# Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations
## 6 Conclusions  
We discuss propulsive configurations. They could improve the thrust-drag balance. This might provide flight benefits, such as energy savings. A series of potential aeropulsive options and samples of future civil aircraft are analyzed. A series of synergistic problems that exist in the integration of large civil aircraft are discussed. These problems involve the configuration schemes selection and multidisciplinary optimization (MDO) of the integration aircraft. They are either in a situation of mutual interference or unable to achieve the effect of a standalone runtime. To this end, the future challenges and trends for various propulsive technologies and MDO approaches that could handle cross-integration are listed. Specific disclosures include the following:  
* Propulsion-airframe: The choice of configuration coupling objects lead to a huge performance difference in the aero-propulsive layout. An incorrect pairing potentially loses the layout benefits from an original configuration. When DP is mounted on the TWB wing, a regional propulsor has to be embedded simultaneously in the aft to rectify the problem of wake losses in the aft. Meanwhile, the larger and more concentrated thrust at the aft either fills or even penetrates the wake. These circumstances cause the DP layout to become invalid and redundant on TWB. Contrarily, the coupling relation between DP and BWB can maintain the original thrust characteristics of DP. It can add some extra benefits to aircraft configuration, such as omitting the area and weight of the rudder, improving the flight performance in the L/D ratio, shortening takeoff distance under specific conditions, etc. However, studies in boundary layer ingestion (BLI) in regional propulsion have shown that TWB has a propulsion efficiency advantage over BWB. The reason is that the current distributed method of BLI is unable to fill the wake loss in BWB completely. Since the drag of BWB is not concentrated on the central axis but is diffused, it makes the uniform thrust of DP unable to achieve the effect of thrust-drag balance after being integrated with BWB. To achieve complete distributed advancement, the distribution of thrust should not depend on the number of propulsors, but should take into account the drag on the aircraft;
* Scheme and design: The layouts and frameworks of 59 aero-propulsive cases from different research units are analyzed. Most layout ideas are constructed and based on the parametric model of the predecessors. The propulsor positions are mostly those predicted to be likely to have high propulsion efficiency. Then, various commercial MDO software is used to optimize the target performance, such as flight range, load, fuel quantity, etc. In fact, scheme selection, evaluation, and analysis of aero-propulsion should be conducted before constructing the parametric model, because conducting the MDO with the existing conventional propulsors and DP layout will not obtain a fresh and effective scheme. From an essential viewpoint of DP, DP does not refer to the distribution of propulsors but the distribution of thrust. These physical limitations lead to the neglect of elongated cross-flow fans that can fully cover the trailing edge of the BWB. Additionally, energy or more advanced propulsion systems that could assist the elongated cross-flow fan are also ignored. Thus, a thrust-drag analysis and calculation model framework for the aircraft's trailing edge is urgently established such that thrust corresponding to the drag can be distributed and coordinated in each trailing part of the aircraft. This configuration could improve layout efficiency and effectiveness to promptly accomplish momentum balance and reduce the energy consumption of flight;
* MDO framework: In terms of multidisciplinary optimization, it can be found that the current goal of disciplinary integration is growing larger. The integrated disciplines include aerodynamics, propulsion, structure, weight, stability, control, and additional duct systems.