List the advantages of a hydraulic system?
- Low weight per unit power
- Low volume
- Low initial cost
- Low maintenance cost
- High reliability
List the mechanical advantages of a hydraulic system?
- The ease with which force can be transmitted over large distances.
- The large gain in mechanical advantage made possible by varying the size of pistons.
Describe the purpose of a pressure relief valve?
- Used to limit the amount of pressure of the fluid in various parts of the system.
- Basically safety valves. Consists of a simple spring-loaded valve, which will operate and relieve pressure to the return line once it exceeds a pre-determined level.
- Also used for temperature regulation and acts as a back up to the pressure regulator.
How can valves be operated with respect to a hydraulics system?
Can be operated:
- Manually
- Electrically
- Hydraulically
- Using a combination of two or more of the above
Describe the function and purpose of a pressure regulator?
- If the output of the pump is able to flow through open lines back to the reservoir, little pressure will be developed.
- It is only when the output of the pump becomes restricted, that pressure will be generated.
- To prevent pressure from building up to where damage can occur, pressure must be regulated by releasing some fluid through a valve and allowing it to flow back through a reservoir.
Describe the function and purpose of an actuator?
- Converts pressure energy in the hydraulic fluid into a mechanical force to do work
- A linear actuator (also called a jack) consists of a cylinder and piston with an attached piston rod
- The cylinder contains a bore within which the piston operates and one or more ports through which the fluid enters and leaves the bore.
What different ‘types’ of rotary actuators are there?
- Piston type
- Vane type
- Rack & pinion type
Describe the principle of operation of a rotary actuator? Also, what are the different arc lengths of a rotary actuator?
- In a rack-and-pinion actuator, a long piston with one side machined into a rack engages a pinion to turn the output shaft. [Figure 12-54].
- One side of the piston receive fluid pressure while the other side is connected to the return.
- When the piston moves, it rotates the pinion.
- Rotary actuators are not limited to the 90° pivot arc typical of cylinders; they can achieve arc lengths of 180°, 360°, or even 720° or more, depending on the configuration.
What are the properties of a hydraulic fluid?
- Low viscosity
- Bulk modulus
- Chemical stability
- Fire resistance
- Operate at high pressure (200 bar) and high temp (below zero to around 200ºC)
Describe the properties of a vegetable base hydraulic fluid? Include its colour
- Castor oil and alcohol
- Used on older aircraft where the hydraulic system requirements are less demanding and natural rubber seals are employed
- Prone to forming sludge and causing corrosion
- COLOURED BLUE
- Highly flammable
Describe the properties of a mineral-based hydraulic fluid? Include its colour
- A kerosene-type petrochemical product that is widely used
- Good lubrication characteristics
- Chemicals to inhibit foaming and prevent corrosion
- Chemically stable
- Little viscosity change with temperature
- Cannot be used with natural rubber seals
- Flammable
- A leak under high pressure will release a spray into the aircraft which causes a fire hazard
- Dyed RED
- Neoprene rubber (synthetic) seals
- Petroleum derivative
Describe the properties of a synthetic-based fluid? Include its colour
- To overcome the flammability limitation of the mineral base fluid and to have a fluid that is better of coping with the higher pressures and temperatures in modern jet aircraft systems, a SYNTHETIC PHOSPHATE-ESTER HYDRAULIC FLUID was developed
- The most commonly used is SKYDROL
- Virtually noninflammable
- Requires very careful handling (has a high flash point)
- It is hydroscopic (tending to draw moisture from the air) and must be kept tightly sealed
- It attacks certain plastics, paint finishes and must not be allowed to drip, especially onto electrical wiring where it will break down the insulation
- The most commonly used is DYED PURPLE, other grades are dyed green or amber
- Doesn’t support combustion
- Uses ETHYLENE PROPYLENE SEALS (SYNTHETIC)
Describe what an accumulator, including its components, and basic principle of operation?
- A pressure vessel which is divided into two parts by a flexible diaphragm, bladder or a sliding piston
- One part will contain compressed air or nitrogen and the other is connected to the hydraulic pressure line
- The gas is compressed to about half the system design pressure
- Hydraulic fluid, when pressurised by the hydraulic pump is forced into the accumulator which will further compress the gas
- Once the gas is pressurised, it is able to provide a reserve of hydraulic pressure when needed
- Accumulators provide a ready store of pressure
- SMOOTHES OUT SHOCKS
Describe the function of an accumulator?
- Dampen pressure surges in the hydraulic system caused by actuation of a unit and the effort of the pump to maintain pressure at a pre-set level.
- Aid or supplement the power pump when several units are operating at once by supplying extra power from its accumulated, or stored, power.
- Store power for the limited operation of a hydraulic unit when the pump is not operating.
- Supply fluid under pressure to compensate for small internal or external (not desired) leaks that would cause the system to cycle continuously by the action of the pressure switches continually kicking in.
What happens when there are no services being used by the accumulator?
When no services are being used, the accumulator is used to hold pressure in the system after the pressure regulator has unloaded the pump.
What does stored pressure in the accumulator reduce?
Peak demand on the pump and helps to smooth out variations in system pressure.
What are the two types of accumulators?
- Bladder type
- Piston type
Describe the principle of operation of a high-pressure pneumatic system? Include the pressure it operates at.
- Engine-driven pumps (compressors) draw ambient air in through a duct, compress it and pass it through an unloading valve.
- The valve maintains the pressure in the system typically between 2900 to 3300 psi.
- When the pressure rises to 3300 psi the valve will trap it in the system and will unload the compressor’s output overboard.
- When the pressure drops to 2900 psi the output of the pump is directed back into the system.
- A ground charging valve allows the system to be pressurised when the engine is not running.
What are the advantages that compressed air has over hydraulic fluid?
- Availability
- Lightweight
- Simplicity
- No fire hazard
What is done to remove moisture from a pneumatic system?
- To prevent any ice particles from entering the system the MOISTURE will remove every bit of water from the air. The separator will collect moisture from the air and will hold onto it until the aircraft is shut down, where it is dumped overboard.
- After having about 98% of its water removed, it passes through a chemical dryer or desiccant (2%) to remove the last bits of water.
- The air is then filtered through a 10-micron sintered metal filter
- After this, the check valves will prevent the backflow of air in the system.
What system does the high-pressure system actuate?
- A smaller emergency bottle is also provided and activated when required to give the emergency lowering of the undercarriage and operation of the brakes.
- Aircraft with pneumatic brakes will have a separate brake bottle, which is charged from the main system for normal operation of the brakes.
- Also flaps.
Selector valves which are used to direct compressed air to the pneumatic jacks are operated in two ways:
- Mechanically
- Electrically
What are the different types of pneumatic jacks?
- Single-acting
- Double acting
What pressure does the medium pressure pneumatic system operate at?
50 to 150 psi
