Page 42 - The Jet Engine

Compressors

Fig. 3-18 An electronically 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 Fig. 3-19 Typical types of fan blades.

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