Integrated reference model for a tilt-rotor vertical take-off and landing transition UAV

This paper introduces an integrated reference model that generates physically feasible trajectories for an incremental nonlinear dynamic inversion (INDI) control strategy for a vertical take-off and landing (VTOL) transition vehicle. The novel feature of this integrated reference model is to ensure a continuous blending between different flight phases – the hover and wing-borne modes. Consequently, no switching procedure is necessary during the transition period. Control Variables are appropriately chosen to ensure that the pseudo controls for the INDI controller remain identical in hover as well as in wing-borne flight. Heading angle rate and velocities in North-East-Down (NED) frame rotated by the heading angle, denoted as Control (C) frame, are used as control variables. Based on the outer loop pseudo control commands, the desired roll and pitch angles are computed in the INDI controller framework and are commanded to the reference model. Special focus has been directed on maintaining intuitively equivalent response of the aircraft with respect to pilot stick commands and disturbances over the complete flight envelope. This characteristic has been illustrated by simulation test results for cross-wind situations in hover and wingborne flights. By accounting for the cross-coupling terms between velocities and body angular rates, it is ensured that the roll angle required for a steady state coordinated turn is always commanded. Furthermore, during cruise flight, feedback of lateral load factor is utilised such that the aircraft performs a crabbed flight maneuver in cross wind conditions. Additionally, flight envelope protections are incorporated to limit the reference model based on flight dynamics restrictions. Pseudo control hedging has been used to include the effect of actuator dynamics in the reference trajectories. Flight test results show that the reference model produced physically meaningful trajectories for different flight maneuvers in hover as well as wing-borne flight leading to good tracking of the reference commands by the aircraft. With the inclusion of these features, an integrated reference model is presented for a tilt-rotor VTOL transition UAV platform. Likewise, the framework of this reference model can be implemented for any VTOL transition aircraft configuration by adapting the corresponding flight envelope parameters.