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Resource Group Mod 15 2024 > 15.7 Exhaust > Flashcards

15.7 Exhaust Flashcards

1
Q

What are the 2 main common functions of the exhaust?

A
  • To direct hot gas flow rearwards to prevent turbulence.
  • To impart a high final/exit velocity on the gases.
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2
Q

What is the main purpose of the exhaust on a turbojet?

A

To accelerate the airflow to generate thrust.

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3
Q

What is the main purpose of the exhaust on a turboprop?

A

To safely guide exhaust gas overboard. The exhaust only makes a small portion of the total thrust.

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4
Q

What is the main purpose of the exhaust in an APU?

A

To send all the exhaust gas overboard.

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5
Q

What else does the APU exhaust do?

A

Reduces APU noise by using mufflers in the exhaust duct.

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6
Q

What are the 3 main components of the exhaust system?

A
  • The cone.
  • The tailpipe.
  • The nozzle.
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7
Q

What’s the main purpose of the exhaust cone?

A

To collect exhuast gases from the turbine section and gradually convert them into a solid flow of gases.

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8
Q

What effect on pressure and velocity can be seen at the exhaust cone area?

A

Slight decrease in velocity and slight increase in pressure.

Due to diverging passage between outer duct and inner core.

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9
Q

What structural components does the exhaust cone consist of?

A
  • Outer duct/shell.
  • Inner core.
  • Radial, hollow struts/fins.
  • Several tie rods.
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10
Q

What is the exhaust cone duct made from?

A

Stainless Steel.

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11
Q

What does the exhaust cone duct do?

A

Collects the exhaust gases and delivers them directly to the exhaust nozzle.

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12
Q

What are the 2 main purposes of radial struts in the exhaust cone?

A
  • Support the inner core.
  • Straightens the swirling exhaust gases.
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13
Q

What does a small hole in the tip of the exhaust cone allow?

A

Cooling air to circulate from the aft end of the cone, up to the face of the turbine wheel.

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14
Q

What is the terminating part of the engine? (where does the engine structure officially end).

A

The exhaust cone.

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15
Q

What type of structure is the exhaust nozzle classed as?

A

The airframe structure, NOT the engine structure.

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16
Q

What is the tailpipe constructed to be?

A

Semi-flexible.

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17
Q

What arrangement does the tailpipe have, to be semi-flexible?

A

Bellows arrangement.

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18
Q

What are the advantages of the semi-flexibility of the bellows arrangement?

A
  • Allows for movement in installation.
  • Easier maintenance.
  • Thermal expansion.
  • Reduces stresses and warping.
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19
Q

What two components protect the fuselage from high heat?

A
  • Insulation blankets.
  • Shrouds.
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20
Q

How are insulation blankets constructed?

A

Several layers of aluminium foil, each seperated by a layer of fibreglass.

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21
Q

What is the primary function of an insulation blanket?

A

To reduce heat loss from the exhaust system. (Improves performance).

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22
Q

What is the secondary function of an insulation blanket?

A

To protect fuselage from heat damage.

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23
Q

What are the two types of exhaust nozzle shape?

A
  • Converging.
  • Converging-Diverging.
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24
Q

How is a converging exhaust nozzle arranged?

A

The first section is divergent to slow and reduce airflow turbulence.

The next section is convergent to increase airflow velocity, to increase thrust.

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25
Q

What speed of airflow would converging exhaust nozzles be used for?

A

Subsonic.

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26
Q

What is a choked nozzle?

A

Where the velocity of the gases at the exhaust nozzle opening become MACH 1. The airflow does not increase or decrease from this speed.

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27
Q

What does a choked nozzle generate?

A

Pressure thrust. There is a pressure differential between inside the nozzle and the ambient pressure.

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28
Q

What is the arrangement of a converging-diverging exhaust nozzle?

A

A converging nozzle, with an extra diverging nozzle section at the aft end.

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29
Q

Where would a converging-diverging nozzle be used?

A

On aircraft where the EPR is high enough to produce exhaust gases that exceed MACH 1 velocities at the nozzle.

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30
Q

How does a converging-diverging nozzle accelerate supersonic gas flow?

A

The divergent nozzle section begins at the location where the gas flow just becomes supersonic.

Supersonic airflow velocity increases with diverging ducts (opposite to subsonic airflow) and thus the supersonic gas velocity is further increased to generate more thrust.

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31
Q

What aircraft would use a converging-diverging nozzle?

A

Very high speed aircraft.

32
Q

What parts of the engine generate noise? (3)

A
  • The inlet.
  • Vibration.
  • Exhaust.
33
Q

What produces the most noise from the aircraft?

A

Exhaust.

34
Q

What causes the noise heard from the exhaust?

A

A high-degree of turbulence of a high-velocity jet stream, moving through a quiet atmosphere.

35
Q

What type of noise is generated just behind the exhaust nozzle?

A

High frequency noise.

36
Q

What type of noise is generated futher from the exhaust nozzle as the jet stream velocity begins to slow?

A

Low frequency.

37
Q

What causes the most audible noise from the exhaust stream?

A

The low frequency noise.

Low frequencies travel further, high frequencies are absorbed by the atmosphere and buldings more easily.

38
Q

How does a low frequency generate more noise?

