Page 57 - The Jet Engine

4. The mean blade speed of a turbine has consid- operating at higher turbine inlet temperatures are
erable effect on the maximum efficiency possible for thermally more efficient and have an improved power
a given stage output. For a given output the gas to weight ratio. By-pass engines have a better
velocities, deflections, and hence losses, are propulsive efficiency and thus can have a smaller
reduced in proportion to the square of higher mean turbine for a given thrust.
blade speeds. Stress in the turbine disc increases as
the square of the speed, therefore to maintain the 5. The design of the nozzle guide vane and turbine
same stress level at higher speed the sectional blade passages is based broadly on aerodynamic
thickness, hence the weight, must be increased dis- considerations, and to obtain optimum efficiency,
proportionately. For this reason, the final design is a compatible with compressor and combustion design,
compromise between efficiency and weight. Engines the nozzle guide vanes and turbine blades are of a

Fig. 5-2 A twin turbine and shaft arrangement.

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