Homing missile autopilot response sensitivity to stability derivative variations

In a recent study, the robustness of a tail controlled homing missile constant gain autopilot to changes in the aerodynamic stability derivative M?? was examined. The constant gains were determined from nominal values of the aerodynamic control derivative, M??, and the stability derivative. The stability derivative was then varied in the positive direction until damping degraded to unacceptable levels and in the negative direction until the system time constant became too slow to achieve a successful intercept against a maneuvering target. Therefore the range of M??´s realized represented an allowable uncertainty in M??. In this paper it is shown that the robustness to variations in M?? of an autopilot designed about a nominal M?? and M?? is strongly dependent on M?? and less dependent on the nominal value of M??. If M?? and M?? values are known accurately and gains can be calculated at each value, then a variable gain flight control system can be designed. It is shown that the range of M?? values which a variable gain flight control system can tolerate is much larger than the range of M?? values which a fixed gain flight control system can handle. The effect of the open loop crossover frequency ??CR and the actuator bandwidth on the allowable region of M?? for both a constant gain and a variable gain flight control system is also discussed. This expansion on the previous study will become more important as improvements in computer technologies such as larger throughput and memory capability allow the autopilot to become more sophisticated.