15.7 Exhaust

Question 1

What is the main task of the exhaust system in a jet engine?

Answer 1

To accelerate the airflow coming from the turbine to efficiently generate thrust.

Question 2

What is the primary function of the exhaust system in a turboprop engine?

Answer 2

To safely guide the exhaust gases overboard.

Question 3

True or False: The exhaust system of the APU absorbs all gas energy for driving a propeller.

Answer 3

False.

Question 4

What is the common function of the components in the exhaust section of a gas turbine engine?

Answer 4

To direct the flow of hot gases rearwards and impart a high final or exit velocity to the gases.

Question 5

List the components of the exhaust section of a gas turbine engine.

Answer 5

• Exhaust cone
• Tailpipe
• Exhaust nozzle

Question 6

What does the exhaust cone do with the exhaust gases from the turbine section?

Answer 6

It collects and converts them into a solid flow of gases, decreasing velocity slightly and increasing pressure.

Question 7

What materials are commonly used for the outer shell or duct of the exhaust cone assembly?

Answer 7

Stainless steel.

Question 8

Fill in the blank: The exhaust cone assembly consists of an outer shell, an inner cone, _______ or fins, and several tie rods.

Answer 8

three or four radial hollow struts

Question 9

What is the purpose of the radial struts in the exhaust duct?

Answer 9

To support the inner cone and straighten swirling exhaust gases.

Question 10

How does cooling air circulate within the exhaust cone?

Answer 10

From the aft end of the cone into the interior and against the face of the turbine wheel.

Question 11

What is the primary construction feature of the tailpipe?

Answer 11

It is usually constructed to be semi-flexible.

Question 12

What is the purpose of insulation blankets in the exhaust section?

Answer 12

To protect the fuselage from heat radiation and reduce heat loss from the exhaust system.

Question 13

True or False: The exhaust nozzle is primarily considered a component of the engine.

Answer 13

False.

Question 14

What layers are used in the construction of an insulation blanket?

Answer 14

• Aluminium foil
• Fibreglass

Question 15

What does the reduction of heat loss from the exhaust system improve?

Answer 15

Engine performance.

Question 16

What is the function of the tie rods in the exhaust cone assembly?

Answer 16

To aid in supporting the inner cone from the outer duct.

Question 17

Fill in the blank: The exhaust cone assembly is the _______ component of the basic engine.

Answer 17

terminating

Question 18

What is the pressure condition at the turbine wheel due to its rotation?

Answer 18

Relatively low.

Question 19

What are the two types of exhaust nozzle designs?

Answer 19

1. Converging design for subsonic gas velocities
2. Converging-diverging design for supersonic gas velocities

Question 20

What types of engines use ducted fan and unducted fan designs?

Answer 20

Turbofan engines

Question 21

How do ducted fan engines direct airflow?

Answer 21

Through closed ducts along the engine

Question 22

What is the thrust contribution percentage of fan airflow in unducted fan engines?

Answer 22

80-85%

Question 23

What is the main function of turboprop exhaust nozzles?

Answer 23

To discharge exhaust gases from the aircraft

Question 24

What percentage of thrust do turboprop exhaust nozzles provide?

Answer 24

10-15%

Question 25

What is the purpose of the tailpipe assembly in turboprop engines?

Answer 25

To add strength to the duct and smooth the gas flow

Question 26

What is the difference between fixed and variable exhaust nozzles?

Answer 26

Fixed nozzles have no moving parts; variable nozzles adjust their opening area

Question 27

What happens when the afterburner is activated in a variable area exhaust nozzle?

Answer 27

The nozzle increases its open area

Question 28

What is the primary function of a convergent exhaust nozzle?

Answer 28

To increase the velocity of exhaust gases and provide increased thrust

Question 29

What are the two limiting factors for the outlet opening of the exhaust nozzle?

Answer 29

1. Nozzle opening too big leads to thrust loss 2. Nozzle opening too small leads to choking

Question 30

What is a choked nozzle?

Answer 30

A nozzle where the flow reaches Mach 1 and cannot increase or decrease

Question 31

What is pressure thrust?

Answer 31

A differential in pressure between the inside of the nozzle and ambient air

Question 32

What is the advantage of a convergent-divergent nozzle?

Answer 32

Higher thrust at high Mach numbers due to higher pressure ratio

Question 33

What does the divergent section of a convergent-divergent nozzle do?

Answer 33

Handles gases and increases their velocity after they become supersonic

Question 34

What is the design purpose of the convergent section in a convergent-divergent nozzle?

Answer 34

To handle subsonic gases and deliver them to the throat at sonic velocity

Question 35

Fill in the blank: The convergent-divergent nozzle is used on _______ aircraft.

Answer 35

[very high-speed]

Question 36

What is the primary reason for noise suppression in aircraft gas turbine engines?

Answer 36

To suppress noise from engine exhaust gases when operating from airports or highly populated areas.

