The Art of the Helicopter John Watkinson - Karatunov.net

232 The Art of the Helicopter
Fig. 6.26 Blade creep takes place in three distinct stages as shown here. Blades thus have a finite life.
are accurately determined such that when at service speed the strain in the root
distributes the thrust over all of the teeth. At rest the blade will be loose in the hub disc.
The rotational stress and temperature experienced by the blade cannot be sustained
indefinitely. Under these conditions, all known materials slowly extend or creep.
Figure 6.26 shows the creep characteristic of a typical blade. The initial extension or
primary creep is rapid, followed by a more stable regime where the secondary creep is
relatively slow. At the end of this stage, tertiary creep sets in, leading to failure. There
are several consequences of this characteristic. The first is that all turbine engines have
a finite service life before the blades need replacing. The second is that considerate
piloting that avoids excessive TOT will improve the reliability of the engine. Finally
in an emergency it is possible to extract substantially more power from a turbine for
a short time than it can reliably sustain. The result will be accelerated creep that will
diminish or even use up the service life of the engine, but the helicopter can still fly to
safety. This topic will be considered further later in this chapter.
6.22 The turbine oil system
The turbine is mechanically straightforward as there are few moving parts. The rotating
parts must be carried on bearings designed to withstand the very high speed. Bearings
adjacent to the power turbines must be protected from the enormous temperature
of the gases flowing past. This is achieved by oil cooling. Heat flows towards the
bearings because of the temperature difference, but the oil carries the heat away at
the rate it arrives and so the temperature can be controlled. As there are no heavy
reciprocating masses, the major function of the oil is cooling and turbine oil is only
about one-tenth the viscosity of piston engine oil so that large volumes can be pumped to
transfer heat. The low viscosity oil is also an advantage when starting in extremely cold
conditions. An oil cooler is used to transfer the heat from the oil to the surrounding air.
In helicopters the oil cooler may be provided with airflow from a transmission-driven
fan. The same fan may also cool the transmission oil. Hot oil from the engine may also
be used for anti-icing. In some engines, the oil tank forms part of the air intake.
It is vital that the correct shutdown procedure is followed for a turbine engine. The
engine must run off load for a time after flight in order to cool down before it is stopped.
If this is not done the heat from the power turbines will soak through to the bearings