I made the trap doors that go in the fuel tanks. The trap doors are a one-way valve that let the fuel flow from the outboard (higher) bays toward the inboard (lower) bay where the fuel pickup line is located and takes fuel to the engine. The traps are hinged and are free moving - but when riveted onto the side of the rib, it only opens one way. The hinged, which is on top is left longer and then bent over as a stop so the flap cant open too far. There are a total of 5 round holes in the 5 "middle" ribs - hence 10 trap doors total are needed for 2 tanks.