Question 37

What are the main sources of engine noise in a gas turbine engine?

Answer 37

Engine fan, compressor, and air discharge from the core of the engine.

Question 38

What is the main cause of noise produced by the engine exhaust?

Answer 38

High degree of turbulence of a high-velocity jet stream moving through a relatively quiet atmosphere.

Question 39

What type of turbulence is associated with the high-speed jet stream immediately behind the engine?

Answer 39

Fine grain turbulence producing relatively high-frequency noise.

Question 40

How does the frequency of noise change as the jet stream slows down?

Answer 40

It produces lower frequency noise as the turbulence becomes coarser.

Question 41

What is the relationship between noise frequency and distance traveled?

Answer 41

Lower frequency noise travels further than high-frequency noise.

Question 42

True or False: High-frequency noise is weakened more rapidly than low-frequency noise.

Answer 42

True.

Question 43

What effect does operating an engine at lower power settings have on noise levels?

Answer 43

It can reduce the noise level considerably.

Question 44

How does a turbofan engine compare to a turbojet in terms of noise during takeoff?

Answer 44

A turbofan is quieter during takeoff than a turbojet.

Question 45

What is a typical method for reducing noise in turbojet aircraft?

Answer 45

Changing the noise pattern or the frequency of the noise emitted by the jet nozzle.

Question 46

What are the two typical types of noise suppressors in use?

Answer 46

* Corrugated perimeter type * Multi-tube type

Question 47

What is the effect of breaking up the main jet exhaust stream into smaller jet streams?

Answer 47

Increases total perimeter of the nozzle area and reduces size of air stream eddies.

Question 48

What is the purpose of acoustic linings in the engine nacelle?

Answer 48

To convert acoustic energy into heat and suppress noise.

Question 49

What is the TAPS combustor designed to reduce?

Answer 49

Exhaust emissions, especially Oxides of Nitrogen (NOX).

Question 50

How does the TAPS combustor improve fuel/air mixing?

Answer 50

By using high-energy swirlers adjacent to the fuel nozzles.

Question 51

What are the benefits of the newly designed combustors besides reducing NOX?

Answer 51

* Lower levels of carbon monoxide * Fewer unburned hydrocarbons

Question 52

Fill in the blank: Most of the NOX is formed by the reaction of _______ and nitrogen at high temperatures.

Answer 52

oxygen

Question 53

What has led to fewer emissions from gas turbine engines?

Answer 53

Increases in gas turbine engine component efficiencies.

Question 54

What is the primary function of thrust reversers?

Answer 54

To reduce the braking distance of an aircraft during landing.

Question 55

What causes the need for thrust reversers on wet, icy, or snow-covered runways?

Answer 55

Reduced braking efficiency and longer stopping distances.

Question 56

How do thrust reversers generate braking force?

Answer 56

By deflecting the exhaust gas stream in the opposite direction.

Question 57

What is the ideal deflection angle for maximum reverse thrust?

Answer 57

180°.

Question 58

What is the typical deflection angle achieved by thrust reversers?

Answer 58

Approximately 45° to 60°.

Question 59

What type of airflow is reversed in modern turbofan engines with a high bypass ratio?

Answer 59

Only the fan airflow.

Question 60

Why are thrust reversers prohibited from being used in flight?

Answer 60

They can cause dangerous aerodynamic changes.

Question 61

What are the two main types of thrust reversers for low bypass ratio turbofan engines?

Answer 61

* Clamshell door type * Bucket door type

Question 62

How are clamshell door thrust reversers operated?

Answer 62

Pneumatically by bleed air from the HP compressor.

Question 63

What type of thrust reverser is often used in modern turbofan engines?

Answer 63

Fan reverser.

Question 64

What is the main difference between fan reversers and turbine reversers?

Answer 64

Fan reversers deflect cold fan air, while turbine reversers deflect hot core engine gas.

Question 65

How do pivoting door thrust reversers function?

Answer 65

They are hydraulically activated and close off airflow when deployed.

Question 66

What are some disadvantages of thrust reverser operation?

Answer 66

* Engine compressor stalls * Reduced braking efficiency at low speeds * Ingested exhaust gases * Whirling up dirt and dust

Question 67

What happens to the airflow when thrust reversers are deployed at low aircraft speeds?

Answer 67

The engine can ingest its own reversed airflow.

Question 68

How can the reversed airflow be optimized on twin-engine aircraft?

Answer 68

By directing the airflow at an optimum angle.

Question 69

What is the key consideration for thrust reversers on four-engine aircraft?

Answer 69

Preventing reversed airflow from being ingested by neighboring engines.

Question 70

What is thrust vectoring?

Answer 70

A technique used in military aircraft to change direction by relocating the exhaust nozzle.

Question 71

By how much can the thrust vectoring nozzle typically be moved?

Answer 71

Up and down by 20°.

Question 72

True or False: A defective thrust reverser system can still allow an aircraft to be released for service.

Answer 72

True.