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Incremental Nonlinear Dynamic Inversion (INDI) is an extension to the Nonlinear Dynamic Inversion (NDI), with more robustness to model parametric uncertainties while still maintaining high performance characteristics. This paper presents the INDI based cascaded control laws designed separately for vertical and forward flight phases of a VTOL quad-plane. A simple strategy of blending the vertical and forward control inputs using sigmoidal control weight allocation is proposed to achieve transition between the flight stages. The derivation of the control input terms for each INDI based control loop is presented. The control laws are implemented on a simulation model of a canard-wing quad-plane UAV. A low-fidelity aerodynamic and propulsive model for the vehicle is built using OpenVSP and available motor/propeller performance data. The simple nominal model is used for robust control law design and simulations. Simulation results indicate good tracking of the reference trajectory commands during all stages of flight including transition.
Bhandari et al. (Sat,) studied this question.