Aero engines

Question 1

Explain the principle behind jet propulsion

Answer 1

• Practical application of the Netwon3
• For aircraft, the “body” is the air that is forced to accelerate when moving through the engine
• The apparatus is accelerated by the equal, oppositely directed force from moving the air

Question 2

What are the differences between jet engines and propeller-engines when producing thrust?

Answer 2

Jet engine: accelerates a small mass flow of air into a high velocity
Prop: accelerating a slow but large amount of air

Question 3

What common factors affect thrust and how?

Answer 3

• Airspeed: for low M, the thrust is decreased. For high M, around 0.8, ram effect counters this
• Pressure: when density increases the thrust increases
• Temperature: with increasing temperature the thrust decreases, as the temperature lowers the air density

Question 4

How is the bypass ratio defined for a turbofan engine?

Answer 4

The ratio of the flow through the bypass ductto the flow at the entry to the engine core

Question 5

What creates the thrust for a turbofan engine?

Answer 5

Both the cold stream thrust and the hot stream thrust

Question 6

What are advantages to the turbofan engine?

Answer 6

Bypass ratio have positive effect on propulsion efficiency and specific fuel consumption

Question 7

What are disadvantages to the turbofan engine?

Answer 7

Reduced optimum FPR and thermal efficiency
High BPR need larger fan diameter:
- Reduced ground clearance, increased risk of FOD
- Risk of supersonic speed at blade tips, which causes normal shock waves that reduces fan efficiency
- Increased noise
- Increased drag and weight

Question 8

How does the turboprop engine produce thrust?

Answer 8

Mainly through the propeller. Necessary to combine shaft power and jet thrust

Question 9

Turboprop engines are ideal for heavy payload and short TO runways, why?

Answer 9

• Delivers more thrust at low speeds
• Large propulsion efficiency

Question 10

What does reversed thrust mean and why is it used?

Answer 10

• Additional method to help brake aircraft on icy/snowy runways
• Exhaust gas stream direction is reversed by deflector doors (turbojet) or blocker door (blocks cold airstream in turbofan engines)
• Turboprops reverses thrust action by pitching prop-blades

Question 11

What is thrust augmentation? Name some common methods

Answer 11

Temporarily increasing the thrust above original design value
Liquid injection
Afterburning/reheating

Question 12

Explain liquid injection and some of its advantages/disadvantages

Answer 12

• Increases TO thrust
• Water is sprayed into compressor inlet
• Evaporation of the water cools the air and increases mass flow
• Requires a lot of liquid (non significant weight penalty if consumed at TO)

Question 13

Explain afterburning

Answer 13

• Additional fuel is burned in the jet pipe after the turbine
• Raises exhaust gas temperature, which reaches a higher velocity when discharged
• Can only be used for limited time because of increases fuel consumption
• Requires a variable area nozzle