip off the covering and start again. Assuming all is well with the shrinking, tie a short piece of monofilament around the figure eight where it crosses over in the middle. This reduces the ability of the wing to get twisted in a crash or other mishap. Double check for any twist in the wing, and correct if necessary, using a little heat, then apply a very small drop of glue (CA works) at the four points where the monofilament passes over the posts, and at the cross-over knot. The result is a very strong, light wing, resistant to warping and able to withstand repeated unexpected encounters with the ground and other static objects.