# 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
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**1 st Wing**} & \multicolumn{2}{c}{**2 nd Wing**} & \multicolumn{2}{c}{**3 rd Wing**} & \multicolumn{2}{c}{**V-Tail**} & \multicolumn{2}{c}{**Fuselage**} & \multicolumn{2}{c}{**Total**} \\ \hline  & Present & Alter. & Present & Alter. & Present & Alter. & Present & Alter. & Present & Alter. & Present & Alter. & Present & Alter. \\ Lift & 48.94 & 45.24 & 52.35 & 48.43 & 59.58 & 53.96 & 2.34 & 1.99 & 2.77 & 2.61 & 165.98 & 152.23 \\ Drag & 18.3 & 16.3 & 26.4 & 22.28 & 4.60 & 3.85 & 0.51 & 0.43 & 1.00 & 1.01 & 10.58 & 9.20 \\ L/D & 26.74 & 27.75 & 19.83 & 21.24 & 12.95 & 14.02 & 4.59 & 4.63 & 2.77 & 2.58 & 15.69 & 16.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: BWA/SL aircraft in cruise flight condition S3 (M = 0.7 at 43,000 ft) with an angle-of-attack \(\alpha\) = 0\({}^{\circ}\). Breakdown of lift and drag forces (in tons) and their ratio, with comparison against the alternative calculation (“aler.” label). Figure 16: BWA/SL aircraft in cruise flight condition S3 (M = 0.7 at 43,000 ft) with an angle-of-attack \(\alpha\) = 0\({}^{\circ}\). Absolute pressure distribution on the aircraft skin. Comparison between the current **(left)** and the alternative **(right)** simulations. Figure 15: BWA/SL aircraft in cruise flight condition S3 (M = 0.7 at 43,000 ft) with an angle-of-attack \(\alpha\) = 0\({}^{\circ}\). Overall lift **(left)** and drag **(right)** coefficients obtained through a grid convergence study. by the planform elements, i.e., the 3 wings. With a respective wetted area that regularly increases (and ignoring sweep or interference effects), the first, second and third wings logically generate an increasing lift force (of approximately 48, 52, and 59 tons, respectively). On the other hand, their associated drag increases even more importantly (with approximately 1.8, 2.6 and 4.6 tons, respectively). As a result, the aerodynamic performance degrades constantly from the first to the third wing (with respective L/D values of 26.7, 19.8 and 12.9). One can notice that, although unoptimized, the first wing exhibits an L/D ratio that is as high as the (highly optimized) wing of the CRM aircraft in its optimal cruise condition, namely S\({}_{1}\) with \(\alpha=2.05^{\circ}\). Conversely, although it is wider by no more than 30%, the third wing generates 2.5 times more drag than the first one. The above observations can be explained by looking more qualitatively at how the lift and drag forces are distributed on the aircraft skin, which is depicted in Figure 17. First, and in contrast to the CRM aircraft (cf. Figure 7), each of the three wings sees its lift and drag forces being roughly equally distributed along its span--which comes from their unoptimized character (see below). This, however, does not hold for some areas. In particular, the first two wings suffer from a loss in lift and an increase in drag at their root. This is logically attributed to the flow interferences coming from the connections (root juncture, pylon) between the wings and the fuselage, as well as to the airflow blockage by the latter. By contrast, the third wing is rather characterized by an increase in both lift and drag, which occurs at its root and tip. The excess lift at the tip region was observed to result from a beneficial side effect incurred by the vertical, backward swept junctures between the second and third wings, which provide the latter with an upward flow in the T-junction area. This results in some unexpected improvement for the outboard section of the third wing, namely a higher lift (and lift-to-drag ratio) than that observed without said vertical tip section of the wing ahead. Such a beneficial effect, however, also comes with a detrimental impact, namely the disruption of the flow by the outboard, straight junction, which then induces some flow swirling--ultimately resulting in vortical patterns that are likely to induce parasitic drag (see Figure 18, left side).