Page 185 - The Jet Engine

Fig. 16-5 Simplified control system.

hydraulic medium, but some systems use fuel. nozzle area restores the P3/P6 ratio and the
Nozzle movement is achieved by the hydraulic pressure ratio control unit alters oil pump output until
operating rams which are pressurized by an oil balance is restored between the hydraulic rams and
pump, pump output being controlled by a linkage the gas loading on the nozzle flaps.
from the pressure ratio control unit. When an
increase in afterburning is selected, the afterburner THRUST INCREASE
fuel control unit schedules an increase in fuel pump
output. The jet pipe pressure (P6) increases, altering 19. The increase in thrust due to afterburning
the pressure ratio across the turbine (P3/P6). The depends solely upon the ratio of the absolute jet pipe
pressure ratio control unit alters oil pump output, temperatures before and after the extra fuel is burnt.
causing an out-of-balance condition between the For example, neglecting small losses due to the
hydraulic ram load and the gas load on the nozzle afterburner equipment and gas flow momentum
flaps. The gas load opens the nozzle to increase its changes, the thrust increase may be calculated as
exit area and, as the nozzle opens, the increase in follows.

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