Unducted aviation propulsion unit for aircraft

Abstract

The invention relates to a propulsion unit comprising: a housing (102); a hub; -a thruster propeller; and-a fixed rectifier mounted on the housing downstream of the propeller propeller along the main axis (X), the fixed rectifier extends around a main axis (X) and comprises at least one stator blade (114) having a variable pitch around a pitch axis (Y '), each variable pitch stator blade (114) having a pitch angle defined as an angle between any plane perpendicular to the main axis (X) and a chord line connecting a leading edge (BA') of the stator blade (114) to a trailing edge (BF ') of the stator blade (114), the chord line is taken at 75% of the outer radius (Re ') of the stator blade (114) with respect to the main axis (X). For at least one value of the pitch angle within the interval [70 DEG; 90 DEG], the leading edge (BA') of the at least one variable pitch stator blade (114) has a tip (BA 'T) located downstream of the pitch axis (Y') at the height of the tip (BA 'T).

Description

TECHNICAL FIELD

[0001] The present invention relates to a non-ducted aerial propulsion unit for an aircraft, such as a turbomachine or turbojet engine or turboshaft engine, and to an aircraft comprising such an aerial propulsion unit. BACKGROUND

[0002] In the prior art, a non-ducted aerial propulsion unit for an aircraft is known, comprising: - a casing; - a hub pivotally mounted with respect to the casing about a main axis extending in an upstream-downstream direction of the aircraft; - a propeller mounted on the hub so as to be able to pivot with respect to the casing; and - a fixed fairing mounted on the casing downstream of the propeller along the main axis, the fixed fairing extending about the main axis and comprising at least one stator blade having a variable pitch about a pitch axis, each pitch- variable stator blade having a pitch angle defined as the angle between, on the one hand, any plane perpendicular to the main axis and, on the other hand, a chord line connecting a leading edge of the stator blade to a trailing edge of the stator blade.

[0003] The propeller generally has rotor blades with variable pitch. The variable pitch system is known in English as the Fan Pitch Actuation System (FPAS) and / or the Pitch Change Mechanism (PCM). To achieve the target thrust, there can be multiple combinations of propeller rotational speed and rotor and stator blade pitch.

[0004] One of the challenges of such a propulsion unit is the certification of the noise level during take-off and landing operations. The noise level emitted by an aircraft is subject to increasingly stringent international regulations aiming to limit the acoustic footprint around the airport. Currently, the OACI (Organisation de l'Aviation Civile Internationale) Volume 1 Annex 16 Chapter 14 sets out valid provisions giving maximum noise levels in EPNL (Effective Perceived Noise Level) according to the flight phase and the weight of the aircraft.

[0005] The main noise sources of a non-ducted propulsion unit are as follows.

[0006] A first source of noise is the interaction noise between the vortex created at the tip of the rotor blade and the wake of the upstream propeller, which interacts with the leading edge of the downstream fairing. This noise source contributes to an increase in broadband noise, as the turbulence rate in the wake is generally very high at the rotor blade tip, and to an increase in tonal noise, which is related to the periodic nature of this vortex as the rotor blade rotates.

[0007] A second source of noise is the inherent noise of the rotor blade and of the stator blade. This inherent noise is related to the fixed loads on the rotor blade, source of tonal noise, and to the development of the boundary layer on the rotor blade and on the stator blade. In this way, when the turbulent boundary layer passes over the trailing edge of the rotor blade and of the stator blade, a broadband noise source is created. Increasing the chord of the rotor blade and / or of the stator blade on an unducted aerial propulsion unit increases the surface area over which the boundary layer develops, thus increasing the broadband noise.

[0008] The aim of the present invention is to improve the acoustic performance of the fairing downstream of the propeller, without reducing the aerodynamic performance, while respecting certain mechanical constraints.

[0009] Furthermore, the patent applications US 2017 0284304 A1 and US 2021 108523 A1 and the article “Cruise noise of an advanced propeller with vortex recovery blades” describe an unducted aerial propulsion unit for an aircraft. SUMMARY

[0010] Thus, an unducted aerial propulsion unit for an aircraft is proposed, comprising: - a housing; - a hub, said hub being pivotally mounted with respect to said housing about a main axis extending in an upstream-downstream direction of said aircraft; - a propeller, said propeller being mounted on said hub so as to be able to pivot with respect to said housing; and - a fixed fairing, said fixed fairing being mounted on said housing downstream of said propeller along said main axis, said fixed fairing extending about said main axis and comprising at least one variable-pitch stator blade about a pitch axis, each variable-pitch stator blade having a pitch angle, said pitch angle being defined as the angle between, on the one hand, any plane perpendicular to said main axis and, on the other hand, a chord line connecting a leading edge of said stator blade to a trailing edge of said stator blade; characterized in that said chord line is taken at 75% of the outer radius of said stator blade with respect to said main axis, and in that, for at least one value of said pitch angle in the interval [70°; 90°], the leading edge of at least one variable-pitch stator blade has a tip, said tip being located downstream of said pitch axis in its height.

[0011] The application can also comprise one or more of the following optional features in any technically possible combination.

[0012] Optionally, the at least one variable-pitch stator blade has an activity factor comprised between 50 and 200, preferably between 90 and 150.

[0013] Optionally, the propeller screw further comprises at least one variable-pitch rotor blade.

[0014] Optionally, the at least one variable-pitch stator blade has a leading edge with a root and a belly, for at least one value of the pitch angle within the interval [70°; 90°], the belly, the root and the pitch axis follow one another in this order along the main axis from upstream to downstream.

[0015] Optionally, for at least one value of the pitch angle within the interval [70°; 90°], the leading edge of the at least one variable-pitch stator blade has a belly between 20% and 70%, preferably between 25% and 55% of the total blade height of the leading edge.

[0016] Optionally, the leading edge and the trailing edge of the at least one variable-pitch stator blade each have a belly, the height and / or the radius of the belly at the leading edge from the main axis X being less than the height and / or the radius of the belly at the trailing edge from the main axis.

[0017] Optionally, the chord length of the cross-section of the at least one variable-pitch stator blade closest to the tip of the leading edge is less than the chord length of the cross-section closest to the root of the leading edge, itself less than the chord length of the cross-section at the belly of the leading edge.

[0018] Optionally, the chord length of the at least one variable-pitch stator blade decreases strictly from 50% to 100% of the total blade height of the leading edge and / or of the trailing edge, preferably from 40% to 100% of the total blade height of the leading edge and / or of the trailing edge.

[0019] Optionally, for the at least one variable-pitch stator blade, the chord length at 95% of the total blade height of the leading edge or of the trailing edge is less than or equal to half the maximum chord length.

[0020] Optionally, for the at least one variable-pitch stator blade, the chord length at 95% of the total blade height of the leading edge or of the trailing edge is less than or equal to half the chord length at 10% of the total blade height of the leading edge or of the trailing edge.

[0021] Optionally, for at least one value of the pitch within the interval [70°; 90°], the at least one variable-pitch stator blade has a sweepback angle greater than 20°, preferably greater than 35°, at a position above 80% of the upstream blade height.

[0022] Optionally, for at least one variable pitch stator blade, for at least one value of the pitch angle comprised between 70° and 90°, the axial distance along the main axis between the tip of the trailing edge and the root of the trailing edge is less than or equal to a coefficient E times the maximum chord length, with E comprised between 0.05 and 0.7, preferably between 0.25 and 0.6.

