The word "hydraulics" is based on the Greek word for water and originally meant the study of the physical behavior of water at rest and in motion. Today, the meaning has been expanded to include the physical behavior of all liquids, including hydraulic fluid. Hydraulic systems are not new to aviation. The first aircraft had hydraulic braking systems. As the planes became more sophisticated, new systems were developed with hydraulic power. Aircraft hydraulic systems provide a means for the operation of aircraft components. Operation of the landing gear, fins, flight control surfaces and brakes is largely done with hydraulic power systems. The complexity of the hydraulic system varies from small aircraft that require fluid only for the manual operation of wheel brakes to large transport aircraft where systems are large and complex. To achieve the necessary redundancy and reliability, the system may consist of several subsystems. Each subsystem has a tank, an accumulator, a heat exchanger, a filtration system, etc. The operating pressure of the system can vary from about two hundred pounds per square inch (psi) in small airplanes and helicopters to 5,000 pounds per square inch in large transports.
Hydraulic systems have many advantages as energy sources for operating several aircraft units; They combine the advantages of light weight, ease of installation, simplification of inspection and minimum maintenance requirements. Hydraulic operations are also nearly 100% efficient, with only negligible losses due to fluid friction.
Hydraulic system fluids are mainly used to transmit and distribute forces to various units to be driven. Liquids are able to do this because they are almost incompressible. Pascal's Law states that the pressure applied to any part of a confined liquid is transmitted with an undiminished intensity to each other. Therefore, if there are several passages in a system, the pressure can be distributed through all of them by means of the liquid. Hydraulic device manufacturers usually specify the type of liquid most suitable for use with their equipment in view of working conditions, required service, expected temperatures inside and outside the systems, pressures to be borne by the liquid, Corrosion possibilities and other conditions that should be considered. If incompressibility and fluidity were the only qualities required, any liquid that was not too thick could be used in a hydraulic system. But a liquid satisfactory for a particular installation must possess a number of other properties. Some of the properties and characteristics that must be taken into account when selecting a suitable liquid for a particular system are discussed in the following paragraphs.
