Fig. 5-10 shows a twelve stage contra-rotating free power turbine driving a contra-rotating rear fan. This design has only one row of static nozzle guide vanes. The remaining nozzle guide vanes are, in effect, turbine blades attached to a rotating casing which revolves in the opposite direction to a rotating drum. Since all but one aerofoil row extracts energy from the gas stream, contra-rotating turbines are capable of operating at much higher stage loadings than conventional turbines, making them attractive for direct drive applications.
Dual alloy discs
Very high stresses are imposed on the blade root fixing of high work rate turbines, which make conventional methods of blade attachment impractical. A dual alloy disc, or ‘blisk’ as shown in fig. 5-11, has a ring of cast turbine blades bonded to the disc. This type of turbine is suitable for small high-power helicopter engines.
