Analytical Initial Sizing Of A Turbogenerator-Battery Hybrid Powerplant For eVTOL Aircraft

Abstract

The increasing demand for long-range electric vertical takeoff and landing (eVTOL) aircraft has underscored the need for efficient hybrid powerplant systems (HPPS) that integrate turbogenerators (TGs) and batteries. This study presents HYSIZE (Hybrid System Initial Sizing Method), an analytical approach for rapidly estimating HPPS weight fraction without iterative performance-weight coupling. By embedding OEI and OBI failure cases into the design logic, the method supports fail-safe architecture essential to eVTOL reliability. Two sizing strategies, targeting either vertical climb or cruise, are introduced, and a threshold is derived to determine whether TG- or battery-dominant configurations are more suitable based on specific power differences. Below the threshold, twin TGs alleviate battery burden under OEI conditions above it, TG count has no effect on system weight. Sensitivity analyses and case studies for 5- and 9-seat aircraft demonstrate the method's scalability and practicality, offering a structured design tool for OEMs to explore viable TG-battery combinations.