To study the effect of geometric parameter on hovering performance of coaxial rotor

the aerodynamic model of coaxial rotor based on free-wake method and second-order lifting-line theory was established

the performance with the rotor's airfoil

the ratio of the upper rotor's radius to the lower rotor's radius

the vertical separation of the rotor

the blade twist angle

and the rotor taper was calculated and analyzed

the comprehensive optimized rotor's geometric parameter was selected and analyzed in hovering based on the upper result at last. The results show that influences of the rotor's airfoil

the ratio of the upper rotor's radius to the lower rotor's radius

the blade twist angle

and the rotor taper are great

while there is almost no influence of the vertical separation on the hovering performance. The tension coefficient of 0.01 of the comprehensive optimized rotor can be reduced by 10.5% and the figure of merit can be increased by 9.5% comparing with the basic rotor.