Elements could have a parent-child hierarchy. The upper-most parent (a root) would be the core of the object. The origin of the world coordinates (point 0, 0, 0), would be in the center of the objects core. A pivot point of the element could be set only inside of his bounding box (of the element). The position and the orientation of the element would be relative to the world (the core of the object) or to his parent (the core or another element inside the object). There could be a limit for the hierarchy depth (2 or 3 should be fine for the folding wings, retractable landing gear and so on).