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Section 1.1 - Introduction to Aircraft Hydraulics

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Aircraft Hydraulics Definition

Aircraft Hydraulics is a means of transmitting energy or power from one place to another efficiently.

What is a hydraulics system?

gear1.gif (139336 bytes)It is a system where liquid under pressure is used to transmit this energy. Hydraulics systems take engine power and converts it to hydraulic power by means of a hydraulic pump. This power can be distributed throughout the airplane by means of tubing that runs through the aircraft. Hydraulic power may be reconverted to mechanical power by means of an actuating cylinder, or turbine.

(1) - A hydraulic pump converts mechanical power to hydraulic power
(2) - An actuating cylinder converts hydraulic power to mechanical power
(3) - Landing Gear
(4) - Engine power (mechanical HP)


If an electrical system were used instead of a hydraulic system, a generator would take the place of the pump and a motor would take the place of the actuating cylinder. 

Advantages of Hydraulic Systems

(over other systems for aircraft use)

  1. It is lighter in weight than alternate existing systems.
  2. It is dead beat, that is, there is an absence of sloppiness in its response to demands placed on the system.
  3. It is reliable; either it works or doesn't.
  4. It can be easily maintained.
  5. It is not a shock hazard; it is not much of a fire hazard.
  6. It can develop practically unlimited force or torque.

   Example: A gun turret must be able to change direction almost instantaneously. This is what is accomplished by this hydraulic system.  In an electrical system, the rotating armature must come to full stop and then reverse direction or else the armature will burn out. This doesn't happen with a hydraulic system because there is no need for a motor in the hydraulic system.
     Example:  In a landing gear the hydraulic motor can produce enough power to pull up the landing gear system without trouble even though air loads act on the system and the slip stream air is impinging against it.   

    The actuating cylinder can change hydraulic power to linear or rotating motion. It has a reduction gear in it to reduce rotating motion to that amount which is needed.  Previously,  systems used to control motion by using steel cables connected by pulleys between the controlling mechanism (such as the pedals) and the controlled surface (such as the rudder).  The cables were affected by expansion rates of the cables due to temperature changes.  Hydraulic systems can control motion without worrying about the effect of temperature since it is a closed system (not open to the atmosphere) compared to a cable system. This means better control of the plane and less lag time between the pilot's movement to control the plane and the response by the control surface.

Some Devices Operated by Hydraulic Systems in Aircraft

  1. Primary control boosters
  2. Retraction and extension of landing gear
  3. Sweep back and forth of wings
  4. Opening and closing doors and hatchways
  5. Automatic pilot and gun turrets
  6. Shock absorption systems and valve lifter systems
  7. Dive, landing, speed and flap brakes
  8. Pitch changing mechanism, spoilers on flaps
  9. Bomb bay doors and bomb displacement gears

Some Devices Operated by the Hydraulic Systems in Spacecraft

  1. Gimbeling of engines and thrust deflector vanes
  2. Thrust reversers and launch mechanisms

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Updated: March 12, 2004