Page 41 - The Jet Engine

Compressors

Fig. 3-17 A pneumatically operated bleed valve system.

32. For casing designs the need is for a light but damage. Earlier designs specified aluminium alloys
rigid construction enabling blade tip clearances to be but because of its inferior ability to withstand damage
accurately maintained ensuring the highest possible its use has declined. Titanium may be used for stator
efficiency. These needs are achieved by using vanes in the low pressure area but is unsuitable for
aluminium at the front of the compression system the smaller stator vanes further rearwards in the
followed by .alloy steel as compression temperature compression system because of the higher
increases. Whilst for the final stages of the pressures and temperatures encountered. Any
compression system, where temperature require- excessive rub which may occur between rotating and
ments possibly exceed the capability of the best static components as a result of other mechanical
steel, nickel based alloys may be required. The use failures, can generate sufficient heat from friction to
of titanium in .preference to aluminium and steel is ignite the titanium. This in turn can lead to expensive
now more common; particularly in military engines repair costs and a possible airworthiness hazard.
where its high rigidity to density ratio can result in
significant weight reduction. With the development of 34. In the design of rotor discs, drums and blades,
new manufacturing methods component costs can centrifugal forces dominate and the requirement is
now be maintained at a more acceptable level in for metal with the highest ratio of strength to density.
spite of high initial material costs. This results in the lightest possible rotor assembly
which in turn reduces the forces on the engine
33. Stator vanes are normally produced from steel structure enabling a further reduction in weight to be
or nickel based alloys, a prime requirement being a obtained. For this reason, titanium even with its high
high fatigue strength when "notched" by ingestion initial cost is the preferred material and has replaced

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