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CHAPTER 4 AIRCRAFT BASIC CONSTRUCTION
CONSTRUCTION MATERIALS

Airman - Aviation theories and other practices
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SHEAR Cutting a piece of paper with scissors is an example of a shearing action. In an aircraft structure, shear (fig. 4-1, view D) is a stress exerted when two pieces of fastened material tend to separate. Shear stress is the outcome of sliding one part over the other in opposite directions. The rivets and bolts of an aircraft experience both shear and tension stresses. BENDING Bending (fig. 4-1, view E) is a combination of tension and compression. For example, when bending a piece of tubing, the upper portion stretches (tension) and the lower portion crushes together (compression). The wing spars of an aircraft in flight are subject to bending stresses. TORSION Torsional (fig. 4-1, view C) stresses result from a twisting force. When you wring out a chamois skin, you are putting it under torsion. Torsion is produced in an engine crankshaft while the engine is running. Forces that produce torsional stress also produce torque. VARYING STRESS All structural members of an aircraft are subject to one or more stresses. Sometimes a structural member has alternate stresses; for example, it is under compression one instant and under tension the next. The strength of aircraft materials must be great enough to withstand maximum force of varying stresses. SPECIFIC ACTION OF STRESSES You need to understand the stresses encountered on the main parts of an aircraft. A knowledge of the basic stresses on aircraft structures will help you understand why aircraft are built the way they are. The fuselage of an aircraft is subject the fives types of stress—torsion, bending, tension, shear, and compression. Torsional stress in a fuselage is created in several ways. For example, torsional stress is encountered in engine torque on turboprop aircraft. Engine torque tends to rotate the aircraft in the direction opposite to the direction the propeller is turning. This force creates a torsional stress in the fuselage. Figure 4-2 shows the effect of the rotating propellers. Also, torsional stress on the fuselage is created by the action of the ailerons when the aircraft is maneuvered. When an aircraft is on the ground, there is a bending force on the fuselage. This force occurs because of the weight of the aircraft. Bending increases when the aircraft makes a carrier landing. This bending action creates a tension stress on the lower skin of the fuselage and a compression stress on the top skin. Bending action is shown in figure 4-3. These stresses are transmitted to the fuselage when the aircraft is in flight. Bending occurs because of the reaction of the airflow against the wings and empennage. When the 4-2 Figure 4-1.—Five stresses acting on an aircraft.







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