## 1 Introduction  
The rotor is the major component of a helicopter to generate lift and operating force, which should satisfy the requirements of the helicopter for hovering, forward flight and maneuvering. When the helicopter is hovering, its performance is limited by the maximum lift of the rotor. Furthermore, the low stall angle of attack and flow separation seriously restrict the flight performance of the helicopter. Thus, as the basic component of the rotor blade, the performance of a rotor airfoil is crucial in terms of high maximum lift coefficient, high lift-to-drag ratio, high stall angle of attack and low zero-lift pitch moment. Therefore, the essential issue is how to improve the lift-drag characteristics and stall characteristics of the rotor airfoil to enhance the aerodynamic characteristics of the rotor and the flight performance of the helicopter. [PERSON] et al. [1] have designed the helicopter blade using global optimization, and [PERSON] et al. [2] have implemented airfoil design optimizations and wind tunnel experiments. However, existing airfoil design and optimization technologies have limited improvements in airfoil performance, while active flow control technology can significantly improve the aerodynamic characteristics of the rotor airfoil. CFJ and DCFJ technologies, proposed and developed by [PERSON] et al. [3; 4; 5], are the novel active flow control approaches which have shown significant performance enhancement with low energy expenditure. An injection slot is created near the leading edge on the airfoil suction side, while a suction slot is created near the trailing edge on the airfoil suction side. A certain amount of mass flow is drawn in through the suction slot, pressured by the fan or pump and injected to the main flow through the injection slot. In this case, the zero net mass flux is achieved consequently. The high-energy can be transported from the jet to the main flow to overcome the inverse pressure gradient and remain the main flow attached at high angles of attack. Moreover, lift coefficients can be increased significantly and drag coefficients can be also decreased dramatically at low angles of attack. The major difference between the CFJ and DCFJ technologies is that the injection slot of CFJ technology is a single open slot, while the injection slot of DCFJ technology is discrete. A large number of numerical simulations on CFJ technology have been carried out worldwide. [PERSON] et al. [6] validated that the CFJ airfoil was not only able to achieve very high maximum lift coefficient, but also able to improve cruise performance at low angle of attack. The enhancement of the transonic supercritical airfoil cruise performance and low speed performance using CFJ technology were proven by [PERSON] et al. [7] and [PERSON] et al. [8], respectively. The influence of the CFJ parameters on airfoil aerodynamic performance at a low Reynolds number came from studies by [PERSON] et al. [9] The effect of CFJ technology on the static and dynamic performance of the wind turbine airfoil was researched by [PERSON] et al. [10; 11] When it was compared with the baseline wind turbine airfoil, the optimum CFJ configuration was able to increase the maximum lift coefficient by 42.3% [12]. [PERSON] et al. [13] explored the effect of CFJ technology on the performance enhancement of the Darrieus vertical axis wind turbine. Results showed that the stall characteristics of the Darrieus wind turbine blade could be significantly improved by using CFJ technology. [PERSON] et al. [14] demonstrated that the CFJ airfoil could achieve super-lift coefficient for takeoff/landing at a very high angle of attack and ultra-high efficiency for cruising at a low angle of attack. The super-lift phenomenon of the CFJ airfoil was validated by [PERSON] et al. [15] using the wind tunnel experiment. The further study and application of the super-lift phenomenon to the VTOL vehicle were conducted by [PERSON] et al. [16], [PERSON] et al. [17] and [PERSON] et al. [18].