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This high voltage requires very good insulation
throughout the circuit. The surface discharge igniter
plug (fig. 11-13) has the end of the insulator formed
by a semi-conducting pellet which permits an
electrical leakage from the central high tension
electrode to the body. This ionizes the surface of the
pellet to provide a low resistance path for the energy
stored in the capacitor. The discharge takes the form
of a high intensity flashover from the electrode to the
body and only requires a potential difference of
approximately 2000 volts for operation.
26. The normal spark rate of a typical ignition
system is between 60 and 100 sparks per minute.
Periodic replacement of the igniter plug is necessary
due to the progressive erosion of the igniter
electrodes caused by each discharge.
27. The igniter plug tip protrudes approximately 0.1
inch into the flame tube. During operation the spark
penetrates a further 0.75 inch. The fuel mixture is
ignited in the relatively stable boundary layer which
then propagates throughout the combustion system.
RELIGHTING
28. The jet engine requires facilities for relighting
should the flame in the combustion system be extinguished
during flight. However, the ability of the
engine to relight will vary according to the altitude
and forward speed of the aircraft. A typical relight
envelope, showing the flight conditions under which
an engine will obtain a satisfactory relight, is shown
in fig. 11-14. Within the limits of the envelope, the
airflow through the engine will rotate the compressor
at a speed satisfactory for relighting; all that is
required therefore, provided that a fuel supply is
available, is the operation of the ignition system. This
is provided for by a separate switch that operates
only the ignition system.
Starting and ignition
131
Fig. 11-13 An igniter plug.
Fig. 11-14 A typical flight relight envelope.
Armstrong Siddeley Sapphire
The Sapphire originated in 1946 with the
Metrovick F9, which was handed over to
Armstrong-Siddeley when Metropolitan-
Vickers withdrew from aviation in 1947. The
Sapphire first ran in October 1948 and the
engine was flight tested in Meteor, Hastings
and Canberra aircraft; before going into
production for the Gloster Javelin and Hawker
Hunter F2.
Rolls-Royce contra-rotating fan (concept)
Contents Page
Introduction 133
Controls 133
Instrumentation 135
Engine thrust
Engine torque
Engine speed
Turbine gas temperature
Oil temperature and pressure
Fuel temperature and pressure
Fuel flow
Vibration
Warning systems
Aircraft integrated data system
Electronic indicating systems
Synchronizing and
synchrophasing 144
INTRODUCTION
1. The controls of the gas turbine engine are
designed to remove, as far as possible, work load
from the pilot while still allowing him ultimate control
of the engine. To achieve this, the fuel flow is automatically
controlled after the pilot has made the initial
power selection (Part 10).
2. All engine parameters require monitoring and
instrumentation is provided to inform the pilot of the
correct functioning of the various engine systems
and to warn of any impending failure. Should any of
the automatic governors fail, the engine can be
manually controlled by the pilot selecting the desired
thrust setting and monitoring the instruments to
maintain the engine within the relevant operating
limitations.
3. The multitude of dials and gauges on the pilot’s
instrument panel may be replaced by one or a
number of cathode ray tubes to display engine
parameters. These are small screens capable of
displaying all of the information necessary to operate
the engine safely.
CONTROLS
4. The control of a gas turbine engine generally
requires the use of only one control lever and the
monitoring of certain indicators located on the pilot’s
instrument panel (fig. 12-1). Operation of the control
(throttle/power) lever selects a thrust level which is
then maintained automatically by the fuel system
(Part 10).
133
12: Controls and instrumentation
Controls and instrumentation
134
Fig. 12-1 Pilot’s instrument panel - turbo-jet engines.
5. On engines fitted with afterburning, single lever
control is maintained, although a further fuel system
is required to supply and control the fuel to the
afterburner (Part 16).
6. On a turbo-propeller engine, the throttle lever is
interconnected with the propeller control unit
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