In order to analyze the aerodynamic characteristics of hovering coaxial rigid rotors in ground effect, a numerical method based on unsteady Reynolds averaged Navier-Stokes equations is established. Moving overset grid is applied to simulate the reversal motion of the coaxial rotors. The non-slip boundary condition is adopted on the ground, and the grid around the rotors and the ground are refined to better capture the details of the flow field and the wake of the rotors. The comparison between the calculations and the experimental results of Lynx tail rotor verifies the effectiveness of the established method. The analyses of aerodynamic characteristics and the flow field of coaxial rigid rotors in ground effect show that the thrust increment of coaxial rotors in ground effect is larger than that of single upper and lower rotors. The increment is caused by the weakening of the interference between the surface of upper and lower blades due to high ground pressure. The ground interference mainly affects the radial position of the wake of the coaxial rotors, but it has little effect on its axial position. Upper and lower rotor wakes fuse and split with each other near the ground, forming complex tip vortex. The radius of the coaxial rotors wake in ground effect is larger than that of single rotor, because the radial jet velocity of the coaxial rotors is larger. With the increase of the height of the rotor from the ground, the aerodynamic interference intensity between the coaxial rotors gradually recovers, making the descending speed of the thrust increment of the lower rotor larger than that of the upper rotor. The relative roll height and the expansion radius of the coaxial rotors tip vortex decrease with the increase of off-ground height.
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