What are the architectures and challenges for hybrid-electric aircraft propulsion?

Hybrid-electric architectures: Series (gas turbine generates electricity for electric motors), Parallel (gas turbine and electric motor both drive propulsor), Turbo-electric (similar to series for distributed propulsion), and Battery-augmented (batteries provide boost for takeoff/climb). Challenges: Battery energy density (current 250 Wh/kg vs needed 500-1000 Wh/kg for regional aircraft), Electric motor power density (targeting 15+ kW/kg), Thermal management of megawatt-class power electronics, Certification of novel systems, and Integration with existing aircraft architectures. Benefits: Improved efficiency through optimal power splitting, Emissions reduction, and Distributed propulsion enabling new configurations. Near-term applications in training aircraft and urban air mobility; regional aircraft by 2035-2040.