## 5 Hybrid Model Analysis  
To further investigate the behavior of the FWT concept, a detailed model of the wingbox was developed using NeOPT [5]. This model is a fully 3D, high-fidelity finite element (FE) representation of the structural box, where the skins, spar webs, and ribs are modeled with composite shell elements (PCOMP applied to CQUAD4/CTRIA elements). The stringers and spar caps are represented by CBAR elements. The FWT hinge kinematics is modeled similarly to the stick model, with coincident nodes connected by MPCs and a dedicated coordinate system to achieve the desired flare angle. Figure 8 shows the high-fidelity model. The stacking sequence of the composite elements is shown in Figure 9 and the material properties are listed in Table 3.  
Figure 6: Transfer function between gust velocity and WRBM at z = 8000 m and \(V_{TAS}=229\) m/s. Figure 7: WRBM time response for different gust gradients at z = 8000 m and \(V_{TAS}=229\) m/s. In the dynamic analysis model, the wing structure is replaced with the detailed wingbox, creating a hybrid model for aeroelastic analysis. In this model, the fuselage and tailplanes are represented by a stick skeleton, while the wingbox is fully detailed. The non-structural mass distribution is still modeled using concentrated masses (CONM2). The aerodynamic model remains unchanged as a DLM, with only the splines updated and connected to the rib nodes. The resulting model is shown in Figure 10.  
NeoCASS cannot handle shell elements, so the hybrid model was analyzed using Nastran. Due to the significantly higher computational cost compared with the stick model, only three gust conditions were considered. These are the same conditions used to generate the time histories in Figure 7, corresponding to three gust gradients (H = 9, 58, 107 m) at an altitude of 8000 m and \(V_{TAS}=229\) m/s. The validation of the hybrid model is presented in Figures 11-13, where the vertical displacements at the wing root, hinge, and wing tip are compared. The comparison shows that the response amplitude is the same for both models, although a time shift is observed. This is due to a slightly different natural frequency of the first wing bending mode, which increases from 1.90 Hz in the stick model to 2.03 Hz in the detailed FE model. However, a 6% difference is acceptable, as no updating of the stick model was performed and the connection between the wing and fuselage cannot be modeled in exactly the same way. Additionally, the 3D model has a mass distribution that differs from the stick model, as the stick model is one-dimensional and condens  
Figure 8: Detailed wingbox close-up with the FWT hinge mechanism highlighted. Figure 9: Stacking sequence of composite components, ply thickness is a percentage of the total thickness. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \(E_{11}\) & \(E_{22}\) & \(v_{12}\) & \(v_{21}\) & \(G_{12}\) & \(G_{13}\) & \(G_{23}\) & \(\rho\) & \(\varepsilon_{1T}\) & \(\varepsilon_{1C}\) & \(\varepsilon_{2T}\) & \(\varepsilon_{2C}\) & \(\gamma_{12}\) \\ \hline GPa & GPa & - & - & GPa & GPa & GPa & \(\frac{\text{kg}}{\text{m}}\) & \(\mu\varepsilon\) & \(\mu\varepsilon\) & \(\mu\varepsilon\) & \(\mu\varepsilon\) & \(\mu\varepsilon\) \\
135 & 10 & 0.3 & 0.022 & 5 & 2.8 & 2.8 & 1500 & 5425 & 2981 & 3974 & 12,829 & 6564 \\ \hline \end{tabular}
\end{table} 
Table 3: Unidirectional carbon fibre structural properties. Full description in the Abbreviation section. Figure 11: Stick vs. hybrid model vertical displacement comparison. H = 9 m, z = 8000 m, \(V_{TAS}=229\) m/s. Figure 12: Stick vs. hybrid model vertical displacement comparison. H = 58 m, z = 8000 m, \(V_{TAS}=229\) m/s. Figure 10: Hybrid aeroelastic model. Figure 13: Stick vs. hybrid model vertical displacement comparison. H = 107 m, z = 8000 m, \(V_{TAS}=229\) m/s. symmetric and balanced, only the FIs for the first four plies from Figure 9 are presented in Figures 15-17.