Hybrid Powertrain Systems for 100 kg Multicopters and Tailsitters

This paper explores the use of hybrid powertrains in vertical takeoff and landing (VTOL) unmanned aircraft configurations using a physics-based conceptual sizing tool. Three multicopter configurations (quadcopter, hexacopter, and octacopter) and their corresponding winged biplane tailsitter variants were sized for 100 kg takeoff mass for mission distances varying from 25 to 250 km. The hybrid powertrain model accommodates power sharing between a battery and an internal combustion engine/generator/rectifier combination. The engine was sized to provide cruise power, while a “boost” battery was sized to provide the necessary additional power required to take off and land vertically. Accommodating two different power sources and their corresponding energy storage systems in a VTOL aircraft affords better cruise efficiency, because the engine is operating at its minimum specific fuel consumption condition, at the cost of additional empty weight. This strategy does not offer noticeable benefits (over powertrains without a boost battery) for multicopter configurations operating in edgewise forward flight because cruise power and hover power are comparable, and peak power for engine sizing is not strongly driven by any one flight condition. For winged VTOL configurations, where cruise power is less than 40% of hover power, the dual-power plant solution enables a twofold increase in payload through both fuel and empty weight savings.