The Art of the Helicopter John Watkinson - Karatunov.net

236 The Art of the Helicopter
Fig. 6.30 In a FADEC system all relevant parameters are sensed and converted to binary codes to form inputs
to the processor. The processor then controls the fuelling.
also a FADEC function, along with automatic power balancing in the case of a twin
or multiple engine installation. FADEC systems may also be able to handle engine
failures by automatically increasing the power of the remaining engine.
Figure 6.30 shows that all of the parameters measured by the instruments and the
fuel control, including N1, N2, PC and TOT are converted to digital codes and fed to
the processor. This will determine what the fuel flow should be as well as controlling
the starter motor, fuel valves and igniters. Needless to say the power supply to the
processor becomes critical. The power controller will be wired to multiple sources so
that it can run from any one.
Additional feedforward signals may come, for example, from a sensor on the collective
lever used by the computer to anticipate a power demand to reduce transient
droop. In a conventional governor if the rotor speed goes above normal the engine
power will be reduced to contain it. If autorotation is entered, the engine power may
be reduced to nearly zero and this will mean that the gas generator spool will be running
very slowly. The turbine engine takes considerable time to return to full power
from this condition. At the end of the autorotation a conventional governor would not
respond until the RRPM had fallen below normal and the result would be droop until
the turbine responded. However, in a FADEC machine the computer could anticipate
the end of the autorotation by detecting the upward movement of the collective lever
and it could start spooling up the gas generator before the RRPM fell below normal.
This would reduce the amount of droop.