15.15 Power Augmentation

Question 1

What are power augmentation systems used for?

Answer 1

To restore or boost power for improved performance

Without these systems, payload, fuel load, take-off, and climb performance would be affected.

Question 2

What is a potential drawback of fitting larger engines for increased performance?

Answer 2

Increased weight, frontal area, and specific fuel consumption

This would consequently reduce the distance the aircraft could fly.

Question 3

What factors affect the maximum power output of a gas turbine engine?

Answer 3

Density or weight of the airflow, air pressure, air temperature

If air pressure decreases or temperature increases, mass flow through the engine decreases.

Question 4

List three methods to compensate for power loss in gas turbine engines.

Answer 4

• Increasing the engine speed
• Increasing the fuel flow
• Increasing the air density

Question 5

Why might increasing the engine speed not always be a viable option?

Answer 5

Due to mechanical limitations and over speed limits

Especially true for older engines.

Question 6

How can air density be increased to compensate for power loss?

Answer 6

By cooling the air using water sprayed into the engine air intake

This cooling increases air density and decreases air temperature in the combustion chamber.

Question 7

What is the effect of decreased combustor air inlet temperature?

Answer 7

More fuel can be burnt before reaching turbine inlet temperatures

This increases engine power and recovers power loss at low air densities.

Question 8

Name a system used to augment engine power on turboprop and turboshaft engines.

Answer 8

Water methanol system

Question 9

What is a common application of water injection systems?

Answer 9

Used on turbojets

Question 10

What is the primary use of afterburners?

Answer 10

Used on military aircraft and was also used on Concorde

Question 11

Which method is most suited for engines with centrifugal compressors?

Answer 11

Compressor Inlet Injection System

Question 12

What is the disadvantage of using only water in the Compressor Inlet Injection System?

Answer 12

Water can freeze during flight at high altitudes

Question 13

What is added to the water in the water/methanol mixture to prevent freezing?

Answer 13

Methanol

Question 14

What is the typical mixture strength of water to methanol used?

Answer 14

Approximately 60% water to 40% methanol by volume

Question 15

How does the water/methanol mixture affect compressor inlet air?

Answer 15

Reduces temperature, causing air density and thrust to increase

Question 16

What does the Combustion Chamber Injection System primarily increase?

Answer 16

Mass flow through the turbine relative to the compressor

Question 17

What is the result of reduced pressure and temperature drop across the turbine?

Answer 17

Greater pressure within the jet pipe, producing more thrust

Question 18

What does the reduction in turbine gas temperature permit?

Answer 18

More heat to be added, resulting in a greater change of momentum to the gas flow

Question 19

What does increased rotational speed in the combustion system provide?

Answer 19

Further additional thrust

Question 20

What is the primary purpose of water/methanol injection systems?

Answer 20

To increase thrust for takeoff

Question 21

What is the term used for the maximum allowable thrust for takeoff when using water injection?

Answer 21

Wet thrust

Question 22

What conditions are required to obtain the wet thrust rating?

Answer 22

Predetermined turbine discharge pressure or engine pressure ratio for the prevailing ambient conditions

Question 23

What limitations are associated with the wet thrust rating?

Answer 23

Time-limited and altitude limitation

Question 24

Why are water injection systems not commonly used on turbine engines today?

Answer 24

Advancement in engine technology

Question 25

How does water injection compensate for reduced thrust on warm days?

Answer 25

By lowering the air temperature and increasing air density

Question 26

What component is actuated by the control shaft when the power lever is moved towards maximum power?

Answer 26

A microswitch in the fuel control

Question 27

What does the water injection speed reset servo do during water injection?

Answer 27

Resets the speed adjustment to a higher value

Question 28

What happens if the water injection system is not armed or if there is no water available?

Answer 28

Nothing happens when the water injection switch is switched on

Question 29

What is the role of methanol in water-methanol injection systems?

Answer 29

Acts as an antifreeze and contributes to thrust production in the combustion chamber

Question 30

Fill in the blank: Water injection systems are typically not used on _______ engines.

Answer 30

high bypass turbofan

Question 31

True or False: Water injection systems can be used at any altitude without limitations.

Answer 31

False

Question 32

What component increases the set speed during water injection?

Answer 32

Speeder spring

Question 33

What is the function of the shuttle valve in the water injection system?

Answer 33

Acted upon by water pressure during water injection

Question 34

What does the water injection speed reset servo displace to increase the force of the speeder spring?

Answer 34

A lever on the cam-operated lever linkage

Question 35

What is afterburning?

Answer 35

A method of augmenting the thrust of an engine to improve takeoff, climb, and combat performance in military aircraft.

Question 36

What are the advantages of afterburning over a larger engine?

Answer 36

It does not increase weight, frontal area, or overall fuel consumption.

Question 37

What is the maximum temperature reached by the engine during afterburning?

Answer 37

More than 1700 °C.

Question 38

How is fuel introduced in the afterburning process?

Answer 38

Fuel is introduced and burned between the engine turbine and exhaust nozzle using unburned oxygen in the exhaust gas.

Question 39

What is the purpose of the afterburning jet pipe's larger area?

Answer 39

To obtain a reduced velocity gas stream.

Question 40

What type of nozzle is fitted to an afterburning jet pipe?

Answer 40

Either a two-position or a variable area propelling nozzle.

Question 41

What happens to the nozzle during non-afterburning operation?

Answer 41

The nozzle is closed.

Question 42

What prevents an increase in pressure in the jet pipe during afterburning?

Answer 42

The nozzle opens to provide an exit area suitable for the increase in the volume of the gas stream.

Question 43

What is the role of the flame stabiliser in afterburner operation?

Answer 43

To provide a region for turbulent eddies to assist combustion and reduce gas velocity for flame stabilization.

Question 44

What initiates combustion in the afterburner?

Answer 44

An igniter.

Question 45

What is the construction of the burner section in an afterburner?

Answer 45

Several circular concentric fuel manifolds supported by struts inside the jet pipe.

Question 46

What materials are used in the construction of the afterburning jet pipe?

Answer 46

Heat-resistant nickel alloy.

Question 47

What is the purpose of insulation in the afterburning jet pipe?

Answer 47

To prevent the heat of combustion from being transferred to the aircraft structure.

Question 48

Describe the construction of the propelling nozzle in an afterburner.

Answer 48

Similar material and construction as the jet pipe, often with two movable eyelids operated by actuators.

Question 49

Fill in the blank: The afterburning process involves the introduction and burning of fuel between the _______ and exhaust nozzle.

Answer 49

[engine turbine]