Aircraft Hydraulic Systems

Most aircraft utilize some form of hydraulic system, which may range from simple hydraulic wheel brakes, to very large and complex systems operating a broad range of services.

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.

 

Helicopter Freewheeling Unit

The free wheel units are fitted so that the main rotor can not drive the engine (torque reversal) in the event of main rotor RPM over-running the engine  This occurs during flaring the main rotors and when the engines fail or have been throttled back for practice Autorotation during any critical situation.

There are two main types, the Roller and Sprag type.Some types can be manually selected for ground running without causing any rotation of the rotor systems.

Roller unit

This unit consists of an inner drive from the engine on which is mounted a cam ring and an outer drive to the rotor.  Circumferential interposed between the two are caged rollers which act as the driving medium.

When the transmission drive from the engine rotates, the rollers ride up the slopes of the cams and are jammed between the transmission drive shaft and the rotor drive forming a positive coupling between them.

Whenever the rotor overruns the engine the rotor outer drive of the free wheel unit is rotating faster than the inner engine drive thus releasing the rollers from their wedging action and making the rotor side of the transmission independent of the engine side.

will rotate without the engine.  The same would happen if the engine stopped.