Dan Tandberg, MD
Sailplane wings that are built up from balsa wood and covering can be a little tricky to repair if the leading edge is sheeted back to the spar. This construction method is usually referred to as a “d-box”. The defect is often the result of hitting a wire fence or tree branch with the leading edge, resulting in a crush type injury. In many cases the main spar is undamaged. Here is my approach to this repair.
First cut away the crushed leading edge and sheeting using a razor saw and model knife using a straight edge. Try to make the defect as close to a rectangle as possible. Save any salvageable ribs pieces. Inspect the spar with good lighting. If there is any cracking, repair it before proceeding.
Make a replacement leading edge, sand it to a perfect fit, and glue it into place. Brace and gusset as necessary. Make new replacement ribs or use the saved rib pieces and glue these carefully into place. Glue balsa “steps” on the back of the leading edge to provide a surface for the front edge of the new sheeting to adhere to. Glue small balsa pieces to the underside of the existing sheeting, also to provide a gluing surface. I hold these in place with bent forceps from underneath and use thin CA to stick them.
Now tape a smoothed piece of aluminum foil over the top wing defect. Use a square ended stick of medium hard balsa to rub along the edges of the defect. This will crease the foil and create a template.
Tape the foil to the balsa sheeting that will be used to cover the defect. Use a pin to perforate the foil at the edge marks every 1/16” or so to serve as a guide to cutting and sanding to fit.
If you cut the balsa carefully, you can achieve a perfect fit with very little edge sanding. It sometimes helps to bevel the front edge of the sheeting.
Once the d-box sheeting fits perfectly, it is glued in place and held with pins and tape as shown. Woodworkers glue works best in my experience since it has a long working time, and sands very easily. The bottom defect in the d-box is repaired in exactly this same way as the top.
After the repair is dry, use balsa filler to fill any minor cracks or blemishes. Masking tape should be used to protect the old covering during careful final finish sanding with a block. You should not be able to feel the seams of the repair with your finger when you've completed sanding. Recover the repaired defect with covering that matches the original as closely as possible. Leave ¼ to ½” overlap to assure good adhesion. Note that in this case I did a poor job matching the balsa sheeting grain and color. Your results can be much better than this.
Ready to fly again!!!