Rotor Blades and Stator Vanes (1021)

The rotor blades are of airfoil section and usually designed to give a pressure gradient along their length to ensure that the air maintains a reasonably uniform axial velocity. The higher pressure towards the tip balances out the centrifugal action of the rotor on the airstream. To obtain these conditions, it is necessary to ’twist’ the blade from root to tip to give the correct angle of incidence at each point. Air flowing through a compressor creates two boundary layers of slow to stagnant air on the inner and outer walls. In order to compensate for the slow air in the boundary layer a localized increase in blade camber both at the blade tip and root has been introduced. The blade extremities appear as if formed by bending over each corner, hence the term ’end-bend’.

The stator vanes are again of airfoil section and are secured into the compressor casing or into stator vane retaining rings, which are themselves secured to the casing. The vanes are often assembled in segments in the front stages and may be shrouded at their inner ends to minimize the vibrational effect of flow variations on the longer vanes. It is also necessary to lock the stator vanes in such a manner that they will not rotate around the casing.

Materials (1837)

Materials are chosen to achieve the most cost effective design for the components in question.

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, because of the high temperature requirements, 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.

Stator vanes are normally produced from steel or nickel based alloys, a prime requirement being high fatigue strength when "notched" by ingestion damage. 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.

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. 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.

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.