# The Effect of Inflatable Pressure on the Strain Deformation of Flexible Wing Skin Film
## 5 Conclusions  
The strain deformation of the flexible inflatable wing film under the biaxial bearing condition is physically coupled with the multiple influencing factors such as internal inflation pressure, environmental pressure and aerodynamic load, structure type, and flexible skin film material. And the excessive local deformation of the wing skin film may cause the failure of the whole wing to be invalid or to seriously affect the aerodynamic shape. Therefore, the internal inflation pressure of the flexible inflatable wing needs to focus on the influence of the inflation pressure to ensure the flexible skin film deformation within a reasonable working range.  
Considering the deformation safety of the flexible wing, it is quite necessary to consider the biaxial deformation of the surfaces of flexible wing along the spanning and the chord-wise direction together with the internal inflation pressure synchronously. The strain deformation of the upper surface gradually decreases in the span-wise direction, but the strain in the vicinity of the wing root occurs with a sudden change in the jump, and there is a large, concentrated load or bending deformation near the wing tip. The deformation of the upper wing surface in the chord direction shows a wave-like variation law. And with the increase in the internal inflation pressure, the strain deformation near the wing root and the wing tip is positively correlated with the internal inflation pressure, and the increase in the internal inflation pressure may cause the more severe local deformation of the wing root to be excessively large and even induce a new failure problem.  
Under the same inflation pressure load conditions, the strain value of the lower surface of the flexible wing increases slightly near the wing root, which is consistent with the constructed theoretical analysis model of biaxial strain deformation. And the change in the strain value of the lower surface is also affected by the deformation of the chord and the spanning biaxial directions. So, while designing the rigidity and strength of the lower surface of the flexible wing, it is quite necessary to consider the maximum strain deformation near the root of the lower wing surfaces to prevent failure due to the excessive local deformation in order to meet the range of allowable strain requirements. However, it is quite beneficial to improve the strain deformation near the wing root position of the wing and improve the bearing stiffness of the whole wing.  
The different uniaxial tensile deformation tests of the flexible inflatable wing skin have verified the failure criteria for the tensile deformation of the flexible inflatable wing skin proposed in this paper. While designing flexible inflatable wing membrane skin, the smaller of the allowable strain of the fiber yarn bundle and the allowable strain of the matrix should be selected according to the different load-bearing deformation conditions.  
Conceptualization, L.L. and M.F.; methodology, X.C.; software, L.L.; validation, M.F. and X.C.; formal analysis, L.L.; data curation, M.F.; writing--original draft preparation, L.L.; writing--review and editing, M.F.; visualization, X.C.; supervision, X.C.; funding acquisition, L.L. All authors have read and agreed to the published version of the manuscript.  
This work was sponsored by the National Natural Science Foundation of China (Grant No. 12372085).  
Not applicable.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Node** & **Inflation** & **Inflation** & **Inflation** & **Pressure** & **Pressure** \\  & **No.** & **Pressure** & **Pressure** & **Pressure** & **2000 Pa** & **2500 Pa** \\ \hline  
\hline \end{tabular}
\end{table}
Table 3: Strain deformation data of the lower wing surfaces along the spanning-wise characteristic line under different inflation pressure loads.