What is hydraulic technology?

In the hydraulic technology we transmit and control forces and velocities by transmitting and controlling pressure and flow. In nearly every kind of technology we use hydraulic drive and control techniques. A few examples are:

mechanical engineering
car technology
agriculture technology
earthmoving and mining technology
ship building technology
offshore-technology
aircraft and spacecraft technology

Advantages of Hydraulic Systems

(over other systems for aircraft use)

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