Aeronautical propulsion unit with improved acoustics

An aeronautical propulsion unit (10) having a longitudinal axis (X) and comprising:

• a plurality of unducted rotor blades (14), mounted to rotate about the longitudinal axis (X) and about a hub (12) of the aeronautical propulsion unit (10), and
• a plurality of stator blades (16), spaced apart from the plurality of unducted rotor blades along said longitudinal axis, fixedly mounted about the longitudinal axis (X) and about a casing (13) of the aeronautical propulsion unit (10), downstream of the rotor blades (14) in the direction of the flow of gases from upstream to downstream along the propulsion unit, the stator blades being unducted, at least two stator blades being identical, two successive stator blades (18) about the longitudinal axis (X) having an azimuthal angular spacing Δφi between them that is defined by an angle between respective reference axes (19), said angle being:
• either for adapting a pitch angle of said two successive stator blades, when these axes are projected in a plane perpendicular to the longitudinal axis (X) and if said two successive blades have a variable pitch angle,
• or transverse to the longitudinal axis (X) and passing through radially inner ends (23) or radially outer ends (25) of said two successive stator blades or through their centers of gravity, respectively, if said two successive blades have a fixed pitch angle,
• or, for one of said respective axes, for adapting a pitch angle of one of said two successive stator blades, when the blade has a variable pitch angle, and for the other, transverse to the longitudinal axis (X) and/or passing through the radially inner end (23) or the radially outer end (25) or through the center of gravity of an adjacent blade, when the blade has a fixed pitch angle,
• the rotor and stator blades each having a free radially outer end (25) that is radially further from the casing (13) than a radially inner end (23) and the propulsion unit being characterized in that:
• along the longitudinal axis (X), at least two stator blades (16) are at different axial positions from one another, and
• for each pair of two stator blades, a ratio between a maximum chord of said two blades is between 0.8 and 1.25, preferably between 0.9 and 1.11, the maximum chord being measured on a radial cross-section of a stator blade.