Model-Based Optimization of Transition Manoeuvres of a Hybrid-Propulsion Compound-Helicopter UAV with Electrically-Driven Rotors

The paper deals with the development of a model-based optimization framework for the design of power management strategies of a lightweight compound helicopter UAV equipped with hybrid propulsion, with focus on the transition manoeuvres from cruise to hover. A high-fidelity multi-physics model of the hybrid propulsion system, composed of a combustion-engine connected to a nose propeller for forward flight phases, and two permanent magnets synchronous machines driving variable-pitch main and tail rotors to provide control during vertical flight phases, is firstly presented. In the model, the aerodynamic loads are obtained by applying an advanced BEMT-based code, including post-stall, tip-loss and low-Reynolds corrections, as well as QPROP-derived induced velocities. The dynamics of the electric machines is simulated via equivalent bi-phase model taking account of the Field-Oriented Control technique used for the three-phase electronic commutation. As a first step of the dynamic optimization process, a so-called fixed-point optimization is obtained, by finding the speeds and blade pitch angles of the rotors that minimize the steady-state electrical power absorption in vertical flight phases. To compare the optimization results with a baseline, a reference manoeuvre is then defined, in which the rotors speeds are ramped from standstill to the optimal fixed-point values, while the blade pitch angles are dynamically adjusted to ensure the UAV vertical equilibrium. Subsequently, a constrained optimization problem is applied by using the high-fidelity model, determining the main rotor commands that minimize the power consumption during transition. Finally, the robustness of the system performances against parameter uncertainties is statistically characterized via Monte Carlo simulations. The results demonstrate the effectiveness of the proposed strategies in reducing the energy consumption and in improving the overall performances with respect to a baseline manoeuvre, highlighting the effectiveness of model-based optimization for the power management design of novel UAV platforms.