Aircraft
Hydraulic Systems
The
operation of aircraft components is made possible by hydraulic systems.
Hydraulic power systems primarily operate the landing gear, flaps, flight
control surfaces, and brakes. The complexity of hydraulic systems vary from
small aircraft, which just need fluid for manual wheel brake function, to huge
cargo aircrafts, which have large and complex systems.
The
large transport and other business jets have complex control systems, heavy
control surfaces that have more dynamic loads, heavy landing gear, brakes, and
cargo doors that are almost impossible to operate without hydraulic power. To
operate their control surfaces and other heavy loads, all large, high-speed
aircraft require hydraulic power.
The
system could have a number of subsystems to achieve the required redundancy and
reliability. A pump, reservoir, accumulator, heat exchanger, filtering system,
etc. are all components of each subsystem. In small aircraft and rotorcraft,
the system operating pressure may range from a few hundred pounds per square
inch (psi) to 5,000 psi, whereas in big transports it may be higher.
Advantage of Hydraulic Power
As
power sources for a variety of aircraft components, hydraulic systems combine
the benefits of being lightweight, simple to instal, easy to examine, and
requiring little maintenance. Additionally, hydraulic processes are almost
entirely efficient, with very little loss from fluid friction.
Depending
on the aircraft, any or all of the following parts may be powered by a single
hydraulic system, or by two or more hydraulic systems that work together:
- Wheel Brakes
- Nose Wheel Steering
- Landing Gear retraction/extension
- flaps and Slats operation
- Thrust reversers
- Spoilers/Speed Brakes
- Flight Control Surfaces
- Cargo doors/loading ramps
- Windshield Wipers
- Propeller pitch control
Hydraulic System Components
The
hydraulic fluid and three key mechanical parts make up a hydraulic system.
These include the "pressure generator," often known as the hydraulic
pump, the hydraulically driven "motor," which drives the affected
component, and the system "plumbing," which holds the fluid and
distributes it as needed throughout the aircraft.
1. Hydraulic Fluid
Based
on the aircraft model, their operating environment, and load factors, the
following three types of hydraulic fluid are used in the aircraft hydraulic
system:
Types of Hydraulic Fluids
The
suitable fluid must be utilised to ensure proper system performance and prevent
damage to the hydraulic system's nonmetallic components. Use the type of fluid
recommended in the aircraft manufacturer's maintenance handbook or on the
instruction plate attached to the reservoir or unit that is being serviced when
adding fluid to a system.
The
following are the most popular three categories of hydraulic fluids:
Minerals
Polyalphaolefins
Phosphate esters
The
technician must be sure to utilise the proper type of replacement fluid when
servicing a hydraulic system. Fluids used in hydraulic systems may not always
mix well. For instance, MIL-H-83282, a fire-resistant fluid, could become
non-fire resistant if MIL-H-5606 was included into the mixture.
Mineral-Based Fluids
The
oldest, MIL-H-5606, is a mineral oil-based hydraulic fluid. It was developed in
the 1940s. It is employed in numerous systems, particularly those where the
risk of fire is minimal. Simply said, MIL-H-6083 is MIL-H-5606 that has had its
rust prevented. They can be used or interchanged in place of one another.
Hydraulic parts are typically shipped with MIL-H-6083 by suppliers. Petrol is
converted into mineral-based hydraulic fluid (MIL-H-5606). It is red-colored
and has a deep oil-like smell. Depending on the type of synthetic hydraulic
fluid, some of them are dyed purple or even green. Petroleum-based fluids are
sealed with synthetic rubber.
Polyalphaolefin-Based Fluids
MIL-H-83282,
a hydrogenated polyalphaolefin-based fluid with fire resistance, was created in
the 1960s to address MIL-flammability H-5606's issues. MIL-H-83282 has a higher
flame resistance than MIL-H-5606, however its high viscosity at low
temperatures is a drawback. Usually, it is restricted to -40 °F. As with
MIL-H-5606, it can be utilised in the same system and with the same hoses,
gaskets, and seals. The rust-resistance variant of MIL-H-83282 is MIL-H-46170.
Most small aircraft utilise MIL-H-5606, however if they can handle the high
viscosity at low temperature, some have upgraded to MIL-H-83282.
Phosphate Ester-Based Fluid
These
highly fire-resistant fluids are utilised in the majority of commercial
transport aircraft. They are not, however, fireproof, and under certain
circumstances, they burn. Additionally, these fluids are highly vulnerable to
contamination from atmospheric water. After World War II, the commercial
aviation industry became more concerned about the rising number of aircraft
hydraulic brake fires, which led to the creation of the first generation of
these fluids. The performance requirements of more modern aircraft designs led
to the gradual development of these fluids. These new generations of hydraulic
fluid, such Skydrol® and Hyjet®, were given performance-based names by the
airframe manufacturers.
