This little triangle fills in the wing root area aft of the wheel well housings. It needs to be thinned down until it is a near wafer, and has to be trimmed back on the side facing the wheel housing to get it to fit flush
with the skin and in line with the rib detail. The style of instructions is not helpful as there is no suggested order of assembly within each section of the instructions. The wheel well housings should be installed first,
using the floor as a guide to alignment. Get them to fit as flush to the wall as you can because any gaps here will prevent the proper fit of the floor and overhead bulkhead. Then the triangle above should be
installed, followed by fitting the floor and the the overhead bulkhead that fits above and between the wheel housings. Dry fitting is needed at each step as the fuselage would not come together freely until the floor
was trimmed a little along the sides an in the cutouts where the wheel housings fit. A little filing was done on the top of the overhead bulkhead, and at the cutouts that fit against the wheel housings.Not much is
needed but if you don't it will result in a difficult fuselage seam. Next the fuselage frames can be installed, and then the heater ducting. Everything was glued to one fuselage half during the fitting process. It was
necessary to remove a little material from the front of the floor before the cockpit and nose wheel assembly would fit properly. Just for insurance about 1/32 was removed from the front of the cockpit assembly to
be sure the nose weight would fit without interference. The cockpit glareshield had to be trimmed on the front and sides, and the area of the fuselage under the front of the windscreen was thinned down
considerably before the fuselage would come together at the top of the nose.