Use of Air in Hydraulic Systems. When subjected to an applied force, a gas (such as air or nitrogen) acts in
a manner similar to a spring: it yields but pushes back with as much force as is being applied to it. This characteristic of
gases makes them useful in aircraft systems.
Gases used. Air is the gas commonly used in hydraulic systems. It is used in accumulators, shock
struts, and emergency systems and for pressurizing system reservoirs. In terms of compressibility, almost any gas could
be used, but many are dangerous because they are flammable or explosive. Pure nitrogen is the only safe substitute for
atmospheric air in hydraulic systems, and it is the only substitute authorized.
. Assuming a constant temperature, the volume of a confined gas changes in opposite
order to changes in pressure. For example, if a given volume of gas is reduced to half its initial size, its pressure doubles
or, if the volume doubles, the pressure is halved. This characteristic of gases is known as Boyle's law and is expressed
by the equation:
V x P = V1 x P1
V = initial volume
P = initial pressure
V1 = changed volume
P1 = changed pressure
If the measurements of any three of these factors are known, the fourth factor can be determined. To illustrate, let us
assume that 30 cubic inches of gas (V) at a constant temperature and under 90-psi pressure (P) is forced into a 15-cubic-
inch space (V1). To find the changed pressure (p1) we substitute in the equation as follows.
90 x 30 = 15 x P1
2700 = 15P1
15 = p1
180 = p1
Uses of Hydraulic Systems in Army Aircraft. Hydraulic systems perform a variety of functions in Army aircraft.
They are used in fixed-wing aircraft for such purposes as changing propeller pitch and operating landing gear, wing flaps,
wheel brakes, and shock struts in helicopters, hydraulic systems start engines and operate brakes, shock struts, dampers,
flight control systems, loading ramps, folding pylons, winch hoists, and hydraulic clutches. There are a number of
reasons why hydraulic systems have been designed for so many uses in aircraft.
Efficiency. A hydraulic system is almost 100 percent efficient. The slight loss of efficiency (a fraction of 1
percent) is due to internal friction in the system machinery.
Ease of Operation. The moving parts of a hydraulic system, being light In weight, can be quickly put into
motion or brought to rest. The valves used in a hydraulic system are capable of quickly starting or stopping the flow of
fluid under pressure, and very little effort is needed to operate them. For these reasons, the system is easy for the
operator to control.
Ease of Installation. Hydraulic lines can be routed almost anywhere. Unlike mechanical systems, which
must follow straight pathways, the lines of a hydraulic system can be easily bent around obstructions. The major parts of
hydraulic systems can be located in a wide variety of places.
Size. Since the components of a hydraulic system are small in comparison with those of other systems, the
space requirement is small.
Lubrication. Most of the parts of a hydraulic system operate in a bath of oil, making the system self-
Types Of Hydraulic Fluid Used in Army Aircraft. Hydraulic fluids are classified generally as vegetable-base,
petroleum-base, and synthetic- base.
Vegetable-Base Fluid. Vegetable-base fluid is no longer authorized for use In Army aircraft.
Petroleum-Base Fluid. Petroleum-base fluid has an odor similar to penetrating oil and is dyed red. Synthetic
rubber seals are used with this fluid.