Pneumatic systems use compressed air as a working fluid and it acts
much like the hydraulic systems we already described previously in our
articles. In comparison with hydraulics there are advantages to be found
by using air in stead of hydraulic fluid to transmit pressure.The principles (Pascal's law) in using air are the same as for
hydraulics but with a remarkable difference. Below we will discuss these
and see where they are used in aircraft.Air is pressurized by an engine driven pump (compressor) and stored in
bottles. This shows the difference with a hydraulic fluid, air is
compressible and this property gives it some unique (dis)advantages.The aircraft pneumatic system provides compressed air for various
requirements for an aircraft. It is one of the three power sources, the
other two are the electrical power and the hydraulic power.
The uses of the compressed air:
Compressed are is mainly used to pressurize the aircraft. As the
modern aircraft can fly to as high as 37,000 feet, the air is too thin
at that altitude. As such the aircraft cabin pressure has to be
maintained high for the comfort and safety of the passengers.
The function to maintain the pressure and the comfortable air
condition is handle in different system. We will briefly discuss the
subject here.
Aircondition:
It is a system to maintain the cabin
air at a specified temperature and humidity and pressurization is the
system to control and maintain the cabin pressure at the predetermined
level.
The air that comes from the engines or APU are higher pressure and
hotter than the cabin. It has to be cooled down by a series of processes
using heat exchanger and expansion process. Smaller aircrafts may use a
system similar to the home refrigerator.
The flying aircraft acts like a balloon. In fact it can be shown that
the diameter of the aircraft increases as it fly higher into the sky.
The pressurization is controlled by controlling the rate of air
discharged from the aircraft.
Pneumatic is used to start a jet engine.
Most jet engines are started using a pneumatic starter. Pneumatic starter had a better power to weight ratio.
3. Use for anti-icing anf deicing.These involve 2 functions, deicing is removing the ice after it had
already formed. Anti-icing is the action taken to prevent the formation
of ice.
The hot air is used to heat up the leading edge of the aircraft’s wing to prevent the formation of the ice.
Sources of the pneumatic
The sources of the compressed air are:
1. The primary source is the engines. As air is being compressed by
the engine, it gets hotter and has a higher pressure, two tapping are
made, one from low pressure position and the other from a higher
pressure area. The outputs are regulated to give the required pressure
and temperature. The is done by changing the ratio of high pressure
bleed to the low pressure bleed.
2. APU – the auxiliary power unit is a small jet engine designed to
provide the compressed air as well as the electrical power. Most
aircraft’s APUs are located at the tail.
3. On ground, the aircraft can be connected to a ground power unit.
Control and Indication.
A simple switches and indication lights are provided in the cockpit
to control the system. The system also has protection against over
temperature.
There are three groups of control and indication. One group for the
sources of the pneumatic air. The second is for the users, the air
condition, anti-icing and engines starter. The third group is for the
pressurization.
System description
In small experimental homebuilt aircraft you will find pneumatic
systems in the form of the gyroscopic vacuum system and the tyres the
aircraft stands on. Some aircraft engines have an airstart system in
where pressurized air in a bottle starts the engine, and as long as the
engine runs it keeps the bottle topped up.
Larger aircraft use bleed air from the engines to actuate a number of
systems from deicing boots, cabin pressurization, backup and main
pneumatic systems. See the image.Backup systems
Used when, for example, the landing gear hydraulic systems fails for a
reason. A backup pneumatic system uses a storage bottle with air and an
actuator as an emergency means to extend the gear.
Low pressure systems
These are pressurized up to about 1000 psi and use an engine driven
vane type pump and they are used to drive the aircon, door seals,
de-icer boots, mainly small low power applications.
High pressure systems
An engine driven compressor feeds air via an unloading valve tot the
system keeping the pressure around 3000 psi, but this may vary from
aircraft manufacturer to another. There will usually also be a ground
valve on the aircraft to enable the system to be pressurized when the
main engines are not running.
You will also find a moisture separator, dryer (desiccant) and filter
to keep the air clean and free from water before it is stored in the
high pressure bottles.
Pressurized air at 3000 psi is reduced before it is routed to valves
and actuators, this reduced pressure is monitored by gauges. Actuators
can be a single acting device where air moves them one way and a strong
spring inside pushes the piston actuator back, or can they be double
acting. These are sometimes used with flap extension systems.
Disadvantages
One disadvantage is that air does not transmit pressure as easy as a
fluid would do. It is springy in nature and therefore not really
suitable for precise control operations. This, however, is an advantage
when air is used in tyres and damping in undercarriage to smoothen the
ride when taxiing.
It is not suitable for large and heavy mechanical devices. Air must
be compressed to a large degree to have enough energy and this would
require large air tanks and actuators with very high working pressures.
Advantages of air
In contrary to the disadvantages there are some properties which make the use of compressed air very desirable, they are:
- No fire problem - Air won't burn by itself, however, bottles can explode and cause damage
- Light weight - Air systems do not need a return line as is the case with hydraulic fluids
- Simple - Pneumatic systems are by design very simple
- Supply - Air can be pumped in from around us and is unlimited in supply
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2 comments:
Is there any further reading you would recommend on this?
Amela
About Bl Pneumatics
At Cooper Freer Compressors, we strive to provide our customers with quality air compressor services at competitive prices. Proudly serving in UK including Leicester, Derby, and Nottingham areas. We offer broad range of air compressor units and service parts.
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