A propulsion system for an aircraft, comprising at least one rotor and a nacelle fairing extending around the at least one rotor with respect to an axis of rotation of the rotor, the nacelle fairing comprising: an upstream section forming an inlet section of the nacelle fairing; a downstream section, a downstream end of which forms an outlet section of the nacelle fairing; and an intermediate section connecting the upstream and downstream sections, wherein the downstream section comprises a radially inner wall and a radially outer wall made of a deformable shape memory material, and wherein the downstream end forming the outlet section has a pneumatic or hydraulic annular actuator extending around the axis of rotation and configured to deform radially under a predetermined control pressure so as to vary an outer diameter of the outlet section between a minimum diameter and a maximum diameter.