The function of the compressor rotor assembly is to increase the pressure of the air stream which is furnished by the air intake. The rotor assembly is the most complex component of the compressor as energies of several ten-thousands of horsepower can be processed in some compressors, especially in those of high-bypass engines. This is the reason why unique methods of rotor construction are required.
Basically, there are two types of jet engine compressor rotor assembly; the drum and the disc type. Being a development of the 1950s, the drum rotor is composed of three elements, which, together with two stub shafts, are joined together by bolts to form a single unit. The discs on which the blades are held are secured to a drum, and not directly to the shaft. The connections are established with fitted bolts which transmit torque from the rear serrated stub shaft that connects to the turbine. A typical drum rotor is that of the Canadian Orenda 14 jet engine, which used to power the F-86 Sabre fighter aircraft.
Below, the General Electric J79's compressor rotor assembly.
In a disc rotor, the blades are set firmly on individual discs, with each one of them being secured directly to the rotor shaft, and not to a drum. The individual discs are in turn separated from each other by spacer rings. Contruction of a disc rotor varies, depending on the engine manufacturer. However, the principle of transmitting torque and axial loads at the same time is characteristic of any axial-flow rotor.
The rotor blades shape, like those of the stator, can be compared to a miniature aircraft wing which features the typical aerofoil section. Nevertheless, unlike an aircraft wing, the rotor blade may be highly twisted from root to tip to get the right “angle-of-attack” to the flow everywhere along the blade length.
The rationale for this twisting shape is that the root section travels much slower than the tip. The need for the blade twist arises from the requirement for constant axial velocity being kept steadily across the flow path. The length of the blades diminishes progressively downstream in the same proportion as the pressure increases.
To hold firmly the rotor blades in place, two types of blade root design are mainly employed; the fir-tree and the dovetail, both of which let blades be firmly attached to the rotor disc and still allow space for expansion during engine operation. The fir-tree design is utilized only where blade loading is high, whereas the simpler dovetail root may be of the axial type, which are mainly used in the front stages.
