Another simple modification to increase the spar holes strength. I notices that after just a few flights (and a few hard landings) that the spar holes in the wing started to stretch. To fix this problem, I reinforced the holes with fiber washers. (Note that these pictures were taken after I had already done this modification)
1. Hobby knife
3. Two fiber washers (5/16" ID x 3/4" OD)
1. Place the spar inside the hole and the washer around the spar so that the washer rest on the root end of the wing. The spar should be fully inserted into the wing so that the washer will be well centered.
2. Make a shallow cut around the washer into the wing root to outline where the washer will be installed.
3. Remove foam as needed so that the washer will sit flush to the root end.
4. Apply CA on the foam where the washer will be located. Install the washer and let dry.