In open skies
For a good flying experience, a sound mechanism
is a must. KNB Panicker explains the
significance of hydraulic systems in an aircraft
The functioning of an aircraft is devoid
without a sound hydraulic system. Since
the advantage of a hydraulic system over
other modes of power transmission, such
as electrical or mechanical, is its ability to control large
power levels with very low controlling power at the
same time utilising high power to weight ratio.
The need of a hydraulic system depends on the
type of aircraft. A smaller aircraft with manually
operated flight control surfaces and fixed landing
gear will not require hydraulics. The same is the
case for an aircraft with retractable landing gear,
where the power required for extension and an
electrical actuator can manage retraction. As the
aircraft becomes bigger, it is not technically feasible
to have manual control or electrical controls for
operating various services. It is here that a hydraulic
system becomes a necessity.
Before going into the details of an aircraft
hydraulic system, let us have a look at the basic
elements of such a system. As any other hydraulic
system this generates and distributes hydraulic power
to all services through tubes with control valves to
operate various services. The above three sections
are elaborated with special reference to aircraft
hydraulic system (Fig 1).
Power generation
Hydraulic power generation essentially consists of
drawing hydraulic fluid from a tank (reservoir) and
pressurising to the required level called system
working pressure. This pressure depends on various
factors as shown below and the value could be 100-
420 bar depending on the requirement. The selection
of hydraulic system pressure for an aircraft system is a
result of optimisation of factors mentioned below:
* Lower the pressure, larger will be the size of the
actuator
* Higher the pressure higher will be the leakage across
orifices
* Higher the pressure, higher will be the temperature
rise of system fluid.
Hydraulic power generators are basically pumps of
various types that are normally driven by aircraft
engine through a gearbox.
Other prime movers as
given above are used depending on the need (Fig 2).
Axial piston pumps are used for pressure levels
over 210 bars and these have the highest overall.
....CONTD
