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Aircraft Systems 1 > Chapter 2 - Aircraft Engines > Flashcards

Chapter 2 - Aircraft Engines Flashcards

1
Q

What is the role of the turbine in a turbofan engine?

A

Turns the fan by extracting a small amount of the exhaust gas energy

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2
Q

What is the role of the turbine in a turboprop?

A

Extracts the majority of the exhaust gas to drive the propeller

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3
Q

What is the overarching purpose of the engine oil system?

A

Lubricate the main bearings

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4
Q

Apart from anti-icing, what else is engine bleed air used for?

A

Pressuring the aircraft

Climate control/ventilation

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5
Q

What attributes do you find in an axial flow compressor?

A

Advantages:
Higher compression ratio

More efficient

Smaller frontal area

Disadvantages:
More expensive

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6
Q

What is the main advantage of a turbofan engine?

A

Increased thrust at low forward speeds

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7
Q

What does an oil tank store?

A

Lubricant

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8
Q

Note - Learn the HP fuel/engine diagrams and be able to label it all.

A

Learn now

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9
Q

What happens to turboprop blades when they exceed 350kts? (strange q from lecturer, included for completeness but research more in manual)

A

Supersonic tips, leads to very high drag and inefficient engine

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10
Q

What is it called when we have a complete brake down of airflow through the engine?

A

Surge

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11
Q

What do we call a partial break down of airflow in an engine?

A

Compressor stall

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12
Q

What does a taco-generator measure?

A

Engine RPM

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13
Q

What are the advantages of a turbofan engine?

A

Increased thrust at low forward speeds

More efficient

Quieter

(Note - Check manual for any more?)

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14
Q

What are the two primary types of internal combustion engine?

A

Four-stroke cycle piston

Gas turbine cycle

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15
Q

In what way do the four-stroke cycle piston and gas turbine cycle methods of internal combustion differ?

A

Four stroke cycle piston:

  • Each step of the cycle is sperate and distinct
  • Ignition is timed to occur at the optimum point during each cycle (only 1/4 of the cycle is actually providing power)
  • Otto cycle

Gas turbine cycle:

  • Phases of power occur simultaneously and continuously
  • Ignition occurs during starting cycle and combustion is then continuous
  • Brayton cycle
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16
Q

What are the stages of the Four-Stroke cycle?

A

Suck

Squeeze

Bang

Blow

(OTTO cycle)

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17
Q

Note - Look at the common cylinder layouts in the piston engine section of the manual and learn if required.

A

Learn now

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18
Q

What are the limitations of Piston engines?

A

RPM:
- Increased RPM = Increased stress = Increased wear on engine components

Temperature:
- High cylinder temperatures lead to a breakdown in cylinder wall lubrication, excessive gas temperatures and distortion

  • High oil temperatures cause failure of cylinder lubrication
  • Oil shortage may cause inadequate lubrication and bearing failure
  • Must have minimum oil and/or cylinder head temperatures (CHT) before applying full power
  • Correct temperatures ensure proper circulation of oil and help to prevent engine damage
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19
Q

What are the main types of Gas Turbine engine?

A

Turbojet

Turbofan

Turboprop

Turboshaft

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20
Q

What is the Gas Turbine work cycle?

A

Intake

Compression

Combustion

Turbine

Exhaust

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21
Q

What is used to explain the forces and work cycle of a gas turbine engine?

A

Brayton Cycle

22
Q

What are the stages of the Brayton Cycle?

A

Induction

Compression

Combustion

Exhaust

23
Q

Explain the intake stage of a gas turbine engine?

A

Air directed by inlet into the compressor.

Supersonic aircraft intakes slow the flow to subsonic speeds.

24
Q

Explain the compression stage of a gas turbine engine?

A

Compressed by an axial or centrifugal compressor.

Increases temp and pressure + decrease voume

25
Q

Explain the combustion stage of a gas turbine engine?

A

Fuel added and burned, further increasing temperature and increases volume.

Note - Some of the compressed air bypasses the burner section and circulates within the engine to provide internal cooling

26
Q

Explain the turbine stage of a gas turbine engine?

A

Rotates due to the expanding hot air and gases, drives the compressor by means of a drive shaft.

Extracts energy, REDUCING Temp and Pressure.

27
Q

Explain the exhaust stage of a gas turbine engine?

A

Hot air and gases accelerated out the rear of the engine to provide thrust

28
Q

How does an axial compressor work?

A

Gas flows parallel to rotation axis through rotors and stators.

29
Q

How does a centrifugal compressor work?

A

Air mass is accelerated by spinning. Faster spin = More acceleration.

Stationary diffuser at outer edge of the impellor. Converts kinetic energy to potential energy.

Resultant airflow is now tangential to the intake.

30
Q

How does an axial compressor work?

A

Gas flows parallel to rotation axis through rotors and stators.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

31
Q

What is primary air?

A

Highly compressed air from the HP compressor.

Fed through main channels in the dome of the combustor and the first set of liner holes. Air is then mixed with fuel to allow combustion.