[0023] Optionally, for at least one variable pitch stator blade, for at least one value of the pitch angle comprised between 70° and 90°, the axial distance along the main axis between the tip of the trailing edge and the root of the trailing edge is less than or equal to a coefficient E times the maximum chord length, with E comprised between 0.05 and 0.7, preferably between 0.25 and 0.6.

[0024] Optionally, for at least one variable pitch stator blade, for at least one value of the pitch angle comprised between 70° and 90°, the axial distance along the main axis between the tip of the trailing edge and the root of the trailing edge is less than or equal to a coefficient E times the maximum chord length, with E comprised between 0.05 and 0.7, preferably between 0.25 and 0.6.

[0025] Optionally, for at least one variable pitch stator blade, for at least one value of the pitch angle comprised between 70° and 90°, the axial distance along the main axis between the tip of the trailing edge and the root of the trailing edge is less than or equal to a coefficient E times the maximum chord length, with E comprised between 0.05 and 0.7, preferably between 0.25 and 0.6.

[0026] Optionally, the trailing edge of the at least one rotor blade and the leading edge of the at least one stator blade are separated along the main axis by a distance taken as 95% of the total blade height of the leading edge or of the trailing edge of the stator blade, this distance being greater than 0.35*Re, preferably greater than 0.5*Re, for at least one value of the pitch angle of the rotor blade comprised between 50° and 80° and for at least one value of the pitch angle of the stator blade comprised between 70° and 90°, where Re is the outer radius of the propeller of the propulsion unit.

[0027] Optionally, the at least two stator blades have at least one different geometrical element among the outer radius, the maximum chord length, the activity factor, the back- sweep angle at 90% of the upstream blade height, the pitch angle and the angular spacing around the main axis.

[0028] It is also proposed an aircraft comprising an aeronautical propulsion unit according to the application. BRIEF DESCRIPTION OF DRAWINGS

[0029] The application will be better understood by means of the following description, given solely by way of example and with reference to the appended drawings, in which: - Figure 1 is a cross-sectional view of an aeronautical propulsion unit according to the application, -Figure 2 is a side view of a rotor blade of a propeller of an aerial propulsion unit according to Figure 1 Figure 3 is a cross-sectional view of a rotor blade in Figure 2 is a similar view showing a sweep angle of a rotor blade, Figure 4 Figure 2 is a front view of a rotor blade in Figure 5 Figure 2 Figure 6 is a side view of a stator blade of a fixed diffuser of an aerial propulsion unit according to Figure 1 Figure 7 is a cross-sectional view of a stator blade in Figure 6 Figure 8 is a similar view showing a sweep angle of a stator blade, Figure 6 Figure 9 Figure 6 Figure 10 is a similar view showing a stator blade with a rounded tip, Figure 6 Figure 11 is a graph showing the evolution of the chord of a stator blade with the blade height, Figure 12 is a cross-sectional view of another aerial propulsion unit according to the invention showing the spacing between the propeller and the fixed diffuser, Figure 13 is a cross-sectional view of a stator blade in Figure 6 Figure 14 is a front view of the trailing edge of a rotor blade and of the leading edge of a stator blade. DETAILED DESCRIPTION

[0030] In the following description, if a feature applies to at least one element, it can also apply to all such elements. Similarly, if a feature applies to at least one value in a range, it can also apply to all values in the range.

[0031] With reference to Figure 1 An aerial propulsion unit 100 to which the invention applies will now be described. The aerial propulsion unit 100 is for example a turbomachine, a turbojet or a turboshaft, since the type of aerial propulsion unit is not decisive here.​​​​​​​​​​​​​​​​​​​​​​​

[0032] The aerial propulsion unit 100 is an unducted single fan (USF) designed to help propel an aircraft.

[0033] The aerial propulsion unit 100 first comprises a casing 102 and a hub 104 mounted to pivot about a main axis X relative to the casing 102.

[0034] In the following, the terms “upstream” and “downstream” will be used to designate the relative position of elements of the aerial propulsion unit 100 along the main axis X in the direction of flow of the air flow PHI when the aircraft is propelled by the aerial propulsion unit 100. For example, the aircraft can be propelled by the aerial propulsion unit 100 in a cruise mode at a flight Mach number greater than 0.7.

[0035] For example, the hub 104 is located upstream of the casing 102.

[0036] The aerial propulsion unit 100 further comprises a propeller 106, which is unducted and mounted on the hub 104 so as to be able to pivot about the main axis X relative to the casing 102. The propeller 106 is designed to drive the air flow PHI downstream to propel the aircraft in flight. To this end, the propeller 106 comprises rotor blades 108, for example 3 to 25, preferably 10 to 16, for example arranged in a single annular row about the main axis X. The rotor blades 108 can for example all be identical and angularly spaced at regular intervals about the main axis X.

[0037] For example, at least one rotor blade 108 has a variable pitch about a respective pitch axis Y. The pitch axis Y can pass through the main axis X or be slightly offset therefrom, for example by up to 10 cm, for example by up to 5 cm, for example by up to 2 cm, for example by up to 1 cm. The pitch axis Y can be perpendicular to the main axis X, or have a slight inclination due to manufacturing tolerances or design intent. Thus, the pitch axis Y can for example be perpendicular within a tolerance range of 5°, 2°, 1°, or 0.1°. In all cases, the pitch axis Y forms a non-zero angle with the main axis X, and thus has a non-zero radial component. As illustrated, the axis Y can thus be radial, i.e. the axis Y forms a 90° angle with the main axis X. The pitch of each variable-pitch rotor blade 108 is defined by a pitch angle C described below. In a preferred embodiment, all rotor blades 108 have a variable pitch.

[0038] The aerial propulsion unit 100 also comprises an engine 110 for driving the hub 104 and thus, via the hub 104, the propeller 106. The engine 110 extends into the casing 102, for example. The engine 110 is located downstream of the propeller 106, for example. This arrangement is referred to as a "puller". The engine 110 is a gas generator, for example, which generally comprises, from upstream to downstream, at least one compressor, at least one combustion chamber and at least one power turbine configured to drive the propeller 106.

[0039] The aerial propulsion unit 100 also comprises a fixed ductless fairing 112 mounted on the casing 102 downstream of the propeller 106. The fairing 112 forms a stator located in the casing 102 which extends around the main axis X but cannot rotate around the main axis X. The fairing 112 comprises stator blades 114 arranged in a single annular row around the main axis X, for example. There are 3 to 25 stator blades, preferably 8 to 14 stator blades 114, for example. Preferably, the number of stator blades 114 is different from the number of rotor blades 108 in order to reduce the noise of the aerial propulsion unit 100. In particular, the number of rotor blades 108 is greater than the number of stator blades 114. Indeed, if the number of rotor blades 108 and the number of stator blades 114 are equal, the rotor blades 108 will then be followed by a wake which will interact simultaneously with a plurality of stator blades 114, which will increase the noise level. The stator blades 114 can be all identical or different, for example, and are angularly spaced around the main axis X in a regular or uneven manner, so that at least two stator blades 114 have a different angular spacing around the main axis X. In particular, in the case of an aerial propulsion unit mounted in an aircraft, the stator blades 114: - can have an outer radius that is smaller than the outer radius of the rotor blades 108 in order to reduce the interaction noise of the tip vortex of the rotor blades 108 with the stator blades 114; this outer radius of the stator blades 114 can be different for each stator blade 114 in order to limit the interaction during the phases of incidence flight, such as landing and takeoff; - can have different geometric characteristics (pitch angle, thickness, chord, etc.) in order to optimize the operation of the stator blades according to the local nature of the flow; and - can have an uneven angular spacing around the main axis X, in particular in the vicinity of the mast or pylon; this allows the operation of the stator blades 114 to be optimized according to their mounting in the aircraft.

