A pneumatic system is a power transmission system that uses compressed air as a working fluid similar to the way hydraulic systems use liquids. Hydraulic systems are more popular, but there are many advantages to using its pneumatic counterpart to transmit power. The principles in using air are the same as those in using hydraulic fluids, but the results are quite different.
In small homebuilt aircraft, you will often find pneumatic systems in the form of the gyroscopic vacuum system and the tires the aircraft stands and rolls on. Additionally, pneumatics are used in the vacuum systems to power the gyroscopes in the instrument panel. There are both advantages and disadvantages to using a pneumatic system. One disadvantage is that air does not transmit as easily as fluid does. It is springy in nature and therefore not ideal for precise control operations. At the same time, this is an advantage when air is used in tires and damping to make a smoother ride when taxiing on rough surfaces.
Another disadvantage of pneumatic power is that it is not suitable for heavy mechanical devices. Air must be compressed to such a degree that it would require large air tanks and actuators with very high working pressures, which is not possible on a small aircraft. Contrary to these disadvantages, there are four main advantages of a pneumatic system. For one, you don’t have to worry about fire, because air will not burn by itself. However, compressed bottles can explode and cause damage. Second, air systems are lightweight, and do not require a return line like hydraulics do. Third, pneumatics systems are intentionally very simple systems to construct and operate. Finally, air can be pumped from virtually anywhere, giving the system a practically endless power supply.
Some aircraft engines have a pneumatic system wherein a bottle of pressurized air starts the engine and keeps it topped up for however long the engine is running. Larger aircraft use bleed air from the engines to actuate a number of aircraft systems including deicing boots, cabin pressurization, as well as backup and main pneumatic systems. Backup pneumatic systems are usually needed when a hydraulic system fails for any reason. These backup systems use storage bottles with air and an actuator to extend the landing gear during an emergency.
There are generally two types of pneumatic systems: low pressure and high pressure. Low pressure systems are pressurized up to around 1,000 psi and use an engine-driven vane pump. These are primarily used in small, low-power applications such as air conditioning, door seals, and de-icer boots. High pressure systems are compressed to approximately 3,000 psi, though this number will vary from aircraft to aircraft, depending on the manufacturer. In these systems, there is a ground valve on the aircraft, allowing the system to maintain pressure even when main engines are not running. High pressure systems are commonly used on flight control surfaces like flap extensions and others.
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