TM 1-1500-204-23-2CHAPTER 2PNEUDRAULICS GENERAL2-1.Pneudraulics Theory and Basic Principles. The aircraft hydraulic system transmits engine power to distantpoints on the aircraft. This force is carried by hydraulic fluid confined In a system of tubing and hosesa.QualitiesofHydraulicFluid. Hydraulic fluid can be described In terms of three physical qualities(1)Incompressibility.For practical purposes, liquids are incompressible This means that even underextremely high pressure a liquid cannot be made much smaller.(2)Expansion and contraction. Liquids expand and contract with changes in temperature. When a liquidin a closed container is heated, the liquid expands and puts pressure on the walls of the container. As the liquid cools,the pressure decreases.(3)Pressure transmission.Pressure applied to a confined liquid is transmitted equally. If an openingexists in a system, such as an actuator, the fluid will act on it, causing it to move.b.ComputationofForce,Area,andPressure. Hydraulic systems are designed to take advantage of thecharacteristics of liquids as they relate to force, area and pressure. These terms as used in hydraulics are defined asfollows:(1) Force.Force is the amount of push or pull applied to an object. The force applied to a piston head, forexample, is the energy applied to the total area of the piston head. In this manual, we measure force in pounds.(2)Area.Area is the measurement of a surface In the aircraft hydraulic system, we are concerned withthe areas of piston heads If we know this area, we can compute the amount of force needed to start a mechanismmoving In this manual, area Is measured in square inches.(3) Pressure. Pressure is the force applied to one unit of area-usually 1 square inch. The pressure on apiston head develops the force that operates a mechanism. In our work, we measure pressure in pounds per square inch(psi).(4)Computation.If you know any two of these factors, you can easily compute the third by using theequation illustrated In figure 2-1. To apply this equation, multiply the two lower factors together to get the top factor, anddivide the top factor by the known lower factor to get the unknown lower factor.Figure 2-1. Computing Force Pressure, and Area2-1