Review of hybrid-electric aircraft technologies and designs: critical analysis and novel solutions

Reducing greenhouse gas emissions has become a priority for civil transport aviation. One of the possible solutions investigated by current aeronautics research is the introduction of electric propulsion, which would drastically reduce greenhouse gas emissions related to flight. This paper addresses this topic in depth; the work is structured in two intertwined parts. The first relates to an extensive review of the state of the art, starting with the analysis of electrical technology enablers for aviation applications, and leading to the investigation of current proposals of aircraft conceptual designs, both for short-medium range and regional class. This review section, which is presented with a critical approach, provides the relevant indications for the definition of the technical framework of the second part of the paper, in which the conceptual development of a novel hybrid-electric aircraft is proposed. Specifically, the outcomes from the analysis of the state of the art suggest that the hybrid-electric aircraft should belong to the regional category, and that energy efficient solutions for the airframe should be considered. Moreover, potentials and limitations of integrating hybrid-electric propulsion are carefully detailed, and reasonably realistic technology levels for the next decade have been selected for the design of the proposed aircraft. A box-wing airframe architecture has been adopted as it has the potential to minimize induced aerodynamic drag while increasing the load transport capacity, thus representing an aerodynamic efficient solution. A design and optimization framework has been developed to evaluate the integration of the hybrid-electric propulsion with the box-wing lifting system. The coupling of these two technologies, together with a paradigm change in the aircraft design approach, allow to identify conceptual solutions that minimize fuel consumption throughout the typical regional mission envelope, leading to a potential emission-free regional aircraft.