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Resource Group Mod 15 2024 > 15.2 Engine Performance > Flashcards

15.2 Engine Performance Flashcards

1
Q

How does thrust relate to Newton’s third law?

A

Thrust is produced as an equal and opposite reaction to the acceleration of air through the engine.

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2
Q

What is the formula for basic thrust, derived from Newton’s second law?

A

Thrust = (mass x acceleration) / gravity

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3
Q

What is the formula for Jet Thrust?
(The main formula used)

A

Fn = Ms (V2 - V1) / g

Where;
Ms = Mass flow (lbs/s)
V2 = Exhaust velocity
V1 = Inlet velocity
g = 32.2 ft/sec² (accel. of gravity)

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4
Q

What is Thrust equal to?

A

The amount of force required to accelerate the air mass through the engine.

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5
Q

What two factors can increase thrust?

A
  • Increasing the mass flow of air into the engine.
  • Increasing the exhaust gas velocity.
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6
Q

What is Gross (Static) Thrust?

A

The thrust that develops when an engine is on the ground and stationary.

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7
Q

What is the definition of Net Thrust?

A

The ‘actual’ thrust at any given moment.

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8
Q

What is the equation for Net Thrust?

A

Net Thrust = Gross Thrust - Ram Drag

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9
Q

What is a Choked Nozzle?

A

The use of a converging duct in the exhaust section, to increase the velocity of the exhaust gas using Bernoulli’s principle.

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10
Q

Why is a Choked Nozzle used?

A

When an exhaust nozzle is convergent, the velocity of gases leaving the rear of the engine increases (Bernoulli). This is required for maximum thrust. The nozzle is said to be choked and ‘choked nozzle thrust’ is produced.

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11
Q

In what part of the engine are divergent ducts used? (Decreasing velocity and increasing pressure).

A

Compressor and diffuser sections.

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12
Q

Equation for Thrust Horsepower?

A

THP = (force x speed) / 375

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13
Q

Why is 375 used in the THP equation?

A

Because 1lb of Thrust develops 1THP at 375mph.

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14
Q

How do the loads shift as they move through and engine?

A

In the forward part of the engine, loads are in the forward direction.

As the gases go throughout the nozzle guide vanes and the turbine, loads shift to the rearward direction.

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15
Q

As power is extracted, what force continues to develop in the engine?

A

Drag, which continues as the gases make their way throughout the exhaust section.

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16
Q

What is Resultant Thrust?

A

The resultant thrust of the engine is the sum of the thrust forces of all the engine main components, including the compressor, combustion chamber, turbine, and exhaust sections.

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17
Q

Which direction should Resultant Thrust act in normal circumstances?

A

The Forward direction.

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18
Q

What is TSFC (Thrust Specific Fuel Consumption) used for?

A

Comparing different jet engines.

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19
Q

What does TSFC show?

A

It shows the amount of fuel an engine needs to make 1 kN of thrust for 1 hour.

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20
Q

How much fuel ‘per hour, per kN of thrust’ is required for a high bypass engine?

A

38kg of fuel, per hour, per kN of thrust that it produces.

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21
Q

Which types of engine require more fuel, per KN of thrust?

A

Turbojet engines, and Low Bypass engines.

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22
Q

What is the equation for TSFC in the International System?

A

38 x (kg / kN x h)

38kg per hour, per kN.

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23
Q

What is the equation for TSFC in the Old Metric System?

A

0.38 x (kg / kP x h)

0.38kg per hour, per Kp.

24
Q

What is the equation for TSFC in the English/American System?

A

0.38 x (lbs / lbs x h)

38lbs per hour, per lb.

25
Q

How is a lot of the energy lost within an engine?

A

From the high temperature of the exhaust gas.

26
Q

Definition of Internal Efficiency?

A

How much of fuel energy is converted into kinetic energy.

27
Q

Internal Efficiency Equation?

A

Internal Efficiency = Kinetic energy / Fuel energy

28
Q

Definiton of External Efficiency?

A

How much of kinetic energy is converted into aircraft movement.

29
Q

Equation for External Efficiency?

A

External Efficiency = Propulsive energy / Kinetic energy

30
Q

Definition of Total Efficiency?

