| Principal Airframe Parts (Airplanes) An airplane's four principal parts are the fuselage, nacelle, wings and empennage. The descriptions in the following paragraphs cover the truss, monocoque, semimonocoque, and reinforced shell constructions for the fuselage; the structural members used; construction for the nacelle; monospar, multispar, and box-beam wing constructions; and empennage constructions. Fuselage. The main structural unit of an airplane is the fuselage. Other structural units are directly or indirectly attached to it. In outline and general design, the fuselage of one airplane is much the same as any other. Designs vary principally in the size and arrangement of the different compartments. On military single-engine airplanes, the fuselage houses the powerplant, personnel, and cargo. The basic fuselage constructions are truss and monocoque. The truss construction, a rigid framework of beams, struts, and bars, shown in Figure 1-3, resists deformation by applied loads. Many smaller general aviation aircraft and a number of older military aircraft have used truss construction. A monocoque fuselage, shown in Figure 1-3, is like a shell in that the skin bears the primary stresses in spite of the formers, frame assemblies, and bulkheads that give the fuselage its shape. The construction strength required depends upon the power used, speed, maneuverability, and design. The full monocoque construction is seldom used because the skin is the principal part of the airframe. The big problem in monocoque construction is maintaining strength and keeping weight down. To overcome this problem, the semimonocoque and reinforced shells were developed. These shells are used in the majority of present-day military aircraft. 
Figure 1-3. Fuselage Construction
The semimonocoque fuselage, in addition to having vertical reinforcements (formers), has the skin reinforced by longitudinal members (stringers and longerons). The reinforced shell has the skin reinforced by a complete framework of structural members. Examples of semimonocoque and reinforced shell constructions are shown in Figures 1-4 and 1-5 
Figure 1-4. Semimonocoque Construction
Structural Members. Formers, frame assemblies, and bulkheads give cross-sectional shape, rigidity, and strength to the fuselage. The shapes and sizes of these members vary considerably, depending on their function and position in the fuselage. Formers are the lightest, and they are used primarily for fillings or skin attachments between the larger members. Frame assemblies are the most numerous and outstanding members in the fuselage in appearance and as strengthening devices. Whenever frame assemblies are used to separate one area from another, they are circular or disc-shaped, reinforced, and equipped with doors or other means of access, and are then called bulkheads. Channel members, hat-shaped sections, and built-up assemblies are inserted to give additional strength. Station webs are built-up assemblies located at various points to attach fittings or external parts, such as empennage surface fittings, engine mounts, wing attachments, and landing gear. 
Figure 1-5. Reinforced Shell Construction
Stringers and longerons are the main lengthwise members in fuselage structures. Notice in Figure 1-5 that the longeron is a fairly heavy member. Usually, several of these run the whole length of the fuselage. The stringers are smaller and lighter, and are used primarily for giving shape to the attached skin. Longerons are stronger and heavier than stringers, and hold the bulkheads and formers, which, in turn hold the stringers. All these joined together make a rigid fuselage framework. Nacelle. The streamlined structures (nacelles) on multiengine aircraft are used primarily to house engines. Figure 1-6 shows the construction of a nacelle in general use. Here also, designs vary depending upon the manufacturer and the use to be made of the nacelle. On twin-engine airplanes, nacelles also house the main landing gear and related equipment. Whether the nacelle houses a reciprocating piston or jet engine, landing gear, or cargo, repair fundamentals are essentially the same as for a fuselage. The nacelle must have sufficient strength to withstand the compression and shear loads it will be subjected to; its weight must be kept within allowable limits; and the exterior must be aerodynamically suited for the nacelle's location on the aircraft. 
