CONTROL SYSTEM

Fig. 16-5 Simplified control system.

15. It is apparent that two functions, fuel flow and propelling nozzle area, must be co-ordinated for sat- isfactory operation of the afterburner system, These functions are related by making the nozzle area dependent upon the fuel flow at the burners or vice- versa. 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.

Fig. 16-6 A simplified typical afterburner fuel control system

16. Since large fuel flows are required for afterburn- ing, 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 incorpo-rates 'fail safe' features in the event of an afterburner malfunction. The interconnection between the control system and afterburner jet pipe is shown diagram- matically in fig. 16-5.

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.

Fig. 16-7 A simplified typical afterburner nozzle 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 hydraulic  medium,  but  some  systems  use  fuel. Nozzle  movement  is  achieved  by  the  hydraulic operating rams which are pressurized by an oil pump, pump output being controlled by a linkage from  the  pressure  ratio  control  unit.  When  an increase in afterburning is selected, the afterburner fuel control unit schedules an increase in fuel pump output. The jet pipe pressure (P6) increases, altering the pressure ratio across the turbine (P3/P6).  The pressure ratio control unit alters oil pump output, causing an out-of-balance condition between the hydraulic ram load and the gas load on the nozzle flaps. The gas load opens the nozzle to increase its exit area and, as the nozzle opens, the increase in nozzle  area  restores  the  P3/P6  ratio  and  the pressure ratio control unit alters oil pump output until balance is restored between the hydraulic rams and the gas loading on the nozzle flaps.