Tiltrotor Control Robustness Assessment To Model Uncertainties In Hover And Near Hover Ground Effect Conditions

Abstract

The paper discusses the developed methodologies for assessing the dynamic stability robustness of tiltrotor aircraft augmented by Flight Control System, in presence of modeling uncertainties, specifically investigating the Hover/near Hover In Ground Effect conditions. The tiltrotor configuration operating in HIGE is characterized by a complex aerodynamic regime, involving important flight dynamics modelling challenges to be tackled and a certain level of model uncertainties, especially when the aerodynamics characterization available is only CFD based (wind tunnel test data not available). This picture is even more demanding considering the Fly-By-Wire nature of tiltrotor vehicles, where the control laws, generally designed and synthetized adopting bare aircraft dynamics model out of ground effect has to guarantee their effectiveness in ground effect as well. The paper specifically addresses, as case study, the Next Generation Civil Tiltrotor Technology Demonstrator (NGCTR-TD). The NGCTR-TD project is developed under the EU Clean Sky 2 program for defining novel technology for future civil tiltrotor platforms. Therefore, to assess the closed-loop dynamic stability of a new Fly-By-Wire rotorcraft for which no experimental flight data exist yet, as part of the flight clearance process, arises a strong need to properly investigate the sources of model uncertainties and verify the control laws robustness to them.