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the jet pipe. Fuel is supplied to the manifolds by feed
pipes in the support struts and sprayed into the flame
area, between the flame stabilizers, from holes in the
downstream edge of the manifolds. The flame
stabilizers are blunt nosed V-section annular rings
located downstream of the fuel burners. An
alternative system includes an additional segmented
fuel manifold mounted within the flame stabilizers.
The typical burner and flame stabilizer shown in fig.
16-4 is based on the latter system.
Jet pipe
12. The afterburning jet pipe is made from a heatresistant
nickel alloy and requires more insulation
than the normal jet pipe to prevent the heat of
combustion being transferred to the aircraft structure.
The jet pipe may be of a double skin construction
with the outer skin carrying the flight loads and the
inner skin the thermal stresses; a flow of cooling air
is often induced between the inner and outer skins.
Provision is also made to accommodate expansion
and contraction, and to prevent gas leaks at the jet
pipe joints.
13. A circular heatshield of similar material to the jet
pipe is often fitted to the inner wall of the jet pipe to
improve cooling at the rear of the burner section. The
heatshield comprises a number of bands, linked by
cooling corrugations, to form a single skin. The rear
of the heatshield is a series of overlapping ’tiles’
riveted to the surrounding skin (fig. 16-4). The shield
also prevents combustion instability from creating
excessive noise and vibration, which in turn would
cause rapid physical deterioration of the afterburner
equipment.
Propelling nozzle
14. The propelling nozzle is of similar material and
construction as the jet pipe, to which it is secured as
a separate assembly. A two-position propelling
nozzle has two movable eyelids that are operated by
actuators, or pneumatic rams, to give an open or
closed position (para. 4.). A variable-area propelling
nozzle has a ring of interlocking flaps that are hinged
to the outer casing and may be enclosed by an outer
shroud. The flaps are actuated by powered rams to
the closed position, and by gas loads to the intermediate
or the open positions; control of the flap
position is by a control unit and a pump provides the
power to the rams (para. 18).
CONTROL SYSTEM
15. It is apparent that two functions, fuel flow and
propelling nozzle area, must be co-ordinated for satisfactory
operation of the afterburner system, These
functions are related by making the nozzle area
dependent upon the fuel flow at the burners or viceversa.
The pilot controls the afterburner fuel flow or
the nozzle area in conjunction with a compressor
delivery/jet pipe pressure sensing device (a pressure
ratio control unit). When the afterburner fuel flow is
increased, the nozzle area increases; when the
afterburner fuel flow decreases, the nozzle area is
reduced. The pressure ratio control unit ensures the
pressure ratio across the turbine remains unchanged
and that the engine is unaffected by the operation of
afterburning, regardless of the nozzle area and fuel
flow.
16. Since large fuel flows are required for afterburning,
an additional fuel pump is used. This pump is
usually of the centrifugal flow or gear type and is
energized automatically when afterburning is
selected. The system is fully automatic and incorporates
’fail safe’ features in the event of an afterburner
malfunction. The interconnection between the control
system and afterburner jet pipe is shown diagrammatically
in fig. 16-5.
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17. When afterburning is selected, a signal is
relayed to the afterburner fuel control unit. The unit
determines the total fuel delivery of the pump and
controls the distribution of fuel flow to the burner
assembly. Fuel from the burners is ignited, resulting
in an increase in jet pipe pressure (P6). This alters
the pressure ratio across the turbine (P3/P6), and the
exit area of the jet pipe nozzle is automatically
increased until the correct PS/PS ratio has been
restored. With a further increase in the degree of
afterburning, the nozzle area is progressively
increased to maintain a satisfactory P3/P6 ratio. Fig.
16-6 illustrates a typical afterburner fuel control
system.
18. To operate the propelling nozzle against the
large ’drag’ loads imposed by the gas stream, a
pump and either hydraulically or pneumatically
operated rams are incorporated in the control
system. The system shown in fig. 16-7 uses oil as the
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