Turbine Engine Performance

Question 1

Power

Answer 1

Amount of torque or thrust being output by the engine.

Question 2

Thrust

Answer 2

Amount of propulsive force being generated by a turbofan or turbojet - F = ma

Question 3

Torque

Answer 3

Amount of useful force =that is being generated by a turboprop or turboshaft engine.

Question 4

Gross thrust

Answer 4

• Total thrust of a jet engine without deduction of the drag caused by the momentum of the incoming air
• AKA static thrust (measured when the engine is not moving)
• used to compare turbine engines, gross thrust is measured at MSL under ISA conditions

Question 5

Net thrust

Answer 5

Gross thrust minus the drag the engine produces and the reverse thrust produced in the compression section. Practicable amount of thrust.

Question 6

Thrust horsepower (THP)

Answer 6

• Thrust on an engine and propeller combination
• Shaft horse power of the engine corrected for propeller efficiency losses
• Only for turboprop and turboshaft - force/velocity equivalent to thrust of a turbofan

Question 7

Shaft horsepower (SHP)

Answer 7

• Horse power generated by a turboprop or turboshaft as measured at the shaft
• Greater than THP

Question 8

Turboprop Turboshaft

Answer 8

Shaft HP - Prop Losses = Thrust HP

Question 9

Turbojets Turbofan

Answer 9

Gross thrust - Drag, Reverse Flow = net thrust

Question 10

Equivalent shaft horsepower (ESHP)

Answer 10

Shaft HP + residual exhaust thrust

Question 11

Specific fuel consumption (SFC)

Answer 11

• Fuel consumer over the amount of thrust generated
• Measure of engine efficiency
• SFC = fuel flow/thrust

Question 12

Airspeed

Answer 12

• Intially engine thrust drops with engine speed
• Due to induction drag at the engine intake
• effect become noticeable above 300kt TAS
• 500hts the increase in air pressure occurring at the inlet caused by ‘ram effect’ offsets the induction drag - recovery thrust

Question 13

Altitude

Answer 13

• Air at high altitude us at a lower pressure than that at sea level
• Density of the air is also less at high altitude

Question 14

Humidity

Answer 14

• Humid air is less dense than dry air
• Thrust lost as a result of high humidity is much less than that for air temperature rise, and in practical terms is negligible and can be disregarded

Question 15

Bleed air

Answer 15

• Reduces efficiency and power

Question 16

Thrust augmentation - water/methanol injection

Answer 16

• Spray of water methanol mix is injected at the compressor inlet and/or the combustion chamber inlet
• reduces the temperature of the air increasing its density and so increasing thrust
• Methanol also burns increasing combustion. To the maximum temperatures without added fuel

Question 17

Thrust augmentation - after burning

Answer 17

• Only used in high performance fighter jets
• Produces large amounts of thrust at the cost of a huge increase in fuel flow
• Mounted after the turbine outlet and injects fuel into the airstream to burn the unburnt oxygen remaining in the air

Question 18

Turboprops

Answer 18

• Efficient at relatively low speeds and altitudes
• Don’t experience ram recovery and increased drag due compressibility at the propeller tip at higher speeds
• Maximum propulsive efficiency is around 80%

Question 19

High bypass ratio turbofan

Answer 19

• More efficient the faster they go
• Mus the balanced against increased drag of flying fast and of approaching the sound barrier
• Propulsive efficiency of around 90%

Question 20

Low bypass ratio turbofan

Answer 20

• More efficient the faster they go
• This must be balanced by the increased drag of flying faster and of approaching the sound barrier
• Produce less drag than high bypass due to their reduced size
• Can reach propulsive efficiency of around 85%

Question 21

Geared turbofan

Answer 21

• Similar to high bypass turbofans but more efficient due to the lower fans speed
• efficient can be over 90%

Question 22

Turbojet

Answer 22

• More efficient the faster you go due to ram rise effect
• Become more efficient than s turbofan they need to be travelling over 1000MPH which is supersonic
• At normal airliner speeds they are around 60% efficient

Question 23

Causes of reduction is SFC with increasing airspeed in turboprop engines

Answer 23

• Reduce in efficiency past around 300MPH due to them not benefiting from ram rise and due to the propellor tip losses at high speed
• Due to compressibility issues when approaching the speed of sound