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Aircraft Pneumatic System for Beginners

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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3 comments:

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Amela Jones said...

Is there any further reading you would recommend on this?

Amela
About Bl Pneumatics

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