Materials

Materials are chosen to achieve the most cost-effective design for the components in question, in practice for aero engine design this need is usually best satisfied by the lightest design that technology allows for the given loads and temperatures prevailing.

For casing designs the need is for a light but rigid construction enabling blade tip clearances to be accurately maintained ensuring the highest possible efficiency. These needs are achieved by using aluminium at the front of the compression system followed by .alloy steel as compression temperature increases. Whilst for the final stages of the compression system, where temperature requirements possibly exceed the capability of the best steel, nickel based alloys may be required. The use of titanium in .preference to aluminium and steel is 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.

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.

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 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.

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 the blade from skins of titanium incorporating a honeycomb core.

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

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.