Engine Exhausts

Question 1

What is the purpose of an exhaust?

Answer 1

Direct the exhaust gasses through the fuselage to the atmosphere in the required direction and at the required velocity

Question 2

What is the temperature of the gas entering the exhaust system?

Answer 2

between 550-850 deg. C

Question 3

What are the 3 main sub assemblies of an exhaust system?

Answer 3

Exhaust unit
Jet Pipe
Fixed Area Propelling nozzle

Question 4

Why is the exhaust cone in the centre of the exhaust unit?

Answer 4

To protect the rear face of the last stage turbine disk from high temperature exhaust gasses

Question 5

What is the function of exhaust support sturts?

Answer 5

To remove residual whirl velocity prior to entering the jet pipe

Question 6

Why is there a divergent duct between the exhaust cone face and the exhaust casing?

Answer 6

To reduce the gas flow velocity to approx Mach 0.5 as well as reduce the pressure and friction losses

Question 7

What is the length of the jet pipe dependent on?

Answer 7

The position of the engine on the fuselage

Question 8

What is the construction of a jet pipe and why?

Answer 8

Twin wall with an annular space between the walls to allow for cooling and insulation as the gases leaving the propelling nozzle induce a flow of air through the annular space

Question 9

Why is a jet pipe lagged?

Answer 9

To prevent any heat being transferred to the surrounding aircraft structure

Question 10

What can the jet pipe be lagged with?

Answer 10

An insulating blanket

Question 11

What shape does a fixed area propelling nozzle take and why?

Answer 11

Convergent duct to ensure gases leaving the turbine are at high velocity, pressure and temperature

Question 12

Why is the size of the propelling nozzle important?

Answer 12

Too Small - exit velocity decreases after a certain point due to back pressure build up causing the aircraft to choke, less air will be going through the intake causing compressor to slow down and can cause stall or surge

Too Big - engine rpm will speed up, more air flow being drawn in, almost 0 back flow causing the temperature of the engine to overheat

Question 13

What does the size of the propelling nozzle affect?

Answer 13

Engine rpm
Mass Airflow
Turbine entry temp
Gas pressure
Thrust

Question 14

What is a disadvantage of a fixed area propelling nozzle and give a solution

Answer 14

Problem - only efficient at low thrust conditions

Solution - Can use Variable Area propelling nozzles

Question 15

Why do engine bays require ventilation systems?

Answer 15

To help cool the engine
Prevent heat transfer to aircraft structure
Protect hardware within the engine bay
Purge flammable vapours