# Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations
## 3 Performance and Coupling Effects of Aero-Propulsion
### Configuration Design of Distributed Propulsion  
The DP system has a variety of configuration methods like thrust split, multi-fan common core, distributed exhaust system, etc. (refer to Figure 2) [5; 10]. Table 1 shows the difference in mission fuel consumption and range of the well-known conceptual aircraft in regional/distributed propulsion, BLI/pylon-above-wing (PA), and single/multi-fan common core system (MPS). It can be seen that the aircraft with DP, BLI, and MPS (N2B) has not achieved a reduction in mission fuel or cumulative noise. These data show that flight performance improves slightly by reducing the wing area and Mach number. Therefore, this configuration is likely feasible. To achieve the benefits of wake filling, overlaying the entire trailing edge of the BWB with an overdraft amount of circular propulsors is unreasonable and energy-consuming [11; 32; 33; 34]. The currently developed DP system has reached the limit in the effectiveness of BWB.  
The cross-flow fans from FanWing Ltd (London, UK) can generate airflow. They evenly cover the entire trailing edge. Their blustering angle is 90\({}^{\circ}\) different from the ordinary fan, consisting of a long strip rotor and blades encased in a customizable rectangular nacelle with an inlet and outlet [39]. This configuration completes the wake ingestion that the circular fan could not reach. It has a relatively complete wake filling, which provides a relevant improvement of the DP. Another promising DP configuration is the homeopathic jet flap. It takes advantage of the exhaust flow. It has a high-velocity thrust from a flap tip, especially when its installation is spanwise along the trailing edge. This could maximize lift and improve take-off and landing. On the other hand, the distributed exhaust system (DES) generally exhausts heat flow, some of which might be exhausted through the central axis of a fan duct connected to an engine. This means that the exhaust ducts allocated between fans are likely to cause an imbalance in the speed and thrust of jet streams between the central  
\begin{table}
\begin{tabular}{l l l l l l l l l l l l} \hline \hline
**Selected** & **Engine** & **Number** & **Mach** & **Wing** & **MTOM** & **Payload** & **Max.** & **L/D** & **Range** & **Cumulative** & **Mission** \\
**Aircraft** & **Location** & **of Engines** & **Number** & **Area** & **(kg)** & **(kg)** & **Sea** & **(_Cruise_)** & **(km)** & **Noise** & **Fuel** \\  & & **(Fan)** & **(m\({}^{2}\))** & **(m\({}^{2}\))** & **(m\({}^{2}\))** & **(m\({}^{2}\))** & **(m\({}^{2}\))** & **(m\({}^{2}\))** & **(m\({}^{2}\))** & **(m\({}^{2}\))** \\ \hline SAX-12 & BLI (IPS) & 4 (4) & 0.80 & - & 154,290 & - & 215 & 21.90 & 14,816 & - & - \\ SAX-40 & BLI (MPS) & 3 (9) & 0.80 & 836 & 150,847 & 23,405 & 215 & 20.10 & 9260 & 185.0 & 33,253 \\ N2A- & PA (IPS) & 2 (2) & 0.81 & 925 & 213,914 & 46,720 & - & 21.42 & 11,112 & 251.0 & 65,453 \\ N2B & BLI (MPS) & 3 (9) & 0.80 & 846 & 216,545 & 46,720 & - & 21.55 & 11,112 & 274.4 & 66,587 \\ N3-X & BLI (MPS) & 2 & (15\(\sim\)21) & 0.84 & - & - & 107,048 & 300 & - & 13,890 & - & - \\ \hline \hline \end{tabular} NOTE: PA = pylons-above-wing, BLI = boundary layer ingestion, IPS = individual core propulsion system, MPS = multi-fan common core propulsion system. Compiled from sources. [3; 35; 36; 37; 38].  
\end{table}
Table 1: Distributed propulsion in multi-fan common core and individual core systems.  
and side of the multi-fan common core system. In addition, the blended effect between the heat and cold flows from the exhaust and fan ducts on propulsion is still unknown [40].  
According to the above inferences, a group of propulsors should be sequenced. This is based on the thrust needed at different positions of the aircraft's trailing edge. If propulsors with the same thrust are rashly sequenced side by side, it would be hard to achieve the goal of reducing energy consumption [14; 41]. Therefore, distributed thrust in a more balanced way to decrease drag is still a challenge.