A weight fraction-based analytical method for initial sizing of hybrid power plant systems in electric vertical take-off and landing aircraft

Hybrid-electric propulsion research for electric vertical takeoff and landing (eVTOL) aircraft requires robust methods that address both vertical flight dynamics and critical failure cases, yet most existing approaches rely on fixed-wing assumptions. This paper presents a hybrid system initial sizing method (HYSIZE), an analytical framework designed for the rapid initial sizing of hybrid power plant systems (HPPS) in eVTOL aircraft. The method is tailored for regional air mobility (RAM) applications where extended range and high reliability are paramount. HYSIZE streamlines the estimation of gross weight and component-level capacities by utilizing a weight fraction-based approach, thereby eliminating the need for iterative calculations. The methodology comprehensively addresses standard operating conditions and critical failure cases, specifically one engine inoperative and one battery pack inoperative, to ensure safety and redundancy from the initial design stages. A key feature of HYSIZE is its determination of the hybridization ratio, based on the relative specific power characteristics of turbogenerators (TGs) and batteries. This enables performance-optimized sizing that adapts to varying levels of technological maturity. Validation against conventional parametric sweep methods demonstrates that HYSIZE achieves comparable accuracy while significantly reducing computational requirements. When applied to a five-seat eVTOL concept, HYSIZE identifies optimal HPPS configurations that preserve the overall system weight and accommodate a flexible range of TG power capacities. These findings provide practical guidance for eVTOL developers seeking to integrate pre-developed TG modules into hybrid eVTOL platforms.