The Art of the Helicopter John Watkinson - Karatunov.net

230 The Art of the Helicopter
Fig. 6.23 The duplex nozzle. (a)At idle the fuel flow is small and spring 2 forces the shuttle 1 to the right
allowing fuel only through the idle nozzle. (b)As flow increases the shuttle admits fuel to the larger nozzle also.
6.21 Power turbines
The power turbine is the most highly stressed part of the engine as, like the compressor,
it has to operate at high RPM but with the additional problems of working with high
temperature gases and at higher power. Turbine inlet temperature may be between 700
and 1200 ◦ C. As the temperature is so high, the speed of sound is correspondingly high
and gas velocities in the turbine may reach 700 metres per second. The power turbines
are almost always axial to provide sufficient flow. Like compressors, power turbines
may have cascaded stages with stator vanes between. The power turbine extracts power
from the gas flow by reducing its velocity, pressure and temperature. As the gases give
up energy, they expand and so the cross-sectional area of the turbine must increase
towards the exhaust. As was seen in Chapter 3, a rotor is at its most efficient with
uniform inflow. One of the functions of the stator is to modify the radial distribution
of gas velocity and pressure so that the gas emerging from the adjacent rotor disc has
uniform pressure and velocity from root to tip. The stator vanes also induce swirl in
the direction of rotation. The rotor blades are not shaped like conventional airfoils, but
have much deeper curvature or camber characteristic of reaction wheels.
One of the greatest challenges in power turbine design is to control the temperature
of parts exposed to the hot gas flow. In general oil cooling cannot be applied to the
rotating blades because of sealing difficulties and the extremely high oil pressure that
would be built up due to the high rotational speed. Thus in practice the parts must be
air cooled. Air from the compressor can be used for this purpose. Figure 6.24 shows
that the stator vanes are hollow and can be fed with cooling air from both ends. The air
can be arranged to exhaust into the gas flow via small drillings in the face of the vane. In
this way an insulating boundary layer of air is provided which protects the thin trailing
edge of the vane.
The rotor blades are subject to extreme temperatures and extend in length considerably
due to expansion. If sufficient tip clearance is provided for expansion, losses due