VTOL propulsion has historically been the constraint that defined what was possible. Helicopters depend on the shaft power of a turbine or piston engine driving a rotor through a mechanical transmission. Conventional fixed-wing aircraft depend on the thrust of a propeller or jet engine. Contemporary VTOL designs have opened up a wider design space by separating the energy source from the propulsion units and by distributing thrust across multiple smaller propulsors driven independently. Three propulsion architectures are now in active development: all-electric, hybrid-electric, and hydrogen-based, each with distinct implications for range, infrastructure, and certification.
