Charlie P.

I ran the two plywood & balsa wing joiner blocks across a table saw to form a slot between the plywood and the balsa about 1/8" wide and 5/8" tall (leaving the plywood and cutting only the balsa). Into these slots I epoxied lengths of fiberglass strips - they were from the sail battens of a windsurfer. Flexible but very strong. Any place that sells small sails - as in for kayaks, windsurfers or sailing dinghys should be able to find you some cheap or free.

The flocking I use is milled fiberglass fibers - basically shredded & powdered tiny glass fibers. Added to epoxy the tiny fibers add quite a bit of mechanical strength; especially shear/tensile. From the images earlier in this thread it looked to me like the wing failures came when the upper surface of the block compressed and the lower stretched - allowing it to shear right along the wing seam. I affixed the fiberglass battens so they resist vertical loads and will not allow that to happen. Carbon fiber is higher tech than I get into. Certainly lighter, but for this application I figured fiberglass is good enough.

I painted the wing sockets and blocks in the epoxy mix with an acid brush and poured in a good amount in the wing, then pressed in the blocks. Quite a bit oozed out but, I hope, the result was no air pockets and a very solid plug. I set the strips in the blocks with 30 minute epoxy but used West System & 205 hardner to set the blocks and along the contact areas of the wing roots. That takes much longer to harden (four hours or more) and soaks into the wood better than the faster setting epoxies.