An air flow straightening assembly for a turbomachine, the turbomachine comprising a pylon for suspending the turbomachine to an aircraft wing, the assembly comprising: - a straightener including vanes, each vane comprising a leading edge and a trailing edge, and - at least one structural arm, wherein the vanes and arm extend radially about an axis (X-X) of the turbomachine, and the structural arm has: - an upstream end part relative to the air flow direction in the turbomachine, comprising a leading edge circumferentially aligned with the leading edge of the vanes, and having a profile identical to the upstream end of the vanes, the upstream end part being delimited downstream by an axial position called an upstream extreme position, - a downstream part, dimensioned to form a shroud of a pylon for suspending the turbomachine, and - an intermediate part connecting the upstream end part to the downstream part, comprising a continuous upper surface wall extending between the upstream extreme point and a downstream extreme point with a determined axial position, wherein the upstream extreme point is located at a distance (X A), in the axial direction, from the leading edge of the arm of between 0.2c and 0.5c, where c is the length of the axial chord of the straightener vanes, the downstream extreme point is located at a distance (X B), in the axial direction, from the leading edge of the arm, greater than the length c of the axial chord of the straightener vanes, and the angle (α A) from the tangent to the upper surface wall to the upstream extreme point is equal to that from the tangent (α B) to the wall to the downstream extreme point to within one degree, characterised in that the downstream extreme point is situated upstream of the upstream end of the pylon, the downstream part comprising an upper surface wall which is convex from the downstream extreme point. - a straightener including vanes, each vane comprising a leading edge and a trailing edge, and - at least one structural arm, - an upstream end part relative to the air flow direction in the turbomachine, comprising a leading edge circumferentially aligned with the leading edge of the vanes, and having a profile identical to the upstream end of the vanes, the upstream end part being delimited downstream by an axial position called an upstream extreme position, - a downstream part, dimensioned to form a shroud of a pylon for suspending the turbomachine, and - an intermediate part connecting the upstream end part to the downstream part, comprising a continuous upper surface wall extending between the upstream extreme point and a downstream extreme point with a determined axial position, the downstream extreme point is located at a distance (X B), in the axial direction, from the leading edge of the arm, greater than the length c of the axial chord of the straightener vanes, and the angle (α A) from the tangent to the upper surface wall to the upstream extreme point is equal to that from the tangent (α B) to the wall to the downstream extreme point to within one degree, characterised in that the downstream extreme point is situated upstream of the upstream end of the pylon, the downstream part comprising an upper surface wall which is convex from the downstream extreme point.