# Computational Investigation of a Novel Box-Wing Aircraft Concept
## 4 Aero-Structural Characterization of BWA/SL Aircraft
### Computational Results and Analysis (Intl. Validation)  
#### 4.3.1 Aerodynamic (CFD) Results  
The CFD computational campaign conducted for the BWA/SL aircraft configuration led to numerous results, only a fraction of which are presented hereafter. As an illustration, the left side of Figure 14 depicts the aerodynamic flow field obtained for the S\({}_{3}\) flight condition (M = 0.7 at 43,000 ft), which is plotted here in terms of steady pressure coefficient. On the right side of the same Figure 14, the computational outputs associated with all flight conditions are plotted in terms of the resulting lift-to-drag ratio (L/D). It appears that the aerodynamic performance of BWA/SL is rather independent of the altitude, while varying more importantly with the airspeed (M) and the aerodynamic settings (\(\alpha\)). The best aerodynamic performance is reached under the S\({}_{3}\) flight condition (M = 0.7 at 43,000 ft), with an L/D value of 15.5 \(\sim\)16 (under an angle of attack of zero or slightly negative value). Notably, this L/D ratio is very similar to that of a Boeing B747-400 in cruise [49], albeit somewhat lower than that of more traditional, biplane BWA aircraft [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] (e.g., L/D \(\sim\) 20). This is rather encouraging, considering that the BWA/SL did not benefit (yet) from any aerodynamic optimization of its planform--as would be needed for its aerodynamic performance to be able to compete with those of more modern aircraft, such as the CRM (whose wings were heavily optimized [46; 47]). This point, which is a matter of an ongoing study by the present authors, is discussed hereafter. In addition, this optimal value in Mach (M = 0.7) fits well with conclusions from previous works [51], according to which BWA aircraft are best suited for shorter range missions and lower cruise speeds, compared to their conventional counterparts. Figure 14: BWA/SL aircraft in various flight conditions. **(Left)**: Steady pressure (C\({}_{\rm p}\) coefficient) on and around the aircraft in cruise flight condition S\({}_{3}\) (M = 0.7 at 43,000 ft) with an angle-of-attack \(\alpha=0^{\circ}\). **(Right)**: L/D values obtained for the various flight conditions and aerodynamic settings (\(\alpha\)). Based on the above, the flight condition S3 is retained as a nominal cruise condition for assessing the BWA/SL merits, and is thus given particular attention. As a preliminary step, this S3 configuration is used for further validating the computational methodology and set-up, which is first achieved by successfully performing a grid convergence study (cf. Figure 15). In a second time, the present simulation results are compared to those coming from an alternative computation, which were obtained under the exact same conditions and following a similar computational approach (RANS with a standard k-\(\varepsilon\) turbulence model) while using a different solver and a denser mesh (with no wall function used). As can be seen in Figure 16, both results are in fairly good agreement, exhibiting very similar patterns of pressure distribution on the aircraft skin. For instance, the maximum wall pressure recorded in both cases differs by no more than 5% (21.8 KPa vs. 20.7 KPa), such discrepancy being attributable to the methodological differences between the two simulations. This good agreement between both simulations is further confirmed by analyzing more closely their breakdown in lift and drag forces (cf. Table 6). Here too, both results match quite closely despite some slight discrepancies, which can again be attributed to the methodological biases. Overall, the L/D ratios recorded by both calculations fall within the same range, with a 6% error bound (L/D = 15.69 vs. L/D = 16.55). This breakdown in lift and drag forces sheds some light on the aerodynamic performance of the BWA/SL aircraft. It is no surprise that, here too, the lift is mostly produced