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AIRCRAFT CONSTRUCTION AND MATERIALS Chapter Objective: Upon completion of this chapter, you will have a basic working knowledge of aircraft construction, structural stress, and materials used on both fixed- and rotary-wing airfraft. One of the requirements of an Aviation Structural Mechanic is to be familiar with the various terms related to aircraft construction. Aircraft maintenance is the primary responsibility of the Aviation Structural Mechanic H (AMH) and Aviation Structural Mechanic S (AMS) ratings. Therefore, you should be familiar with the principal aircraft structural units and flight control systems of fixed and rotary-wing aircraft. While the maintenance of the airframe is primarily the respon-sibility of the AMS rating, the information presented in this chapter also applies to the AMH rating. The purpose, locations, and construction features of each unit are described in this chapter. Each naval aircraft is built to meet certain specified requirements. These requirements must be selected in such a way that they can be built into one machine. It is not possible for one aircraft to have all characteristics. The type and class of an aircraft determine how strong it will be built. A Navy fighter, for example, must be fast, maneuverable, and equipped for both attack and defense. To meet these requirements, the aircraft is highly powered and has a very strong structure. The airframe of a fixed-wing aircraft consists of five principal units. These units include the fuselage, wings, stabilizers, flight control surfaces, and landing gear. A rotary-wing aircraft consists of the fuselage, landing gear, main rotor assembly, and tail rotor. A furtherbreakdown of these units is made in this chapter. This chapter also describes the purpose, location, and construction features of each unit. FIXED-WING AIRCRAFT Learning Objective: Identify the principal structural units of fixed-wing and rotary-wing aircraft. There are nine principal structural units of a fixed-wing (conventional) aircraft: the fuselage, engine mount, nacelle, wings, stabilizers, flight control surfaces, landing gear, arresting gear, and catapult equipment. FUSELAGE The fuselage is the main structure or body of the aircraft to which all other units attach. It provides spare for the crew, passengers, cargo, most of the accessories, and other equipment. Fuselages of naval aircraft have much in common from the standpoint of construction and design. They vary mainly in size and arrangement of the different compartments. Designs vary with the manufacturers and the requirements for the types of service the aircraft must perform. The fuselage of most naval aircraft are of all-metal construction assembled in a modification of the monocoque design. The monocoque design relies largely on the strength of the skin or shell (covering) to carry the various loads. This design may be divided into three classes: monocoque, semimonocoque, and longitudinal members, that is, stringers and longerons, but has no diagonal web
members. The reinforced shell has the shell reinforced by a complete framework of structural members. The cross sectional shape is derived from bulkheads, station webs, and rings. The longitudinal contour is developed with longerons, formers, and stringers. The skin (covering) which is fastened to all these members carries primarily the shear load and, together with the longitudinal members, the loads of tension and bending stresses. Station webs are built up assemblies located at intervals to carry concentrated loads and at points where fittings are used to attach external parts such as wings alighting gear, and engine mounts. Formers and stringers may be single pieces of built-up sections.
Figure 1-1.-Semimonocoque fuselage construction. The semimonocoque fuselage is constructed primarily of aluminum alloy; however, on newer aircraft graphite epoxy composite material is often used. Steel and titanium are found in areas subject to high temperatures. Primary bending loads are absorbed by the "longerons," which usually extend across several points of support. The longerons are supplemented by other longitudinal members, called "stringers." Stringers are lighter in weight and are used more extensively than longerons. The vertical structural members are referred to as "bulkheads, frames, and formers." These vertical members are grouped at intervals to carry concentrated loads and at points where fittings are used to attach other units, such as the wings, engines, and stabilizers. Figure 1-1 shows a modified form of the monocoque design used in combat aircraft. The skin is attached to the longerons, bulkheads, and other structural members and carries part of the load. Skin thickness varies with the loads carried and the stresses supported. There are many advantages in the use of the semimonocoque fuselage. The bulkheads, frames, stringers, and longerons aid in the construction of a streamlined fuselage. They also add to the strength and rigidity of the structure. The main advantage of this design is that it does not depend only on a few members for strength and rigidity. All structural members aid in the strength of the fuselage. This means that a semimonocoque fuselage may withstand considerable damage and still remain strong enough to hold together. On fighters and other small aircraft, fuselages are usually constructed in two or more sections. Larger aircraft may be constructed in as many as six sections.
Various points on the fuselage are heated by station number. Station 0 (zero) is usually located at or near the nose of the aircraft. The other fuselage stations (FS) are located at distances measured in inches aft of station 0. A typical station diagram is shown in figure 1-2. On this particular aircraft, station 0 is located 93.0 inches forward of the nose. Quick access to the accessories and other equipment carried in the fuselage is through numerous doors, inspection panels, wheel wells, and other openings. Servicing diagrams showing the arrangement of equipment and the location of access doors are supplied by the manufacturer in the maintenance instruction manuals and maintenance requirement cards for each model or type of aircraft. Figure 1-3 shows the access doors and inspection panels for a typical aircraft. 
In aircraft construction, a longeron, or stringer or stiffener, is a thin strip of material to which the skin of the aircraft is fastened. In the fuselage, stringers are attached to formers (also called frames) and run in the longitudinal direction of the aircraft. They are primarily responsible for transferring the aerodynamic loads acting on the skin onto the frames and formers. In the wings or horizontal stabilizer, longerons run spanwise and attach between the ribs. The primary function here also is to transfer the bending loads acting on the wings onto the ribs and spar. Sometimes the terms "longeron" and "stringer" are used interchangeably. Historically, though, there is a subtle difference between the two terms. If the longitudinal members in a fuselage are few in number and run all along the fuselage length (usually 4 to 8), then they are called "longerons". The longeron system also requires that the fuselage frames be closely spaced (about every 4 to 6 in or 10 to 15 cm). If the longitudinal members are numerous (usually 50 to 100) and are placed just between two formers/frames, then they are called "stringers". In the stringer system the longitudinal members are smaller and the frames are spaced farther apart (about 15 to 20 in or 38 to 51 cm). Generally, longerons are of larger cross-section when compared to stringers. On large modern aircraft the stringer system is more common because it is more weight-efficient, despite being more complex to construct and analyze. Some aircraft  use a combination of both stringers and longerons. Longerons often carry larger loads than stringers and also help to transfer skin loads to internal structure. Longerons nearly always attach to frames or ribs. Stringers often are not attached to anything but  the skin, where they carry a portion of the fuselage bending moment through axial loading. It is not uncommon to have a mixture of longerons and stringers in the same major structural component.