A hybrid INDI control for ensuring flying qualities in failures of Xcg measurement subsystem

Modern version of fighter aircraft has a wide range of longitudinal center-of-gravity (Xcg) travel due to wide-range fuel tanks layout, various weapons, many payload stations, and so on. The wide Xcg travel degrades flying qualities of a closed-loop control system; moreover, a catastrophic accident can be caused by loss of stability due to the wide Xcg travel in severe cases. For this reason, the inner-loop control law generally employs additional feedback variables such as weight and Xcg position to guarantee the flying qualities and satisfy stability requirements. Nevertheless, this design approach can seriously deteriorate the flight safety and stability of the aircraft in the event of fuel mismanagement and Xcg measurement subsystem failures. Therefore, additional design technique should be considered to ensure the flight safety in response to the situation of the failure conditions. This paper proposes two types of a hybrid Incremental Nonlinear Dynamic Inversion control to guarantee the minimum flight safety for return to base in case of fuel mismanagement and Xcg measurement subsystem failures. The frequency-domain linear analysis and time-domain simulations were performed based on a supersonic trainer mathematical model to evaluate the performances of the proposed control methods. The evaluation results confirm that the proposed control method satisfies the level of the required flying qualities and ensure flight safety even in the event of the subsystem failures for the Xcg measurement.