Multibody Simulation of Integrated Tiltrotor Flight Mechanics, Aeroelasticity, and Control

This work describes the application of an adaptive control algorithm based on the generalized predictive control paradigmtoadetailednonlinearaeroelasticmodelofatiltrotoraircraftthatincludesrigid-bodydegreesoffreedom, restrictedtotheplaneofsymmetry.Theanalysisisbasedonanoriginalmultibodydynamicsformulation.Themodel is augmented by an autopilot and a stability augmentation system; in relevant cases, a model of the passive biomechanics of the pilot is considered as well. The interaction of rigid-body and deformable degrees of freedom alters the aeroelastic behavior of the system compared with that of the clamped wing rotor, discussed in a previous work. The flutter mechanism, originally consisting in whirl flutter, changes after considering rigid-body degrees of freedom;the short-period flight mechanics mode nowdominates high-speedstability. Thisissue ismainlyaddressed by the stability augmentation system, whereas the generalized predictive control is mainly used to reduce gustinducedwingloads,althoughitsinterventionforwhirl-fluttersuppressionisstillneededtoextendthe flightenvelope at very high speed. The capability of the adaptive regulator to reduce wing loads is assessed in hover and in highspeed forward flight. In the latter condition, the capability to suppress whirl flutter is assessed as well. HE use of active control in aerospace promises structural loads reduction and flutter suppression. However, setting aside all issues related to reliability, redundancy, and certification for a moment, modern aircraft (and rotorcraft) may not allow control designers to rely on the (sometimes abused) frequency separation paradigm that separately considers the dynamics of the aeroelastic and flight mechanics problems, thus significantly simplifying the analysis, since linearized models usually suffice for aeroelastic problems. In fact, airframe elastic modes with low frequencies interacting with low-frequency rotor modes may result in significant dynamic loading and reduced aeroelastic stability. This work focuses on the numerical analysis of the active aeroelastic control of a tiltrotor aircraft by means of an adaptive controller based on generalized predictive control (GPC), as discussed in [1], considering its interaction with flight mechanics dynamics, including the automatic flight control system (AFCS) and the passive dynamics of the pilot. Current generation tiltrotor aircraft mount two rotors on tilting nacelles that are located at the wing tips of an otherwise relatively conventional fixed wing aircraft design. Nacelle tilt changes the orientation of the rotor thrust about the pitch axis of the aircraft. As a consequence, tiltrotor aircraft can fly in two operating modes. In the helicopter mode (Fig. 1a) the nacelles are in the vertical position to provide the rotor thrust required to lift the aircraft. In helicopter mode, pitch control moments are generated by symmetrically tilting the rotor disks longitudinally, like conventional helicopters. Yaw control moment is produced in helicopter mode by differentially

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