Aircraft Hydraulic System
Welcome to part two of the miniseries Aircraft Power Systems, today we'll talk about the basics of Hydraulics.
General description
A hydraulic system is one of the aircraft power systems, its purpose is to transfer energy from a source to different aircraft components using pressurized hydraulic fluid. This system is especially useful when an aircraft component requires a large amount of force to move.
Why do aircraft use hydraulics?
As aircraft became larger, heavier and faster, some components became too difficult to operate using only mechanical systems. For example, aerodynamic forces acting on flight-control surfaces can become extremely large.
They are very useful because they can produce large forces while keeping the components relatively small and light. They also react quickly, allow smooth control and can transfer energy to parts of the aircraft that are difficult to reach.
Hydraulic systems have been used in aircraft since around 1930, initially for operating landing gear. They were later introduced into flight-control systems and hydraulic boosters.
What can Hydraulic system operate?
Hydraulic power can be used for many important aircraft functions, including landing gear, wheel brakes, flaps, spoilers/aerodynamic brakes and flight control surfaces.
Let's take L-410 as an example here, it's normal hydraulic system operates the landing gear, flaps, interceptors, roll-control surfaces, nose-wheel steering, main-wheel brakes and even windshield wipers.
So, let's say when the pilot selects GEAR DOWN, the pilot does not physically push the landing gear into position. The command controls the hydraulic system, which directs pressurized fluid to an actuator that produces the mechanical movement.
How does a hydraulic system work?
The basic principle is relatively simple. A pump supplies hydraulic fluid under pressure. The pressurized fluid travels through pipes and valves to a hydraulic actuator, such as a hydraulic cylinder or hydraulic motor. The actuator converts the pressure energy of the fluid into mechanical movement.
The reservoir stores hydraulic fluid. The pump creates the required flow and pressure. Filters remove contamination, while valves control where the fluid travels and protect the system against excessive pressure. Finally, the actuator converts hydraulic energy into useful mechanical work .
Hydraulic fluid
The hydraulic fluid is extremely important because it is the actual carrier of pressure energy through the system.
Aircraft hydraulic fluid should have good lubricating properties, remain relatively stable as temperature changes, resist corrosion and maintain suitable physical properties during operation. This is particularly important because aircraft can encounter very large temperature differences.
The fluids can be mineral or synthetic.
Main types of hydraulic circuits
Pressure/energy circuit
- converts mechanical energy into hydraulic pressure energy
Emergency pressure circuit
- allows hydraulic functions to continue when the normal pressure circuit fails
Working circuit
- converts hydraulic pressure energy back into mechanical energy at the component which is being operated
E.g. The working circuit can operate the flaps, aerodynamic brakes or landing gear.
Why are there several hydraulic circuits?
Reliability is extremely important because hydraulics can control components necessary for safe flight and landing. For that reason, important hydraulic systems are redundant.
Modern aircraft can have three independent pressure circuits, the most important hydraulic actuators can receive power from all three, while less critical components may use only one or two.
Hydraulic flight controls
As said before, the aerodynamic force becomes too great in larger aircraft that the pilot can't comfortably overcome them manually.
A hydraulic servomechanism, also called a hydraulic booster or amplifier, increases the available control force. Their use became necessary as aircraft speeds and masses increased.
In other words, the pilot makes a relatively small control input, the hydraulic system supplies the large force and control surface moves, easy.
This allows the pilot to control very large surfaces such as an elevator, rudder or aileron without physically producing the enormous forces required to move them.
Advantages and disadvantages
Hydraulics are really valuable because they can provide very large forces, fast response, smooth control and efficient energy transmission, while hydraulic components can remain relatively compact. The fluid also provides natural damping and oil-based fluids can lubricate parts of the system.
But of course, there are disadvantages too. Hydraulic systems require pipes, valves, seals, filters and hydraulic fluid, making the distribution network more complicated than an electrical installation. Maintenance and inspection can also be more demanding and challenging.
Summary
An aircraft hydraulic system is used when the aircraft needs a lot of force from a relatively compact and lightweight system, it uses pressurized fluid to transmit energy and generate the large forces required to operate components such as flight controls, landing gear, breaks and flaps.
It looks like this is completely everything from this system, but no, I made this significantly shorter than it was supposed to be and left out some uninteresting parts, to not get completely tangled into it and to prevent writing a complete nonsense.
Well, I hope you enjoyed this another lesson, next time we'll take a look on pneumatic system. Have a great day, rest of your day or night. Take care and see you sometime else.