Fluids
of types IV and V are utilised nowadays. Based on their density, type IV fluids
can be divided into two groups: class I fluids have a low density, while class
II fluids have a standard density. Compared to class II, class I fluids offer
advantages in terms of weight savings. Type V fluids are being created in response
to industry needs for a more thermally stable fluid at higher working
temperatures in addition to the type IV fluids that are already in use. In
comparison to type IV fluids, type V fluids will be more resistant to
hydrolytic and oxidative breakdown at high temperatures.
Characteristics of Hydraulic Fluid
The
medium through which a hydraulic system transfers its energy is fluid, and
theoretically, any fluid might be used. However, considering the working
pressure (between 3000 and 5000 psi) that the majority of aircraft hydraulic
systems produce, as well as the climatic circumstances and stringent safety
requirements, the hydraulic fluid used should have the following
characteristics.
High Flash Point
Fluid
ignition shouldn't happen in the event of a hydraulic leak at the surrounding
components' typical operating temperatures. For usage in aircraft, special
hydraulic fluids with fire-resistant characteristics have been created. Unlike
hydraulic fluids based on mineral oil, these fluids are phosphate esters and
are extremely difficult to ignite at room temperature. The fluid will continue
combustion, nevertheless, if heated to temperatures higher than 180 degrees C.
Most aviation hydraulic fluids have an auto-ignition temperature in the range
of 475 degrees C.
Fire Point
The
temperature at which a substance releases enough vapour to ignite and burn when
exposed to a spark or flame is known as the fire point. Desirable hydraulic
liquids must have a high fire point, similar to flash point requirements.
Chemical Stability and Viscosity
Another
feature that is crucial in choosing a hydraulic liquid is chemical stability.
It is the liquid's propensity to withstand oxidation and degradation over an
extended length of time. Under difficult operating conditions, all liquids
frequently experience undesirable chemical changes. For instance, when a system
runs for a long time at a high temperature, this is the situation. The life of
a liquid is greatly impacted by high temperatures. It should be noted that the
temperature of the liquid in a hydraulic system's reservoir does not
necessarily reflect the actual state of the system's working circumstances. Localized
hot spots can appear on bearings, gear teeth, or where a liquid is driven
through a small opening under pressure. Even while the liquid in the reservoir
may not show signs of an abnormally high temperature, continuous flow of a
liquid across these locations may result in local temperatures high enough to
carbonise or sludge the liquid.
Hydraulic
systems in aircraft must function effectively throughout a wide range of
temperatures. The employed fluid must maintain acceptable viscosity at high
temperatures while still flowing smoothly at very low temperatures. The
freezing and boiling points of the ideal hydraulic fluid will be extremely low
and high, respectively.
Lubricant Properties
The
system's motors, actuators, and pumps are all lubricated by the hydraulic
fluid. The fluid must be both thermally stable and anti-corrosive.
Thermal Capacity/Conductivity
System
cooling is accomplished via hydraulic fluid. Heat must be easily absorbed and
released by the fluid.
2. Hydraulic Pumps
Aviation applications use a range of hydraulic pumps that are powered by various energy sources. Pumps consist of:
Gear Pumps
Gear
pumps move fluid by utilising meshing gears. Because they move a certain volume
of fluid every rotation, gear pumps fall under the category of fixed
displacement pumps. Gear pumps are typically not appropriate for high pressure
applications, however they may be utilised on low pressure systems (under 1500
psi).
Fixed Displacement Piston Pumps
A
piston that moves inside a cylinder is used in piston pumps to pressurise a
fluid. Each stroke of a fixed displacement pump moves a predetermined volume of
fluid.
Variable Displacement Piston Pumps
On
large aircrafts, this kind of pump is the most prevalent. The pump can adjust
its fluid flow to match changes in system demand thanks to its variable
displacement design. This makes it possible to maintain a nearly constant
system pressure.
These pumps' motive power can be produced in a wide range of ways, including:
Manual operation
A
manual hydraulic pump is commonly used in light aircraft to supply pressure for
the expansion and retraction of flaps or wheel brakes.
Pumps
powered by engines are typically positioned on the accessory gearbox for
engines.
Electric
Hydraulic
pumps can be driven by both AC and DC motors, with three phase AC motors being
the most popular.
Pneumatic
Some
aeroplanes use motors that are powered by bleed air to operate hydraulic pumps.
Hydraulic
Without
any hydraulic fluid being transferred, a Power Transfer Unit (PTU) enables the
hydraulic pressure of one hydraulic system to drive a pump to pressurise another
hydraulic system. A PTU can be either single- or bi-directional, depending on
the implementation.
Ram Air Turbine
Some
aeroplanes include a Ram Air Turbine (RAT) that can be extended into the
airstream to produce hydraulic pressure in an emergency.
