CHAPTER 7 BASIC HYDRAULIC/PNEUMATIC AND EMERGENCY POWER SYSTEMS

CHAPTER 7 BASIC HYDRAULIC/PNEUMATIC AND EMERGENCY POWER SYSTEMS Chapter Objective: Upon completion of this chapter, you will have a working knowledge of the normal and emergency hydraulic and pneumatic power systems. You should also be able to identify and maintain the various components in these systems. The Navy uses hydraulic and pneumatic power systems extensively in naval aircraft. These systems have a number of favorable characteristics; they eliminate the need for complicated systems of gears, cams, and levers. Also, they transmit motion without the slack or delay inherent in the use of solid machine parts. The fluids used are not subject to breakage as are mechanical parts, and the mechanisms are not subjected to great wear. The different parts of a fluid power system can be conveniently located at widely separated points, since the forces generated are rapidly transmitted over considerable distances with small loss. These forces can be conveyed up and down or around corners with small loss in efficiency and without complicated mechanisms. Very large forces can be controlled by much smaller ones, and can be transmitted through comparatively small lines and orifices. If the system is well adapted to the work it is required to perform, and if it is not misused, it can provide smooth, flexible, uniform action without vibration, and it is unaffected by variation of load. In case of an overload, an automatic release of pressure can be guaranteed, so that the system is protected against breakdown or strain. Fluid power systems can provide widely variable motions in both rotary and straight-line transmission of power. ‘he need for control by hand can be minimized. In addition, fluid power systems are economical to operate. The question may arise as to why hydraulics is used in one application, pneumatics in another, or a combination of hydraulics and pneumatics, also known as hydropneumatics, in still another application. Many factors are considered by the user and/or the manufacturer when determining which type of system to use in a specified application. There are no hard and fast rules to follow; however, past experience has provided some sound ideas that are usually considered when such decisions are made. If the application requires speed, a medium amount of pressure, and only a fair amount of control, a pneumatic system may be used. If the application requires only a medium amount of pressure and a more accurate control, a combination of hydraulics and pneumatics may be used. If the application requires a great amount of pressure and/or extremel y accurate control, a hydraulic system should be used. TYPES OF POWER SYSTEMS Learning Objective: Identify the two types of power systems used on naval aircraft. Hydraulic and pneumatic systems in aircraft contain power systems and several subsystems, the number depending upon the design of the aircraft The power systems are sometimes called the heart of the system, and the subsystems are known as the muscle. The power systems include all the components normally installed in the system, from the reservoir to, but not including, the selector valve. In pressurized reservoir systems, this also includes all components used to control and direct the pressurizing agent to the reservoir. The utility hydraulic system includes systems used for landing gear, arresting gear, nosewheel steering, and many other systems that will be discussed in chapter 12 of this TRAMAN. In accordance with military specifi- cations, which set up the requirements for aircraft hydraulic systems, all hydraulically operated systems considered essential to flight safety or landing must have provisions for emergency actuation. The hydraulic/pneumatic and emergency power systems are discussed in this chapter. HYDRAULIC/PNEUMATIC POWER SYSTEMS System design must prevent the failure of a single part, such as a pump, pressure line, or filter, from disabling the aircraft. Special consideration is given to the hydraulic flight control system. System design specifications require two separate systems for operating the primary flight controls. All aircraft that use hydraulically operated flight controls have at least two hydraulic power systems. The systems supply pressure to the utility or normal system in addition to the flight controls. The flight control portion is given pressure priority by an isolation valve. This design feature isolates nonessential flight functions and 7-1