A civil aircraft fuselage is designed to carry revenue-generating payloads, primarily passengers but the cargo version can also carry containers or suitably packaged cargo. It is symmetrical to a vertical plane and maintains a constant cross-section with front and aft-end closures in a streamlined shape. The aft fuselage is subjected to adverse pressure gradients and therefore is prone to separation. This requires a shallow closure of the aft end so that the low-energy boundary layer adheres to the fuselage, minimizing pressure drag (see Section 3.3). The fuselage also can produce a small amount of lift, but this is typically neglected in the conceptual stages of a configuration study. The following definitions are associated with the fuselage geometry (Figure 3.49).
1.23.1 Fuselage Axis/Zero-Reference Plane
Fuselage axis is a line parallel to the centerline of the constant cross-section part of the fuselage barrel. It typically passes through the farthest point of the nose cone,
facilitating the start of reference planes normal to it. The fuselage axis may or may not be parallel to the ground. The principal inertia axis of the aircraft can be close to the fuselage axis. In general, the zero-reference plane is at the nose cone, but designers can choose any station for their convenience, within or outside of the fuselage. This book considers the fuselage zero-reference plane to be at the nose cone, as shown in Figure 3.49.