Limiters and Non-Linear Aspects
Figure 13.1 shows the relationship between fan speed and throttle servo demand. The severe non-linearity is due to the engine governor cutting in at high fan speeds. Note, that as well as the change in gain there is also a change in the dynamics, they being much faster at high engine speeds. An engine controller has been developed to compensate for this large non-linearity. Refer to the block diagram in appendix B where the engine controller is shown within a dotted box. It gives an approximately linear relationship between the demand APF and achieved fanspeed (which in turn reflects achieved thrust). Note that the compensator relies on knowing the fan speed at which the governor cuts in which in practice will not be known exactly. The engine controller has been left parametric so that the values can be tuned to the actual aircraft if necessary. FORTRAN variables ENJA, ENJB, ENJC, ENJD and ENJE correspond to the values A, B, C, D and E in figure 13.1.
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