Control capability: From two-loop autopilot to three-loop autopilot

The essential control characteristics for a two and three-loop acceleration autopilot of tactical missile were presented from a frequency analysis perspective. The basic limitation of right-half-phase pole and zero from airframe aerodynamics was analyzed using an open-loop crossover frequency inequality, which showed a consistent conclusion with the final overall stability condition in nature. To reveal the feedback control capability, the rate feedback, rate plus attitude feedback, and linear acceleration feedback, were calculated and simplified based on engineering requirement, respectively. Then an equivalent actuator dynamics were introduced to compute the stability condition at high frequency. Such that the global stability condition was obtained in explicit formulation. The results showed that the autopilot control capability is dominated by actuator bandwidth, and a compromise should be determined between the flight performance and the actuator requirement for a statically instable missile.