Optimization of Electrified, Variable Speed Drivetrain Concepts for Enhanced Vehicle Performance

Hybrid-electric propulsion could provide numerous benefits for full-size rotorcraft, including reduced peak turbine power demand, reduced transmission system weight and complexity, and reduced operating costs. Variable speed electric motors, furthermore, could be configured to enable continuously variable rotor speed. Achieving these benefits requires accounting for coupling between the hybrid-electric drivetrain and vehicle performance within a large, unexplored design space. This paper presents a framework for simultaneous optimization of vehicle and electrified powertrain conceptual design using Geometric Programming (GP) methods. Four hybrid-electric powertrain architectures are evaluated relative to a baseline non-electrified powertrain for single main rotor, compound coaxial-rotor, and tiltrotor configurations. For designs with an upper limit on turbine power, electrification increases the maximum cruise speed for the compound coaxial-rotor configuration. Variation of the rotor speed by 15% allows the vehicle to carry 8% more fuel, relative to the non-electrified baseline, and 1,246 lb of battery. Operating the rotor at optimal speeds across the mission results in increased off-design mission performance, most notably a 43% increase in transport radius relative to a baseline powertrain. The results demonstrate the utility of the design optimization framework for exploration of novel hybrid-electric concepts as well as the challenges associated with incorporating electrical components into the drivetrain.