# Computational Investigation of a Novel Box-Wing Aircraft Concept
## 4 Aero-Structural Characterization of BWA/SL Aircraft
### Computational Results and Analysis (Intl. Validation)
#### 4.3.2 Structural (FSI) Results  
The structural analysis of the BWA/SL aircraft is first conducted for its nominal cruise condition S\({}_{3}\) (M = 0. 7 at 43,000 ft) with an angle of attack \(\alpha\) = 0\({}^{\circ}\). Figure 23 depicts the corresponding FSI results, plotted here in terms of wing deformation and associated stress ([PERSON] criteria). As for the CRM aircraft (cf. Figure 10), the wing deformation gradually increases from root to tip, which translates the deflection induced by the lift forces. As shown on the left side of Figure 23, the maximum displacement occurs at the tip of the third (and wider) wing, which is deflected up by 0.57 m. This value is considerably lower than those usually recorded for typical tube-wing commercial airliners. For instance, this deflection of the BWA/SL third wing represents less than 20% of the one by the CRM main wing (cf. SS 3.3.2), even though the former has a 30% higher span than the latter (80 m and 60 m, respectively). This fact can be attributed to both the aerodynamic (lift distribution) and structural (closed frame) specificities of the BWA/SL planform. First, although the overall lift produced by the BWA/SL planform is 25% higher than that of its CRM counterpart (160 and 127 tons, respectively), it is almost equally distributed over the three wings (each of which is thus exerting between 2 and 2.5 times less lift than the CRM wing alone). Second, the closed frame of the BWA/SL planform naturally translates into a structure that is much stiffer (i.e., more likely to resist deformations) than its CRM counterpart. As a logical consequence, the BWA/SL planform exhibits much lower structural stress, with a maximum Von Mises stress that is 7 times lower than that recorded for the CRM wing (compare, for instance, the left sides of Figures 10 and 24). Although they are rather small, the higher stresses experienced by the BWA/SL planform are located over the first half of the first wing. This may be due to the fact that the first wing is highly twisted near its root (see left side of Figure 21), which makes it more likely to generate higher lift locally, thereby inducing larger flexural stresses. Even though they are even less important, the secondary stresses appear to concentrate at the juncture of the third wing and V-tail (cf. left sides of Figure 23). This is likely due to the overly turbulent flow originating from the rather rough design of such juncture (cf. SS 4.3.2), whose optimization to come will hopefully minimize the impact. Figure 23: BWA/SL aircraft in cruise flight condition S\({}_{3}\) (M = 0.7 at 43,000 ft) with an angle-of-attack \(\alpha\) = 0\({}^{\circ}\). (**Left**): Stress distribution (left wing) and deflection (right wing) exerted by the aircraft planform. (**Right**): Deflection of the three wings (visual rendering is here exacerbated on purpose). In a second time, the structural analysis is performed for an alternative (and likely unrealistic) flight condition, i.e., S\({}_{4}\) (M = 0.7 at sea level) with an angle of attack \(\alpha=0^{\circ}\). Compared to the nominal cruise condition S\({}_{3}\) of unitary load factor (n = 1), this flight condition entails an overall lift of around 2.5 times the BWA/SL's expected maximum take-off weight (MTOW \(\sim\) 300-450 tons), thereby inducing a load factor n = 2.5. In such an extreme scenario, the maximum stress (Von Mises) recorded on the planform does not exceed 163 MPa, i.e., about 26% of the one reached by the CRM aircraft in its optimal cruise condition (n = 1). Moreover, the maximum deflection occurring at the tip of the third wing now becomes 1.06 m (see Figure 24), which represents roughly 35% of its CRM counterpart in cruise. Although preliminary, this FSI analysis highlights the structural merits of the BWA/SL, which takes full benefit from the advantages offered by the Box-Wing Aircraft concept, namely a stiffer airframe that is thus capable of enduring higher aerodynamic loads.