Aircraft Structural Considerations
Creating just the aircraft shell, satisfying only aerodynamic needs, has consequences during manufacture. It is simple to create the drawings but not as easy to produce the hardware. During the conceptual study phase, it is routine procedure in the industry to obtain the valued opinion of production engineers in an IPPD environment. Compromises may be made in shaping an aircraft if doing so facilitates manufacturability, which in turn saves cost – more so in the commercial aircraft business, where operational economic gains are more important than in pure aerodynamics.
Manufacturing philosophy is associated with the choice of materials, machining routine, forming, fabrication, and assembly-tool (i. e., jigs and fixtures) concepts (see Chapter 17). Typically, the aim is to shape components as simply as possible with fewer parts and faster assembly time. Attention is given to minimizing complex 3D curvatures; applying more circular shapes than complex, convoluted curves; maintaining commonality of geometry; and providing accessibility for maintenance.
Therefore, it is suggested that a second-term project be assigned to introduce the structural philosophy in harmony with the manufacturing philosophy, beginning with internal structural arrangements in simple line diagrams, as shown for an aft – fuselage in Figure 15.13a. Similar line drawings for the wing and empennage are not shown herein. The advantages of using CAD are discussed in previous chapters of this book, which are apparent, as shown for a typical military aircraft fuselage in Figure 15.13b. If a basic aircraft configuration is created in CAD, then the external aircraft contour lines guide the shape of the internal arrangement, with the added benefit of being able to examine accessibility and production complexity to establish manufacturing philosophy.
Table 15.7. Aircraft door types
* If Type II is located over the wing, it can have an inside step up of 10 inches and an outside step down of 17 inches.
** The types of doors are related to the minimum number of passengers carried. The higher the number of passengers, the larger is the door size.
The strategy is to lay out the main internal structural arrangements, such as the position of the ribs, spars, longerons, bulkheads, wing box-fuselage, flap – empennage attachment, engine attachment, and fuel tank. At this stage, it is not detailed component design. In the next phase (i. e., Phase 2, project definition), line diagrams are developed into shapes after stressing to consolidate the manufacturing philosophy and, if necessary, to prepare for the bidding process to subcontract work. During Phase 3 (i. e., detailed design), the parts are developed into detailed production drawings ready for manufacture. The use of CAD avoids duplication in generating components and the subassemblies outline. CAD is capable of making the procedures paperless. Reference  provides a good description and analyses of aircraft structural design.