# Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations
## 4 Multidisciplinary Analysis and Optimization in Aero-Propulsive Design
### Wake Ingestion and Propulsion Technologies  
The operation mode of BLI technology starts when the low-energy boundary layer flow through the fuselage is absorbed into the engines of the DP with BLI. BLI technology mainly absorbs and reuses the stream generated by the trailing edge to reduce the energy loss from the wake and then produces the same thrust with less fuel consumption in the same flight condition. Applying BLI technology in some specific conditions can reduce fuel consumption by 5.5-10% [69; 70; 71; 72]. When the BLI technology is integrated with ultra-high bypass turbofan engines, it can improve fuel efficiency by min. 3% to max. 9% [67; 73; 74]. Figure 4: Aero-propulsive interaction of BWB and DP. The performance of this technology (BLI) can be raised dramatically by 25-30% when the propulsors are placed at the trailing edge [75; 76]. The above studies are from a partial research study based on BLI. An independent BLI could fully cover its drag to achieve high efficiency and energy-saving. However, when multidisciplinary is considered, the expected results cannot be achieved. When the DP is disabled to cover the wake seamlessly and achieve the thrust-drag balance, these BLIs would continuously disperse propulsion and energies. Such uncoordinated thrust-drag is unable to achieve the expected favorable advancement effect (as shown in Figure 5). On the other hand, when the airflow into the engine becomes distorted, the performance of the blades is degraded under additional pressure and a fatigue scenario [69]. The aerodynamic blockage related to the boundary layer in the body is much larger than that caused by the channel boundary layer [70]. The non-uniformity circumstance affects the thrust momentum at the exit of the nozzle, which is much greater than with a conventional propulsion system [77]. In the BLI outlet, the most important objective is to arrange the propulsor position until the thrust achieves a balance on the horizon of the trailing edge. When part of the thrust is converted from energy into \"anergy\", this leads to a dilemma, which has a negative impact on the propulsion operation [41]. This is no longer a problem that can be solved by unilateral research and operation. The trailing edge must be precisely designed and calculated. This is to ensure the balance of thrust and drag. Nonetheless, a limited number of propulsors could not compensate for the wake caused by excessive wing area. These conflicts will lead to the stagnation of DP technologies in fuel efficiency, energy conservation, and carbon emission reduction. A summary of the propulsive layout that affects DP performance is as follows:  
1. [label=()]
2. Propulsion layout: In terms of the wingspan samples, distances between propulsors are all arranged in an even manner. Their thrust distributions are not arranged by considering the drag on each domain of an aircraft. This arrangement would cause a high drag domain (the central axis of an aircraft) to propel with low thrust and a low drag domain (the domain farther from the center axis) to propel with high thrust. If propulsor location is not considered according to the magnitude of the drag, the thrust might be dispersed and inadequate. It would cause the trailing edge to fail to reach the state of thrust balance, further causing the wake to dissipate. Ultimately, more propulsors would be requested, leading to higher energy consumption;
3. Dilemmas in aero-propulsive integration: From the individual BLI local optimization to the aero-propulsive overall optimization, the difficulty of simulating and optimizing the performance of an overall aircraft is still sustained. The first reason is that local  
Figure 5: Benefit of wake ingestion in various aero-propulsive configurations. optimization results would mismatch the local optimization results of the overall optimization results. The second is that the computational methods and tools used to execute the overall aircraft optimization are more complex than the process for local optimization.