The Art of the Helicopter John Watkinson - Karatunov.net

214 The Art of the Helicopter
Turbochargers only became economic when suitable materials were found which could
withstand this kind of treatment for a reasonable lifespan.
A turbocharged engine without some form of control system could be dangerous.
The more power the engine develops, the more turbocharging can be achieved, which
develops even more power and so on. In practice this is prevented by a system controlling
the induction pressure. The exhaust turbine may be bypassed by a valve known as a
wastegate 6. If the induction pressure exceeds the design limits, the wastegate actuator 5
begins to operate to cut the power received by the exhaust turbine. At sea level the air
is sufficiently dense that only a little exhaust energy is needed for charging and the
wastegate will open relatively wide. As the machine climbs, air pressure and density
fall, and the induction pressure tends to fall with it. The control system senses the fall
in pressure and starts to close the wastegate. This diverts more of the exhaust through
the turbine and drives the compressor faster until the pressure is brought back to the
sea-level value. Clearly engine power is maintained with increasing altitude until the
wastegate is completely closed, and after that, power will naturally fall off.
The compression process in the induction system raises the air temperature and
reduces its density. A further power increase can be obtained by cooling the induction
air between the turbocharger and the engine. This is done by an intercooler 9 that is
a form of heat exchanger. These are uncommon on helicopters because of the extra
complexity, but provided the weight is moderate the effects are beneficial.
It is sometimes claimed that turbocharged helicopter engines are less reliable because
the turbo stresses the engine. Whilst this is true of racing car engines, where brute power
for one race is the goal, the helicopter engine does not use the turbo so much to increase
power as to maintain the same power under all conditions. This can only be a good
thing.
From the pilot’s point of view the turbocharger does not cause any difficulty. In
the cockpit the only change is that the figures on the manifold pressure gauge can go
above sea-level pressure. There may be a boost pressure limit denoted by a red line. An
overboost warning light is sometimes found. Turbocharged machines display transient
droop. When more manifold pressure is called for, the wastegate has to close down
to increase the turbo power, and it takes a finite time for the turbocharger RPM to
increase.
Part of the pre-flight check is to inspect the turbocharger for mounting security (when
it is cold!). The rotor bearing 11 needs a supply of oil from the engine oil system 12,
and the small pipes should be checked for cracks or leaking unions.
Power loss due to high altitude or air temperature has less effect on a turbocharged
machine and when in-flight power checks are performed it will be seen that there is
more reserve power than in a naturally aspirated machine.
6.13 Gasoline engine instruments
In air-cooled gasoline engines, the cylinder head temperature (CHT) gauge is used
after starting to determine if the engine is warm enough to run properly with the rotors
engaged. After flight it is used to check that the engine has cooled down sufficiently to
prevent heat soak when it is switched off. In flight CHT rises with the power the engine
is asked to develop and if it reaches the red line on the gauge the power should be
reduced. If the CHT reaches an unreasonable value for the applied load the possibility
of an excessively weak mixture or a blockage or other defect in the cooling should be
considered.