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can be made in temperatures as low as -40 deg. C.
without having to pre-heat the oil.
34. Turbo-propeller engines use a slightly higher
viscosity oil due to the additional requirements of the
reduction gear and propeller pitch change
mechanism.
Lubrication
83
Rolls-Royce RB162-86
Armstrong Siddeley Mamba
The Mamba axial-flow turbo-prop was
conceived in 1945 as a 1000 hp engine. First
run in April 1946, the single Mamba eventually
went into service with the Short Seamew at
1770 ehp. A further development was the
Double Mamba, a combination of two single
Mambas in one power unit. Providing up to
3875 ehp, the Double Mamba saw service
with the Fairey Gannet.
9: Internal air system
Contents Page
Introduction 85
Cooling 86
Turbine cooling
Bearing chamber cooling
Accessory cooling
Sealing 89
Labyrinth seals
Ring seals
Hydraulic seals
Carbon seals
Brush seals
Hot gas ingestion
Control of bearing loads 91
Aircraft services 93
INTRODUCTION
1. The engine internal air system is defined as
those airflows which do not directly contribute to the
engine thrust. The system has several important
functions to perform for the safe and efficient
operation of the engine. These functions include
internal engine and accessory unit cooling, bearing
chamber sealing prevention of hot gas ingestion into
the turbine disc cavities, control of bearing axial
loads, control of turbine blade tip clearances (Part 5)
and engine anti-icing (Part 13). The system also
supplies air for the aircraft services. Up to one fifth of
the total engine core mass airflow may be used for
these various functions.
2. An increasing amount of work is done on the air,
as it progresses through the compressor, to raise its
85
pressure and temperature. Therefore, to reduce
engine performance losses, the air is taken as early
as possible from the compressor commensurate with
the requirement of each particular function. The
cooling air is expelled overboard via a vent system or
into the engine main gas stream, at the highest
possible pressure, where a small performance
recovery is achieved.
COOLING
3. An important consideration at the design stage of
a gas turbine engine is the need to ensure that
certain parts of the engine, and in some instances
certain accessories, do not absorb heat to the extent
that is detrimental to their safe operation. The
principal areas which require air cooling are the
combustor and turbine. Refer to Part 4 for combustor
cooling techniques.
4. Cooling air is used to control the temperature of
the compressor shafts and discs by either cooling or
heating them. This ensures an even temperature distribution
and therefore improves engine efficiency by
controlling thermal growth and thus maintaining
minimum blade tip and seal clearances. Typical
cooling and sealing airflows are shown in fig. 9-1.
Turbine cooling
5. High thermal efficiency is dependent upon high
turbine entry temperature, which is limited by the
turbine blade and nozzle guide vane materials.
Continuous cooling of these components allows their
environmental operating temperature to exceed the
material’s melting point without affecting the blade
and vane integrity. Heat conduction from the turbine
blades to the turbine disc requires the discs to be
cooled and thus prevent thermal fatigue and uncontrolled
expansion and contraction rates.
Internal air system
86
Fig. 9-1 General internal airflow pattern.
Internal air system
87
Fig. 9-2 Nozzle guide vane and turbine blade cooling arrangement.
6. An air cooled high pressure nozzle guide vane
and turbine blade arrangement illustrating the
cooling airflow is shown in fig. 9-2. Turbine vane and
turbine blade life depends not only on their form but
also on the method of cooling, therefore the flow
design of the internal passages is important. There
have been numerous methods of turbine vane and
turbine blade cooling which have been used
throughout the history of gas turbines. Generally,
single pass internal (convection) cooling was of great
practical benefit but development has lead to multipass
internal cooling of blades, impingement cooling
of vanes with external air film cooling of both vanes
and blades, these are shown in fig. 9-3. and fig. 9-4.
7. The ’pre-swirl nozzles’ (fig. 9-2) reduce the
temperature and pressure of the cooling air fed to the
disc for blade cooling. The nozzles also impart a
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