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HYDRAULICALLY OPERATED FLIGHT CONTROL SYSTEM
Elevator aft bobweight and damper assembly

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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(or power control cylinder) built into the control linkage. PRIMARY FLIGHT CONTROL SYSTEMS Learning Objective: Recognize the functions of the three primary flight control systems (longitudinal, lateral, and directional) and the maintenance associated with each system. Different aircraft manufacturers call units of the primary flight control system by a variety of names. The types and complexity of control mechanisms used depend on the size, speed, and mission of the aircraft. A small or low-speed aircraft may have cockpit controls connected directly to the control surface by cables or pushrods. Some aircraft have both cable and a pushrod system. See figure 9-1. The force exerted by the pilot is transferred through them to the control surfaces. On large or high-performance aircraft, the control surfaces have high pressure exerted on them by the airflow. It is difficult for the pilot to move the controls manually. As a result, hydraulic actuators are used within the linkage to aid the pilot in moving the control surface. Figure 9-2 shows a mechanically controlled, hydraulically assisted system. Because these systems reduce pilot fatigue and improve system performance, they are now commonly used. Such systems include automatic pilot, automatic landing systems, and stability augmentation systems. Navy specifications require two separate hydraulic systems for operating the primary flight control surfaces. Current specifications call for an independent hydraulic power source for emergency operation of the primary flight control surfaces. Some manufacturers provide an emergency system powered by a motor-driven hydraulic pump. Others use a ram-air-driven turbine emergency system pump. LONGITUDINAL CONTROL for operating the SYSTEMS Longitudinal control systems control pitch about the lateral axis of the aircraft. Many aircraft use a conventional elevator system for this purpose. Aircraft that operate in the higher speed ranges usually have a movable horizontal stabilizer. Elevator Control System The elevator control system, shown in figure 9-2, is typical of many conventional elevator systems. It operates by the control stick in the cockpit and is hydraulically powered. The operation of the elevator control system starts when the control stick is moved fore or aft. The movement of the stick transfers through the control cables to move the elevator control bell crank. The bell crank transmits the movement to the hydraulic actuating cylinder through the control linkage. The hydraulic actuating cylinder operates a push-pull tube, which deflects the elevators up or down. The elevator system uses forward and aft bobweights. The bobweights induce a load on the control stick during pitching and vertical acceleration and prevent pilot-induced oscillations through the elevator controls. If the gravity force is increased on the bobweights, the induced load tends to return the control stick to the neutral position. Viscous dampers on the bobweight assemblies retard control stick movement to prevent overcontrol. Overcontrol could cause airframe overstress. The elevator forward bobweight serves to help recenter the control stick when a heavy gravity load pulls against the airframe. The forward bobweight and damper assembly is in a housing forward of the control stick in the cockpit. See figure 9-3. The Figure 9-3.—Elevator forward bobweight and damper assembly. 9-3







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