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(P.C.U.), thus maintaining single lever operation of
the engine. Similarly, the throttle control lever of a
helicopter is interconnected with the collective pitch
lever, so ensuring that an increase in pitch is
accompanied by an increase in engine power,
7. The fuel system (Part 10) incorporates a high
pressure fuel shut-off cock to provide a means of
stopping the engine. This may be operated by a
separate lever, interconnected with the throttle lever,
or electrically actuated and controlled by a switch on
the pilot’s instrument panel.
8. A turbo-jet engine fitted with a thrust reverser
usually has an additional control lever that allows
reverse thrust to be selected (Part 15). On a turbopropeller
engine, a separate control lever is not
required because the interconnected throttle and
P.C.U. lever is operated to reverse the pitch of the
propeller.
INSTRUMENTATION
9. The performance of the engine and the operation
of the engine systems are shown on gauges or by
the operation of flag or dolls-eye type indicators. A
diagrammatic arrangement of the control and instrumentation
for a turbo-jet engine is shown in fig. 12-2.
Controls and instrumentation
135
Fig. 12-2 Diagrammatic arrangement of engine control and instrumentation.
Controls and instrumentation
136
Fig. 12-3 Electro-mechanical E.P.R. transmitter.
Engine thrust
10. The thrust of an engine is shown on a thrustmeter,
which will be one of two basic types; the first
measures turbine discharge or jet pipe pressure, and
the second, known as an engine pressure ratio
(E.P.R.) gauge, measures the ratio of two or three
parameters. When E.P.R. is measured, the ratio is
usually that of jet pipe pressure to compressor inlet
pressure. However, on a fan engine the ratio may be
that of integrated turbine discharge and fan outlet
pressures to compressor inlet pressure.
11. In each instance, an indication of thrust output is
given, although when only the turbine discharge
pressure is measured, correction is necessary for
variation of inlet pressure; however, both types may
require correction for variation of ambient air
temperature. To compensate for ambient
atmospheric conditions, it is possible to set a
correction figure to a sub-scale on the gauge; thus,
the minimum thrust output can be checked under all
operating conditions.
12. Suitably positioned pilot tubes sense the
pressure or pressures appropriate to the type of
indication being taken from the engine. The pilot
tubes are either directly connected to the indicator or
to a pressure transmitter that is electrically
connected to the indicator.
13. An indicator that shows only the turbine
discharge pressure is basically a gauge, the dial of
which may be marked in pounds per square inch
(p.s.i.), inches of mercury (in. Hg.), or a percentage
of the maximum thrust.
14. E.P.R. can be indicated by either electromechanical
or electronic transmitters. In both cases
the inputs to the transmitter are engine inlet pressure
(P1) and an integrated pressure (PINT) comprised of
Controls and instrumentation
137
Fig. 12-4 A simple torquemeter system.
fan outlet and turbine exhaust pressures. In some
cases either fan outlet pressure or turbine exhaust
pressure are used alone in place of PINT.
15. The electro-mechanical system indicates a
change in pressure by using transducer capsules
(fig. 12-3) to deflect the centre shaft of the pressure
transducer causing the yoke to pivot about the axis
A.A. This movement is sensed by the linear variable
differential transformer (L.V.D.T.) and converted to an
a.c. electrical signal which is amplified and applied to
the control winding of the servo motor.
16. The servo motor, through the gears, alters the
potentiometer output voltage signal to the E.P.R.
indicator and simultaneously drives the gimbal in the
same direction as the initial yoke movement until the
L.V.D.T. signal to the motor is cancelled and the
system stabilizes at the new setting.
17. The electronic E.P.R. system utilizes two
vibrating cylinder pressure transducers which sense
the engine air pressures and vibrate at frequencies
relative to these pressures. From these vibration
frequencies electrical signals of E.P.R. are computed
and are supplied to the E.P.R. gauge and electronic
engine control system (Part 10).
Engine torque
18. Engine torque is used to indicate the power that
is developed by a turbo-propeller engine, and the
indicator is known as a torquemeter. The engine
torque or turning moment is proportional to the
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