## 1 Introduction  
In forward flight conditions, the freestream velocity of a helicopter rotor blade in the advancing side is larger than that of the retreating side due to the effect of forward flight. As a result, the inflow velocity of rotor airfoil varies with the rotor azimuth. Therefore, investigations on unsteady aerodynamic characteristics of rotor airfoil under oscillating freestream velocity are important, especially for the dynamic stall phenomenon [1; 2]. To discover the physical essence of a rotor airfoil dynamic stall, a lot of research under steady freestream velocity (SFV) conditions has been accomplished by using the experimental method [3; 4; 5; 6], theoretical method [7; 8; 9; 10; 11; 12], and numerical method [13; 14; 15]. Hence, the dynamic stall of airfoil under SFV conditions is well-studied at present. However, the dynamic stall of rotor airfoil under oscillating freestream velocity (OFV) conditions was seldom taken into account in early researches. In order to discover the physical phenomenon of dynamic stall under OFV condition, [PERSON] [16] preliminarily researched dynamic stall characteristics of the NACA0012 under the condition of oscillating velocity by using the experimental method. After that, the aerodynamic loads of the NACA0012 airfoil were measured by [PERSON] [17; 18] at a fixed angle of attack (AoA) under unsteady freestream conditions in a vertical unsteady water tunnel with a cross-sectional area of \(45.7\times 45.7\) cm. Based on this experiment, it was found that the lift coefficient (\(C_{l}\)) can be one order of magnitude higher under OFV conditions than that under SFV conditions. [PERSON] [19] investigated the dynamic small of NACA0012 airfoil under variational velocity by employing the ANSYS Fluent software, and the results were indicated that the leading edge vortex (LEV) formation could be obviously affected by the phase angle between the oscillating pitch and the oscillating freestream velocity. However, the maximum AoA and maximum velocity of airfoil were superimposed in this research, which is much different from the actual working environment of a helicopter rotor in forward flight. As a result, the conclusions of the dynamic stall characteristics of two-dimensional (2D) airfoil may not coincide with that of a helicopter rotor airfoil. At the same time, a new experiment [20; 21] was performed by the researchers of Ohio State University (OSU) to test the dynamic stall characteristics of SSC-A09 airfoil under oscillating velocity conditions. By comparing the measured \(C_{l}\) and \(C_{m}\) (pitching moment coefficient) under OFV conditions and SFV conditions, it was found that the lift curve slope and stall angle increased under the OFV conditions. However, due to the restriction of the experimental equipment, the maximum oscillating velocity (0.08 Ma) was much smaller than the normal forward flight velocity of the helicopter (Mach number of 0.1-0.3). Moreover, the aspect ratio of the blade model used in these experiments was only 1.0, which would generate obvious three-dimensional (3D) effects, and the characteristics of dynamic stall vortices would be influenced seriously. What is more, the added-mass effect due to the velocity variation would also influence the measured results in this research as mentioned in another study [22]. At the same time, [PERSON] et al. [23; 24; 25] researched the dynamic stall characteristics of an airfoil under OFV conditions in horizontal free-surface water tunnel of the U.S. Air Force Research Laboratory. However, these studies were focused on aerodynamic characteristics of airfoil at fixed AoAs, which was much different from the working environment of a helicopter rotor. Meanwhile, the freestream velocities were also smaller than the rotational velocity of the helicopter rotor due to the restriction of water tunnel. Therefore, the dynamic stall characteristics of the rotor airfoil under the real environment of helicopter are still not included in these studies, and this issue is valuable to research more deeply.