Spiral arc propagation in meridional view, only taking into account the orbital motion of the stars. The ratio of the maximum height, reached by a wind particle sent out in the z-direction, to the maximal distance travelled by a particle sent out in the orbital plane direction is the flattening ratio R_f given by Eq. (12). The angle with the orbital plane of the estimated path of a particle that reaches a maximal height is θ_f given by Eq. (11).