# 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
From a more global perspective, these localized swirls can also be seen as a by-product of the downwash effect that is unavoidably entailed by each wing. The subsequent flow deflection is being counteracted by the clean airflow that is located on the outboard part of the wing, which then results in circulation and vortical effects. This is illustrated on the left side of Figure 20, which depicts the flow circulation experienced by the BWA/SL aircraft in its nominal cruise flight condition (\(\mathbb{S}_{3}\)), as plotted in the vertical planes passing by the bottom, bent tip junctions of either the first or second wing. Figure 20 also reveals the relative importance of the downwash effect, which is here measured as the ratio between the vertical and axial components of the airflow velocity. As can be seen, each one of the three wings logically entails some downwash, deflecting the flow downward. Even though this deflected flow does not directly impinge on the downstream wing (which is purposely located above), it is likely to degrade its aerodynamic performances. Such degradation, however, is difficult to assess. For instance, it was seen that the lift regularly increases (by approximately 7%) from one wing to its aft counterpart (including when they are of the same span, e.g., the first two wings). This tend to indicate that the flow degradation entailed by the successive downwashes is not too critical, even though it undoubtedly exists. At this stage, it is worth mentioning that one cannot easily discriminate the various aerodynamic side-effects identified above. In particular, it is difficult to isolate their respective (negative) contribution to the overall aerodynamic performance, e.g., the regular decrease in the L/D ratio from one wing to another. This being said, these aerodynamic side-effects of the present BWA/SL planform are expected to be partly mitigated through further optimization of its planform, which is the subject of an ongoing study. This primarily concerns the design of the BWA/SL wings section, which should be improved. Indeed, whereas the highly optimized CRM wing involves supercritical airfoils, its BWA/SL counterparts rely on much simpler airfoils, of handmade design. Moreover, both the CRM and BWA/SL wings vary considerably in their respective twist (see left side of Figure 21). Indeed, the CRM wing exhibits a twist angle whose evolution along the span is constantly varying, and whose value is prominently negative (such a feature being desirable for both structural and stability reasons, e.g., enhanced washout effect providing the aircraft continued alieron control and some resistance to spin). Conversely, the BWA/SL wings are characterized by a quasi-constant twist, which is always positive (thereby producing more lift at zero angle-of-attack) and can be rather high (i.e., more prone to induce flow separation and, thus, higher drag). Figure 20: BWA/SL aircraft in cruise flight condition \(\mathbb{S}_{3}\) (M = 0.7 at 43,000 ft) with an angle-of-attack \(\alpha=0^{\circ}\). (**Left**): Flow circulation (iso-lines) and corresponding downwash effects (color code) occurring in the vertical plane passing by the first (left half) and second (right half) bent tip wing junctions (left and right half, respectively). (**Right**): downwash effects occurring in the vertical plane passing by the spanwise location \(\mathbb{b}_{\text{Ref}}/2\). As a result, whereas the wall pressure distribution evolves along the CRM wingspan, it remains almost unchanged along each of the BWA/SL wings. This may explain why the aerodynamic performance of BWA/SL aircraft is more \"conservative\" than that of its CRM counterpart, as revealed by Figure 22. This figure compares the L/D ratio obtained for both aircraft when flying (at their respective cruise altitudes) over a range of speeds and angles-of-attack. As can be seen, the good performance of the CRM aircraft is reached only under very specific conditions (M = 0.85, \(\alpha\) = 2.05\({}^{\circ}\)), being much less impressive otherwise.