What are the basic operating principles of hydraulic in aircraft systems?

Hydraulic systems are commonly used in aircraft to move and articulate landing gear, flaps, and brakes. Larger aircraft will also use them to handle flight controls such as ailerons and flaps, spoilers, thrust reversers, and more. Hydraulics are popular because they transmit high amounts of pressure and force with a small volume of liquid, which is very useful in the aviation industry, where space and weight aboard an aircraft are at a premium.

Hydraulics use principles of hydrodynamics, the study of how fluids can be used to move mechanical components. One of these principles is Pascal’s Law, which states that pressure exerted on a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio remains the same. This means that small amounts of force applied to hydraulic fluid can translate to high amounts of pressure elsewhere, creating a highly efficient method for actuating components.

Hydraulic fluids must be able to transmit pressure, lubricate moving parts of the hydraulic system, and act as a coolant at the same time. Oil with these qualities will have a low viscosity for easier filling of all lines and aluminum pipes, but must retain these properties over a wide temperature range. The three types of hydraulic fluid currently used are vegetable-based, mineral-based, and synthetic-based.    

  • Vegetable-based is colored blue, and made from castor oil and alcohol. It is mostly used in older aircraft where natural rubber seals are used, until they are replaced with synthetic types. The problem with vegetable-based hydraulic fluid is that it can cause sludge and corrosion build-up.
  • Mineral-based hydraulic fluid is colored red and made from kerosene petrochemical products. This type has good lubrication properties, and additives that prevent corrosion and foaming. While it is very stable and has a low viscosity change with temperature change, it cannot be used with natural rubber and is flammable.
  • Synthetic hydraulic fluids are colored purple, green, or amber, and are not flammable. Based on man-made phosphate esters, synthetic fluid must be carefully stored and managed, as it can degrade plastics, paints, and aircraft wiring by damaging the insulation.

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March 20, 2023

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