Paladin:

Thanks for the info.
I doubled the rib thickness in the area of the split flaps, and then added 1/64 ply gussets. Maybe if I had used the ply throughout I wouldn't have had the problem.

I guess so much depends on the grain of the sheeting. In some areas my construction was fine.

I think next time I will either use solid tapered stock or add the same thickness balsa sheeting underneath the overhang.

Were both kits, the Zero and the FW, designed for split flaps, or did you kit bash?