Hybrid-Electric Design Studies for Long-Endurance Tailsitter Concept

A conceptual-level study was conducted to compare suitability of four different propulsion systems for long-endurance tailsitter aircraft. Diesel, parallel hybrid diesel, turboshaft, and advanced series hybrid solid oxide fuel-cell designs were considered. Each configuration was optimized, and loitering time was varied to quantify performance and high-level design characteristics as the mission changed. Design excursions showing gross weight sensitivity to varying range were also performed. The hybrid-electric configurations allowed for greater design flexibility by partially decoupling hover power requirements from energy conversion system size, resulting in substantially differing optimal airframes and forward flight speeds compared to the single engine aircraft. Results were dependent on best-in-class electric motor technology, batteries concurrently possessing high C-rate and high pack-level specific energy, and a duty cycle consisting of a short hover period and sustained operation at part power. A key insight from this study is that, with the appropriate battery and electric motor technology, hybrid-electric propulsion architectures can enable vertical-takeoff-and-landing-capable, long-range, and fuel-efficient aircraft for low-speed applications by pairing fuel-efficient engine choices with power-dense electric motors and batteries. Solid oxide fuel cells in particular exhibit promising characteristics for these use cases, but widespread adoption in aviation applications requires the maturation of emerging higher specific power technology or combined cycle approaches.