6 - Hydraulics

Question 1

Vegetable based fluid (colour, composition, seals required, general use)

Answer 1

Blue coloured
Castor oil and alcohol
Used in older a/c
Requires natural rubber seals
Highly flammable

Question 2

Mineral based fluid (colour, composition, seals required, general use)

Answer 2

Red coloured
Petroleum derivative
Used widely in GA
Requires neoprene rubber (synthetic) seals
Flammable

Question 3

Synthetic fluid (colour, composition, seals required, general use)

Answer 3

Purple coloured
Civil transport a/c
Phosphate ester based
Very high flash point (will not combust)
Skydrol/aeroshell fluid 31

Question 4

Hydraulic fluid requirements

Answer 4

Low viscosity
Bulk modulus (resists change in volume under pressure/incompressible)
Chemical stability
Fire resistant

Operate at high pressure and temperature (200 bar (3000 psi), below 0 to 200 degrees)

Question 5

What materials are hydraulic lines made of?

Answer 5

Al-alloy, stainless steel, titanium

Question 6

What are the types of hydraulic systems? (3)

Answer 6

Open centre
Closed centre
Power pack

Question 7

How does an open centre system operate?

Answer 7

The system has fluid flow but no pressure when the actuating mechanisms are idle, the pump circulates fluid from the reservoir, through the selector valves and back to the reservoir in series.

When a selector valve is opened to operate a actuator the flow of fluid through the valve to the reservoir is blocked, pressure builds up in the system to overcome the resistance and move the piston of the actuating cylinder.

Question 8

What kind of pump would an open centre system use?

Answer 8

Constant delivery

Question 9

How many systems can operate at one time for open centre?

Answer 9

One

Question 10

What is the advantage of using an open centre system?

Answer 10

Prevents pump overloading

Question 11

How does a closed centre system operate?

Answer 11

Fluid is under pressure whenever the pump is operating, the actuators are arranged in parallel and multiple can operate at one time.

If a constant delivery pump is used the pressure in the system must be regulated by an unloading valve when no functions are operating (relief valve as backup).

If a variable displacement pump is used the system pressure is controlled by the pumps integral pressure mechanism.

Question 12

What is a power pack?

Answer 12

Essentially a condensed closed centre system, pump is usually driven by a reversible dc electric motor

Question 13

What are the requirements for primary and secondary system redundancy in heavy aircraft?

Answer 13

The probability of failure of a single hydraulic channel is 10^3

Primary systems are required to have a failure probability of less than 10^9 (3 channels)

Secondary systems are required to have a failure probability of less than 10^6 (2 channels)

Question 14

What type of pump is typical of large aircraft? how does it function?

Answer 14

Variable displacement, senses system pressure and adjusts delivery rate to keep it constant

Question 15

What types of constant delivery pumps are there? (3)

Answer 15

Gear type - low/med pressure/flowrate
Vane type - medium pressure
Radial or axial piston type - high pressure

Question 16

What forms of pressure controls are there? (3)

Answer 16

Relief valve
Pressure regulator
Pressure reducer

Question 17

What are the purpose of valves and what kind are there? (2)

Answer 17

Control direction and rate of flow

Selector valve (poppet or piston type)
Check valve (non return, restrictor)

Question 18

What types of actuators are there? (2)

Answer 18

Linear actuators
Rotary actuators