Unsteady Trim for the Simulation of Maneuvering Rotorcraft with Comprehensive Models † Carlo L. Bottasso 1, * , Alessandro Croce 2 , Domenico Leonello 2 , Luca Riviello 2 1 Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA, USA 2 Dipartimento di Ingegneria Aerospaziale, Politecnico di Milano, Milano, Italy key words: Flight mechanics, aeroelasticity, maneuvers, comprehensive analysis, multibody dynamics, trajectory optimization, model predictive control, rotorcraft vehicles. SUMMARY We propose a methodology for extending the applicability of comprehensive analysis rotorcraft codes to the maneuvering flight regime. Our approach can be interpreted as a generalization of the classical steady flight trim procedures, implemented in all rotorcraft codes, to the problem of time dependent trim in unsteady flight. Rotorcraft maneuvers are here mathematically described in a concise yet completely general form as optimal control problems, each maneuver being defined by a specific form of the cost function and by suitable constraints on the vehicle states and controls. In principle, by solving the maneuver optimal control problem, one could determine the flight trajectory and the control time histories that fly the vehicle model along it, while minimizing the cost and satisfying the constraints. Unfortunately, optimal control problems are prohibitively expensive to solve for detailed comprehensive models of rotorcraft denoted by a large number of structural degrees of freedom and possibly very sophisticated aerodynamics. In order to make the problem computationally tractable, our formulation makes use of two models of the same vehicle. A coarse level flight mechanics model is used for solving the trajectory optimal control problem. Being based on a reduced model of the vehicle with only a few degrees of freedom, the resulting non-linear multi-point boundary value problem is computationally feasible. Next, the fine scale comprehensive model is steered in closed loop, tracking the trajectory computed at the flight mechanics level using a receding horizon model predictive controller. This amounts to a standard time marching problem for the comprehensive model, which is therefore also computationally feasible. The flight mechanics model is iteratively updated for ensuring close matching of the trajectories flown by the two models. This two-level procedure enables the simulation using comprehensive models of arbitrary complexity of highly unsteady maneuvers of possibly long duration, with general constraints on the vehicle inputs and outputs. We demonstrate the proposed approach studying the take-off of a helicopter in the one-engine failure case under Category-A certification requirements, and an obstacle avoidance problem involving a violent pull-up/pull-down. † Under review in: Journal of the American Helicopter Society. * Correspondence to: Carlo L. Bottasso, Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, 270 Ferst Dr., Atlanta, GA 30332-0150, USA. Email: Carlo.Bottasso@ae.gatech.edu