[0040] The rectifier 112 is designed to rectify at least a portion of the air flow PHI passing through the propeller 106 to improve the performance of the aerial propulsion unit 100. More specifically, the purpose of the rectifier 112 is to absorb the flow swirl induced by the propeller 106 to improve the performance of the unducted configuration. However, the presence of the rectifier induces a major noise source generated by the interaction with the wake of the propeller 106 (and also with the blade tip vortex when the truncation of the stator blades 114 is not sufficient). Therefore, it is important to reduce the noise generated by the rectifier 112 and its interaction with the wake of the propeller 106 while maintaining good aerodynamic performance, since reducing noise emissions and fuel consumption are major challenges for unducted engine architectures.

[0041] For example, at least one stator blade 114 has a variable pitch around a respective axis Y’. The pitch axis Y’ can pass through the main axis X or be slightly offset from the main axis X, for example by up to 10 cm, for example by up to 5 cm, for example by up to 2 cm, for example by up to 1 cm. The pitch axis Y’ can be perpendicular to the main axis X, or have a slight inclination due to manufacturing tolerances or design intent. Thus, the pitch axis Y’ can for example be perpendicular within a tolerance range of 5°, 2°, 1°, or 0.1°. In all cases, the pitch axis Y’ forms a non-zero angle with the main axis X, and thus has a non-zero radial component. As shown, the axis Y’ can thus be radial, i.e. the axis Y’ forms a 90° angle with the main axis X. The pitch of each variable-pitch stator blade 114 is defined by a pitch angle C’, which will be described in more detail below. In a preferred embodiment, all stator blades 114 have a variable pitch.

[0042] In the case where one of the stator blades is stationary (for example for integration constraints, for example if there is not enough space below the hub to integrate a variable-pitch system or to reduce weight), the axis Y’ of the blade can be defined by a line perpendicular to the main axis X passing through the leading edge BA’ at the root of the blade. In this case, the blade has a fixed pitch angle C’.

[0043] The aerial propulsion unit 100 also comprises an air inlet 116, for example for supplying the engine 110 with a main flow of air. This air inlet 116 is for example provided between the propeller 106 and the rectifier 112.

[0044] With reference to Figure 2 The rotor blade 108 first comprises a leading edge BA at which the air flow PHI arrives (i.e. the front of the rotor blade 108 facing the fluid (or first in contact with the fluid)), and a trailing edge BF at which the air flow PHI exits (i.e. the rear of the rotor blade 108 in the direction of flow).

[0045] The leading edge BA extends from the root BA_P (the point on the leading edge BA closest to the hub 104) to the tip BA_T (the highest point on the leading edge BA or the point with the largest radius relative to the main axis X on the upstream portion of the rotor blade 108). For at least one value of the pitch angle C in the interval [50°; 80°], the leading edge BA has a constant curvature between the root BA_P and the tip BA_T, i.e., without any inflection points (in other words, along the leading edge BA, the height h of the rotor blade 108 is constant). BA The function of the axial position x of the leading edge. BA Regarding h BA The second derivative of is not zero, that is, , and h BA It is the height from the root BA_P). The curvature of the leading edge BA is also regular, i.e., there are no discontinuities. The leading edge BA also includes the ventral BA_V, and in particular, for at least one value of the pitch angle C in the interval [50°; 80°], the ventral BA_V is the upstream point of the leading edge BA (the smallest {x BA}).

[0046] Similarly, the trailing edge BF extends from the root BF_P (the point on the trailing edge BF closest to the hub 104) to the tip BF_T (the highest point on the trailing edge BF or the point with the largest radius relative to the main axis X on the rear portion of the rotor blade 108). For at least one value of the pitch angle C within the interval [50°; 80°], the trailing edge BF has a constant curvature between the root BF_P and the tip BF_T, i.e., no inflection point (in other words, along the trailing edge BF, the height h of the rotor blade 108 is constant). BF The function of the trailing edge BF, axial position x BF Regarding h BF The second derivative of is not zero, that is, , and h BF It is the height from the root BF_P). The curvature of the trailing edge BF is also regular, i.e., without discontinuities. The trailing edge BF also includes the ventral BF_V, specifically for at least one value of the pitch angle C in the interval [50°; 80°], where the ventral BF_V is the most upstream point of the trailing edge BF (minimum {x BF}).

[0047] The rotor blade 108 can also be truncated at its free end, as in the example shown, i.e., there is a truncated segment 202, for example a straight line, connecting the tips BA_T and BF_T. In this case, there is a curvature discontinuity at the tip BA_T between the leading edge BA and the truncated segment 202, and another curvature discontinuity at the tip BF_T between the truncated segment 202 and the trailing edge BF. Alternatively, the propeller blade can be untruncated, in which the tips BA_T and BF_T are combined.

[0048] In the following, the term "height" when applied in the context of the rotor blade 108 will refer to the distance between two points along the radial component of the pitch axis Y, i.e. the distance between the orthogonal projections of these points on the radial component of the pitch axis Y.

[0049] Therefore, an upstream blade height H 上游 can be defined for a position on the leading edge BA. 上游 is the ratio between the height h BA from the root BA P and the total height H BA of the leading edge BA (between the root BA P and the tip BA T): H 上游 = h BA / H BA . Therefore, the upstream blade height H 上游 can be expressed in percentage and varies between 0% (position at the root BA P) and 100% (position at the tip BA T). Similarly, a downstream blade height H 下游 can be defined for a position on the trailing edge BF. 下游 is the ratio between the height h BF from the root BF P and the total height H BF of the trailing edge BF (between the root BF P and the tip BF T): H 下游 = h BF / H BF . Therefore, the downstream blade height H 下游 can be expressed in percentage and varies between 0% (position at the root BF P) and 100% (position at the tip BF T).

[0050] Furthermore, the propeller 106 has an outer radius Re, defined as equal to the height or distance between the main axis X and the point on the rotor blade 108 that is farthest from the main axis X, in particular for at least one value of the pitch angle C within the interval [50°; 80°]. In the example shown, this farthest point is the tip BA T of the leading edge BA.

[0051] Figure 3 is the cross section of the rotor blade 108 (also known as aerodynamic profile) at a certain height, perpendicular to the radial component of the pitch axis Y.

[0052] As can be seen, the rotor blade 108 has a pressure side 302 and a suction side 304 that are concave and convex, respectively, and the pressure side 302 and the suction side 304 are connected to each other by a leading edge BA and a trailing edge BF. Therefore, the leading edge BA separates the pressure side 302 and the suction side 304 in the upstream portion of the rotor blade 108, while the trailing edge BF separates the pressure side 302 and the suction side 304 in the rear portion of the rotor blade 108.

