# Computational Investigation of a Novel Box-Wing Aircraft Concept
## 3 Aero-Structural Characterization of CRM Aircraft
### Computational Results and Analysis (Incl. Validation)  
#### 3.3.1 Aerodynamic (CFD) Results  
The CFD computational campaign conducted for the CRM aircraft led to numerous results, only a fraction of which are presented hereafter. As an illustration, the left side of Figure 6 depicts the aerodynamic flow field obtained for the S\({}_{1}\) flight condition and optimal aerodynamic settings (\(\alpha\) = 2.05\({}^{\circ}\)), which is plotted here in terms of steady pressure coefficient. On the right side of the same Figure 6, the computational outputs associated with all flight conditions are plotted in terms of resulting lift-to-drag ratio (L/D). Notably, the S\({}_{1}\) flight condition under a nominal angle of attack of \(\alpha\) = 2.05\({}^{\circ}\) (\(\mathrm{C_{L}}\) = 0.45) exhibits an L/D value of 19.74, which is similar to that obtained through simulations (L/D = 19.56) and experiments (L/D = 20.47) by DLR and ETW for the same conditions ([PERSON], 1965). Here, it is worth mentioning that such an important L/D value is typical of most modern aircraft (e.g., L/D = 19.3 for Boeing B777 and L/D = 19.2 for Airbus A340) ([PERSON], 1965), which are highly optimized in terms of aerodynamic performance. The slightly more favorable L/D recorded here can be attributed to the fact that the present CRM configuration lacks both its vertical tail and engines, whose presence would otherwise entail a drag penalty (for no extra lift produced). \begin{table}
\begin{tabular}{c c c c} \hline
**Altitude (ft)** & **Aispeed (Mach)** & **Re (\(\mathrm{c_{ref}}\) = MAC = 7 m)** & **AoA (\(\alpha\))** \\ \hline
27,300 & 0.86 & \(61\times 10^{6}\) & \(1^{\circ};2^{\circ};3^{\circ}\) \\ \hline
37,000 & 0.85 & \(43\times 10^{6}\) & \(2.05^{\circ};2.25^{\circ}\) \\ \hline
38,500 & 0.85 & \(40\times 10^{6}\) & \(0^{\circ};1^{\circ};2.05^{\circ};3^{\circ}\) \\ \Here, the horizontal tail also induces a penalty on the L/D value, simply because it produces a negative lift. This can be seen in Table 3, which provides a breakdown of the lift and drag forces exerted on the main parts of the airframe in this cruise condition. One can here notice the very beneficial effect of the wings on the overall L/D, as well as the contribution by the fuselage to the overall lift produced by the aircraft. A more qualitative description of the aerodynamic performance by the CRM aircraft in such a cruise flight condition S\({}_{1}\) with \(\alpha=2.05^{\circ}\) is given in Figure 7. This figure depicts the distribution of both the lift and drag forces on the aircraft skin. In particular, one can notice how the lift is mostly produced by the wings (as expected), being distributed rather heterogeneously over their area. On the other hand, the drag is mostly generated by those regions of the wings' suction side that are located close to the fuselage--which is due to the interference effect produced by the latter. Figure 6: CRM aircraft in various flight conditions. (**Left**): Steady pressure (C\({}_{\rm p}\) coefficient) on and around the aircraft in cruise flight conditions S\({}_{1}\) (M = 0.85 at 44,000 ft) with an angle-of-attack \(\alpha=5.5^{\circ}\). (**Right**): L/D values obtained for the various flight conditions and aerodynamic settings (\(\alpha\)). Figure 7: CRM aircraft in cruise flight condition S\({}_{1}\) (M = 0.85 at 44,000 ft) with an angle-of-attack \(\alpha=2.05^{\circ}\). Distribution of the lift (**left**) and drag (**right**) forces (in Newtons) exerted on the suction (**(top)** halves) and pressure (**(bottom)** halves) sides of the aircraft. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Main Wing** & **Horizontal Tail** & **Fuselage** & **Total** \\ \hline Lift & 126.63 & \(-6.03\) & 17.48 & 138.08 \\ Drag & 4.82 & 0.54 & 1.62 & 6.99 \\ L/D & 26.25 & \(-11.11\) & 10.74 & 19.74 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: CRM aircraft in cruise flight condition S\({}_{1}\) (M = 0.85 at 44,000 ft) with an angle-of-attack \(\alpha=2.05^{\circ}\). Breakdown of lift and drag forces (in tons), with their ratio.