The aileron and flap servos have to be installed before the bottom skin can be installed. Of course that involves building the ailerons and flaps themselves. As Gordon noted, building them on the Proskin sheeting is easy and they come out *very* stiff (so be sure to get them straight).
The flap servo is accessible from the wheel well and a hatch to access the aileron servo will be made in the bottom skin after it is installed. The aileron servo is buried in the wing so that only the aileron horn and a short section of the pushrod are outside the wing.
The flaps are hinged with the flat hinges supplied in the kit. The ailerons were hinged using Robart hinge points. I was uncomfortable with only two hinges on the aileron, as Gordon was, so I used three. I installed balsa blocks at the locations of the hinges.