[0053] The leading edge BA is, for example, a point on the cross-section of the upstream portion of the rotor blade 108, having a local minimum radius of curvature. Similarly, when the downstream portion of the rotor blade is rounded, the trailing edge BF is, for example, a point on the cross-section of that downstream portion, exhibiting a local minimum radius of curvature. To simplify the manufacturing method of the rotor blade 108, the downstream portion of the rotor blade can be truncated. In the case where the downstream portion of the rotor blade is truncated, the trailing edge BF is, for example, the midpoint of the truncated portion.

[0054] When the leading edge BA and the trailing edge BF exist in the cross section under consideration (i.e., at a height not too close to the root BA_P and below the truncated section 202), the leading edge BA and the trailing edge BF can be connected by a chord 306, the orientation of which varies depending on the height under consideration. The leading edge BA and the trailing edge BF are separated by a distance on the chord 306, referred to as chord length L, which varies depending on the height under consideration.

[0055] Therefore, an angle A exists between any plane P perpendicular to the main axis X and the chord 306 at a certain height. This angle A can therefore be varied depending on the height considered. To explicitly identify the pitch, the pitch angle C is chosen as the previous angle A at a height of 75% of the outer radius Re of the rotor blade 108 (see...). Figure 2 The pitch angle C is measured, for example, on the upstream side of plane P, with its positive direction being from plane P to chord 306. This direction coincides with the direction from pressure side 302 to suction side 304.

[0056] Reference Figure 4 Furthermore, the sweep angle F of the rotor blade 108 can be defined, which is based on the blade height H. 上游 And it changes. By definition, the sweep angle F is the radial component of the pitch axis Y intersecting the line 402 in the plane formed by the radial component of the pitch axis Y and the principal axis X. Figure 4 The angle between the projections on the drawing plane, line 402 connects the blade height H under consideration. 上游 Point 404 on the leading edge BA and at the considered blade height H 上游 Add the total height H BA Point 406 on the leading edge BA at 1%.

[0057] ReferenceFigure 5 Furthermore, the dihedral angle D of the rotor blade 108 can be defined at the leading edge BA. BA The dihedral angle is based on the blade height H. 上游 However, this changes. By definition, the dihedral angle D at the leading edge BA... BA The pitch axis Y (or more generally, the radial component of the pitch axis Y) and line 402 lie in a plane perpendicular to the main axis X and containing the pitch axis Y (or more generally, the radial component of the pitch axis Y). Figure 5 The angle between the projections on the paper plane. As mentioned above, line 402 connects the leading edge BA at the blade height H under discussion. 上游 Point 404 and leading edge BA at the blade height H under discussion 上游 Add the total height H BA Point 406 at 1% of the value. Similarly, the dihedral angle D can be defined at the trailing edge BF. BF .

[0058] Reference Figure 6 The stator blade 114 first includes the leading edge BA' (i.e. the front part of the stator blade 114 facing the fluid (or initially in contact with the fluid) reached by the airflow PHI from the propeller 106) and the trailing edge BF' (i.e. the rear part of the stator blade 114 in the flow direction) where the airflow PHI leaves.

[0059] The leading edge BA' extends from the root BA'P (the point on the leading edge BA' closest to the outer casing 102) to the tip BA'T (the highest point of the leading edge BA' or the point with the largest radius relative to the main axis X on the upstream portion of the stator blade 114). Specifically, for at least one value of the pitch angle C' within the interval [70°; 90°], the leading edge BA' has a constant curvature between the root BA'_P and the tip BA'_T, i.e., without any inflection points (in other words, along the leading edge BA, as the height h' of the stator blade 114). BA The function, the axial position x' of the leading edge BA' BA Regarding h' BA The second derivative of is not zero, that is, , and h' BA It is the height from the root BA'_P). The curvature of the leading edge BA' is also regular, i.e., without discontinuities. The leading edge BA' also includes the ventral BA'_V, and in particular, for at least one value of the pitch angle C' in the interval [70°; 90°], the ventral BA'_V is the upstream point of the leading edge BA' (minimum {x' BA}).

[0060] Similarly, the trailing edge BF' extends from a root BF'_P (point on the trailing edge BF' closest to the casing 102) to a tip BF'_T (point on the trailing edge BF' having the highest elevation or the largest radius with respect to the main axis X on the rear portion of the stator blade 114). In particular, for at least one value of the teaching angle C' in the interval [70°; 90°], between the root BF'_P and the tip BF'_T, the trailing edge BF' has a curvature of constant orientation, i.e. without inflection points (in other words, along the trailing edge BF', as a function of the height h' BF of the stator blade 114, the axial position x' BF of the trailing edge BF' has a second derivative with respect to h' BF not equal to zero, i.e. , and h' BF is the height from the root BF'_P). The curvature of the trailing edge BF' is also regular, i.e. without discontinuity. The trailing edge BF' also comprises an abdomen BF'_V, in particular for at least one value of the pitch angle C' in the interval [70°; 90°], the abdomen BF'_V being the most upstream point of the trailing edge BF' (minimum {x'BF}).

[0061] The stator blade 114 can also be truncated, as in the example shown, i.e. there is a truncated segment 602, for example straight, connecting the tips BA'_T, BF'_T. In this case, there is a curvature discontinuity at the tip BA'_T between the leading edge BA' and the truncated segment 602, and also at the tip BF'_T between the truncated segment 602 and the trailing edge BF'. Alternatively, the stator blade can be non-truncated, in which case the tips BA'_T, BF'_T would be merged.

[0062] In the following, the term "height" will refer, when applied in the context of the stator blade 114, to the distance between two points along the radial component of the pitch axis Y', i.e. the distance between the orthogonal projections of these points on the radial component of the pitch axis Y'.

[0063] It is thus possible to define an upstream blade height H' 上游 for positioning on the leading edge BA'. The upstream blade height H' 上游 is thus the ratio between the height h' BA from the root BA'_P and the total height H' BA of the leading edge BA' (between the root BA'_P and the tip BA'_T): H' 上游 = h' BA / H' BA . The upstream blade height H' 上游This can be expressed as a percentage and varies between 0% (at the root BA'_P) and 100% (at the tip BA'_T). Similarly, the height H' of the downstream blade located on the trailing edge BF' can be defined. 下游 Therefore, the downstream blade height H' 下游 It is the height h' from the root BF'_P BF The total height H' of the trailing edge BF' BF The ratio between (the root BF'_P and the tip BF'_T): H' 下游 =h' BF / H' BF Therefore, the downstream blade height H' 下游 It can be expressed as a percentage and varies between 0% (at the root BF'P) and 100% (at the tip BF'T).

[0064] Furthermore, specifically for at least one value of the pitch angle C' within the interval [70°; 90°], each variable-pitch stator blade 114 of the rectifier 112 has an outer radius Re', which is defined as equal to the height or distance between the main axis X and the point on the stator blade 114 furthest from the main axis X. In the example shown, this furthest point is the tip BF'_T of the trailing edge BF'. Alternatively, this furthest point could be the tip BA'_T of the leading edge BA'.

[0065] Each variable-pitch stator blade 114 also has an inner radius Ri' at its leading edge BA'. BA The inner radius Ri' BA It is the distance between the principal axis X and the root BA'_P of the leading edge BA, and has an inner radius Ri' at the trailing edge BF'. BF The inner radius Ri' BF It is the distance between the principal axis X and the root BF'_P of the trailing edge BF'.

[0066] Figure 7 and Figure 13 The cross-sections of the stator blade 114 at different heights are shown, with the radial component perpendicular to the pitch axis Y'.

