Abstract: This paper presents the analyses of the aerodynamic inter action between the two rotors of a coaxial rotorhelicopter and the change of inter action with the incr ease of the advance ratio. Wind-tunnel exper imental resultsshow that the effect of the upper rotor on the lower rotor is larger than t he effect of the lower rotor on the upper rotor in hover. As t he thrust coefficient incr eases, the mutual int eraction incr eases also. In forward flight the upper rotordecreases its effect on the lower rotor while the lower rotor increases its effect on t he upper rotor. For the advance ratio between 0 to 0. 05, the upper rotor has its largest effect on the lower rotor, while the lower affects the upper thegreatest when the advance ratio is 0.1. The mutual influence appears the most evident in the range of the advancer at io from 0 to 0. 1.

Key words: coaxial rotor, aerodynamic interaction, upper rotor, lower rotor, thrust coefficient, torque coefficient, advance ratio, wind tunnel experiment