Figure 1-6. Nacelle Construction
Wings. Airplane surfaces designed to give lifting force when moved forward rapidly through the air are wings. Wing design for any given airplane depends upon size, weight, and use of the airplane; desired speed in flight and at landing; and the desired rate of climb. Wings are designated as left and right, corresponding to the left and right hands of the pilot seated in the cockpit. Variations in design give a wing its particular features. The wing tip may be square, rounded, or tapered. Both the leading edge and the trailing edge of the wing may be straight or curved. Many types of modern airplanes have swept-back wings. Wings on military airplanes are generally of cantilever design; that is, no external bracing is needed. Wings of this design are usually of the stressed-skin type. This means that the skin is part of the wing structure and carries part of the wing stresses. Spar and Box-Beam Wings. In general, monospar, multispar, and box-beam are the three basic wing-construction designs. Modifications of these designs may be used by various manufacturers. A separate description of each basic design is given in the paragraphs that follow. The monospar wing has only one main longitudinal member in its construction. Ribs or bulkheads supply the necessary contour or shape to the airfoil. The strict monospar wing is not in common use. However, this design is modified by adding fake spars or light shear webs along the trailing edge to support the control surfaces. The multispar wing has more than one main longitudinal member in its construction. To give the wing contour and relieve stress on the wing's skin, ribs or bulkheads are often included. This construction, or some modification of it, is used in lighter airplanes. The box-beam wing uses two main longitudinal members with connecting bulkheads to give additional strength and contour to the wing. A corrugated sheet of aluminum alloy may be placed between the bulkheads and the smooth outer skin so that the wing can better carry tension and compression loads. Sometimes, heavy longitudinal stiffeners are substituted for the corrugated sheets. A combination of corrugated sheets on the upper surface of the wing and stiffeners on the lower surface is sometimes used. Wing Spars. Figure 1-7 shows spars, ribs, bulkheads, stringers, and stiffeners. These, the wing's main structural components, are riveted or welded together. 
Figure 1-7. Wing Construction
Spars are the principal structural members of the wing. They correspond to the longerons of the fuselage. Spars run from the base of the wing toward the tip and are usually attached to the fuselage by wing fittings, plain beams, or part of a truss system. The I-beam construction for a spar consists of a web, a deep wall plate, and capstrips. These are either extrusions or formed angles, as shown at Detail A in Figure 1-7. The web, the principal depth portion of the spar, is attached to the capstrips that carry the loads caused by the wings bending. When joined, the web and capstrips form a foundation for attaching the skin. Stiffeners give additional strength to the spar structure. These may be beads pressed into the web or extrusions or formed angles riveted to the web vertically or diagonally. Wing Ribs. In the framework of a wing, ribs are the crosspieces running from the leading edge to the trailing edge of the wing. The ribs give the wing its contour and shape and transmit the load from the skin to the spars. Ribs are also used in ailerons, elevators, fins, and stabilizers. Figure 1-7 shows three general rib constructions: the former, reinforced, and truss ribs. Each type is discussed separately in the following paragraphs. Former ribs, located at frequent intervals throughout the wing, are made of formed sheet metal and are very lightweight. The bent-up portion of a former rib is the flange and the vertical portion is the web. The latter is generally made with beads pressed between the lightening holes. These holes lessen the rib's weight without decreasing its strength. Lightening hole area rigidity is ensured by flanging the edges of the holes. The reinforced rib is similar in construction to the spar, consisting of upper and lower capstrips joined by a web plate. Vertical and diagonal angles between the capstrips reinforce the web plate. The reinforced rib is used more frequently than the truss rib. Vertical and diagonal cross members only are used to reinforce and join the capstrips in constructing truss ribs. These and reinforced ribs are heavier than former ribs and are used only at points where the greatest stresses are imposed. Empennage. The aft end of the fuselage, or tail section of the aircraft, includes the rudder or rudders, elevators, stabilizers, and trim tabs, and it is called the empennage. Figure 1-8 shows the empennage construction. Airplane stabilizing units consist of vertical and horizontal surfaces at the aft end of the fuselage. In many respects, construction features are identical with those of wings. Empennage components are usually of all-metal construction and cantilever design. Both monospar and multispar construction are commonly used. Ribs develop the cross-sectional shape, and fairings are used to streamline angles between these surfaces and the fuselage. The vertical stabilizer, in addition to being the base for attaching the rudder, assists in maintaining the airplane's directional stability in flight. On propeller-driven airplanes, the vertical stabilizer is sometimes offset from the centerline to compensate for the torque developed by the engine and propeller. The horizontal stabilizer helps to maintain stability about the airplane's lateral axis, and it is the base for attaching the elevators. As with wings, many variations in size, shape, and placement, as well as number of components, are used by manufacturers in making an empennage. 
Figure 1-8. Empennage Construction
|
