Aircraft hydraulic system consists of – hydraulic reservoirs, hydraulic pipelines, hydraulic actuators and hydraulic pumps.
- Hydraulic Reservoir
Some hydraulic reservoirs are pressurised whereas some are unpressurised. Unpressurised reservoirs are good for low performace aircraft. Pressurised reservoirs are good for high performance aircraft like fighter aircrafts. Most of the modern aircraft are unpressurised.
The reservoir has five ports, namely –
- Outlet for emergency pump
- Outlet for hand driven pump (main pump)
- Return line
- Vent valve
- Filter neck
Oulet for emergency pump is used when the main pump is leaked out. Sight gauge indicates the level of hydraulic fluid in the reservoir. Baffle slows down or restricts the momentum of hydraulic fluid.
Range of pressure in the hydraulic reservoir is 190 to 200 kg. It is important to maintain the temperature of hydraulic fluid i.e. 85-90 degree Celcius otherwise it changes its properties. As the temperture rises, supply is turned off for a particular time period.
- Pressure Pipelines – Hydrualic fluid which is under pressure flows through these pipelines. These are particularly made strong enough to withstand the pressure.
- Return Pipelines – The hydraulic fluid which isn’t under pressure flows through these pipelines.
For selecting a hydraulic pipelines, cost is an important factor.
Hydraulic actuators help to operate the wheel brakes, retractable landing gears, etc. Hydraulic energy is transformmed into rotated or deflected work or power.
It consists of an actuating cylinder and a piston. The actuating cylinder transforms energy in the form of fluid pressure into mechanical force. The piston extends when the landing gears, etc are to be operated.
There are two types of Hydraulic actuators –
- Single Acting Actuator
- Double Acting Actuator
Hydraulic pumps move non-compressible fluid through an aircraft hydraulic system. This power is mainly used on small aircrafts to operate wheel brakes, retractable landing gear, and some constant-speed propellers.
There are two types of hydraulic pumps –
- Gear Type Hydraulic Pump
Gear type pump is also called Positive Displacement Pump because each revolution delivers a given amount of fuel. It consists of two ports – outlet and inlet, spur gears. One of the spear gears is driven by the electric motor/ engine and the other one is driven by the first spear itself.
Fluid enters the pump from inlet and is carried through the gears and gets compressed. As the fluid is compressed and under pressure, it goes to the hydraulic system. Gears are closely meshed so as to have no leakage of fluid.
- Vane Type Hydraulic Pump
Vane type consists of two ports – inlet and outlet, rotor, shaft and rectangular vanes. The slotted rotors are located off-centre within the cylindered pump body with rectangular vanes free to move radially in each slot. As the rotor rotates, due to the centrifugal force, the vanes move outward and comes in contact with the smooth surface of the inner casing.
Since the rotor is eccentric with respect to the casing, the vanes from the chamber develops difference in the pressure due to contraction and extension of the chamber. The fluid enters the chamber when the developed pressure is low that is when the chamber is extended. As the chamber contracts, due to high pressure fluid moves to the hydraulic system.