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now more common; particularly in military engines
where its high rigidity to density ratio can result in
significant weight reduction. With the development of
new manufacturing methods component costs can
now be maintained at a more acceptable level in
spite of high initial material costs.
33. Stator vanes are normally produced from steel
or nickel based alloys, a prime requirement being a
high fatigue strength when "notched" by ingestion
damage. Earlier designs specified aluminium alloys
but because of its inferior ability to withstand damage
its use has declined. Titanium may be used for stator
vanes in the low pressure area but is unsuitable for
the smaller stator vanes further rearwards in the
compression system because of the higher
pressures and temperatures encountered. Any
excessive rub which may occur between rotating and
static components as a result of other mechanical
failures, can generate sufficient heat from friction to
ignite the titanium. This in turn can lead to expensive
repair costs and a possible airworthiness hazard.
34. In the design of rotor discs, drums and blades,
centrifugal forces dominate and the requirement is
for metal with the highest ratio of strength to density.
This results in the lightest possible rotor assembly
which in turn reduces the forces on the engine
structure enabling a further reduction in weight to be
obtained. For this reason, titanium even with its high
initial cost is the preferred material and has replaced
Compressors
31
Fig. 3-17 A pneumatically operated bleed valve system.
the steel alloys that were favoured in earlier designs.
As higher temperature titanium alloys are developed
and produced they are progressively displacing the
nickel alloys for the disc and blades at the rear of the
system.
35. The high by-pass ratio fan blade (fig. 3-19) only
became a design possibility with the availability of
titanium, conventional designs being machined from
solid forgings. A low weight fan blade is necessary
because the front structure of the engine must be
able to withstand the large out of balance forces that
would result from a fan blade failure. To achieve a
sufficiently light solid fan blade, even with titanium,
requires a short axial length (or chord). However,
with this design, the special feature of a mid-span
support (’snubber’ or ’clapper’) is required to prevent
aerodynamic instability. This design concept has the
disadvantage of the snubber being situated in the
supersonic flow where pressure losses are greatest,
resulting in inefficiency and a reduction in airflow.
This disadvantage has been overcome with the
introduction of the Rolls-Royce designed wide chord
fan blade; stability is provided by the increased chord
of the blade thus avoiding the need for snubbers.
The weight is maintained at a low level by fabricating
Compressors
32
Fig. 3-18 An electronically operated bleed valve system.
Fig. 3-19 Typical types of fan blades.
the blade from skins of titanium incorporating a
honeycomb core.
36. Centrifugal impeller material requirements are
similar to those for the axial compressor rotors.
Titanium is thus normally specified though aluminium
may still be employed on the largest low pressure
ratio designs where robust sections give adequate
ingestion capability and temperatures are acceptably
low.
BALANCING
37. The balancing of a compressor rotor or impeller
is an extremely important operation in its
manufacture. In view of the high rotational speeds
and the mass of materials any unbalance would
affect the rotating assembly bearings and engine
operation. Balancing on these parts is effected on a
special balancing machine, the principles of which
are briefly described in Part 25.
Compressors
33
Rolls-Royce RB211 Trent
Rolls-Royce RB41 Nene
On 17 March 1944 Rolls-Royce commenced
work on the RB40 as the result of a
Government request for a turbo-jet of 4200 lb
thrust. After discussions with Supermarine,
the airframe designers, the engine was scaled
down to produce 3400 lb. The resulting Nene
was eventually rated at 5000 lb and powered
the Hawker Sea Hawk and Supermarine
Attacker.
4: Combustion chambers
Contents Page
Introduction 35
Combustion process 36
Fuel supply 38
Types of combustion chamber 38
Multiple combustion chamber
Tubo-annular combustion chamber
Annular combustion chamber
Combustion chamber
performance 41
Combustion intensity
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