Hydraulics

three types of hydraulic reservoir found on aircraft?

• Unpressurised (gravity fed) – Normally fitted vertically and higher than the EDP’s, relying on gravity to feed the pumps. Often fitted to the transport fleet or helicopters.

• Pressurised (gas) – Regardless of altitude or attitude pump is supplied. Pressurised using engine bleed air or charged via nitrogen on the ground.

• Pressurised (hydraulic) – system pressure acts upon a small piston. This pressure acts on the large piston. The transmitted force supplies a low pressure within the fluid. As system pressure is applied to the small piston, the piston assy tries to move. As it moves it pressurises the fluid in the reservoir. Usually a 60:1 ratio between pistons.

Out Valve

The ACOV regulates system pressure by offloading flow from the pump to return when there is no demand from the operating services.

It is sensitive to system pressure and when system pressure is attained the ACOV will operate, trapping fluid downstream of the ACOV and offloading flow back to the reservoir.

• (variable delivery) pump during low and high system demands?
  
  

• The cylinder block containing pistons and the drive shaft rotate together. As it rotates the pistons move around the angled yoke and are drawn in and out once per revolution. This action displaces fluid in the system. The angle in the yoke can be varied to increase or decrease the pump stroke so constant pressure is maintained.

• Low pressure – spring pressure on the control piston moves the yoke to its maximum angle. The pistons move to full stroke and maximum output is delivered to the system.

• High Pressure – Pressure acting against the control piston moves the yoke back towards the minimum stroke angle. In this position a small flow through the pump is maintained for lubrication and heat dissipation.

• Providing an emergency store of hydraulic energy which normally requires an NRV to be attached to an accumulator.

• Giving initial Impetus to hydraulically operated components.

• Damping out pump oscillations.

• Compensating for temporary drop in supply pressure on selection of a service.

• Preventing chatter in the ACOV

• Compensating for thermal expansion of the fluid.
  
  

• tasks and procedures, it is essential to ensure that system pressure is dissipated. Describe two methods employed to facilitate this?
  
  

  • Pressure release valves allow system pressure to be dissipated to return and are manually operated.

  • Operating the brakes or control surfaces multiple times once hyd pumps are off will also dissipate pressure. Potentially flying controls.