Hydraulic systems are used on aircraft to move and actuate landing gear, flaps and brakes. Larger aircraft use these systems also on flight controls, spoilers, thrust reversers and what not.
The reason to use hydraulics is because they are able to transmit a very high pressure or force with a small volume of fluid (hydraulic oil). Power assisted brakes is one application where you will see hydraulics the most in small experimental, lsa and ultralight aircraft.
Hydraulic systems are very reliable, its fluid is virtually incompressible and is able to transmit high pressures but its not lightweight compared to a pneumatic system.
Usually aircraft builders will put together their aircraft brake systems themselves so it is helpful to gain an understanding of how hydrodynamics work and how to handle it safely during installation and maintenance.
Pneumatic systems are not very common on experimental homebuilt aircraft but we will discus these anyway as these have their own advantages. Some manufacturers use a pneumatic system to start the engine, YAK is a good example.
Based on the Greek word for water, hydromechanics is the study on how fluids can be used to help in moving mechanical heavy or difficult to move components like ailerons, rudder or flaps against aerodynamic forces during flight.
Pascal's principle of transmission of fluid pressure states that "pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same." (Bloomfield, Louis (2006). How Things Work: The Physics of Everyday Life (Third Edition)
A good example of this is shown in the image to the right. The force (F) on the piston is given by the equation F = P × A. Thus: if F1 = 10 kg and A1 = 1 cm2 then P (fluid pressure) = F / A or 10 kg/cm2.
According to Pascal's law we have the same fluid pressure but now the area (A2) is 3 times larger. According to F = P × A it follows that force F2 = 10 kg/cm2 × 3 cm2 = 30 kg, thus 3 times larger.
This illustrates Pascal's law in a hydraulic press, and the same principle is also used in aircraft toe or hand brakes. Applied this way, you create a mechanical advantage where a small force is used to gain a larger force somewhere else.
The fluid used must transmit the pressure, lubricate the parts of the hydraulic system and act as a coolant at the same time. The oil used with these qualities has a low viscosity for easy filling of all lines and aluminum pipes. But, as the oil will heat up it must retain these properties over a wide temperature range.
Three types of hydraulic oil are in use nowadays: vegetable, mineral and synthetic based. Each of these have their own properties and requirements for special applications.
Its colored blue and made from castor oil and alcohol. It is mostly used on older aircraft where natural rubber seals are used (until they are replaced with synthetic types). The use of this fluid can also cause sludge and corrosion.
Colored red and based on a widely used kerosene petrochemical product. It has good lubricating properties with additives inhibiting corrosion and foaming. Very stable and low viscosity change with temperature changes. Note that mineral oil may not be used in combination with natural rubber and be careful: it is flammable.
This type of fluid is colored purple and less flammable than the mineral type. Base on synthetic (man made) phosphate ester and tends to attract water (moisture) and attacks certain plastics and paint. It can degrade aircraft wiring by eating away the insulation. It is known as Skydrol and other synthetic grades are dyed green or amber.