Engines 5: Hydraulic Systems

Question 1

Pascal’s Law

Answer 1

Pressure applied to a confined liquid transmits equally in all directions (without the loss of any pressure) and acts with equal force on all surfaces

Container shape has no effect on pressure or force

Question 2

Pascal’s Law Formula

Answer 2

P = F/A

Pressure in PSI. Force in pounds. Area in inches squared.

Question 3

Force

Answer 3

A push or pull

Question 4

Pressure

Answer 4

The amount of force per unit area

Question 5

What is the relationship between linear displacement and change of force between the input and output pistons?

Answer 5

The relationship depends on the surface area of the piston faces

If the pistons have the same area, then the output force is equal to the input force.

If the pistons have different areas, then the output force is protional (or inversely proportional) to the difference between the two areas.

Question 6

What is the main purpose of the hydraulic system?

Answer 6

To multiply force/provide mechanical advantage

Question 7

What is the usual output pressure of most aircraft hydraulic systems?

Answer 7

3000 psi

Question 8

What are the usual components of a hydraulic system?

Answer 8

• A fluid supply (reservoir)
• A power supply (pump)
• Other components up to (but not including) the selector valves

Question 9

How many hydraulic systems do aircraft with hydraulic actuated flight controls usually have?

Answer 9

Two hydraulic systems

One system is for flight controls only. The other is for flight controls and utility systems (landing gear, wheel brakes)

Question 10

What is the purpose of the reservoir?

Answer 10

• Store fluid
• Heat Dissipation
• Purge air bubbles/removal of air bubbles
• Removal of FOD from system
• Thermal expansion

Question 11

What are the different types of reservoirs?

Answer 11

Pressurized and non-pressurized

Question 12

Describe a non-pressurized reservoir.

Answer 12

Vented to ambient air to prevent formation of a vacuum within the reservoir. Usually used on utility aircraft not meant for violent maneuvers

Can be installed in a pressurized part of the aircraft if the aircraft will be at higher altitudes

Question 13

Describe a pressurized reservoir.

Answer 13

Pressurized to ensure positive fluid flow at any aircraft attitude.

Question 14

What is used to pressurize a pressurized reservoir?

Answer 14

Compressor bleed air or hydraulic system pressure

Question 15

What is the purpose of a pump?

Answer 15

To generate fluid pressure in hydraulic systems

Question 16

What are the two types of pumps?

Answer 16

Hand pumps
Power pumps

Question 17

What are the two types of power pumps?

Answer 17

Variable displacement pump
Constant displacement pump

Question 18

What does a variable displacement pump do?

Answer 18

Regulates volume delivery based on system flow demands

Question 19

What does a constant displacement pump do?

Answer 19

Delivers constant fluid output for any rotational speed

Uses a pressure regulator or unloader valve

Question 20

What is the purpose of pressure regulator/unloader valves? What are they used with?

Answer 20

To regulate the hydraulic system pressure to normal limits

Used with constant displacement pumps

Question 21

What is the purpose of the accumulator?

Answer 21

• S = Acts as a shock absorber by absorbing system pressure surges
• E = Stores fluid for emergency use
• A = assists with system pressure during peak operation
• S = shutdown

Question 22

What is the purpose of a check valve?

Answer 22

To ensure one-way fluid flow

Also works with the accumulator to maintain system pressure during engine shutdown

Question 23

What is the purpose of the a selector valve?

Answer 23

To redirect fluid flow for hydraulic system operation

Question 24

What is the purpose of a relief valve?

Answer 24

To be a safety backup in case of over-pressurization in the hydraulic system

To prevent pressure build up that could lead to burst seals or damage

Question 25

What is the purpose of a hydraulic fuse?

Answer 25

To guard against leaks by isolating parts of the hydraulic system

Question 26

What is the purpose of an actuating cylinder?

Answer 26

To convert fluid energy into mechanical energy

Question 27

What is the purpose of a filter?

Answer 27

To ensure the delivery of contaminant free hydraulic fluid by removing foreign debris

Question 28

What is the purpose of a pressure gauge? A pressure switch?

Answer 28

To indicate the amount of pressure in the hydraulic system

To indicate when the amount of pressure in the hydraulic system falls below allowable limits