# Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations
## 3 Performance and Coupling Effects of Aero-Propulsion
### Aero-Propulsive Coupling Effects  
Aero-propulsion could gain from coupling aerodynamics with the thrust stream in certain conditions [46]. Specific types of equipment with low defect rates that could significantly impact aero-propulsion are discussed (refer to Table 2). Generally, the unconstrained geometric BWB can equip DP arbitrarily. Its thrust can be adjusted according to the position requirement of the discretely distributed multi-vector propulsors to achieve synergy effects and thrust balance on the overall trailing edge. This thereby reduces the crisis caused by an engine stall and also improves the safety and reliability of the aircraft. The advantage of  
Figure 2: Distributed propulsion systems in BWB. this synergy can improve the aerodynamic characteristics, propulsion efficiency, and aero-elasticity of BWB [14; 15]. As for TWB, a well-known example is the encircled aft propulsor of the STARC-ABL, which is designed to re-energize airflow from the boundary layer and release thrust along the wake generated by the fuselage [47]. This setup favors regional propulsion over DP, since its thrust is eventually concentrated on the \"tube\". Moreover, although the thrust of ECO-150 is distributed well on both wing sides, its crucial aft is not equipped with a centralized thrust. If it has an aft-mounted propulsor, the residual velocity might be severe on both wing sides. This propulsion configuration has higher wake loss than conventional aircraft. In the wing part, BLI propulsors mainly absorb and reduce the energy loss of the wake. They use less fuel to produce the same thrust at the same flight conditions. This improves fuel efficiency and cuts energy costs. Since the BLI engine is now closer to the central axis after being embedded in the wing, it could reduce the extra pitching moment from the pylon propulsor. This would cut the control surface size, skin friction drag, and balance the thrust-drag requirements. \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Ref.** & **Section** & **Specific Condition** & **Benefits** & **Defects** \\ \hline
[48] & Fuselage/ body & TWB & \(\bigstar\) Flight performance is easier to discriminate by accumulated long-term experiences. & \(\bigstar\) The fuselage has to be elongated to embed a circle aft propulsion system, resulting in higher costs with low benefits. & \(\bigstar\) The non-integrated “body” is difficult to deploy DP systems. \\  & & & & \(\bigstar\) The load distribution is more uniform and significant in load advantages. & \(\bigstar\) Maneuverability problems have to be addressed for aircraft without rudder and flaps. \\  & & & \(\bigstar\) Easier to implant DP in an integrated platform. & \(\bigstar\) The interaction between wing, control surfaces, and DP adds complexity to the design. \\ \hline \end{tabular}
\end{table} 
Table 2: Benefits and defects of Aero-propulsive configuration in specific conditions. Figure 3: Wake dissipation of kinetic energy with/without an extra aft-mounted engine. Additionally, BLI with an embedded duct could encapsulate a high bypass ratio engine in a body, which reduces and shields engine noise from the wings. The shape design parameters that could directly affect the mentioned performance of BLI include the width to height ratio, the shape of the embedded nacelle, the bump air inlet, and swept lip technology [65]. Regarding the propulsion system, the thrust distribution of the DP system with smaller engines can contribute to increase the maximum take-off weight for a large thrust-to-weight ratio. A systematic research study from BWB Boeing company shows that aft-mounted propulsion could save 3-5% of fuel consumption compared with the pylon-mounted propulsion [5; 7; 8; 47]. Due to the small size engine, a profusion of high-strength composite materials can be used to reduce engine weight and save manufacturing costs. However, it also faces some technical challenges, such as low Reynolds number flow effects, strict engine manufacturing tolerance, and lower combustion efficiency [66].