Closed-Loop Rotorcraft Simulations with Coupled Flight Dynamics and State-Space Free-Vortex Wake

Abstract:
This paper presents the integration and use of state-space free-vortex wake models within closed-loop rotorcraft flight dynamics simulations. The free-vortex wake models are formulated in state-variable form, such that they constitute a system of nonlinear, time-varying ordinary differential equations in first-order form that augment the baseline rigid-body and rotor dynamics. The wake models considered include a tip-vortex-only formulation, as well as a formulation combining a vortex-lattice near wake with a tip-vortex representation of the far wake. Following trimming, linearization, and model-order reduction of the flight dynamics at discrete increments in flight speed, Dynamic Inversion (DI) flight control laws are synthesized to enable automatic transition from hover to cruise flight. Two-way-coupled losed-loop simulations are then performed for a generic utility helicopter representative of an H-60 in transition from hover to forward flight using three inflow models: (i) Pitt-Peters, (ii) tip-vortex-only wake, and (iii) vortex-lattice near wake with tip-vortex far wake. These simulations are compared both for validation purposes and to assess whether the different wake representations lead to significant differences in the predicted closed-loop response.

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