Who hasn’t dropped a wing on landing or landed out and treed your composite model, snapping the boom? My experience is that most mishaps where the model is substantially damaged seem to also break the boom generally midway between the wing’s TE and the stabilizers.
Being so far behind the CG, it is important that the least amount of weight is added during the repair or your model is going to be compromised in performance and need added nose weight to compensate.
Let’s discuss a repair procedure that is adequately strong, yet adds less than 3/4 ounce to the overall weight of the model.
The first step is to assess whether the stabilizer and rudder pushrods need to be replaced. I typically remove the old ones and replace them in any case, because the originals are often .050-inch stainless steel hypodermic tubing and they get bent during crashes. New hypodermic-tube pushrods can be obtained from Graves RC (see “Sources”).
I use .050 x .030 x 60-inch tubing. Disconnect the clevis at the servo end and either cut or de-solder the clevis end to allow you to remove the pushrods from the rear of the fuselage. Fold your rudder back against the fin and tape it there so you can remove an inch of the balsa rudder post in the back of the fin to gain access to the stabilizer bellcrank. Unhitch the clevis and remove the stabilizer pushrod.
Inspect the pushrod outer sleeves and replace the sleeves with new ones available from CST-The Composites Store if your new pushrods don’t slide freely through when the broken fuselage is aligned.
The first step is to accurately align the two broken fuselage halves and temporarily bond them in place with thick CA. I use Zap thick-viscosity glue.
Attach the wing’s center section to the fuselage and suspend it above a bench or table with a box under each side so the wing is parallel with the bench surface and the towhook is not touching down. Attach the stabilizers and set the rear piece up with the correct thickness spacers over your bench so you can align the two halves.
Tack the break with thick CA on the top and then, using a straightedge to check alignment, tack the bottom. Before tacking on the side, use the straightedge to ensure the center seam is aligned and double check it by measuring from the ends of the center section to the tips of the stabilizers.
After you are sure the two halves are perfectly aligned, thoroughly glue the broken area with CA and spray it with accelerator before dismantling the model from your fixtures.
Remove the stabilizers and center section and carefully use a Dremel tool to grind or sand the area to remove excess or protruding CA from the joint area. Tape off the fuselage with two layers of masking tape, at least one inch wide on either side of the break.
With 360-grit sandpaper, carefully sand the paint from the fuselage between the masking tapes. Using a Sharpie, mark 12 lines evenly spaced around the fuselage, across the break and parallel with the fuselage.
Use your Dremel tool with a 1/2 to 5/8-inch diameter, and 1/8-inch wide grinding wheel to grind a shallow groove along each of the lines. Cut 12 pieces of carbon tow the same length as your grooves and using CST West System G-Flex epoxy in each groove, affix each piece of carbon tow in place. Blot the excess epoxy with a paper towel and tape the area with electrical tape.
After the epoxy has cured, remove the electrical tape and carefully sand the repaired area with 600-grit sandpaper to take off any high spots.
To complete the repair, a single wrap of carbon cloth should be cut and applied over the joint area with epoxy, making sure that you blot up all excess epoxy. Finally, wrap the joint area neatly with insulation tape and allow it to cure.
After it has cured, the insulation tape can be removed. Sand the area lightly with 600-grit and then 1,000-grit sandpaper. Be sure not to sand through the masking tape adjacent to the repair.
The result is now stronger than the original fuselage and, providing you were diligent with blotting excess epoxy, it will require little nose weight to rebalance the model.
Reinstall your pushrods and go fly.
Go downwind and soar.
