Modeling and Control of Advanced Tiltrotor Aircraft in Airplane Mode

The paper discusses the control law synthesis problem for the Advanced Tiltrotor Aircraft (ATA), specifically considering the lateral-directional dynamics in Airplane mode. The aircraft under investigation is a new tiltrotor concept developed by Leonardo Helicopters. The ATA aims at positioning itself as a new evolution of the tiltrotor concept, following the experience matured with existing aircraft configurations, as the AW609 and the NGCTR. During the concept design phase of a complex vehicle such as a tiltrotor, it is neither possible nor encouraged to rely on high-fidelity flight mechanics models (e.g. multibody), due to their computational cost and the large number of design parameters required, which are not typically known with sufficient accuracy in this project stage. Therefore, technology validation activities in the field of flight dynamics and control should rely on lean and flexible simulation tools, based on reduced set of design parameters but yet capable of predicting most important dynamic characteristics in the frequency range of interest for flight controls. The paper first addresses the inclusion of rotor gimbal states in the original ATA 6 dof flight dynamics code, which based on Tiltrotor experience are considered crucial for a proper identification of dominant aircraft dynamics in high inflow conditions, i.e. Airplane mode. The outcomes of lateral-directional model trimming and linearization procedures are then assessed, and a novel basic lateral-directional Command Augmentation System scheme designed and tested, in both linear and non-linear framework, against prescribed MIL-STD-1797B performance and dynamic stability requirements and SAE-AS-94900 robust stability criteria. The results show that proposed control strategy is a viable approach for further investigations and the adopted flight physics modelling complexity adequate for the addressed control problem.