# Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations
## 1 Introduction  
The current methods to cut energy use in new planes often rely on innovative propulsion systems. This includes blended wing body (BWB) designs, distributed propulsion (DP), and boundary layer ingestion (BLI) [1]. A NASA report shows that at least 70% of carbon emissions from civil aircraft can be reduced by replacing fuel with electricity [2]. However, carbon emissions (CO\({}_{2}\)) and energy consumption (\(J\)) are two different physical quantities. Using different energy sources (fuel, electricity, or hybrid) does not reduce total energy use. One potential method for reducing energy consumption involves recharging the jet flow on the boundary layer into the free stream to backfill the overall wake of an aircraft. This approach aims to improve thrust efficiency and contribute to the goal of green aviation [3]. To backfill the overall wake of an aircraft, numerous small engines must be placed evenly along the trailing edge as part of the distributed propulsion (DP) system to achieve wake recovery effects during propulsion [4, 5]. This technology is considered a revolutionary configuration for future aircraft propulsion systems. In 1929, the aviationindustry began to test DP aircraft designs. Only recently, its refined design and research work have started as a new potential type of aircraft configuration [6]. In recent years, the National Aeronautics and Space Administration (NASA) has achieved numerous successful discoveries in aero-propulsive technology, such as propulsive fuselage, turboelectric DP, non-planar systems, BWB-DP, etc. [4; 5; 7; 8; 9]. Pioneering institutions such as the University of Cambridge and the Massachusetts Institute of Technology (MIT) have successively explored some advanced DP technology and its corresponding aircraft configurations, including the silent aircraft program and the energy sources of future aircraft [4; 10; 11]. The concepts of these configurations are mainly to reduce the noise generated by large turbine engines and to reduce harmful emissions generated by combustion processes. Further research in this sector demonstrates a feasible integrated scheme of distributed electric propulsion [12]. It means that the all-electric aircraft will no longer be a distant dream [9]. However, the specific total amount of energy storage, the hazards of conductive materials emission, and the technical challenges of the integrated layout have not yet been fully specified. Among the numerous aero-propulsive configurations, the most revolutionary is undoubtedly the BWB-DP configuration scheme. The development of DP has initially been concentrated on large-width BWB. The purpose is to maximize the carrying capacity of an aircraft while reducing wake losses caused by regional thrust. The synergy of BWB-DP can improve the lift-to-drag ratio by approximately 20%. It has a large capacity volume, small surface friction drag, and low manufacturing cost [7; 8]. A study found that a 16-propeller aircraft could reduce its take-off distance by at least 50% compared to a 2-propeller one [13]. However, the total amount of energy consumption is unspecified. Moreover, there are still some technical difficulties with the integration of BWB with DP. For instance, gaps between circular propulsors cause wake loss and energy waste. Wake losses can be reduced with an expanded nacelle propulsion system, but more propulsors have to be disposed to cover the trailing edge of the BWB. This aero-propulsive configuration would exhaust more energies, which would eventually not be worth the candle. Therefore, some researchers shifted the focus of DP technology to the low laterally surface tube wing body (TWB) aircraft. Additionally, the performance of each configuration needs to be known, such as propulsion and energy efficiency, maneuverability, control capabilities, structural strength, and aero-elasticity [14; 15]. The complex coupling between these variables involves multidisciplinary aspects. It is hard to analyze the performance without considering the effects of different disciplines.