Control for optimal energy regeneration from autorotation in UAVs

The paper considers autorotative descent in electrically-driven rotorcraft as the analog of regenerative braking in electric vehicles, focusing on its potential to improve energy efficiency through control. Specifically, an aerodynamics model for the propeller in vertical flight based on momentum and blade element theories is coupled to a regenerative electromechanical drive with energy storage capability. The drive introduces a modulating control input that can be used to manipulate rotor torque while maximizing the amount of energy recovered from autorotation. An internal, or storage-centric energy balance equation is derived that provides the basis for optimization. An external, or whole-system energy balance maps the distribution of energy and losses and can be used for verification. A nonlinear constrained optimization problem is formulated as the maximization of stored energy with respect to descent velocity, given a fixed descent distance and constraints associated with the validity of momentum theory. Simulation results suggest that there is a significant potential for energy efficiency improvement from energy extraction from controlled autorotative descents, motivating further study.