Chapter 4 - Hydraulics

Question 1

Purpose of Hydraulic Systems?

Answer 1

• Relieve Pilots of fatigue

- Move heavy control surfaces

Question 2

What is Hydrostatic Pressure?

Answer 2

• The weight force of the fluid acting on itself.

- Acts in all directions.

Question 3

Hydraulics: Areas of use?

Answer 3

Ailerons
Yaw
Brakes
Flaps
Elevators
Under Carriage
Spoilers
Cargo doors and Ramps
Engine Thrust Reversers
Nose Wheel Steering

Question 4

Atmospheric pressure at sea level? (AGK)

Answer 4

1 bar (14.8psi) at 15 degrees C.

Question 5

What’s the usual range of PSI for aircraft to operate hydraulics at?

Answer 5

2000-5000psi

Question 6

What happens to pressure exerted by a liquid when it becomes pressurised?

Answer 6

It spreads in ALL directions equally.

Question 7

Pascals Law

Answer 7

• Refers to fluid as incompressible.

- Liquids are ALMOST incompressible (Up to 32Tons inches squared.

Question 8

Force Formula?

Answer 8

Force = Pressure x Area

Force - Energy Applied (Lbs)
Pressure - Energy Created (PSI)
Area - The surface of a piston (Lb Inches Sq)

Question 9

Area Formula?

Answer 9

Area = Force / Pressure

Force - Energy Applied (Lbs)
Pressure - Energy Created (PSI)
Area - The surface of a piston (Lb Inches Sq)

Question 10

Pressure Formula?

Answer 10

Pressure = Force / Area

Force - Energy Applied (Lbs)
Pressure - Energy Created (PSI)
Area - The surface of a piston (Lb Inches Sq)

Question 11

Surface area of the _____, dictates the amount of _____.

Answer 11

Surface area of the PISTON, dictates the amount of FORCE.

Question 12

Input / Output Stroke Formula

Answer 12

Input Force x Input Stroke = Output Force x Output Stroke

Question 13

Passive System (Hydraulics)

Answer 13

Powered by Human Effect - Foot on brake in car.

Question 14

Active System (Hydraulics)

Answer 14

Constant hydraulic pressure.

- Pump fed all the time.

Question 15

Closed Centred System (Hydraulics)

Answer 15

Selectors are in parallel.
- Can be used at the same time

Used in larger aircraft.

Question 16

Open Centred System (Hydraulics)

Answer 16

Selectors are in series.
- Can only use one system at a time.

Not used on larger aircraft.

Question 17

Low pressure is considered? (Hydraulics)

Answer 17

2000PSI and under

Question 18

High Pressure is considered? (Hydraulics)

Answer 18

3000-4000PSI

Question 19

Properties of Hydraulic Fluid?

Answer 19

• Low viscosity (Moves easier)
• Resistance to foaming
• Lubricator and Corrosion Resistance
• Resistance to Heat (Thermally conductive)
• Chemically stable
• Low Freezing Point (-70degrees)
• High Boiling Point (80degrees)
• Non-Flammable Flash point over 100
• Good storage properties

Question 20

Advantages of Hydraulic System over Mechanical?

Answer 20

• Leaks are easy to spot
• Reduced weight as less mechanical equipment required.
• Ability to operate heavy services with ease.

Question 21

Disadvantages of Hydraulics

Answer 21

• Highly corrosive substance
• Susceptible to fluid contamination.
• Constant exposure to Hydraulic fluid or fumes can be health risk.
• Environmentally hazardous.

Question 22

2 Commonly used Hydraulic Fluids?

Answer 22

Mineral - DTD 858 (Military)

Synthetic - Skydrol (Commercial)

Question 23

Colour of Mineral DTD 858?

Answer 23

Red

Question 24

Colour of Synthetic Skydrol?

Answer 24

Purple

Question 25

Difference between Mineral and Synthetic Hydraulic Fluid?

Answer 25

Mineral
- Synthetic Rubber Seals and Hoses only.
RED

Synthetic
- Fire resistant (Does not support combustion)
- Butyl rubber and Teflon seals
- 500 500A 700
PURPLE

Question 26

Can Hydraulic Fluids be used interchangeably?

Answer 26

No, they can’t. Different systems require the specified fluid. AFM authority.