Page 63 - The Jet Engine

the serrations. The blade is free in the serrations capable of operating at much higher stage loadings
when the turbine is stationary and is stiffened in the than conventional turbines, making them attractive
root by centrifugal loading when the turbine is for direct drive applications.
rotating. Various methods of blade attachment are
shown in fig. 5-9; however, the B.M.W. hollow blade Dual alloy discs
and the de Laval bulb root types are not now 22. Very high stresses are imposed on the blade
generally used on gas turbine engines. root fixing of high work rate turbines, which make
conventional methods of blade attachment
20. A gap exists between the blade tips and casing, impractical. A dual alloy disc, or 'blisk' as shown in
which varies in size due to the different rates of fig. 5-11, has a ring of cast turbine blades bonded to
expansion and contraction. To reduce the loss of the disc. This type of turbine is suitable for small high
efficiency through gas leakage across the blade tips, power helicopter engines.
a shroud is often fitted as shown in fig. 5-1. This is
made up by a small segment at the tip of each blade COMPRESSOR-TURBINE MATCHING
which forms a peripheral ring around the blade tips.
An abradable lining in the casing may also be used 23. The flow characteristics of the turbine must be
to reduce gas leakage as discussed in Part 9. Active very carefully matched with those of the compressor
Clearance Control (A.C.C.) is a more effective to obtain the maximum efficiency and performance of
method of maintaining minimum tip clearance the engine. If, for example, the nozzle guide vanes
throughout the flight cycle. Air from the compressor is allowed too low a maximum flow, then a back
used to cool the turbine casing and when used with pressure would build up causing the compressor to
shroudless turbine blades, enables higher tempera- surge (Part 3); too high a flow would cause the
tures and speeds to be used. compressor to choke. In either condition a loss of
efficiency would very rapidly occur.
Contra-rotating turbine
21. Fig. 5-10 shows a twelve stage contra-rotating MATERIALS
free power turbine driving a contra-rotating rear fan.
This design has only one row of static nozzle guide 24. Among the obstacles in the way of using higher
vanes. The remaining nozzle guide vanes are, in turbine entry temperatures have always been the
effect, turbine blades attached to a rotating casing effects of these temperatures on the nozzle guide
which revolves in the opposite direction to a rotating vanes and turbine blades, The high speed of rotation
drum. Since all but one aerofoil row extracts energy which imparts tensile stress to the turbine disc and
from the gas stream, contra-rotating turbines are blades is also a limiting factor.

Fig. 5-9 Various methods of attaching blades to turbine discs.

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