Cutting the slot and the spar

I had originally thought of adding a wing spar for extra strength, and have now decided to do so. Jack Devine says it's OK to do, but not necessary on this size plane, but I want a little extra insurance since I'm putting a G62 on this bird. Due to the route that I had to run my retract airlines, I couldn't go quite as long with the spar as I normally would. I ended up with a 17" long spar, which should be plenty and won't weigh too much. It crosses the main wing joing in the center as well as the two inboard-outboard joints, and is the full depth of the wing, so it should be fine.

First step was to cut a 1/8" x 17" slot out of the balsa sheeting on the bottom side of the wing. It runs parallel to the wing and sets just behind the rear end of the wheel wells, adding some strength to that cutout area. Next I used a hot wire made from coat hangar heated with a blow torch, and melted out a groove/slot in the wing. The slot is the full 17" length of the spar and goes nearly all the way to the top side of the wing.

I then cut a 17" spar from 1/8" aircraft ply, making it follow the same basic dihedral angle of the wing, which is the tricky part. It took a little trial and fitting, but I eventually got it there by measuring angles, melting foam, checking depth and repeating until it was right. I made the spar about 1/2 inch taller/deeper than the wing so that I'd have excess sticking out of the spar slot.

Next I slide the spar in as deep as it will go and mark a line along its length showing where the wing bottom surface meets the spar. This would show me where to cut the excess off.

After cutting the excess from the spar, I masked all the way around the spar slot in the wing to minimize glue overflow on the balsa. I coated the spar with Gorilla Glue, misted it with water and slid it down into the slot. The excess glue oozes out of the slot onto the masking tape and can be easily wiped off right away.

Once the water starts the Gorilla Glue to "kick", it will begin foaming up dramatically. This ensures that it will fill all the voids and gaps in the slot, and actually penetrates "into" the styrofoam itself making a good solid joint.

While the glue foams up out of the slot, I just keep an eye on it for a while, scraping the foam off with a small card (playing cards work fine for this). Eventually the glue will stop foaming and you can pull the masking tape back off. Be sure the glue is done foaming, but not fully dry or it will get hard and make it difficult to remove the tape