TENSION VARYING STRESS Tension may be defined as “pull.” It is the stress of stretching an object or pulling at its ends. An elevator control cable is in additional tension when the pilot moves the control column. Tension is the resistance to pulling apart or stretching, produced by two forces pulling in opposite directions along the same straight line. COMPRESSION If forces acting on an aircraft move toward each other to squeeze the material, the stress is called compression. Compression is the opposite of tension. Tension is a “pull,”   and compression is a “push.” Compression is the resistance to crushing, produced by two forces pushing toward each other in the same straight line. While an airplane is on the ground, the landing gear struts are under a constant compression stress. SHEAR Cutting a piece of paper with a pair of scissors is an example of shearing action. Shear in an aircraft structure 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 in an aircraft experience both shear and tension stresses. BENDING Bending is a combination of tension and compression. Consider the bending of an object such as a piece of tubing. The upper portion stretches (tension) and the lower portion crushes together (compression). The wing spars of an aircraft in flight undergo bending stresses. TORSION Torsional stresses are the result of 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 cause torsional stresses produce torque. All materials arc somewhat elastic. A rubberband is extremely elastic, whereas a piece of metal is not very elastic. All the structural members of an aircraft experience one or more stresses. Sometimes a structural member has alternate stresses. It is under compression one instant of time 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 should understand the stresses encountered on the main parts of an aircraft. A knowledge of the basic stresses on aircraft structures helps you understand why aircraft are built the way they are. The fuselage of the aircraft encounters the five types of stress-torsion, bending, tension, shear, and compression. Torsional stress in a fuselage is created in several ways. An example of this stress is encountered in engine torque on turboprop aircraft. Engine torque tends to rotate the aircraft in the direction opposite to that in which the propeller is turning. This force creates a torsional stress in the fuselage. Figure 1-20 shows the effect of the rotating propellers. Another example of torsional stress is the twisting force in the fuselage due to 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 itself. Bending greatly 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. This bending action is shown in figure 1-21. These stresses are also transmitted to the fuselage when the aircraft is in flight. Bending occurs due to the reaction of the airflow against the wings and empennage. When the aircraft is in flight, lift forces act upward against the wings, tending to bend them upward. The wings are prevented from folding over the fuselage by the resisting strength of the wing structure. This bending action creates a tension stress on the bottom of the wings and a compression stress on the top of the wings. MATERIALS OF CONSTRUCTION Learning Objective: Recognize and identify the properties of the various types of metallic and nonmetallic materials used in aircraft construction. 1-20