## 3 Discussions of Experimental Results
### Effect of the Obstruction Factor  
The _RUL_ is fixed to 0.025 (the blocking unit length is 20 mm). As shown in Figure 11, the DCFJ rotor airfoils with \(OF=1/3\), \(1/2\) and \(2/3\), which are named DCFJ312-OF1/3, DCFJ312-OF1/2 and DCFJ312-OF2/3, respectively, are selected for this trade study. Figure 12 shows the aerodynamic coefficient comparisons of DCFJ312 airfoils with various \(O\)Fs. Table 7 shows the lift coefficients of DCFJ312 airfoils with various \(O\)Fs at \(AoA=10^{\circ}\) and \(20^{\circ}\). It can be seen from Figure 12a and Table 7 that when the \(AoA\) is less than \(5^{\circ}\), lift coefficients of DCFJ312 airfoils with various \(O\)Fs are also almost equal, and slightly larger than that of the OA312 airfoil. When the \(AoA\) is greater than \(5^{\circ}\), lift coefficients of DCFJ312 airfoils with various \(O\)Fs are significantly improved in comparison with the OA312 airfoil. As the \(OF\) increases, the lift coefficient of the DCFJ312 airfoil first increases and then decreases. The DCFJ312-OF1/2 airfoil has the highest lift coefficient among the three DCFJ312 airfoils. At \(AoA=10^{\circ}\), the lift coefficient increment of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is \(3.2\%\) greater than that of the DCFJ312-OF1/3 airfoil over the OA312 airfoil, and \(6.4\%\) greater than that of the DCFJ312-OF2/3 airfoil over the OA312 airfoil. At \(AoA=20^{\circ}\), the lift coefficient increment of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is \(7.6\%\) greater than that of the DCFJ312-OF1/3 airfoil over the OA312 airfoil, and \(9.5\%\) greater than that of the DCFJ312-OF2/3 airfoil over the OA312 airfoil. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Airfoils**} & \multicolumn{2}{c}{\(C_{M}\)} & \multicolumn{2}{c}{\(\bigtriangleup\)\(1\,C_{M}\)\(\bigtriangleup\)\(1\,\)} \\ \cline{2-5}  & \(AoA\) = \(10^{\circ}\) & \(AoA\) = \(20^{\circ}\) & \(AoA\) = \(10^{\circ}\) & \(AoA\) = \(20^{\circ}\) \\ \hline OA312 & \(-0.0099\) & \(-0.0310\) & — & — \\ CFJ312 & \(0.0116\) & \(-0.0257\) & \(17.2\%\) & \(-17.1\%\) \\ DCFJ312-OF1/2 & \(0.0078\) & \(-0.0251\) & \(-21.2\%\) & \(-19.0\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparisons of moment coefficients of DCFJ312 and CFJ312 airfoils at typical \(AoA\)s.  
Table 8 shows the maximum lift coefficients and stall _AoAs_ of DCFJ312 airfoils with various OFs. When compared with the OA312 airfoil, the maximum lift coefficients and stall _AoAs_ of DCFJ312 airfoils with various OFs are significantly improved. As the OF increases, the maximum lift coefficient of the DCFJ312 airfoil first increases and then decreases. The DCFJ312-OF1/2 airfoil has the highest maximum lift coefficient among the three DCFJ312 airfoils. The maximum lift coefficient increment of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is 5.4% greater than that of the other two DCFJ312 airfoils over the OA312 airfoil. The stall _AoAs_ of the three DCFJ312 airfoils with various OFs are the same, and 12\({}^{\circ}\) greater than that of the OA312 airfoil. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Airfoils**} & \multicolumn{2}{c}{\(\mathbf{C_{L}}\)} & \multicolumn{2}{c}{\(\triangle\mathbf{C_{L}}\)} \\ \cline{2-5}  & _AoA_ **= 10\({}^{\circ}\)** & _AoA_ **= 20\({}^{\circ}\)** & _AoA_ **= 10\({}^{\circ}\)** & _AoA_ **= 20\({}^{\circ}\)** \\ \hline OA312 & 0.63 & 1.06 & — & — \\ DCFJ312-OF1/3 & 0.77 & 1.25 & 22.2\% & 17.9\% \\ DCFJ312-OF1/2 & 0.79 & 1.33 & 25.4\% & 25.5\% \\ DCFJ312-OF2/3 & 0.75 & 1.23 & 19.0\% & 16.0\% \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparisons of lift coefficients of DCFJ312 airfoils with various OFs at typical _AoAs_. Figure 12: Aerodynamic coefficients comparisons of DCFJ312 airfoils with various OFs. (**a**) Left coefficient; (**b**) lift-to-drag ratio; (**c**) moment coefficient. Table 9 shows the drag coefficients and lift-to-drag ratios of DCFJ312 airfoils with various \(O\)Fs at \(AoA=20^{\circ}\).