$^{ }$Hydraulic Power Systems Evolution of Hydraulic Systems Closed-Center Hydraulic Systems Hydraulic Power Pack System 12-7 Figure 12-4.  A basic closed-center hydraulic system with a variable displacement pump.  Static Suction Pressure Return Reservoir Pump Relief valve Actuating unit Actuating unit Actuating unit Selector valve (closed) A Selector valve (operating) B Selector valve (operating) C A hydraulic power pack is a small unit that consists of an  electric pump, filters, reservoir, valves, and pressure relief  valve.  [Figure 12-5]  The advantage of the power pack  If a variable displacement pump is used, system pressure  is controlled by the pump’s integral pressure mechanism  compensator. The compensator automatically varies the  volume output. When pressure approaches normal system  pressure, the compensator begins to reduce the flow output  of the pump. The pump is fully compensated (near zero flow)  when normal system pressure is attained. When the pump  is in this fully compensated condition, its internal bypass  mechanism provides fluid circulation through the pump  for cooling and lubrication. A relief valve is installed in the  system as a safety backup.  [Figure 12-4]  An advantage of the  open-center system over the closed-center system is that the  In the closed-center system, the fluid is under pressure  whenever the power pump is operating. The three actuators are  arranged in parallel and actuating units B and C are operating  at the same time, while actuating unit A is not operating. This  system differs from the open-center system in that the selector  or directional control valves are arranged in parallel and not in  series. The means of controlling pump pressure varies in the  closed-center system. If a constant delivery pump is used, the  system pressure is regulated by a pressure regulator. A relief  valve acts as a backup safety device in case the regulator fails.  Many modern aircraft use a power supply system and fly-by- wire flight control. The pilot input is electronically sent to the  flight control servos. Cables or push rods are not used. Small  power packs are the latest evolution of the hydraulic system.  They reduce weight by eliminating hydraulic lines and large  quantities of hydraulic fluid. Some manufacturers are reducing  hydraulic systems in their aircraft in favor of electrically  controlled systems. The Boeing 787 is the first aircraft designed  with more electrical systems than hydraulic systems. Smaller aircraft have relatively low flight control surface  loads, and the pilot can operate the flight controls by hand.  Hydraulic systems were utilized for brake systems on early  aircraft. When aircraft started to fly faster and got larger in  size, the pilot was not able to move the control surfaces by  hand anymore, and hydraulic power boost systems were  introduced. Power boost systems assist the pilot in overcoming  high control forces, but the pilot still actuates the flight  controls by cable or push rod.  The manually engaged and pressure disengaged type of  selector valve is similar to the valve previously discussed.  When the actuating mechanism reaches the end of its cycle,  the pressure continues to rise to a predetermined pressure.  The valve automatically returns to the neutral position and  to open center flow. The relief valve unseats and allows the fluid to flow back to  the reservoir. The system pressure remains at the relief valve  set pressure until the selector valve is manually returned to the  neutral position. This action reopens the open center flow and  allows the system pressure to drop to line resistance pressure.  continuous pressurization of the system is eliminated. Since  the pressure is built up gradually after the selector valve is  moved to an operating position, there is very little shock from  pressure surges. This action provides a smoother operation of  the actuating mechanisms. The operation is slower than the  closed-center system, in which the pressure is available the  moment the selector valve is positioned. Since most aircraft  applications require instantaneous operation, closed-center  systems are the most widely used.