Conceptual design and comparison of hybrid electric propulsion systems for small aircraft

This paper presents a novel conceptual design method for electric and hybrid electric propulsion systems in small aircraft. The effects of key design parameters on the propulsion system performance are analyzed and the advantages and drawbacks of the investigated propulsion systems are discussed on the basis of two sets of thrust requirements. First, the general conceptual design algorithm is outlined. This is followed by a description of the three propulsion systems investigated: the fully electric; the parallel hybrid; and the conventional internal combustion engine. Scalable models of all required propulsion system components are presented, including weight estimation and operating characteristics. Afterwards, the conceptual design algorithm is exemplified for a reference two-seater motorized glider with a cruising speed of 140 kt and a maximum take-off mass of 1000 kg. Key design parameters are identified and their impact on propulsion system mass and cruise efficiency discussed. This study suggests that the parallel hybrid propulsion system is advantageous for high power ratios between take-off and cruise. For a power ratio of 4.5, either a relative cruise efficiency advantage of 12% or a maximum system mass advantage of 10% can be expected, depending on the propeller design. For the chosen cruise range of 300 km, the system mass of the fully electric propulsion system is at least 2.37 times higher when compared to the conventional propulsion system. In summary, a design method for hybrid electric propulsion systems is presented here which may be used for conceptual design. Furthermore, the suitability of the propulsion systems under investigation for different sets of thrust requirements is assessed, which may be helpful for aircraft designers.