[0067] As can be seen, the stator blade 114 has a pressure side 702 that is concave and a suction side 704 that is convex, respectively, and the pressure side 702 and the suction side 704 are connected to each other by a leading edge BA' and a trailing edge BF'. Therefore, the leading edge BA' separates the pressure side 702 and the suction side 704 in the upstream portion of the stator blade 114, while the trailing edge BF' separates the pressure side 702 and the suction side 704 in the rear portion of the stator blade 114.

[0068] The leading edge BA' is, for example, a point on the cross-section of the upstream portion of the stator blade 114, having a local minimum radius of curvature. Similarly, when the downstream portion of the stator blade is rounded, the trailing edge BF' is, for example, a point on the cross-section of that downstream portion, exhibiting a local minimum radius of curvature. To simplify the method for manufacturing the stator blade 114, the downstream portion of the stator blade can be truncated. In the case where the downstream blade of the stator blade is truncated, the trailing edge BF' is, for example, the midpoint of that truncated portion.

[0069] When the leading edge BA' and trailing edge BF' are present in the cross-section under consideration (i.e., below the truncated section 602, for example, in the example shown), the leading edge BA' and trailing edge BF' can be connected by a chord 706, the orientation of which varies depending on the height under consideration. The leading edge BA' and trailing edge BF' are separated on the chord 706 by a distance referred to as the chord length L', which can vary depending on the height under consideration.

[0070] Therefore, an angle A' exists between any plane P' perpendicular to the main axis X and the chord 706 at a certain height. This angle A' can therefore be varied depending on the height considered. To explicitly identify the pitch, the pitch angle C' is chosen as the previous angle A' at a height of 75% of the outer radius Re' of the stator blade 114 (see [reference]). Figure 6 The pitch angle C' is measured, for example, on the upstream side of plane P', with its positive direction being from plane P' to chord 706. This direction coincides with the direction from pressure side 702 to suction side 704.

[0071] Reference Figure 8 Furthermore, the sweep angle F' of the stator blade 114 can be defined, which is based on the blade height H'. 上游 And it changes. By definition, the sweep angle F' is the angle between the radial component of the pitch axis Y' and the projection of line 402 onto the plane formed by the radial component of the pitch axis Y' and the principal axis X, with line 402 connecting the blade height H' under consideration. 上游 Point 804 on the leading edge BA' and at the considered blade height H' 上游 Add the total height H' BA Point 806 on the leading edge BA' at 1%.

[0072] Reference Figure 9 Furthermore, the dihedral angle D' of the stator blade 114 can be defined at the leading edge BA'. BA' The dihedral angle is based on the blade height H' 上游 And change. By definition, the dihedral angle D' at the leading edge BA'... BA'is the angle between the radial component of the pitch axis Y' and the projection of the line 802 in a plane perpendicular to the main axis X and containing the radial component of the pitch axis Y'. Similarly, a dihedral angle D' can be defined at the trailing edge BF' BF' .

[0073] The shape of the stator blade 114 of the rectifier 112 is designed to reduce noise pollution while ensuring good aerodynamic performance.

[0074] Therefore, for at least one value of the pitch angle C' in the interval [70°; 90°], the tip BA'_T of the leading edge BA' is located downstream of the pitch axis Y' along a straight line parallel to the main axis X and passing through the tip BA'_T. In this way, the free end of the stator blade 114 is moved away from the propeller 106, which reduces the noise over a wide range of pitch angles C'. The positioning of the free end of the stator blade 114 downstream enables to reduce the interaction noise between the wake of the propeller 106 and the leading edge BA' of the stator blade by increasing the distance between the trailing edge BF of the rotor blade 108 and the leading edge BA' of the stator blade 114, making the noise sources along the leading edge BA' decorrelated and dissipating the wake of the propeller 106.

[0075] Preferably, to facilitate the positioning of the free end of the stator blade 114 downstream, for at least one value of the pitch angle C' in the interval [70°; 90°], the belly BA'_V, the root BA'_P and the pitch axis Y' follow each other in this order along the main axis X from upstream to downstream. In other words, for at least one value of the pitch angle C' in the interval [70°; 90°], x BA'_V <x BA'_P <x Y' . Therefore, it is possible to increase the sweep angle F' at the upper portion of the stator blade 114, in particular near the free end of the stator blade 114. When the pitch axis Y' is inclined or not perpendicular to the main axis X, x Y' corresponds to the axial position of the pitch axis Y' of the stator blade at the casing 102. Furthermore, for at least one value of the pitch angle C' in the interval [70°; 90°], the belly BA'_V, the root BA'_P and the tip BA'_T preferably follow each other in this order along the main axis X from upstream to downstream.

[0076] Even more preferably, to facilitate the positioning of the free end of the stator blade 114 downstream, the belly BA'_V is located between 20% and 70% of the total blade height H' BA , preferably between 30% and 60% of the total blade height H' BAThe sweep angle is between 25% and 55%. This allows the belly BA'_V to be positioned close to the outer casing 102, and thus increases the sweep angle F' from a relatively low radial position, which reduces interaction noise. Therefore, a large sweep angle F' can be achieved in the upper portion of the stator blade 114, particularly near the free end of the stator blade 114. Furthermore, this positioning of the belly BA'_V causes the mass in the lower portion of the stator blade 114 to move upstream, which allows the mass in the upper portion of the stator blade 114 to move downstream (and therefore towards the free end) without significantly altering the center of mass of the stator blade 114 and having little effect on the aerodynamic torque about the pitch-changing axis. This is beneficial for the size and mechanical balance of the stator blade 114.

[0077] Utilizing this positioning of the ventral part BA'_V of the leading edge BA', the ventral part BF'_V of the trailing edge BF' is preferably located at the total blade height H' of the trailing edge. BF The total blade height H' is preferably between 20% and 70% of the trailing edge. BF Between 30% and 60%. This ensures that the belly BF'_V of the trailing edge BF' is located at a height relatively close to the belly BA'_V of the leading edge BA', which may be useful for the mechanical strength of the stator blade 114. Similar to the position of the belly BA'_V of the leading edge BA', this position of the belly BF'_V of the trailing edge BF' causes the mass in the lower portion of the stator blade 114 to move upstream, which makes it possible to move the mass in the upper portion of the stator blade 114 downstream (and therefore towards the free end) without significantly altering the center of mass of the stator blade 114.

[0078] In one embodiment, the height or radius (height or distance from the main axis X) of the ventral BA' of the leading edge BA' is smaller than the height or radius (height or distance from the main axis X) of the ventral BF' of the trailing edge BF'. This improves the mechanical strength of the blade and makes the chord L' more evenly distributed along the span (see...). Figure 11 ).

[0079] Furthermore, for at least one value of the pitch angle C' within the range [70°; 90°], by utilizing this positioning of the belly BA'V of the leading edge BA', stator blades 114 that satisfy one or more of the following three criteria can be obtained, thereby enabling an acceptable aeroacoustic and mechanical compromise.

[0080] According to the first criterion, the ventral part BA'_V of the leading edge BA' and the tip BF'_T of the trailing edge BF' are separated along the principal axis X by a distance greater than or equal to a coefficient K multiplied by the maximum chord length L' (maximum {L'}), where K is between 1 and 2, preferably between 1.2 and 1.6. The following equation summarizes this first criterion: x BF'_T -xBA'_V ≥ K * max{L'}, with x M being the axial position of the point M along the main axis X.