3. Hydraulic Motors and Cylinders
Pressurized
fluid is used by hydraulic motors and cylinders to do mechanical work.
Hydraulic Motors
A
hydraulic motor is a mechanical device that transforms hydraulic flow and
pressure into rotation and torque. There are several types of hydraulic motors,
including gear, vane, and radial piston motors. Hydraulic motors are frequently
used on aircraft to power jackscrews, which in turn can power flaps, stabiliser
trim, and some applications for vertically extending landing gear, such as
those seen on the LOCKHEED C-130 Hercules aircraft.
Hydraulic Cylinders
A
hydraulic cylinder, also known as a linear hydraulic motor or a hydraulic
actuator, is a type of mechanical actuator that delivers a single-direction,
reversible force. The hydraulic cylinder consists of a barrel-shaped cylinder
inside of which a piston attached to a piston rod moves back and forth using
hydraulic pressure. Applications for aircraft include moving the flight control
surfaces and extending and retracting the landing gear.
4. System "Plumbing" Components
In
general, "open loop" aviation hydraulic systems pull fluid from a
reservoir, pressurise it, and then make it available to the various user
components before returning the fluid to the reservoir. The following are the
main parts of the hydraulic system's "plumbing" segment:
Reservoir
Most
aircraft systems need for hydraulic fluid reservoirs, which must hold a range
of fluid volumes and serve as a ready source of fluid for the hydraulic
pump(s). This variation is due to the thermal expansion or contraction of the
fluid as well as the differential actuator volume (which depends on whether the
actuator is extended or retracted). Only the amount of fluid required for
proper operation is carried since the reservoir size is optimised. To help
minimise hydraulic pump cavitation, bleed air is frequently utilised to
pressurise or "bootstrap" the reservoir.
Filters
Clean
hydraulic fluids are necessary for a system to operate properly. The hydraulic
system includes in-line filters to purge any impurities from the fluid.
Shut Off Valves
In
most cases, hydraulic shut-off valves are mounted at the engine firewall. The
shutdown valve is closed in the case of an engine fire to avoid the hydraulic
fluid from possibly igniting.
Control Valves
An
connected control valve for hydraulic motors and actuators is positioned in
response to a manual or automatic system choice, such as raising the flap
lever. In response to that choice, the control valve adjusts its position to
let pressurised hydraulic fluid enter the motor or actuator in the desired
direction.
Pressure Relief Valve
Pressure
relief valves are sometimes included in systems to make sure that the nominal
system pressure is not exceeded, especially in systems that use fixed
displacement pumps. Fluid is restored to the reservoir when the relief valve
opens due to excessive system pressure.
Hydraulic Fuses
By
automatically cutting off a hydraulic line if pressure drops too low, hydraulic
fuses are in-line safety devices.
Accumulators
An
external source of energy maintains the pressure of hydraulic fluid in a
hydraulic accumulator, which is a pressure storage reservoir. A compressed gas
or a spring may serve as the external source. A hydraulic system can respond
more quickly to a brief demand and handle extremes of demand with the help of
an accumulator. It also dampens pulsations, acting as a system shock absorber.
The energy in an accumulator can only deliver a certain amount of brake
applications after landing in the event of a hydraulic pump failure.
Hydraulic System Redundancy
There are two main ways to achieve hydraulic system redundancy: by using several systems and multiple pressure sources inside a single system.
Multiple Pressure Sources
There
are frequently multiple pumps available to pressurise hydraulic systems. One or
more engine-driven pumps and one or more electric pumps are extremely prevalent
in systems. A manual pump is occasionally also included. Some systems only
employ manual or electric pumps while the engines are not running when they are
on the ground. Others employ the electric pump(s) as the main pressure source
in the event that the engine-driven pump fails or as an extra pressure source
during high demand circumstances like gear retraction (s). When an electric
pump is utilised as the main source of hydraulic pressure, the system may also
include a second electric pump or a Ram Air Turbine as a backup. In order to
prevent the failure of the complete hydraulic system due to a single component,
numerous pressure sources are provided.
Multiple Hydraulic Systems
Flight
control surfaces are frequently hydraulically activated in aircraft. In these
situations, it is crucial to have numerous actuators on each surface, each
powered by a different hydraulic system, to prevent loss of control in the
event of a hydraulic system failure. Three independent hydraulic systems are
frequently used to power the flight control surfaces in contemporary commercial
aircraft. Two of the systems could fail according to the control surface
architecture without compromising control.
Conclusion
Particularly,
aviation hydraulic systems are employed to assist in managing and controlling
apparatus like brakes, flaps, thrust reversers, flying controls, and of course,
landing gear. Because they provide the ideal amount of pressure to operate
these devices, hydraulic systems are the recommended system.
Also Read
Full Authority Digital Engine Control (FADEC) System Description & Operation
0 Comments