Hydraulic Power Systems

Simple Hydraulic Systems

  • Simple Hydraulic systems consist of:
    • Vented Reservoir
    • Hand-operated pump w/ check valve
      • Pump does nothing more than move the non-compressible fluid through the hydraulic system. There is no pressure generated until this flow of fluid is opposed.
      • Selector valve - directs fluid either from the pump into the actuator or from the actuator back into the reservoir.
    • Basic system has two limitations:
      • It requires manual operation of the pump
      • It can apply hydraulic pressure to only one side of the actuator.
  • Double-Acting Actuator and Two-Way Selector Valve
    • Double-Acting Actuator - uses hydraulic pressure on both sides of the piston, the actuator can operate under hydraulic pressure in both directions.
    • Two-Way Selector Valve - has four ports, and two positions.
      • In one position it directs fluid from the pump to the upper side of the piston while the lower side is connected to the reservoir.
      • In the other position the lower side of the piston is connected to the pump outlet and the fluid from the upper side is returned to the reservoir.
  • Engine-Driven Pump and Pump-Control Valve
    • Engine-Driven Pump - The pump is coupled directly to the engine and there is no way to disconnect it. It needs very little power when it is not moving the fluid against an opposition.
    • Pump-Control Valve - Installed between the pump outlet and the return line into the reservoir.
      • Can be opened when pressure is not needed, and fluid flows from the bottom of the reservoir, through the pump and back into the top.
    • To actuate this system the pilot puts the selector valve in the desired position and then closes the pump-control valve.
    • Can operate both the landing gear and the flaps or even more actuators if needed.
  • Unloading Valve and Accumulator
    • Unloading Valve - opens when system pressure reaches the desired value then opens the return line to the reservoir and unloads the pump.
    • Accumulator - having two compartments separated by a movable partition; either a piston, a diaphragm, or a bladder.
      • One compartment is connected directly to the pressure manifold
      • The other is sealed and filled with either compressed air or nitrogen.
        • The air pressure is initially about one-half of the system operating pressure.
        • When fluid enters the other side it becomes equal in pressure to that of the fluid.
      • The shock absorbing action of the accumulator will prevent the pressure from changing so rapidly that it could damage the system.
  • Hand Pump and Standpipe
    • Hand Pump - provides pressure for the brakes before the first engine is started.
    • Standpipe - A pipe in the bottom of the reservoir that provides the fluid to the pump.
      • The engine driven pump can exhaust the reservoir only down to the top of the standpipe.
  • Filters and Thermal Relief Valves
    • Filters - Remove particulate matter from the hydraulic system
      • usually located in the return line to the reservoir
      • They are equipped with a bypass valve so that if the filter plugs up, the fluid will open the valve and return to the reservoir without doing any damage.
      • Best way to minimize contamination is to cap all lines and fitting as soon as they are opened.
      • Rated by the size of particles it will remove
        • Sizes are measure in microns which are one millionth of a meter.
        • Micronic filters are replaced rather than cleaned.
Types Filter Rating Description
Pleated * Made of specially treated paper folded into pleats
Mesh .0055 Made of stainless wire woven into a mesh and wrapped around a wire frame
Two stage 0.4-3 / 1.5-15 Used in the return line of large aircraft hydraulic systems. Allows the use of extremely fine element at low flow rates without causing excessive pressure drop. During high flow operations, the flow is divided with up to five gallons per minute passing through both elements. Included are differential pressure indicators that indicate when replacement is required.
Cuno * Used on the pressure side of the system and is made up of a stack of discs and spacers mounted on a rod, with a cleaner blade between each of the discs. Clean contaminants from the bowl by turning the handle connected to the rod which turns the whiper arm between the discs.

Hydraulic Systems

  • Open-Center System - utulizies two selector valves in series with one another.
    • Open-Center valves serve the functions of the selector valve and unloading valve
  • Hydraulic Power Pack System - Utilizes an electric motor to drive the hydraulic pump.
    • Incorporates in a single unit the:
      • Reservoir
      • Control Valve
      • And auxiliary valves
    • The pump is driven by a reversible DC motor in the appropriate direction to lower the landing gear.
      • There are no mechanical uplocks in this system and the gear is held up by hydraulic pressure.

Hydraulic System Components

  • Reservoirs - Store the aircrafts supply of hydraulic fluid.
    • Integral Reservoir - Has no housing of its own but is merely a space set aside within some major component to hold a supply of operational fluid.
    • In-line - Has its own housing, is complete within itself, and is connected with other components in a system by tubing or hose.
    • In all reservoirs, a space is provided above the normal level of the fluid for fluid expansion and the escape of entrapped air.
    • Unpressurized Reservoir - Used by low altitude aircraft.
      • The filter cap on an un-pressurize reservoir may be opened while the system is operating.
        • The level should only be measure with the system at rest.
      • Pressurized Reservoir - Used on high altitude aircraft to assure a positive feed of fluid to the pump.
        • Three ways of pressurizing the reservoir are:
          • Variable displacement hydraulic pumps
            • Aspirators - A venturi draws either cabin or ambient air pressure by jet action into the reservoir thereby pressurizing it.
              • Installed in the return line to the reservoir.
            • A pressure regulator maintains a pressure between 30 and 35 psi in the reservoir.
          • Engine bleed air
            • Maintains a pressure of 40 to 45 psi on the reservoir.
            • The air must be vented before removing the reservoir cap.
          • Or hydraulic system pressure acting on a piston in the reservoir.
  • Pumps
    • Hand Pumps
      • Single-action Pumps - Move fluid only on one stroke of the piston.
      • Double-action Pumps - Move fluid on both strokes of the piston.
        • Most commonly used on aircraft.
        • Piston rod displacement pump - pumping action is created by the difference in area between the two sides of the piston.
Type Valve Description Location
Two-Way Selector Valve Used to direct fluid between the reservoir and the actuator. Allows selection of either side of the actuator. Place in-line with the pressure and return of the system between the reservoir and actuator.
Pump-Control Valve Allows fluid to flow from the bottom of the reservoir, through the pump, back into the top. Installed between the pump outlet and the return line into the reservoir.
Relief Valve Allows fluid to return to the reservoir and maintain a pressure below the bursting pressure of the lines, or below the pressure that could damage the pump. Installed any where pressure may become to great and cause failure or lines or components.
Unloading Valve opens the return line to the reservoir and unloads pump pressure In the line between the pump and the selector valves (can dispense with pump control valve)
Check Valve Allows fluid flow in only one direction. Installed any where in the system where fluid may flow into the wrong part of the system.
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