Turbine Engine Performance

Question 1

What components of a turbine engine contribute to forward acting propulsive forces?

Answer 1

The compressor, diffuser, combustion chamber and exhaust section all contribute to forward acting propulsive forces.

Question 2

What components of a turbine engine contribute to rearward acting forces?

Answer 2

The turbine section and the propelling nozzle (if fitted) contribute to rearward acting forces.

Question 3

What factors will increase the thrust/torque output of a turbine engine?

Answer 3

1. An increase in air intake pressure/density (mass).

2. An increase in fuel flow to the combustors (acceleration).

Question 4

How does RAM effect vary a turbine engine’s thrust output?

Answer 4

Initially as airspeed increases RAM drag reduces thrust, but at higher airspeeds RAM rise (RAM effect) overcomes this and thrust increases.

Question 5

What is the result of RAM effect?

Answer 5

The kinetic energy of the intake air is converted into pressure (potential) energy, thereby increasing the mass flow of air into the engine.

Question 6

Why does an increase in fuel flow increase thrust output?

Answer 6

Because the temperature of the combustion gases increases leading directly to an increase in turbine rpm and exhaust gas velocity.

Question 7

What is the effect of increasing the thrust levers of a turbofan aircraft on takeoff?

Answer 7

Fuel flow increases causing an increase in the EGT and engine rpms (and EPR if fitted).

Question 8

What is the effect of air temperature on static (gross) thrust?

Answer 8

Increased ambient air temperature reduces static (gross) thrust.

Question 9

What is the effect of air pressure on static (gross) thrust?

Answer 9

Increased air pressure increases static (gross) thrust.

Question 10

How can the thrust output performance of a turbine engine be increased?

Answer 10

By increasing the maximum operating temperature, thereby increasing the maximum rpm.

Question 11

What limits the output of a turbine engine?

Answer 11

Temperature and rpm.

Question 12

Where is design rpm?

Answer 12

Slightly less than the maximum rpm.

Question 13

What is the effect of operating a turbine engine at design rpm?

Answer 13

SFC is minimised.

Question 14

How does an increase in altitude affect the maximum thrust output performance of a turbofan?

Answer 14

The maximum thrust output performance progressively reduces up to the tropopause, then because the ambient temperature remains constant, the rate of thrust reduction increases as the aircraft climbs through the tropopause.

Question 15

At a constant flight level, how does the maximum thrust output performance of a turbofan engine vary between 180 knots and 450 knots TAS?

Answer 15

The maximum thrust output performance initially reduces slightly due to RAM drag and then recovers to progressively increase due to RAM rise (RAM effect).

Question 16

How does an increased pressure altitude affect a turbofan engined aircraft’s takeoff distance required (TODR)?

Answer 16

TODR increases.

Question 17

How does the use of bleed air services, such as anti-icing or air-conditioning, affect the performance of a turbofan engine during a take-off?

Answer 17

For a given set of takeoff performance requirements, the required EPR and/or N1 will be increased and the EGT will be higher.

Question 18

What is the effect of an increase in the relative humidity of the intake air on engine thrust?

Answer 18

Thrust will decrease because the density of the air will decrease.

Question 19

How does thrust and fuel flow vary, at a constant engine rpm, if the air temperature increases?

Answer 19

Thrust and fuel flow will decrease because density reduces.

Question 20

When is the thrust output from a turbojet or turbofan engine maximum?

Answer 20

The thrust output from a turbojet or turbofan engine is maximum at full power stationary on the ground.

Question 21

How much power is a turbojet or turbofan engine generating at maximum thrust?

Answer 21

The maximum thrust a turbojet or turbofan engine is generating no power at all, as it is stationary it is doing no work.

Question 22

What is equivalent horsepower?

Answer 22

Shaft horsepower plus the effect of any residual jet thrust. Used in turboprops.

Question 23

What is power?

Answer 23

Power is the rate of doing work, which effectively equated to thrust times TAS.

Question 24

What is thrust horsepower (THP)?

Answer 24

A force, equating to mass x acceleration.

Question 25

How is the power/thrust output of a turbine engine controlled?

Answer 25

The power/ thrust output of a turbine engine is controlled by varying the fuel flow to the combustion chamber.

Question 26

What is normal power lever limit during a take-off in a turboprop aircraft?

Answer 26

Either the Ng; turbine temperature or torque limit, whichever occurs first.

Question 27

How do thrust and fuel flow vary in the climb at a constant engine rpm and IAS?

Answer 27

Thrust/torque and fuel flow decrease.

Question 28

How does N1 vary in the climb at constant thrust?

Answer 28

N1 increases

Question 29

Why does range increase at high ( up to optimum) altitude?

Answer 29

Because the engine operates at its design rpm at the most efficient angle of attack.

Question 30

What is specific fuel consumption (SFC)?

Answer 30

SFC is the fuel flow divided by the thrust output of an engine. That is, units of fuel used per unit of thrust per hour.

Question 31

What does a low SFC mean?

Answer 31

A low SFC means that less fuel is being used for a given thrust output, so the engine’s efficiency is high.

Question 32

What is the specific fuel consumption (SFC) if an engine in the cruise produces 5000lb of thrust and burns 2000lb of fuel over a 30 minute period?

Answer 32

2000 x 2 = 4000 lb/hr, divided by 5000 lb of thrust = a SFC of 0.8.

Question 33

What is the effect on Specific Fuel Consumption (SFC) if the compression ratio increases?

Answer 33

Specific thrust increases and SFC decreases.

Question 34

What are the two factors which determine a turbine engine’s propulsive efficiency?

Answer 34

Temperature and rpm

Question 35

What type of turbine engine is preferred for operating at cruise speeds above about 350 knots TAS, and why?

Answer 35

Turbofan engines, because turboprop engines lose efficiency at higher airspeeds.

Question 36

Which type of turbine engine has the highest propulsive efficiency in the medium to high subsonic speed ranges ( >350 knots) ?

Answer 36

A high bypass turbofan.

Question 37

Why can a ducted fan operate at high speed more efficiently than a propeller?

Answer 37

Because the ducting reduces tip losses and more blades means more power can be developed with a smller diameter.

Question 38

Why does the propulsive efficiency of a turboprop reduce with increasing airspeed?

Answer 38

Because of increasing compressibility effects on the propeller blades.

Question 39

Why is turboprop is more efficient at low airspeed than and turbojet of equal thrust?

Answer 39

Because turboprop accelerates a higher air mass to a lower velocity.

Question 40

Why can a turboprop not operate at the high airspeeds of a turbofan?

Answer 40

Because shockwaves from at the propeller blade tips at higher Mach numbers.

Question 41

What operating parameters best suit a turboprop engined aircraft?

Answer 41

Relatively shorter routes flown at relatively lower altitudes and airspeeds.

Question 42

What environmental conditions are most likely to require the use of thrust augmentation (water/methanol)?

Answer 42

High DA - i.e. high pressure altitude and high ambient temperature.

Question 43

What is the effect of injecting water/methanol into the combustion chamber?

Answer 43

It cools the air, thereby increasing the weight (mass) of the gas flow through the engine.

Question 44

Why is methanol mixed with injected water?

Answer 44

To resist the water freezing at altitude.