In the aerospace control domain, Nonlinear Dynamic Inversion (NDI)-based control laws are widely spread. As a variation to Incremental Nonlinear Dynamic Inversion (INDI), the sensory Nonlinear Dynamic Inversion (sNDI) method was recently developed. Both methods rely on replacing model knowledge with sensor measurements. However, the methods differ in how the pseudo-controls are allocated: INDI allocates them incrementally, while sNDI allocates them globally, with corresponding advantages and disadvantages. While INDI requires a restoring mechanism in the control allocation due to path dependency issues in overactuated nonlinear systems, sNDI does not experience this problem. In addition to the comparison, the paper demonstrates that both methods lead to identical results if restoring is applied in the control allocation of INDI. Even though sNDI and INDI with restoring can lead to limit cycles for theoretical non-linear overactuated systems, the practical applicability of this approach to transition electrical vertical take-off and landing vehicles (eVTOL) is demonstrated in flight tests of the Variable Skew Quad Plane.

Incremental Nonlinear Dynamic Inversion (INDI) has become a popular control strategy for unmanned aerial vehicles due to its disturbance rejection capabilities and minimal model reliance. However, its standard formulation neglects actuator dynamics, leading to undesired coupling in systems with heterogeneous actuator characteristics such as in the Variable Skew Quad Plane (VSQP). This paper analyzes the limitations of INDI in such scenarios and demonstrates how Actuator Nonlinear Dynamic Inversion (ANDI) solves them. The closed-loop transfer function analysis shows how ANDI eliminates cross-axis coupling by directly incorporating actuator dynamics into the control allocation process. Additionally, ANDI is compared to a modified INDI approach that employs lead-lag filters to homogenize actuator behavior. While this approach is effective in decoupling, it unnecessarily slows down system response and worsens saturation handling. Simulation and flight test results using the VSQP validate the theoretical findings, confirming that ANDI offers improved control accuracy on coupled axes without compromising performance on decoupled axes.