Hybrid-electric VTOL aircraft combine a combustion engine with electric propulsion, with the combustion engine serving primarily as an onboard energy source rather than as the direct mechanical driver of the propulsion units. The architecture aims to combine the range and energy density of liquid fuel with the operational advantages of electric propulsion.
Three configurations are commonly distinguished. In a series hybrid, the combustion engine drives a generator that produces electrical power for the electric motors at the propulsion units. There is no mechanical connection between the engine and the propellers, which allows the engine to run continuously at its most efficient operating point, decoupled from instantaneous propulsion demand. In a parallel hybrid, both the combustion engine and an electric motor are mechanically coupled to the propulsion system. In a series-parallel hybrid, both arrangements are present and the system can operate in either mode depending on flight phase.
For VTOL aircraft, series hybrid architectures are the most common contemporary choice. The distributed electric propulsion that makes tiltrotor, lift-and-cruise, and multicopter configurations practical depends on independent electric motors at the propulsion units, which fits naturally with a series architecture where electrical power is distributed from a central generator. Battery storage in a series hybrid can be sized for peak power demand (typically the hover phase) rather than for total mission energy, which reduces battery mass substantially compared to an all-electric design of comparable range.
The combustion engine in a hybrid VTOL can be a gas turbine, a piston engine, or a rotary engine. Turbogenerator architectures are dominant in higher-power applications because of the favourable power-to-weight ratio of small turbines, the range of certified aviation fuels (including Jet-A and increasingly sustainable aviation fuel) they can burn, and the ability to use existing airport fuel infrastructure. Piston-electric architectures are used at lower power levels where the lighter installation weight and broader fuel flexibility of a piston engine are advantageous.
A related architecture sometimes confused with series hybrid is turbo-electric, in which a gas turbine drives a generator to power electric motors directly, but without battery storage in the energy path. The absence of an energy buffer distinguishes turbo-electric from hybrid systems and affects both peak power capability and certification basis. Some hybrid VTOL concepts also support plug-in operation, in which the battery can be charged from ground infrastructure between flights as well as from the onboard generator.