1. JET PROPULSION
Part 3
The Jet Engine

2. The Jet Engine The jet engine used in this section
is the Rolls-Royce Avon.
It is a relatively simple engine
known as a ‘Turbo-jet’.

3. The Jet Engine External Features
Intake
Anti-Icing Tube
Rear Compressor Case
Combustion Chambers (Cans)
Jet Pipe (Exhaust)
Oil Tank
Turbine Section
Front Compressor Casing
Carries hot air from the rear of the compressor,
to the outer casing, front vanes bullet,
as ice can build up and restrict airflow
to the engine, reducing efficiency and thrust.
Accessory Gearbox
Fuel, Oil & Hydraulic Pumps, Electrical Generators,
RPM Governor.

4. The Jet Engine External Features
Intake
Anti-Icing Tube
Compressor Handling Bleed Valve
Rear Compressor Case
Combustion Chambers (Cans)
Jet Pipe (Exhaust)
Oil Tank
Turbine Section
Front Compressor Casing
Accessory Gearbox
Allows compressor air to bleed overboard.
This prevents engine surging
in what is known as the
‘Off Design Condition’
Fuel, Oil & Hydraulic Pumps, Electrical Generators,
RPM Governor.

5. The Jet Engine External Features
Intake
Anti-Icing Tube
Compressor Handling Bleed Valve
Front Engine Mount
Rear Compressor Case
Combustion Chambers (Cans)
Jet Pipe (Exhaust)
Oil Tank
Turbine Section
Rear Engine Mount
Front Compressor Casing
Accessory Gearbox
Used to fix the engine into the aircraft,
the front mount is flexible to allow expansion
and contraction due to temperature changes.
Fuel, Oil & Hydraulic Pumps, Electrical Generators,
RPM Governor.

6. The Jet Engine
The Static Assembly

7. The Jet Engine The Static Assembly
FRONT BULLET
& SUPPORT VANES
COMBUSTION INNER CASING
COMBUSTION OUTER CASINGS
COMPRESSOR OUTER CASINGS
COMBUSTION FLAME TUBES
REAR CONE AND SUPPORT VANES
The casing is the structural frame work of the engine.
It has to be strong enough to carry the weight of all the external
fitments and accessories and to withstand the internal temperatures
and pressures (‘hoop-stress’).

8. The Jet Engine The Static Assembly
SUPPORT BEARINGS
STATOR VANES
NOZZLE
GUIDE VANES (NGV’S)
The power to weight ratio is very important, the heavier the engine,
the less payload the aircraft can carry.
Designers strive to get he most power out of
the lightest engine design.

9. The Jet Engine The Rotating Assembly
Compressor Blades
Turbine Blades
Shaft Assembly
The rotating assembly - compressor, shaft and turbine –
is carried on bearings and is known as a ‘spool’.
Located in the casings via the three bearing assemblies,
the shaft rotates at whatever rpm the combustion gases drive it.
Compressor blades generally use a ‘Dove-tail’ root fixing,
and turbine blades generally use a ‘Fir-Tree’ root fixing.

10. The Jet Engine The Rotating Assembly
Compressor Blades
Turbine Blades
Shaft Assembly
The compressor and turbine ‘drums’ are a series of discs
fixed together at the rims by bolting or welding together.
The turbine absorbs just sufficient energy from the gas stream
to drive the compressor,
leaving the remainder to provide the thrust.

11. The Jet Engine The Gas Path and Temperatures
The shaded area is called the ‘GAS PATH’.
As the air passes through the engine,
its temperature is changed,
first by compression, then by adding and burning fuel,
the burnt gasses then pass through the turbine section,
and finally through the exhaust to the atmosphere.

12. The Jet Engine Problem ! The Gas Path and Temperatures
Ambient temp
450 Deg C
2,500 Deg C
1,000 Deg C
Above ambient
Problem !
Temperature changes cause the shaft
and casings to expand and contract.
But by different amounts !
When the engine is started, the casings expand
before the rotor does.

13. The Jet Engine The Gas Path and Temperatures
Ambient temp
450 Deg C
2,500 Deg C
1,000 Deg C
Above ambient
These sections expand first
And this could put tremendous
unwanted forces on the casings, shaft and rotor,
leading to failures.

14. The Jet Engine The solution is in the bearings
The Gas Path and Temperatures
Front Bearing Chamber
Centre Bearing Chamber
Rear Bearing Chamber
Roller Bearing
Ball Bearing (Location Bearing)
Roller Bearing
The solution is in the bearings

15. Centre Bearing Chamber
The Jet Engine
The Engine Bearings
Remember the Problem
Temperature changes cause the shaft
and casings to expand and contract.
Casing
Front Bearing Chamber
Centre Bearing Chamber
Rear Bearing Chamber
Shaft
Front Bearing Chamber
Rear Bearing Chamber
Centre Bearing Chamber
The solution is in the bearings

16. The Jet Engine The Engine Bearings
Ball Bearings provide positive Axial Location
and don’t allow lateral movement between the casings and the rotor
Casing
Shaft
The bearings are selected to allow expansion
(and contraction on cool down)
without causing undue stresses.

17. The Jet Engine The Engine Bearings
These bearings use Rollers instead of ball bearings.
The outer track prevents the rollers from moving laterally.
The inner track allows lateral movement without undue stress.
Casing
Shaft
The bearings are selected to allow expansion
(and contraction on cool down)
without causing undue stresses.

18. Then, with ‘heat soak’ the shaft expands
The Jet Engine
The Engine Bearings
The casing expands first, pushing the rear outer bearing with it
Casing
Hot section
Then, with ‘heat soak’ the shaft expands
Shaft
Shaft and Casing have no differential movement at this point

19. Then, with ‘heat soak’ the shaft expands
The Jet Engine
The Engine Bearings
The same happens at the front with pressured air and heat sink from
the combustor heating the hardware
The casing expands first, pushing the rear outer bearing with it
Casing
Hot section
Then, with ‘heat soak’ the shaft expands
The engine is actually designed to dimensions
based on normal operating temperatures
Shaft
Shaft and Casing have no differential movement at this point

20. Check of Understanding
The compressor handling bleed valve
allows compressor air to bleed overboard
and so prevent engine surging.
What is this surging known as?
Hoop Stress
Off Design Condition
Root Fix Condition
Induced Flow Condition

21. Check of Understanding
What are the front and rear
engine mounts used for?
To fix the engine into the aircraft
To hold the shaft assembly in place
To allow expansion without stress
To stop ice build-up in the engine

22. Check of Understanding
Where would you expect
the fuel, oil & hydraulic pumps,
electrical generators and RPM governor
to be situated on a jet engine?
In the front bullet
In the accessory gearbox
In the rear compressor case
In the turbine section

23. Check of Understanding
Which of these does not form part of
the static assembly?
Rear support vanes
Combustion flame tubes
Compressor blades
Stator vanes

24. Check of Understanding
The casing the engine has to be strong enough
to withstand the internal pressures.
What are these pressures known as?
Ambient Pressure
Fir Tree Pressure
Root Fix Stress
Hoop Stress

25. Check of Understanding
The shaft assembly and turbine blades
are part of which engine assembly?
Rotating Assembly
Outer Casing Assembly
Static Assembly
Bearing Assembly

26. Check of Understanding
Which of these statements applies to the
centre bearing chamber?
Allows lateral movement
without undue stress
Prevents rollers from
moving laterally
Ball Bearings provide
positive axial location
Use rollers instead
of ball bearings

27. Questions?

28. JET PROPULSION
End of Presentation