Hydraulic actuation offers many advantages over conventional mechanical and electrical systems, the major advantages being:
- Provision of smooth and steady movement
- Hydraulic power is confined to pipelines and components, and does not require widespread significant structural strengthening.
- The installation of hydraulic systems and components is simpler than mechanical power transmission systems.
- Variations in speed and power output can be made without the need for complex and heavy gearboxes etc
- Power for hydraulic systems can be provided from many separate sources, for both normal and emergency operation.
There are a number of fluids that have been used in aircraft systems over the years, they are in the form of an oil, which usually have a natural, mineral or synthetic base. There are advantages and disadvantages for each of these fluid types, and selection of the fluid used in a particular aircraft or system will depend upon which properties are required. Listed below are the properties required of an ideal hydraulic fluid, but it must be remembered that no one fluid provide all of them.
- It should be incompressible.
- It should have a reasonable density with little or no variation due to temperature changes.
- Its viscosity should have a low rate of change with changes in temperature.
- It should have a large working temperature range.
- It must provide good lubrication properties.
- It should not present a significant health hazard to operators.
- The flash point should be above 100°C, but should preferably be non‑flammable.
- It should not foam.
- It should be chemically stable under all operating conditions.
- It should neither harm, nor be harmed by materials used in the system pipelines and components.
- It should have a good storage life.
