Hydraulic systems(power and user)

Question 1

Hydraulic principle

Answer 1

Pascal’s principle In an enclosed system, the pressure of the liquid is felt undiminished on all surfaces

Question 2

Hydraulic power generation

Answer 2

hydraulic pumps provide hydraulic power pumps can be driven by the engine or by an electric motor

Question 3

Different types of pumps used

Answer 3

Gear pump Fixed displacement pump Variable displacement pump

Question 4

Gear pumps

Answer 4

used in low-pressure applications relevant with benevolent wear characteristics

Question 5

Fixed displacement pump

Answer 5

moves a fixed amount of fluid at each stroke axial pumps are an example used in aircraft

Question 6

Variable displacement pump

Answer 6

pump can change the volume of fluid moved with each stroke easy to achieve with aerial pump-change swashplate angle

Question 7

Hydraulic user systems

Answer 7

landing gear retraction and doors brakes nose wheel steering primary/secondary flight controls ramps and cargo doors flight refuelling probe stowage

Question 8

Design requirements of a hydraulic user system

Answer 8

pressure requirement type of fluid/properties redundancy fluid direction space/weight

Question 9

Properties of a hydraulic fluid

Answer 9

non-corrosive light non-compressable appropriate viscosity non-flammable

Question 10

Simple power system

Answer 10

schematic

Question 11

Basic components

Answer 11

reservoir filter pump heat exchanger

Question 12

Reservoir

Answer 12

stores fluid cools fluid de-aerates returning fluid

Question 13

Filter

Answer 13

5-10 micro-filtration rate

Question 14

Pump

Answer 14

constant pressure, variable volume maintains system working pressure either 207,276 or 345 bar

Question 15

Heat exchanger

Answer 15

controls temperature of hydraulic fluid

Question 16

Safety devices

Answer 16

NRV PRV

Question 17

NRV

Answer 17

no return valves simple spring-loaded valve only allows flow in one direction prevents backflow through a failed pump

Question 18

PRV

Answer 18

pressure relief valves simple spring-loaded valve opens when the fluid force generated by fluid pressure overcomes the spring force opens to restrict absolute maximum pressure

Question 19

Future MEA/AEA

Answer 19

MEA more decentralised hydraulics and electro hydrostatic actuation AEA no hydraulic power electro-mechanical actuation

Question 20

Linear actuation

Answer 20

used for primary flight controls, landing gear

Question 21

Balanced actuator

Answer 21

equal output of force for a given pressure due to equal piston area e.g. primary flight controls

Question 22

Double-acting actuator

Answer 22

unequal output force for a given pressure due to the unequal piston area e.g. landing gear retraction, spoilers, air brakes

Question 23

Single-acting actuator

Answer 23

hydraulic pressure opposes datum spring which acts on spring e.g. landing gear doors

Question 24

Control

Answer 24

need to control hydraulic flow from a centralised power system to provide required actuator movement

Question 25

Servo-valve

Answer 25

a control device that works by comparing the input with output data from a feedback loop, any difference creates an error signal to drive the device when the error is zero, the device stops operating provides both directional and rate of movement control

Question 26

Selector valve

Answer 26

provides only directional control rate of movement fixed by design using flow restrictors

Question 27

Hydraulic accumulator

Answer 27

schematic When pressurised, the gas is compressed and when depressurised, the gas expands

Question 28

Benefits of hydraulic accumulator

Answer 28

emergency power source smooth out pressure fluctuations from power system provide initial impetus for the operation of the system allow for thermal expansion of the fluid