The fuselage should carry the payload, and is the main body to which all parts are connected. It must be able to resist bending moments (caused by weight and lift from the tail), torsional loads (caused by fin and rudder), and cabin pressurization. The structural strength and stiffness of the fuselage must be high enough to withstand these loads. At the same time, the structural weight must be kept to a minimum.
Figure 4.12: Semimonocoque Airbus A340 rear fuselage, seen from inside. © Sovxx / Wikimedia Commons / CC-BY-SA-3.0.
In transport aircraft, the majority of the fuselage is cylindrical or near-cylindrical, with tapered nose and tail sections. The semi-monocoque construction, which is virtually standard in all modern aircraft, consists of a stressed skin with added stringers to prevent buckling, attached to hoop-shaped frames. See Figure 4.12.
The fuselage has also elements perpendicular to the skin that support it and help keep its shape. These supports are called frames if they are open or ring-shaped, or bulkheads if they are closed.
Disturbances in the perfect cylindrical shell, such as doors and windows, are called cutouts. They are usually unsuitable to carry many of the loads that are present on the surrounding structure. The direct load paths are interrupted and as a result the structure around the cut-out must be reinforced to maintain the required strength.
In aircraft with pressurized fuselages, the fuselage volume both above and below the floor is pressurized, so no pressurization loads exist on the floor. If the fuselage is suddenly de-pressurized, the floor will be loaded because of the pressure difference. The load will persist until the pressure in the plane has equalized, usually via floor-level side wall vents. Sometimes different parts of the fuselage have different radii. This is termed a double-bubble fuselage. Pressurization can lead to tension or compression of the floor-supports, depending on the design.
Frames give the fuselage its cross-sectional shape and prevent it from buckling when it is subjected to bending loads. Stringers give a large increase in the stiffness of the skin under torsion and bending loads, with minimal increase in weight. Frames and stringers make up the basic skeleton of the fuselage. Pressure bulkheads close the pressure cabin at both ends of the fuselage, and thus carry the loads imposed by pressurization. They may take the form of flat discs or curved bowls. Fatigue-critical areas are at the fuselage upper part and at the joints of the fuselage frames to the wing spars.
Figure 4.13: Structural wing sketch.
Figure 4.14: Structural wing torsion box. Adapted from © User Kadellar / Wikimedia Commons / / CC-BY-SA-3.0.