## 7 Conclusions  
For the parametric design of the medium thickness airfoils, an integrated design method of wind turbine airfoil based on aerodynamic performance and blade structural stiffness is proposed in this work. A multi-objective optimal mathematic model with the objective functions of aerodynamic performance and blade cross-section stiffness is established. A new WQ-B300 airfoil with high aerodynamic and cross-section stiffness performance was optimized by using a genetic algorithm coupled with the functional integrated expression of airfoil and the calculation program of blade cross-sectional stiffness matrix. Then, the aerodynamic performance and structural stiffness characteristics of the WQ-B300 airfoil were compared to those of the DU97-W-300 and WQ-A300 airfoils. Compared with that of the DU97-W-300 airfoil, the maximum lift coefficient of the WQ-B300 airfoil is 1.823 and 1.295, increased by 11.8% and 2.1%, respectively, under the smooth condition and rough conditions. The maximum lift-drag ratio of the WQ-B300 airfoil is 139.647 and 64.798, increased by 13.9% and 4.7%, respectively, under the smooth condition and rough conditions. In addition, compared with the airfoil with 30% thickness in literature [32], the aerodynamic performance of the WQ-B300 airfoil exhibits a higher maximum lift-drag ratio in the smooth condition, which is increased by 8.7%. Compared with that of the DU97-W-300 airfoil, the blade cross-sectional flapwise and edgewise stiffness is increased by 6.2% and 8.4%, respectively. The main reason is that the aerodynamic and stiffness characteristics of the WQ-B300 airfoil are taken into account in the process of optimal design. However, the WQ-A300 airfoil is designed mainly considering the aerodynamic performance improvement. In summary, the traditional design of airfoils with the medium thickness (the maximum relative thickness is 25%-35%) often neglects the structural characteristics of the blades and focuses on the improvement of aerodynamic performance. However, the influence of the blade cross-sectional stiffness is ignored. For the design of the medium thickness airfoils, not only the aerodynamic performance of the blade should be considered to improve the utilization of wind energy, but also the stiffness performance of the blade should be taken into account to improve the strength and stability of the blade. \begin{table}
\begin{tabular}{c c c c} \hline
**Stiffness** & **DU97-W-300 Section** & **WQ-A300 Section** & **WQ-B300 Section** \\ \hline Flapwise \(\text{EI}_{\text{x}}\) (N\(\cdot\)m\({}^{2}\)) & \(1.12\times 10^{9}\) & \(1.13\times 10^{9}\) & \(1.19\times 10^{9}\) \\ Edgewise \(\text{EI}_{\text{y}}\) (N\(\cdot\)m\({}^{2}\)) & \(3.57\times 10^{8}\) & \(3.48\times 10^{8}\) & \(3.87\times 10^{8}\) \\ \hline \end{tabular}
\end{table} 
Table 6: Stiffness comparisons of the new section and the DU97-W-300 blade section. \begin{table}
\begin{tabular}{c c c} \hline
**Layer Parameters** & **DU97-W-300 Section** & **WQ-B300 Section** \\ \hline Uniaxial thickness (mm) & 0.892 & 0.855 \\ \hline Biaxial thickness (mm) & 0.555 & 0.643 \\ \hline Triaxial thickness (mm) & 0.906 & 0.987 \\ \hline Uniaxial layer of leading and trailing edge & 22 & 20 \\ \hline Triaxial layer of leading and trailing edge & 2 & 3 \\ \hline Uniaxial layer of main beam & 55 & 58 \\ \hline biaxial layer of main beam & 2 & 4 \\ \hline triaxial layer of main beam & 2 & 4 \\ \hline Main beam position & 0.20 C & 0.185 C \\ \hline Main beam position & 0.55 C & 0.522 C \\ \hline \end{tabular}
\end{table} 
Table 5: Comparisons of the layer parameters of the DU97-W-300 and WQ-B300 blades. **Author Contributions:** Conceptualization, Q.W. and J.W. ; methodology, Q.W. ; software, P.H. ; validation, Q.W., P.H. and D.G. ; formal analysis, Q.W. ; investigation, P.H. ; resources, Q.W. ; data curation, D.G. ; writing--original draft preparation, Q.W. ; writing--review and editing, J.W. ; visualization, P.H. ; supervision, D.G. ; project administration, Q.W. ; funding acquisition, Q.W.