Wing Spars Two or more spars are used in the construction of a wing. They carry the main longitudinal -butt to tip - load of the wing. Both the spar and a compression rib connect the wing to the fuselage. Compression Ribs Compression ribs carry the main load in the direction of flight, from leading edge to trailing edge. On some aircraft the compression rib is a structural piece of tubing separating two main spars. The main function of the compression rib is to absorb the force applied to the spar when the aircraft is in flight. Former Ribs A former rib, which is made from light metal, attaches to the stringers and wing skins to give the wing its aerodynamic shape. Former ribs can be classified as nose ribs, trailing edge ribs, and mid ribs running fore and aft between the front and rear spar on the wing. Formers are not considered primary structural members. Stringers Stringers are made of thin sheets of preformed extruded or hand-formed aluminum alloy. They run front to back along the fuselage and from wing butt to wing tip. Riveting the wing skin to both the stringer and the ribs gives the wing additional strength. Stress Plates Stress plates are used on wings to support the weight of the fuel tank. Some stress plates are made of thick metal and some are of thin metal corrugated for strength. Stress plates are usually held in place by long rows of machine screws, with self-locking nuts, that thread into specially mounted channels. The stressplate channeling is riveted to the spars and compression ribs. Gussets Gussets, or gusset plates, are used on aircraft to join and reinforce intersecting structural members. Gussets are used to transfer stresses from one member to another at the point where the members join. Wing Tips
The wing tip, the outboard end of the wing, has two purposes: To aerodynamically smooth out the wing tip air flow and to give the wing a finished look. Wing Skins Wing skins cover the internal parts and provide for a smooth air flow over the surface of the wing. On full cantilever wings, the skins carry stress. However, all wing skins are to be treated as primary structures whether they are on braced or full cantilever surfaces. Fuselage Assemblies. The largest of the aircraft structural components, there are two types of metal aircraft fuselages: Full monocoque and semimonocoque. The full monocoque fuselage has fewer internal parts and a more highly stressed skin than the semimonocoque fuselage, which uses internal bracing to obtain its strength. The full monocoque fuselage is generally used on smaller aircraft, because the stressed skin eliminates the need for stringers, former rings, and other types of internal bracing, thus lightening the aircraft structure. The semimonocoque fuselage derives its strength from the following internal parts: Bulkheads, longerons, keel beams, drag struts, body supports, former rings, and stringers. Bulkheads A bulkhead is a structural partition, usually located in the fuselage, which normally runs perpendicular to the keel beam or longerons. A few examples of bulkhead locations are where the wing spars connect into the fuselage, where the cabin pressurization domes are secured to the fuselage structure, and at cockpit passenger or cargo entry doors. Longerons And Keel Beams Longerons and keel beams perform the same function in an aircraft fuselage. They both carry the bulk of the load traveling fore and aft. The keel beam and longerons, the strongest sections of the airframe, tie its weight to other aircraft parts, such as powerplants, fuel cells, and the landing gears. Drag Struts And Other Fittings
Drag struts and body support fittings are other primary structural members. Drag struts are used on large jet aircraft to tie the wing to the fuselage center section. Body support fittings are used to support the structures which make up bulkhead or floor truss sections. Former rings and fuselage stringers are not primary structural members. Former rings are used to give shape to the fuselage. Fuselage stringers running fore and aft are used to tie in the bulkheads and former rings. FUSELAGE FUSELAGE The fuselage includes the cabin and/or cockpit, which contains seats for the occupants and the controls for the airplane. In addition, the fuselage may also provide room for cargo and attachment points for the other major airplane components. Some aircraft utilize an open truss structure. The truss-type fuselage is constructed of steel or aluminum tubing. Strength and rigidity is achieved by welding the tubing together into a series of triangular shapes, called trusses.
[Figure 1-2 The Warren truss] Construction of the Warren truss features longerons, as well as diagonal and vertical web members. To reduce weight, small airplanes generally utilize aluminum alloy tubing, which may be riveted or bolted into one piece with cross-bracing members. As technology progressed, aircraft designers began to enclose the truss members to streamline the airplane and improve performance. This was originally accomplished with cloth fabric, which eventually gave way to lightweight metals such as aluminum. In some cases, the outside skin can support all or a major portion of the flight loads. Most modern aircraft use a form of this stressed skin structure known as monocoque or semimonocoque construction.
The monocoque design uses stressed skin to support almost all imposed loads. This structure can be very strong but cannot tolerate dents or deformation of the surface. This characteristic is easily demonstrated by a thin aluminum beverage can. You can exert considerable force to the ends of the can without causing any damage. However, if the side of the can is dented only slightly, the can will collapse easily. The true monocoque construction mainly consists of the skin, formers, and bulkheads. The formers and bulkheads provide shape for the fuselage.
[Figure 1-3 Monocoque fuselage design.] Since no bracing members are present, the skin must be strong enough to keep the fuselage rigid. Thus, a significant problem involved in monocoque construction is maintaining enough strength while keeping the weight within allowable limits. Due to the limitations of the monocoque design, a semimonocoque structure is used on many of today’s aircraft. The semi-monocoque system uses a substructure to which the airplane’s skin is attached. The substructure, which consists of bulkheads and/or formers of various sizes and stringers, reinforces the stressed skin by taking some of the bending stress from the fuselage. The main section of the fuselage also includes wing attachment points and a firewall.
[Figure 1-4 Semi-monocoque construction. ] Truss—A fuselage design made up of supporting structural members that resist deformation by applied loads. Monocoque—A shell-like fuselage design in which the stressed outer skin is used to support the majority of imposed stresses. Monocoque fuselage design may include bulkheads but not stringers. Skin Former Bulkhead Figure 1-3. Monocoque fuselage design. Bulkheads and/or Formers Stressed Skin Wing Attachment Points Firewall Stringers Figure 1-4. Semi-monocoque construction. Semi-Monocoque—A fuselage design that includes a substructure of bulkheads and/or formers, along with stringers, to support flight loads and stresses imposed on the fuselage. On single-engine airplanes, the engine is usually attached to the front of the fuselage. There is a fireproof partition between the rear of the engine and the cockpit or cabin to protect the pilot and passengers from accidental engine fires. This partition is called a firewall and is usually made of heat-resistant material such as stainless steel