A

The slower air generates large turbulent swirls, where a greater portion of the energy is converted into noise energy.

39
Q

What is noise generated proportional to?

A

Thrust.

The amount of work done by the engine as the air passes through it.

40
Q

What’s louder, a larger engine at partial-thrust, or a smaller engine at full thrust?

A

A smaller engine at full thrust is louder. The exhaust velocity is the key factor in noise production.

41
Q

Why are fan engines usually quiter than turbojets?

A

They use large turbines with an addtional stage, which slows exhaust gas velocity.

Most of the thrust is also produced by fan-air (high bypass engines).

42
Q

How do turbojets reduce their noise?

A

By altering the noise pattern, or frequency.

43
Q

What two methods are used to alter the noise generated by turbojets?

A
  • Corrugated perimeter type.
  • Multi-tube type.
44
Q

How do corrugated perimeters and multi-tubes, reduce turbojet noise?

A

They break the single exhaust stream into several smaller streams.

45
Q

How does breaking a single exhaust stream into smaller streams, alter the noise pattern?

A
  • The total perimter of the nozzle area is increased.
  • Air stream eddie size reduced.
46
Q

What’s the overall effect to the noise produced from turbojets, when using these noise reducing methods?

A

Teh total noise energy remains the same, but the frequency is increased.

47
Q

What two benefits does increasing the noise frequency of turbojet exhaust, have?

A
  • The noise frequency is increased to above the point of human hearing.
  • The noise frequency is increased as higher frequencies travel shorter distances.
48
Q

What is used in the engine nacelle of turbofans, to help reduce noise?

A

Accoustic linings.

49
Q

How do accoustic linings reduce noise?

A

Convert the noise energy into heat energy.

50
Q

How are accoustic linings constructed?

A

A porous skin is supported by a honeycomb backing, which produces a seperation between the fact sheet and engine duct.

51
Q

How is optimum sound supression achieved in accoustic linings?

A

By matching the accoustic properties of the skin and the liner.

52
Q

What is a Twin-Annular Premixing Swirler (TAPS)?

A

A unique combustor design that reduces emissions.

53
Q

What’s the main component feature of a TAPS?

A

Two swirlers, adjacent to fuel nozzles, swirl the air from the HP compressor.

54
Q

How does a TAPS reduce emissions?

A

The swirled air produces a leaner fuel-air mixture, allowing it to burn at lower temperatures than in previous designs.

55
Q

What is the main principle of Thrust reversal?

A

To produce a braking force by deflecting the exhaust gas stream forwards, reducing braking distance.

56
Q

What angle is the airflow typically deflected at when using reverse thrust?

A

45-60 degrees. (180 is the ideal angle, but would never work asit would cause engine problems).

57
Q

What is true about reverse thrust when compared to the forward thrust?

A

The reverse thrust force produced is much smaller than the forward thrust.

58
Q

What airlfow is reversed in modern turbofan engines?

A

The fan (bypass) air.

59
Q

When must thrust reversers only be operated.

A

On the ground.

60
Q

On what aircraft type are clamshell type thrust reversers used?

A

Older jet engines with low bypass ratios.

61
Q

How is the Clamshell thrust reverser operated?

A

Pneumatically operated using bleed air from HP compressor.

62
Q

On what type of aircraft are Bucket thrust reversers used?

A

Older jet engines with low bypass ratios.

63
Q

How are Bucket thrust reversers operated?

A

Hydraulically.

64
Q

What are the 3 main components of a typical turbofan ‘fan-reverser’?

A
  • Translating sleeves.
  • Blocker doors.
  • Cascade vanes.
65
Q

How does a fan-reverser, reverse the fan air?

A

The translating sleeves move rearwards, uncovering exit ducts through cascade vanes.

The blocker doors block the fan duct, and direct the air to discharge through the cascade vanes and out of the exit ducts.

66
Q

How is the fan-reverser operated?

A

Pneumatically or hydraulically.

67
Q

What is the other type of fan-reverser method?

A

Pivoting door type.

68
Q

How is the pivoting door fan reverser operated?

A

Hydraulically actuated.

69
Q

How do pivoting doors reverse the fan-air?

A

They open and deflect the fan-air forwards, blocking-off the fan duct.

70
Q

What negative effects can thrust reversal cause?

A
  • As the airflow is disrupted, it can cause the compressor to stall.
  • Can create an air cushion under the wings, reducing the load on the MLG brakes.
  • Whirls up FOD.
71
Q

What can happen when using thrust reversers at very low aircraft speeds?

A

The aircraft can ingest its own reversed air, causing a rise in EGT and compressor stall.

72
Q

What can be a problem on 4-engine aircraft?

A

One engine ingesting the reversed thrust of another engine on the same wing.

73
Q

How are the negative effects of thrust reversal prevented?

A

By directing the reversed thrust away from the MLG wheel well, and away from other engines (4-engine aircraft).

74
Q

Can an aircraft be released into service with a defectice reverse thrust system.

A

Yes, so long as the system is secured.

(Reverse thrust is classifed as only an addtional braking system).

75
Q

What angle can a thrust-vectoring nozzle move by?

A

20 degrees, up or down.

Resource Group Mod 15 2024 (13 decks)

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