Thrust

Question 1

Explain the relationship between thrust and altitude

Answer 1

Thrust decreases significantly with increased altitude due to decrease air pressure and density
Specific thrust will increase due to inlet temperature effects
Initially thrust decreases due to increased momentum drag then increases due to beneficial effects of the ram pressure rise

Question 2

How can engine thrust be measured?

Answer 2

In a test cell with a strain gauged load cell

Engine thrust cannot be measured during flight

Question 3

What is the engine pressure ratio (EPR)?

Answer 3

The ratio of Turbine exit to Compressor inlet pressure (P04/P01)

Question 4

What is the ram pressure ratio (RPR)?

Answer 4

P01/Pa

Question 5

What are the options for increasing engine thrust?

Answer 5

Re-design to allow for increased mass flow or increased T3

Temporary increase of thrust (augmentation)

Question 6

What are the methods for augmentation?

Answer 6

Liquid injection (methanol injection)
Afterburning

Question 7

Describe methanol injection

Answer 7

Spray water methanol mixture at compressor inlet during take-off and climb
Equivalent drop in compressor temperature (needs less compressor work, generates more thrust)
Secondary effect: thrust increases since mass flow increases
Partly outdated method
Weight of water not a problem - it’s used up during take off and not carried further

Question 8

Describe afterburning

Answer 8

Burn additional fuel in the jet pipe
No rotating parts therefore maximum allowable temperature is higher. Typically around 2000+ K.
Accept penalty in SFC
Moves us to right in TS diagram
Greater enthalpy difference is available so increases exit jet velocity
Greater thrust

Question 9

The additional equipment in the jet pipe causes…

Answer 9

… lots of pressure losses.

To reduce losses, air velocity is lowered by expanding the flow area in this region of engine