In this research, a small-scale, 4-Kw, variable-voltage, hybrid-electric powertrain was constructed and tested to understand the fundamental behavior of such a system. The powertrain is meant for distributed propulsion for a multirotor electric vertical take-off and landing (eVTOL) aircraft. The powertrain was examined component by component, as well as in combination with one to four distributed rotors. Steady-state mathematical models for the engine, generator, and motor were developed for performance and weight. The component models were calibrated with the test data. It was found that a simple physics-based model of brake specific fuel consumption (bsfc) is possible to build if certain thermal efficiency constants could be calibrated with test data. The impact of various losses on the electric motor efficiency plots were revealed. Statistical weight models were developed by gathering a database of commercial reciprocating engines, electric motors, and power electronics, which were accurate to within ±30%. However, given the importance of electric motors for eVTOL design, a geometry- and material-based model was also developed. This model was accurate to within a 6% average error. The principal findings of this work are that the generator voltage is a key parameter in a hybrid-electic powertrain, and engine efficiency is closely coupled to the controls and aeromechanics of an eVTOL aircraft. The ability to vary the generator voltage with operating state appears crucial for the optimal specific fuel consumption. Generator voltage is a function of engine speed. For any operating state—defined by rotor torque and RPM—the generator voltage should be minimized as far as possible. However, generator voltage limits the maximum rotor RPM; so not all rotor RPM can be achieved at the same generator voltage. Hence, the optimal generator voltage will vary with rotor RPM as needed during specific mission segments. This implies for an RPM controlled aircraft, generator voltage is not simply a criteria for design but also for control, if the optimal bsfc is to be achieved. Additionally, for any generator voltage and rotor RPM, there is a rotor torque that maximizes overall efficiency, i.e minimizes bsfc. A method for adjusting the rotor torque, such as collective pitch control, is also desired if the optimal bsfc is to be achieved. Therefore the most efficient change is power results from a combination of reducing engine RPM and increasing rotor torque and vice versa. In summary, a hybrid-electric powertrain is an attractive option for multi-rotor aircraft as long as it is judiciously designed together with the platform. It is hoped that the data generated in this dissertation and the accompanying understanding will help the future designer accomplish this task.