[0081] According to a second criterion, the root BA'_P and the tip BA'_T of the leading edge BA' are separated along the main axis X by a distance smaller than or equal to a coefficient B times the maximum chord L' (max{L'}), with B being comprised between 0.5 and 1.2. The following equation summarizes this second criterion: x BA'_T - x BA'_V ≤ B * max{L'}.

[0082] According to a third criterion, the root BF'_P and the tip BF'_T of the trailing edge BF' are separated along the main axis X by a distance greater than or equal to a coefficient E times the maximum chord L' (max{L'}), with E being comprised between 0.05 and 0.7, preferably between 0.25 and 0.6. The following equation summarizes this third criterion: x BF'_T - x BF'_P ≤ E * max{L'}.

[0083] Even more preferably, in order to facilitate the positioning of the free end of the stator blade 114 downstream, the chord L' of the cross section closest to the tip BA'_T of the leading edge BA' is smaller than the chord L' at the root BA'_P of the leading edge BA' and / or at the root BF'_P of the trailing edge BF', the chord at the root of the leading edge and / or at the root of the trailing edge itself being smaller than the chord L' of the cross section at the belly BA'_V of the leading edge BA' ("smaller" means "of smaller length"). Increasing the chord L' at the belly BA'_V and decreasing the chord L' at the tip BA'_T increases the sweep angle F' at the upper portion of the stator blade 114 and thus moves the free end of the stator blade 114 further downstream, which is advantageous for noise reduction. Moreover, in case the chord L' at the root BA'_P is greater than the chord L' at the tip BA'_T, the mechanical strength of the blade (e.g. after ingestion by a bird) is improved. Furthermore, the smaller the chord L' at the tip BA'_T, the lighter the free end of the stator blade 114 and thus the more likely it is to place this free end downstream without moving the center of mass of the stator blade 114 too much.

[0084] For example, the sweep angle F' at the leading edge BA' of the stator blade 114 is greater than 20°, preferably greater than 35° in the region above 80% of the upstream blade height H' 上游 and even more preferably greater than 45° in the position above 90% of the upstream blade height H' 上游 and even more preferably greater than 45° in the position above 90% of the upstream blade height H' 上游even more preferably greater than 50°. This reduces the component of flow velocity normal to the trace of the leading edge BA' of the cross section, thus reducing interaction noise.

[0085] Even more preferably, in order to facilitate the positioning of the free end of the stator blade 114 downstream, for at least one value of the pitch angle C' within the interval [70°; 90°], the belly BF'_V is located upstream of the root BF'_P, itself located upstream of the tip BF'_T. In other words, for at least one value of the pitch angle C' within the interval [70°; 90°], x BF'_V <x BF'_P <x BF'_T .

[0086] With reference to Figure 10 , a non-truncated stator blade 114 is shown, having a rounded tip.

[0087] With reference to Figure 11 , preferably, the chord length L' is strictly decreasing from the upstream blade height H' 上游 and / or the downstream blade height H' 下游 of 40% to 100%, preferably from 50% to 100% (thus in Figure 11 , the reference H' can denote H' 上游 or H' 下游 ). This aspect makes it possible, on the one hand, to increase the distance between the trailing edge BF of the rotor blade 108 and the leading edge BA' of the stator blade 114, and on the other hand, to increase the sweep angle F' towards the free end of the stator blade 114, in particular when there are integrated or mechanical constraints that prevent the trailing edge BF' of the stator blade 114 from being modified or offset. Indeed, offsetting the trailing edge BF' to the upper portion of the blade (for example on the tip cross section 602) can increase the moment of the aerodynamic forces around the pitch axis Y' and thus have a significant impact on the mechanical dimensions of the pitch variation system. A parameter providing a first estimate of the chord length distribution along the span of the stator blade 114 is its activity factor (AF), defined as follows: where Ri' is the inner radius Ri BA of the stator blade 114 at the leading edge BA' or the inner radius Ri BF of the stator blade 114 at the trailing edge BF' (see Figure 6 ); Re' is the outer radius Re BA of the stator blade 114 at the leading edge BA' or the outer radius Re BF ; denoting the radial distance from the main axis X, other than the outer radius Re'; L'(ξ) denotes the chord length at said radial distance the chord length between the leading edge BA' and the trailing edge BF' of the cross section (or aerodynamic profile) of the stator blade 114 in a plane perpendicular to the radial component of the pitch axis Y'.

[0088] The activity factor of the stator blade 114 is preferably between 50 and 200, more preferably between 90 and 150. With values such as these, it is possible to ensure that the chord length L' in the lower portion of the stator blade 114 is sufficient, which contributes to the mechanical strength of the blade, as well as to reduce the chord length L' in the upper portion of the stator blade 114, which increases the sweep angle in the upper portion of the blade and thus reduces the noise.

[0089] Even more preferably, the chord length L' at 95% of the upstream blade height H' 上游 and / or of the downstream blade height H' 下游 is less than or equal to half of the maximum chord length L'max{L'} of the stator blade 114: L'(H' = 95%) < 0.5 * max{L'}, with H' = H' 上游 or H' 下游 This makes it possible to increase the sweep angle F' with respect to the position in the middle of the span and close to the blade belly, which is beneficial from an aerodynamic acoustic point of view, as well as to reduce the mass of the blade in the upper portion, which is beneficial from a mechanical point of view, as well as to reduce the inertia in the event of a blade tip loss (for example, in the event of bird ingestion) or a blade loss event (Fan Blade Out, FBO).

[0090] Even more preferably, the chord length L' at 95% of the upstream blade height H' 上游 and / or of the downstream blade height H' 下游 is less than or equal to half of the chord L' at 10% of the upstream height H' 上游 and / or of the downstream height H' 下游 : L'(H' = 95%) < 0.5 * L'(H' = 10%), with H' = H' 上游 or H' 下游 This reduces the inertia in the event of a blade tip loss (for example, in the event of bird ingestion) or in the event of a blade loss.

[0091] With reference to Figure 12 , the positioning of the free end of the stator blade 114 downstream makes it possible to maintain a large distance between the trailing edge BF of the rotor blade 108 and the leading edge BA' of the stator blade 114, thus reducing the noise of the aeronautical propulsion unit 100. In particular, for at least one value of the pitch angle C in the interval [50°; 80°] and at least one value of the pitch angle C' in the interval [70°; 90°], the distance between the trailing edge BF of the rotor blade 108 and the leading edge BA' of the stator blade 114 is greater than 10% of the chord L' at 95% of the upstream blade height H' 上游 and / or of the downstream blade height H'下游 At 95%, the axial distance s (along the main axis X) between the trailing edge BF of the rotor blade 108 and the leading edge BA' of the stator blade 114 is preferably greater than 0.35 times the Reynolds number (Re) of the propeller 106, and even more preferably greater than 0.5 times the Reynolds number.

[0092] Figure 14 This is shown in a plane perpendicular to the principal axis X ( Figure 14 The projection y of the trailing edge BF of rotor blade 108 on the plane of the paper BF The projection y of the leading edge BA' of stator blade 114 BA' .

[0093] It can be seen that at least one stator blade 114 has a dihedral angle D' at 95% of its outer radius Re'. BA' For at least one value of the pitch angle C' within the interval [70°; 90°], the dihedral angle D' BA' The absolute value is greater than 1°, preferably 3°, and even more preferably 10°.

