A model-based design framework for rotorcraft trim control laws

Abstract

Generally speaking, the trim problem is that of finding an equilibrium solution corresponding to an assigned set of well-posed constraints defining the current flight condition of the aircraft. The trim problem appears in different instances in helicopter practice, both in simulation environment and in the field, and analyzing it is a twofold process. Firstly, it is necessary to find an equilibrium condition in terms of rotorcraft states and controls. Secondly, one needs to keep the helicopter in the trimmed condition, which can be accomplished by means of dedicated trim control laws. This paper explores the opportunity to exploit a linearized, airspeed-scheduled high-fidelity model of a helicopter for this twofold task. An analytic linearized model has been defined for hover, feeding the linearized equations of the helicopter dynamics with data obtained from ad-hoc simulations on a detailed multi-body model of a specific testbed, considered also for the testing phase. For higher airspeeds, an approach through model identification has been envisaged for characterizing a suitable system accounting for changes in the dynamics due to forward flight. Subsequently, the models have been used to design control laws based on different strategies, capable of successfully keeping the machine in trimmed flight under various testing conditions. This in turn allows trimming the machine at even higher speeds, thus allowing to identify further models, capturing the dynamics of the system over larger portions of the operating envelope of the rotorcraft. The availability of the linearized model has been fully exploited implementing an automatic procedure for optimal gain tuning, with application to multiple control laws.