A

The product of its internal and external efficiency.

31
Q

Equation for Total Efficiency?

A

Total Efficiency = Propulsive energy / Fuel energy

32
Q

What ratio is high bypass and above?

A

8:1

33
Q

Equation for Bypass Ratio?

A

Bypass Ratio = Mass flow through engine bypass duct / Mass flow through engine core

34
Q

How is engine pressure ratio (EPR) displayed?

A

An EPR gauge in the flight deck.

35
Q

How is EPR calculated?

A

EPR = Engine exit pressure / Engine inlet pressure

36
Q

What is EPR used to set, in low bypass ratio aircraft?

A

For setting thrust levels and monitoring performance.

37
Q

At what speed is the exhaust gas in most operating conditions?

A

The speed of sound.

38
Q

What is the propelling nozzle said to be, when the exhaust gas reaches the speed of sound?

A

Choked.

39
Q

In a choked propelling nozzle, how can gas velocity be further increased?

A

ONLY by an increase in temperature.

40
Q

What speed does the gas exit the propelling nozzle at, in low thrust conditions?

A

Subsonic speed.

41
Q

What is Pressure Thrust?

A

The thrust produced by static pressure difference across the propelling nozzle.

The exhaust gases are at a higher static pressure than the ambient atmospheric pressure.

42
Q

What are the effects of a smaller propelling nozzle?

A

Increase in pressure, temperature, and thurst.
But an increase in chance of engine surges.

43
Q

What are the effects of a larger propelling nozzle?

A

Pressure, temperature, and thrust values may be too low.

44
Q

How is an engine rated?

A

By Thrust Performance. It’s divided into 5 catagories;

‘Maximum’:
1) T/O Thrust.
2) Climb Thrust.
3) Cruise Thrust.
4) Continuous Thrust.
5) Go-around Thrust.

45
Q

Definiton of Max Take-off Thrust?

A

Highest thrust permitted for T/O. Only permitted for a few minutes.

46
Q

Definition of Max Climb Thrust?

A

Highest thrust permitted for climb, used during climb until cruise speed is reached.

47
Q

Definition of Max Cruise Thrust?

A

Highest thrust permitted for normal cruise operation. (Max Climb N1 minus 1.9%)

48
Q

Definition of Max Continuous Thrust?

A

Highest thrust permitted for continued use, without a given time limitation. Only used for emergency conditions.

49
Q

Definition of Max Go-around Thrust?

A

Highest thrust permitted for Go-around situations. Has the same rating as Max Take-off thrust and can also only be used for a few minutes.

50
Q

What is Static Thrust?

A

The Thrust produced whilst the engine is stationary.

(Usually the thrust that builds up when intiating the take-off roll, before the aircraft actually starts to move forward).

51
Q

What is a Flat Rating?

A

The thrust performance guaranteed by the manafacturer under specific conditions.

52
Q

What are the benefits of having a Flat Rating? (5)

A
  • Prevents the crew from using max possible thrust on a cold day.
  • Makes sure that the necessary thrust for safe take-off and climb is available.
  • Increases the service life of the engine.
  • An engine of high horsepower can be restricted to a lower horsepower rating.
  • Allows aircraft to operate under more demanding conditions, without the need for extra structural strengthening.
53
Q

At a given throttle position, what effect does a hot or cold day have on the thrust produced?

A

An engine produces more thrust on a cold day, and less thrust on a warm day.

54
Q

What is the Flat Rate Temperature?

A

An upper temperature limit, where a flat-rated engine gives a constant thrust at all ambient conditions below this temperature.

55
Q

Up to what temperature do most modern aircraft give max take-off thrust to?

A

30 - 40 degrees C.

56
Q

What happens to optimum aircraft performance as temperature and altitude increase?

A

The optimum aircraft performance decreases.

Therefore, high altitude T/O and landings on a hot day, at a high altitude airport, drastically reduce performance.

57
Q

How does high humidity affect the performance of the aircraft?

A

Less power overall. The high level of water vapour in the air reduces the amount of air available for combustion.

Resource Group Mod 15 2024 (13 decks)

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