Longitudinal Decoupling Control of Altitude and Velocity for a Fixed-Wing Aircraft

The longitudinal altitude and airspeed coordinated control is critical for unmanned fixed-wing aircrafts, especially during the taxiing landing process However, due to the strongly coupled relation of kinetic energy and gravitational potential energy, the precision longitudinal control is difficult. The traditional longitudinal control method based on PID (Proportional Integral-Differential) usually adopts different control parameters at different flight stages. But the regulation of the control structure is complex, and some fluctuation may exist during the transition process, which will affect the control performance. The TECS (Total Energy Control System) method adopts the decoupling strategy that the throttle controls the total energy and the pitch angle controls the energy balance between kinetic and potential. However, the parameters-adjusting process is relatively complex and the adaptability to model nonlinearity is imperfect. Based on the INDI (Incremental Nonlinear Dynamic Inversion) method and the trim relation of the aircrafts, a longitudinal altitude/airspeed decoupling control method is proposed in this paper. Firstly, the process of control law design is systematically described. The core points of states balance and transition are described for the design of the method. Secondly, the feasibility and basic control effects of the control method are verified by simulations. During the simulation, a variable coefficient PI pseudo is proposed, to accommodate the requirements of accurate gliding and smooth grounding. Finally, to further verify the effectiveness and superiority of the proposed method, comparisons between the proposed method and the TECS method is carried out. From the simulation results, the in the sense of precise altitude control during auto landing process.