###### Abstract
Other more complex configurations have been proposed in the scientific literature, but they are unsuitable for aerospace applications, where it is mandatory to reduce the mass, bulk, and complexity. Both the Honda Hybrid System (HHS) and the THS allow the optimization of the engine working point; however, in the planetary systems, the engine is rigidly connected to the propulsion system to the detriment of driving comfort in the case of changing the engine start-stop mode (typical of the automotive applications). In a previous investigation [2] performed on the same vehicle, the advantages of using a series/hybrid electric configuration to increase the safety and reduce fuel economy were addressed. This study adapted the THS system to an ultralight aerial vehicle and compared it with a conventional thermal powertrain adopting either a piston or a rotary engine. As in the original Toyota system [17], the engine is coupled with the carrier, the electric machine EM1 with the sun gear, while the electric machine EM2 is on the outer gear (see Figure 1). The same electric machines of the original THS were considered, while the piston engine was replaced by a rotary thermal machine. A Wankel engine [2] was preferred for this application because of its high power/weight ratio and simple design. Similar to all rotary engines, the thermodynamic cycle of a Wankel is not obtained from the reciprocating movement of a piston in a cylinder. The four stages of intake, compression, ignition, and exhaust, in fact, are achieved by the movement of an eccentric rotor within an epi-trochoidal housing. Wankel engines are characterized by a low maintenance cost thanks to the reduced number of moving parts and the absence of intake and exhaust valves, while the rotary kinematics decrease noise and vibrations. The drawbacks of Wankel engines are related to the large surface-to-volume ratio and to the slow rotation of the rotor chamber (one-third of the eccentric shaft speed). These features intensify the wall heat flux and reduce the thermal efficiency of the engine. Moreover, Wankel engines suffer from sealing problems and low power outputs at low speeds [18]. These problems can be partially solved by the use of a hybrid electric power system, especially in a series configuration [19]. The results of the previous study releveled that, even accounting for a larger take-off mass and despite the lower efficiency of the Wankel engine, the proposed HEPS allowed an improvement in the fuel consumption of about 20%. Compared with a non-hybrid Wankel engine arrangement, the series/parallel HEPS improved the fuel economy by 28%. These fuel saving figures were estimated with a backward simulation approach starting from a typical mission and calculating the consumption of fuel and electricity (in the hybrid case). Stationary performance curves and efficiency maps were used for the propeller, the engine, and the electric machines. A planetary gear configuration allows high torque transmission and a large speed reduction to be achieved with relatively small gears, as required by the automotive application. However, its complex design and manufacturing tend to make it a more expensive solution than other gearbox types and its configuration makes heat dissipation more difficult. Other disadvantages are the high bearing load, inaccessibility, and design complexity [20]. Moreover, it requires a complex energy management strategy. For these reasons and for the fact that it is impossible to disengage the engine in case of failure, the Honda system was adopted in this investigation and a preliminary Honda-type series/parallel hybrid configuration was designed. To the authors' knowledge, this system has never been considered for aerospace applications. Another contribution of this investigation is represented by the reliability analysis.