A weak coupling method between the dynamics code HOST and the 3D unsteady Euler code WAVES. This paper presents a weak coupling method between the HOST dynamics code and the WAVES Euler aerodynamic solver for computing the trim of a flexible rotor in steady forward flight. The coupling can be applied on the lift, the pitching moment and the drag variables which are introduced as corrections to the simplified aerodynamics used in HOST. On the other hand, the blades motion and deformation are taken into account in WAVES by a deforming grid approach. Applications to the 7A and 7AD model rotors show that a simultaneous coupling on these 3 parameters is required in order to obtain a converged solution independent of the simplified aerodynamic model used in HOST. The coupled solution provides significant improvements on the pitching moment and torsion prediction, but further work is necessary to obtain an improved solution for the remaining parameters.