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horse-power and is transmitted through the propeller
reduction gear.
19. A torquemeter system is shown in fig. 12-4. In
this system, the axial thrust produced by the helical
gears is opposed by oil pressure acting on a number
of pistons; the pressure required to resist the axial
thrust is transmitted to the indicator.
20. In addition to providing an indication of engine
power; the torquemeter system may also be used to
automatically operate the propeller feathering
system if the torquemeter oil pressure falls due to a
power failure. It is also used, on some installations,
to assist in the automatic operation of the water
injection system to restore or boost the take-off
power at high ambient temperatures or at high
altitude airports (Part 17).
Engine speed
21. All engines have their rotational speed (r.p.m.)
indicated. On a twin or triple-spool engine, the high
pressure assembly speed is always indicated; in
most instances, additional indicators show the speed
of the low pressure and intermediate pressure
assemblies.
22. Engine speed indication is electrically
transmitted from a small generator, driven by the
engine, to an indicator that shows the actual
revolutions per minute (r.p.m.), or a percentage of
the maximum engine speed (fig. 12-5). The engine
speed is often used to assess engine thrust, but it
Controls and instrumentation
138
Fig. 12-5 Engine speed indicators and
generator.
does not give an absolute indication of the thrust
being produced because inlet temperature and
pressure conditions affect the thrust at a given
engine speed.
23. The engine speed generator supplies a threephase
alternating current, the frequency of which is
dependent upon engine speed. The generator output
frequency controls the speed of a synchronous
motor in the indicator, and rotation of a magnet
assembly housed in a drum or drag cup induces
movement of the drum and consequent movement of
the indicator pointer,
24. Where there is no provision for driving a
generator, a variable-reluctance speed probe, in
conjunction with a phonic wheel, may be used to
induce an electric current that is amplified and then
transmitted to an indicator (fig. 12-6). This method
can be used to provide an indication of r.p.m. without
the need for a separately driven generator, with its
associated drives, thus reducing the number of
components and moving parts in the engine.
25. The speed probe is positioned on the
compressor casing in line with the phonic wheel,
which is a machined part of the compressor shaft.
The teeth on the periphery of the wheel pass the
probe once each revolution and induce an electric
current by varying the magnetic flux across a coil in
the probe. The magnitude of the current is governed
by the rate of change of the magnetic flux and is thus
directly related to engine speed.
26. In addition to providing an indication of rotor
speed, the current induced at the speed probe can
be used to illuminate a warning lamp on the
instrument panel to indicate to the pilot that a rotor
assembly is turning. This is particularly important at
engine start, because it informs the pilot when to
open the fuel cock to allow fuel to the engine. The
lamp is connected into the slatting circuit and is
illuminated during the starting cycle.
Turbine gas temperature
27. The temperature of the exhaust gases is always
indicated to ensure that the temperature of the
turbine assembly can be checked at any specific
operating condition. In addition, an automatic gas
temperature control system is usually provided, to
ensure that the maximum gas temperature is not
exceeded (Part 10).
28. Turbine gas temperature (T.G.T.) sometimes
referred to as exhaust gas temperature (E.G.T.) or jet
pipe temperature (J.P.T.), is a critical variable of
engine operation and it is essential to provide an
indication of this temperature. Ideally, turbine entry
temperature (T.E.T.) should be measured; however,
because of the high temperatures involved this is not
practical, but, as the temperature drop across the
turbine varies in a known manner, the temperature at
the outlet from the turbine is usually measured by
suitably positioned thermocouples. The temperature
may alternatively be measured at an intermediate
stage of the turbine assembly, as shown in fig. 12-7.
29. The thermocouple probes used to transmit the
temperature signal to the indicator consist of two
wires of dissimilar metals that are joined together
inside a metal guard tube. Transfer holes in the tube
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