STRUCTURAL STRESS

Figure 1-19.—Five stresses acting on an aircraft. rotary rudder head is driven by the tail gearbox. Change in blade pitch is accomplished through the pitch change shaft that moves through the horizontal shaft of the tail gearbox. As the shaft moves inward toward the tail gearbox, pitch of the blade is decreased. As the shaft moves outward from the tail gearbox, pitch of the blade is increased. The pitch control beam is connected by links to the forked brackets on the blade sleeves. A flapping spindle for each blade permits flapping of the blade to a maximum of 10 degrees in each direction. Rotary Rudder Blades The blades are on the rotary rudder head. Each blade consists of the following: • Aluminum spar • Aluminum pocket with honeycomb core • Aluminum tip cap • Aluminum trailing edge cap • Abrasion strip In addition, those blades that have deicing pro- visions have a neoprene anti-icing guard, embedded with electrical heating elements. The root end of the blade permits attaching to the rotary rudder head spindles. The abrasion strip protects the leading edge of the blade from sand, dust, and adverse weather conditions. The skin is wrapped completely around the spar, and the trailing edge cap is installed over the edges of the skin at the trailing edge of the blade, The tip cap is riveted to the outboard end of the blade. STRUCTURAL STRESS Learning Objective: Identify the five basic stresses acting on an aircraft. Primary factors in aircraft structures are strength, weight, and reliability. These three factors determine the requirements to be met by any material used in airframe construction and repair. Airframes must be strong and light in weight. An aircraft built so heavy that it could not support more then a few hundred pounds of additional weight would be useless. In addition to having a good strength-to-weight ratio, all materials must be thoroughly reliable. This reliability minimizes the possibility of dangerous and unexpected failures. Numerous forces and structural stresses act on an aircraft when it is flying and when it is static. When it is static, gravity force alone produces weight. The weight is supported by the landing gear. The landing gear also absorbs the forces imposed during takeoffs and landings. During flight, any maneuver that causes acceleration or deceleration increases the forces and stresses on the wings and fuselage. These loads are tension, compression, shear, bending, and torsion stresses. These stresses are absorbed by each component of the wing structure and transmitted to the fuselage structure. The empennage, or tail section, absorbs the same stresses and also transmits them to the fuselage structure. The study of such loads is called a “stress analysis.” The stresses must be analyzed and considered when an aircraft is designed. These stresses are shown in figure 1-19. 1-19