32
Q

What is secondary air?

A

Air injected into the combustion zone via the second set of liner holes.

This air completes the reaction processes, cooling the air and diluting the high concentrations of carbon monoxide and hydrogen.

33
Q

What is Dilution air?

A

Airflow injected through the holes in the liner at the END of the combustion chamber.

Helps cool the air before it reaches the turbine stages.

Air is used to produce the uniform temperature profile desired in the combustor.

34
Q

What is turbine creep?

A

Lengthening (creep) of turbine blades at very high temperatures and centrifugal loads.

Can ultimately cause failure if RPM and temperature limits are exceeded.

35
Q

What is turbine stress?

A

Uneven expansion and contraction due to rapid temperature changes increases stress in a turbine blade.

Minimising power changes and allowing temperatures to stabilise before shutdown can reduce thermal stress and prolong blade life.

36
Q

How does a turbofan engine differ from a turbojet engine?

A

Has a High Pressure (HP) stage and a Low Pressure (LP) stage.

37
Q

How does a turbofan engine differ from a turbojet engine?

A

Has a High Pressure (HP) stage and a Low Pressure (LP) stage.

HP Stage:

  • HP compressor
  • Combustion chamber
  • HP turbine
LP Stage:
- Fan
- LP compressor
- LP turbine
HP compressor and HP turbine are mounted one a separate shaft to the LP compressor and LP turbine
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
38
Q

What are the advantages of a turbofan engine?

A

Increased thrust at LOW speeds - better T/O performance

By-pass air is used for cooling

Ground clearances are generally greater than a turboprop (but not as good as a turbojet due to increased engine diameter)

Increased operating economy and range (compared to a turbojet) - More efficient

39
Q

What is the role of the Fan in a turbofan engine?

A

Compresses and accelerates the air rearwards.

40
Q

Explain how a turboprop engine works?

A

In a turboprop the turbine absorbs the majority of the gas energy to drive the propeller, leaving only a small residual jet efflux that leaves the system via the exhaust.

By comparison, in a turbojet the turbine only extracts sufficient energy from the gas to drive the compressor, the rest is used to provide a high velocity propulsive jet.

Less kinetic energy waste makes turboprops the most efficient use of a gas turbine at low/medium altitudes and speeds (up to 350kts where prop efficiency deteriorates rapidly to due the development of shock waves on the blade tips).

41
Q

Explain how a turboshaft engine works?

A

All the useful gas energy is absorbed by a turbine to produce rotary shaft power.

Lack of propulsive thrust allows the engine to be fitted anywhere in the airframe.

Engine design and layout can be compact.

Used on helicopters and hovercraft + APUs on larger aircraft

42
Q

Name a common type of turboshaft engine and the associated advantages and disadvantages.

A

Free-wheel turboshaft

Power to run the rotor is separate to the gas turbine (compressor) - I.e. 2 turbines

Adv:

  • No need for clutch
  • More versatile load bearing

Disadv:
- Slight loss of thrust caused by the exhaust stream being re-circulated

43
Q

How does the propulsive efficiency of turbojet, turbofan and turboprop engines compare?

A

Turbojet - Increases with velocity

Turbofan - Increases with velocity (steeper curve than a turbojet as it is more efficient, i.e. higher propulsive efficiency at a given velocity)

Turbo-fan - Initially the highest/steepest of all three curves, but soon drops off and then decreases when a certain velocity is reached (350kts)

NOTE - Learn and be able to label the graph in the course manual, look now.

44
Q

What is a compressor stall?

A

Occurs when the compressor operates outside its RPM pressure ratio and mass flow limits.

Air flows back on itself in the engine.

45
Q

What causes a compressor stall?

A

Airflow impact the compressor blades at the wrong angle - compressor blade ‘stalls’.

[Due to an imbalance between the airflow supply and the airflow demand, i.e. pressure ratio is incompatible with the engine RPM]

All blades in a stage are at the same angle, therefore the stage stalls.

Disrupts airflow to the next compressor stage. Can cause the next stage to stall.

46
Q

What is an engine surge?

A

If all stages of the compressor stall, the pressure in the combustion chamber is high enough to force the flame the wrong way and out the front of the engine.

Reversed flow all the ay through the engine.

47
Q

What can cause an engine surge?

A

Over fuelling

48
Q

How can engine stalls and surges be prevented?

A

Variable inlet guide vanes and variable stator vanes

Multi-spool compressors

Compressor bleeds

49
Q

What is the role of the Fan in a turbofan engine?

A

Compresses and accelerates the air rearwards.

Provides thrust from air which has bypassed the internal combustion pathway of the engine (bypass air).

50
Q

What are the indications of a compressor stall?

A

Increase in engine vibration and Turbine Gas Temperature (TGT).

51
Q

What are the indications of a compressor surge?

A

Audible noise (ranges from muffled rumblings to an abrupt explosion) and vibration

Large rise in TGT

Loss of thrust

Possible catastrophic compressor failure

Aircraft Systems 1 (6 decks)

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