Abstract: An iterative free-wake computational method has been developed for the prediction of aerodynamic interaction characteristics between the twin rotors of a tandem helicopter.In this method,the mutual interaction effects among the rotor and wake as well as the wake and wake are included,and a rotor wake model,blade aerodynamic model and rotor trim model are coupled to solve the aerodynamic force and momentum of twin rotors.A new dual-rotor trim method has been given so as to be suitable for the aerodynamic interaction calculation of tandem twin rotors.By the method,the distribution of velocity and the rotor performance on a tandem twin-rotor at the condition of interactions are calculated,and the comparisons with the available experimental data are also made to show the capability of the present method.Then,the effects of such parameters as the longitudinal spacing and axial spacing between twin rotors are emphatically analyzed.As a result,the performance of both front and rear rotors is slightly worse than that of single rotor in hover.Furthermore,the effects of such parameters as the longitudinal separation and axial separation between twin rotors are emphatically analyzed.The results show that the tandem rotors need 8.5% additional power compared to single rotor when the longitudinal separation is 1.3R,and with the longitudinal separation increases,the additional power are reduced.The additional power is minimum when the longitudinal separation is 1.85R.