[0094] Preferably, for at least one value of the pitch angle C' within the interval [70°; 90°], the dihedral angle D' is located at 95% of the outer radius Re'. BA' The absolute value of the dihedral angle D of the leading edge BA' of stator blade 114 at 50% of the outer radius Re' is equal to that of the angle between the two sides. BA' The angle is at least 1°, preferably at least 3°, or even more preferably at least 10°.

[0095] Even more preferably, for at least one value of the pitch angle C' within the interval [70°; 90°], the dihedral angle D on the leading edge BA' of the stator blade 114 BA' The absolute value of is strictly monotonic for radial positions greater than 90% of the outer radius Re'.

[0096] Even more preferably, for at least one value of the pitch angle C' within the interval [70°; 90°], the projection y of the leading edge BA' of the stator blade 114 onto a plane perpendicular to the main axis X BA' It has at least one local maximum and / or minimum value.

[0097] Even more preferably, for at least one value of the pitch angle C' within the interval [70°; 90°], the projection y of the leading edge BA' of the stator blade 114 onto a plane perpendicular to the main axis X BA' It has at least one inflection point PI.

[0098] In addition, the protrusion y at the trailing edge BF of rotor blade 108 BF The protrusion y at the leading edge BA' of stator blade 114 BA'They move in the opposite direction as they approach their free ends. At least for the blade height H' of stator blade 114. 上游 Greater than 80%, for rotor blade 108, the pitch angle C is at least one value within the interval [50°; 80°], and for stator blade 114, the pitch angle C' is at least one value within the interval [70°; 90°], this is by assuming the dihedral angle D of the leading edge BA' of stator blade 114 is greater than 80%. BA' This is achieved by having the opposite sign to the dihedral angle DBF of the trailing edge BF of the rotor blade 108. Therefore, the leading edge BA' of the stator blade 114 is no longer aligned with the trailing edge BF of the rotor blade 108 from this height (80%), and thus no longer aligned with the wake of the propeller 106. In this way, the propeller 106 and rectifier 112, which form noise sources, are uncorrelated with each other, making it possible to reduce the interaction noise between the propeller 106 and rectifier 112 from this height.

[0099] Preferably, for at least one value of the pitch angle C of the rotor blade 108 within the interval [50°; 80°] and for at least one value of the pitch angle C' of the stator blade 114 within the interval [70°; 90°], the dihedral angle D' of the leading edge BA' of the stator blade 114... BA' The absolute value is greater than the dihedral angle D of the trailing edge BF of the rotor blade 108. BF 20% of the absolute value, i.e., |D' BA' |>0.2*|D BF |

[0100] More preferably, the dihedral angle D' of the leading edge BA' of the stator blade 114 BA The dihedral angle D with the trailing edge of rotor blade 108 BF They are not equal; or, for at least one value of the pitch angle C of the rotor blade 108 in the interval [50°; 80°] and for at least one value of the pitch angle C' of the stator blade 114 in the interval [70°; 90°], at any radial position (height or distance from the main axis X), the dihedral angle D' of the leading edge BA' of the stator blade 114 is... BA' The absolute value is greater than the dihedral angle D of the trailing edge BF of the rotor blade 108. BF 130%; or, for at least one value of the pitch angle C of the rotor blade 108 in the interval [50°; 80°] and at least one value of the pitch angle C' of the stator blade 114 in the interval [70°; 90°], at any radial position, the dihedral angle D' of the leading edge BA' of the stator blade 114 BA' The dihedral angle D of the trailing edge BF of the rotor blade is less than 108. BF 50%.

[0101] The dihedral angle D' of the leading edge BA' of stator blade 114BA' The optimal value depends on the dihedral angle D of the trailing edge BF of rotor blade 108. BF When the wake of rotor blade 108 interacts with stator blade 114, this maximizes the phase shift. This is particularly important in the upper portion of the blade near its free end, where the noise source is more intense.

[0102] Even more preferably, the dihedral angle D' of the leading edge BA' of the stator blade 114 BA' The absolute value is at 95% of the upstream blade height H' 上游 At an angle greater than 15°, or even more preferably at 95% of the upstream blade height H' 上游 The angle is greater than 30°. For the rotor blade 108, the pitch angle C is at least one value in the interval [50°; 80°] and for the stator blade (114), the pitch angle (C') is at least one value in the interval [70°; 90°], the dihedral angle D of the leading edge BA' of the stator blade 114 is greater than 30°. BA’ The size is relatively large at the blade tip, which is useful for reducing the sound level by increasing the phase shift of the noise source.

[0103] As can be further seen, the projection y of the trailing edge BF of rotor blade 108 onto a plane perpendicular to the main axis X BF With a lateral deviation Δy along the lateral direction T BF The lateral direction T is perpendicular to the radial component of the main axis X and / or the pitch axis Y. Similarly, the projection of the leading edge BA' of the stator blade 114 onto a plane perpendicular to the main axis X has a lateral deviation Δy along the lateral direction T' perpendicular to the radial component of the main axis X and / or the pitch axis Y'. BA' .

[0104] Preferably, for at least one value of the pitch angle C' within the interval [70°; 90°], the lateral deviation Δy of the leading edge BA' of the stator blade 114 BA' The radius of the stator blade 114 is greater than 1% of the outer radius Re' of the stator blade and less than 10% of the outer radius of the stator blade. This ensures that the stator blade 114 does not tilt too much in the lateral direction T', which may be beneficial to the blade's balance and / or mechanical strength.

[0105] Preferably, for at least one value of the pitch angle C of the rotor blade 108 within the interval [50°; 80°] and for at least one value of the pitch angle C' of the stator blade 114 within the interval [70°; 90°], the deviation Δy of the leading edge BA' of the stator blade 114 BA' For all operation points, the distance Δy is less than the distance Δy BFThis maximizes the phase shift and / or interaction between the wake of the propeller 106 (generated at the trailing edge BF of the rotor blades 108) and the leading edge BA' of the stator blades 114 downstream. Indeed, the deviation Ay BA' The greater the deviation Ay, the greater the dihedral angle on the blades, in particular at the blade tips.

[0106] In any case, it should be noted that the present application is not limited to the above-described embodiments. Indeed, various modifications can be made to the above-described embodiments, according to the teachings just disclosed, for the person skilled in the art.

[0107] In the foregoing detailed description of the application, the terms used are not to be interpreted as limiting the application to the embodiments disclosed in the specification, but as including all equivalents the skilled person would be able to obtain by applying his common general knowledge to the implementation of the teachings just disclosed.

