Experimental Study on Dynamic Stall of Airfoil in Rotor Reverse Flow Region

The large reverse flow region of the rotor with a high advance ratio is an important factor that affects the performance improvement of helicopters. This paper examines the dynamic stall of the airfoil in the reverse flow region of the rotor to develop a set of dynamic pressure measurement systems for the airfoil in the reverse low region. The pitch oscillation experiment of the NACA0018 airfoil in and out of the rotor reverse flow region has been carried out, and the influence of the airfoil motion parameters on dynamic stalls has been studied. The results show that the airfoil is more likely to stall in the reverse flow region under static conditions and that stall is invariant to the Reynolds number. After a stall occurs, the lift coefficient decreases more slowly with the increase of the angle of attack (about 10% of that in the forward flow state). Under the dynamic environment, in a pitching cycle, the quarter chord moment coefficient of the airfoil alternates between positive and negative frequently during reverse-blowing. The pitching oscillation in the reverse flow region is more sensitive to the changes in airfoil parameters than that under normal flow, and there is a marked difference. The alternating moment of the airfoil in the reverse flow region is significant: the greater the pitching amplitude, the greater the alternating moment. In the reverse-blowing state, with the increase in reduced frequency, the dynamic stall angle of attack increases more significantly.