# Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations
## 2 Development and Tendency of Aero-Propulsion  
Conventional aero-propulsive designs consist of TWB and few concentrated engines. Novel aero-propulsive designs generally profit from the integration between aerodynamics and propulsion systems. In the context of novel propulsion systems, distributed propulsion (DP) refers to maximizing the distribution of thrust across an aircraft to minimize wake losses caused by its configuration. This approach aims to achieve the optimal balance between aerodynamic thrust and drag [10; 16]. In other words, the magnitude of kinetic energy and stream velocity must be consistent as much as possible after flitting over the trailing edge. The reason for arranging the propulsion in this way has been proven in marine propulsion, where inefficiencies in propulsion are found when wake losses occur at the trailing edge of a moving craft. This is due to an uneven thrust distribution on the trailing edge of the object [17]. When multiple propulsors are distributed, the differences in locations, Mach numbers, and exhaust port shapes might all be reasons for an uneven thrust distribution. When the thrust distribution is uniform, DP could provide more lift with less surface to increase the lift coefficient for a given flight speed and wing planform area [7; 18; 19; 20]. The flight noise can also be highly reduced due to the propulsor size and the arrangement of boundary layer ingestion (BLI) [21; 22; 23]. For aerodynamics, we must evenly distribute the aircraft's weight. This reduces energy loss between thrust and lift. In 1986, [PERSON] re-explored BWB to reduce an aircraft's weight [24]. This was a trend in developing future civil aircraft. The overly concentrated weight on a tube has been identified as one of the causes of uneven DP, leading to energy losses. A huge portion of drag on conventional aircraft comes from the middle aff. The conventional side thrust cannot cover it. In 2005, [PERSON] illustrated that friction drag constitutes more than half of the total TWB transport aircraft drag. The percentages of friction drag in the fuselage (including the horizontal tail and fin), wings, and nacelles are 29%, 18%, and 3%, respectively. The idea of reducing drag on wing area by laminarity is possible, but it is impossible to distribute it in a fuselage with a very large Reynolds number [25]. As opposed to TWB, BWB could reduce takeoff weight by 15%, fuel consumption per seat per mile by 27%, empty weight by 12%, and increase the lift-to-drag ratio by 20% [26]. It is believed that DP could cover the drag from uniform weight. Figure 1 shows that regardless of which aircraft configuration is integrated with the DP, the problem of an imbalanced thrust-drag distribution still exists on the trailing edge. It is worth noting that most of the propulsors in active-duty TWB are placed on the wings, where the aff has typically been ignored. This configuration is probably the solution adopted after measuring the gains and losses of thrust and load in the presence/absence of aff wakes [24; 27; 28; 29]. Assuming that the thrust is distributed evenly on the wings of the BWB and TWB, attributable to geometric issues, the thrust of the TWB has to be divided into three sections (two semi-span wings and a fuselage), which leads to a more complicated propulsor installation. Conversely, BWB can easily achieve a smooth integrated trailing edge on the Y-axis. In the absence of BLI, both aircraft configurations present a side with a potential severe wake dissipation at the Z-axis. Even if a circumferential propulsor is implanted on the trailing edge of TWB, the wake dissipation may occur at the exact center point of the circumferential propulsor, which is detrimental to flight performance. In short, the main purpose of DP is not to distribute the propulsors in fancy locations but to distribute the thrust of the aircraft's trailing edge in the most coordinated and balanced possible way.