Claims

1. Unducted aerial propulsion unit (100) for an aircraft, comprising: - a casing (102); - a hub (104) pivotally mounted with respect to the casing (102) about a main axis (X) extending in an upstream-downstream direction of the aircraft; - a propeller (106) mounted on the hub (104) so as to be able to pivot with respect to the casing (102); and - a fixed fairing (112) mounted on the casing (102) downstream of the propeller (106) along the main axis (X), the fixed fairing (112) extending about the main axis (X) and comprising at least one stator blade (114) having a variable pitch about a pitch axis (Y’), each variable-pitch stator blade (114) having a pitch angle (C’) defined as the angle between, on the one hand, any plane (P’) perpendicular to the main axis (X) and, on the other hand, a chord (706) connecting a leading edge (BA’) of the stator blade (114) to a trailing edge (BF’) of the stator blade (114); the propeller (106) comprising at least one rotor blade (108) of variable pitch. characterized in that said chord (706) is taken at 75% of an outer radius (Re') of said stator blade (114) with respect to said main axis (X), wherein, for at least one value of said pitch angle (C') within the interval [70°; 90°], a leading edge (BA') of at least one variable pitch stator blade (114) has a tip (BA'_T) at the height of which said tip is downstream of said pitch axis (Y'), and for at least one variable pitch stator blade (114), a chord length (L') at 95% of a total blade height (H' BA ) of said leading edge (BA') or of said trailing edge (BF') is less than or equal to half of a maximum chord length (L').

2. The aerial propulsion unit (100) of claim 1, wherein, The at least one variable pitch stator blade (114) has an activity factor comprised between 50 and 200, preferably between 90 and 150, the activity factor being defined as follows: wherein Ri' corresponds to the inner radius (Ri') of the stator blade (114) at the leading edge (BA') or to the inner radius (Ri') of the stator blade (114) at the trailing edge (BF'); Re' corresponds to the outer radius (Re') of the stator blade (114) at the leading edge (BA') or to the outer radius (Re') of the stator blade (114) at the trailing edge (BF'); BA BF ; and BA BF ; represents the radial distance with respect to the main axis (X) divided by the outer radius (Re'); L'(Ri') represents the chord of the cross section of the stator blade (114) between the leading edge (BA') and the trailing edge (BF') at the radial distance ; and L'(Re') represents the chord of the cross section of the stator blade (114) between the leading edge (BA') and the trailing edge (BF') at the radial distance​​ 3. The aerial propulsion unit (100) according to claim 1 or 2, wherein The leading edge (BA’) of at least one variable-pitch stator blade (114) has a root (BA’_P) and a belly (BA’_V), the belly (BA’_V), the root (BA’_P) and the pitch axis (Y’) following one another in this order from upstream to downstream along the main axis (X) for at least one value of the pitch angle (C’) in the interval [70°; 90°].

4. The aerial propulsion unit (100) according to any one of claims 1 to 3, wherein, The leading edge (BA’) and the trailing edge (BF’) of at least one variable-pitch stator blade (114) each have a belly (BA’_V, BF’_V), the belly (BA’_V) at the leading edge (BA’) having a height and / or a radius from the main axis (X) smaller than the belly (BF’_V) at the trailing edge (BF’) from the main axis X.

5. The aerial propulsion unit (100) according to any one of claims 1 to 4, wherein, For at least one value of the pitch angle (C') within the interval [70°; 90°], the leading edge (BA') of the at least one variable-pitch stator blade (114) has a belly (BA'_V) between 20% and 70%, preferably between 25% and 55%, of the total blade height (H BA ) of the leading edge (BA').

6. The aerial propulsion unit (100) according to any one of claims 1 to 5, wherein, The chord length (L’) of the cross section of the tip (BA’_T) of at least one variable-pitch stator blade (114) closest to the leading edge (BA’) is smaller than the chord length (L’) of the cross section of the root (BA’_P) closest to the leading edge (BA’), the chord length of the cross section of the root closest to the leading edge itself being smaller than the chord length (L’) of the cross section at the belly (BA’_V) of the leading edge (BA’).

7. The aerial propulsion unit (100) according to any one of claims 1 to 6, wherein, For at least one variable-pitch stator blade (114), for one value of the pitch angle (C’) between 70° and 90°, the axial distance along the main axis (X) between the tip (BF’_T) of the trailing edge (BF’) and the belly (BA’_V) of the leading edge (BA’) is greater than or equal to a coefficient K multiplied by the maximum chord length (L’), with K being between 1 and 2, preferably between 1.2 and 1.

6.

8. The aerial propulsion unit (100) according to any one of claims 1 to 7, wherein, The chord length (L') of the at least one variable pitch stator vane (114) preferably strictly decreases from 50% to 100% of the total blade height (H' BA , H' BF ) of the leading edge (BA') and / or trailing edge (BF').

9. The aerial propulsion unit (100) according to any one of claims 1 to 8, wherein, For at least one variable pitch stator vane (114), a chord length (L') at 95% of a total blade height (H' BA ) of the leading edge (BA') or a total blade height (H' BF ) of the trailing edge is less than or equal to half of a chord length (L') at 10% of the total blade height (H' BA ) of the leading edge (BA') or the total blade height (H' BF ) of the trailing edge.​​​​ 10. The aerial propulsion unit (100) according to any one of claims 1 to 9, wherein, For at least one value of said pitch angle (C') within the interval [70°; 90°], the back- sweep angle (F') of the at least one variable-pitch stator blade (114) is greater than 20°, preferably greater than 35°, at a position more than 80% of the upstream blade height (H 上游 ).

11. The aerial propulsion unit (100) according to any one of claims 1 to 10, wherein, For at least one value of said pitch angle (C') within the interval [70°; 90°], said back- slope angle (F') of at least one variable-pitch stator blade (114) is greater than 45° at a position more than 90% of said upstream blade height (H 上游 ) and / or greater than 50° at a position more than 95% of said upstream blade height (H 上游 ).

12. The aerial propulsion unit (100) according to any one of claims 1 to 11, wherein, ​ 13. The aerial propulsion unit (100) according to any one of claims 1 to 12, wherein, For at least one variable-pitch stator blade (114), for at least one value of said pitch angle (C') between 70° and 90°, the axial distance along said main axis (X) between the tip (BA'_T) of said leading edge (BA') and the ventral part (BA'_V) of said leading edge (BA') is less than or equal to a coefficient B times said maximum chord (L'), wherein B is comprised between 0.5 and 1.

2.

14. The aerial propulsion unit (100) according to any one of claims 1 to 13, wherein, For at least one variable-pitch stator blade (114), for at least one value of said pitch angle (C') between 70° and 90°, the axial distance along said main axis (X) between the tip (BA'_T) of said leading edge (BA') and the ventral part (BA'_V) of said leading edge (BA') is less than or equal to a coefficient B times said maximum chord (L'), wherein B is comprised between 0.5 and 1.

2.

15. The aerial propulsion unit (100) according to any one of claims 1 to 14, wherein, The trailing edge (BF) of the at least one rotor blade (108) and the leading edge (BA’) of the at least one stator blade (114) are spaced apart along the main axis (X) by a distance (s) taken as 95% of the total blade height (H BA ) of the leading edge (BA’) or the total blade height (H BF ) of the trailing edge of the stator blade (114) for at least one value of the pitch angle (C) of the rotor blade (108) between 50° and 80° and at least one value of the pitch angle (C’) of the stator blade (114) between 70° and 90°, the distance being greater than 0.35*Re, preferably greater than 0.5*Re, where Re is the outer radius (Re) of the propeller (106).

16. The aerial propulsion unit (100) according to any one of claims 1 to 15, wherein, At least two stator blades (114) have at least one different geometric element among an outer radius (Re') of the stator blade (114), the maximum chord length (L'), the activity factor (FA), a sweepback angle (F') at 90% of an upstream blade height (H' 上游 ), the pitch angle (C'), and an angular spacing around the main axis (X).

17. An aircraft comprising an aeronautical propulsion unit (100) according to any one of claims 1 to 16.

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