Design Space Exploration for Small Aircraft with Hybrid-Electric Powertrains and Distributed Electric Propulsion

Abstract

Due to the ongoing technological progress in the field of electric components, such as motors, batteries and power electronics, the electrification of aircraft powertrains is becoming increasingly relevant. In these days, this is particularly true when it comes to general aviation class aircraft. The prospect of designing and building better aircraft in terms of efficiency and operating costs due to electric components in aircraft powertrains has resulted in a recent increase in development projects of aircraft. While first companies have already demonstrated the flight capabilities of hybrid- and all-electric aircraft, several others are set to start developing and testing. Many of those new vehicles are designed for on-demand air mobility (ODAM) transportation options, which are envisioned to provide affordable air transportation on short- and regional-distance missions. While most vehicles for ODAM for intra-urban routes are designed with vertical takeoff and landing capability, it is expected that aircraft for regional-distance missions will still need to be designed as fixed-wing aircraft for conventional takeoff and landing procedures in the future. This is mainly due to efficiency reasons. One of the ideas to take advantage of the benefits of electric propulsion for small fixed-wing aircraft is sizing the wing for cruise flight conditions instead of high-lift conditions, resulting in a smaller wing due to higher airspeeds in this design point. The smaller wing is more efficient during cruise flight, which is usually the longest mission segment, while the necessary high-lift during takeoff and landing can be provided by an active high-lift system. This active high-lift system consists of several propellers attached to the wing's leading edge. The most popular example for a configuration like this is NASA's X-57 Maxwell aircraft. In recent months, the Institute of Aerospace Systems' (ILR) multidisciplinary integrated conceptual aircraft design and optimization environment MICADO has been extended to model small aircraft with conventional, hybrid- and all-electric powertrains. This also includes the capability of modeling active high-lift systems based on distributed electric propulsion (DEP) technology. The first part of this paper gives an overview of the process of modeling small aircraft with DEP technology within MICADO. A special focus is on electric components of the powertrain and the propeller-wing-interaction model. In the second part, the results of a multi-dimensional study on the design space for small aircraft with hybrid- and all-electric powertrains and DEP as active high-lift system are presented. Study parameters comprise the degree of hybridization, the number of high-lift propellers, design maximum high-lift coefficient, cruise speed, wing aspect ratio and the influence of battery energy density.