## 3 Discussions of Experimental Results
### Comparisons of Aerodynamic Characteristics of the DCFJ312 and CFJ312 Airfoil  
The typical DCFJ rotor airfoil with OF = 1/2 and \(RUL\) = 0.025 (the blocking unit length is 20 mm), which is named DCFJ312-OF1/2, is selected for comparison with the CFJ312 airfoil. Figure 10 shows the aerodynamic coefficient comparisons of the DCFJ312 and CFJ312 airfoils. Table 3 shows the lift coefficients of the DCFJ312 and CFJ312 airfoils when the angle of attack (\(AoA\)) is 10\({}^{\circ}\) and 20\({}^{\circ}\). It can be seen from Figure 10 and Table 3 that when compared with the OA312 airfoil, the lift curve slopes of the DCFJ312-OF1/2 and CFJ312 airfoils are obviously increased. When the \(AoA\) is less than 18\({}^{\circ}\), lift coefficients of DCFJ312-OF1/2 and CFJ312 airfoils are also almost equal, but greater than that of the OA312 airfoil. The lift coefficient increment of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is 1.6% greater than that of the CFJ312 airfoil over the OA312 airfoil at \(AoA\) = 10\({}^{\circ}\). When the \(AoA\) is greater than 18\({}^{\circ}\), the lift coefficient of the DCFJ312-OF1/2 airfoil is greater than that of the CFJ312 airfoil, and significantly greater than that of the OA312 airfoil. The lift coefficient increment of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is 5.7% greater than that of the CFJ312 airfoil over the OA312 airfoil at \(AoA\) = 20\({}^{\circ}\). The DCFJ312 airfoil has better lift characteristics. Figure 10: Aerodynamic coefficients comparisons of DCFJ312 and CFJ312 airfoils. (**a**) Lift coefficient; (**b**) lift-to-drag ratio; (**c**) moment coefficient. Table 4 shows the maximum lift coefficients and stall \(AoA\)s of DCFJ312 and CFJ312 airfoils. When compared with the OA312 airfoil, the maximum lift coefficients and stall \(AoA\)s of DCFJ312-OF1/2 and CFJ312 airfoils are significantly improved. The maximum lift coefficient increment of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is 11.5% greater than that of the CFJ312 airfoil over the OA312 airfoil. The stall \(AoA\) of the DCFJ312-OF1/2 airfoil is 12\({}^{\circ}\) greater than that of the OA312 airfoil, and 2\({}^{\circ}\) greater than that of the CFJ312 airfoil. The stall characteristics of the DCFJ312 airfoil is also better than that of the CFJ312 airfoil. Table 5 shows the drag coefficients and lift-to-drag ratios of DCFJ312 and CFJ312 airfoils at \(AoA=20^{\circ}\). It can be seen from Table 5 that when compared with the OA312 airfoil, the drag coefficients of DCFJ312-OF1/2 and CFJ312 airfoils are significantly decreased. When the \(AoA\) is less than 12\({}^{\circ}\), the drag coefficients are decreased to negative values. The drag coefficient decrement of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is 4.7% greater than that of the CFJ312 airfoil over the OA312 airfoil at \(AoA=20^{\circ}\). The lift-to-drag ratio increment of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is 20.4% greater than that of the CFJ312 airfoil over the OA312 airfoil at \(AoA=20^{\circ}\). The drag characteristics and the lift-to-drag ratio characteristics of the DCFJ312 airfoil are better than those of the CFJ312 airfoil. Table 6 shows the moment coefficients of DCFJ312 and CFJ312 airfoils at \(AoA=10^{\circ}\) and \(20^{\circ}\), where the reference point of the moment coefficient is located at 25% of the chord. It can be seen from Figure 3 and Table 6 that the absolute values of moment coefficients of DCFJ312 and CFJ312 airfoils are gently increased after the OA312 airfoil stalls. At \(AoA=10^{\circ}\), the absolute value of the moment coefficient of the DCFJ312-OF1/2 airfoil is decreased by 21.2% when compared with the OA312 airfoil, while that of the CFJ312 airfoil is increased by 17.2%. The absolute value decrement of the moment coefficient of the DCFJ312-OF1/2 airfoil over the OA312 airfoil is only 1.9% greater than that of the CFJ312 airfoil over the OA312 airfoil at \(AoA=20^{\circ}\). The moment characteristics of the DCFJ312-OF1/2 airfoil are slightly better than the CFJ312 airfoil.