Aircraft Sub-systems: Hydraulic Systems

Introduction to Hydraulic Systems in Aircraft

• Hydraulic systems are used in aircraft due to their ability to provide efficient power amplification, precise control, and smooth power output
• The largest aircraft, such as the A380, require extensive use of hydraulics, while smaller aircraft, like the Cessna 172, have limited hydraulic applications

Hydraulic Architecture

• Hydraulic systems are effective and efficient due to their ability to:
  • Amplify power
  • Control load rate, position, and magnitude
  • Handle multiple loads simultaneously
  • Provide smooth, vibration-free power output

Typical Users of Hydraulic Systems

• Landing gear: extension, retraction, locking, steering, and braking
• Primary flight controls: rudder, elevator, aileron, and active (multi-function) spoiler
• Secondary flight controls: high lift (flap/slat), horizontal stabilizer, spoiler, and thrust reverser
• Utility systems: cargo handling, doors, ramps, and emergency electrical power generation

Concept and Function

• Based on Pascal's Law: pressure exerted on a fluid in an enclosed container is transmitted equally and undiminished to all parts of the container
• Function: to control movable parts by transforming hydraulic energy (pressure and volume) into mechanical energy (force and stroke)

Basic Hydraulic System and Operation

• The pump converts mechanical power from the prime mover (electric or diesel motor) to hydraulic power at the actuator
• Valves control the direction of pump flow, power level, and fluid pressure supplied to the actuator
• Different fluid flow rates are required for different flight profiles

Main Components of Hydraulic Systems

• Pump: supplies pressurized hydraulic fluid for flight control systems and utility systems
• Reservoir: supplies fluid for operating needs, replenishes lost fluid, and serves as an overflow basin for excess fluid
• Filter: filters hydraulic fluid before it leaves the reservoir
• Pressure Relief Valve: limits pressure exerted on a confined liquid to prevent failure of components or rupture of hydraulic lines
• Check Valve: controls fluid flow in one direction
• Accumulator: an energy storage device that provides transient flow, maintains constant pressure, and serves as an emergency power supply

Accumulator Types

• Diaphragm: consists of two hollow, hemispherical metal sections with a synthetic rubber diaphragm dividing the accumulator into two sections
• Bladder: operates on the same principle as a diaphragm but varies in construction
• Cylinder: consists of a cylinder and piston assembly with an internal piston separating the fluid and air/nitrogen chambers