Abstract: A numerical study solving the Navier-Stokes equations for the unsteady massively separated flows around an airfoil section at -90° angle of attack is performed based on the SA-DES approach. Four passive flow control strategies, namely trailing edge flap, Kruger flap, droop-nose and spoiler are used to research the download (vertical drag) reduction for tiltrotor aircraft in hover. The effects of different factors are also discussed. The results indicate that when the trailing edge flap deflection angle is 60°, the airfoil has the minimum download; the minimum downloads of configurations with the Kruger flap, the droop-nose and the spoiler are generated at a Kruger flap angle of 85°, a droop-nose deflection angle of 45°, and a height-to-chord ratio of h/c=0.05, respectively. The results show that the drag of the optimal assembly configuration, with a trailing edge flap deflection angle of 60° and a Kruger flap angle of 85°, is reduced by 55.4% compared with the basic airfoil.