The invention relates to systems and methods for manufacturing composite parts for aircraft. In one innovative aspect, a system and method for adjusting a location of a center of gravity of an aircraft relative to a center of lift of a wing of the aircraft is disclosed, wherein a modular spar tunnel mold is provided and configured to be detachably fixed at a plurality of selected locations within a main mold for a fuselage portion of the aircraft. In another innovative aspect, a movable detail or mold is used for manufacturing complex composite parts for an aircraft. In yet another innovative aspect, a movable detail is used to fabricate a common feature for a plurality of different parts for one or more aircraft or a family of aircraft of similar design.