UNFACED AERONAUTICAL PROPELLER

Abstract

The unfaired aeronautical propulsion system comprises a propeller extending around a main axis and having at least one variable-pitch rotor blade around a rotor pitch axis. For each variable-pitch rotor blade, the relative position of maximum thickness (x°_EPmax) decreases from a first relative height (R1) between 20% and 40% to a second relative height (R2) between 55% and 100%, where the relative position of maximum thickness (x°_EPmax) is between 15% and 35%, preferably between 20% and 30%. (See Figure 5 for abbreviations.)

Description

Title of the invention: UNFACED AERONAUTICAL PROPELLER Technical field of the invention

[0001] The present invention relates to an unfaired aeronautical propulsion system for an aircraft. Technological background

[0002] The blades of an unfaired aircraft propeller (also called an "open fan" or "open rotor") may be subjected to an upstream flow that is not uniform and parallel to the engine axis. This is known as angle-of-attack flight, occurring particularly during the aircraft's takeoff or landing phases and / or with a crosswind. For example, when the aircraft propeller is installed under the wing, the presence of the wing can induce an updraft at the propeller, which increases the angle of attack of the flow experienced by the propeller blades. In this case, increased forces and moments appear in the propeller plane. These forces and moments are called "IP loads" (or "p-factor") and can be significant from a mechanical point of view.

[0003] At an angle of attack, the propeller not only provides thrust, but also vertical and lateral forces and moments which, in some cases, can reach the same value as the thrust. These forces are unsteady and depend on the azimuthal position around the principal axis of the blade in question. When the aircraft propulsion system is installed under the wing, the downward-moving propeller blade (the 3 o'clock blade in the case of a clockwise-rotating propeller viewed from the front) experiences an increased angle of attack and is therefore subjected to increased forces, unlike the upward-moving blade (the 9 o'clock blade in the case of a clockwise-rotating propeller viewed from the front), on which reduced forces are exerted. Consequently, over one engine revolution, the same propeller blade is subjected to variable forces depending on its azimuthal position.

[0004] The aeronautical propulsion system may also have a stator (or propeller) located downstream of the propeller and whose blades may also undergo a variable incidence depending on their azimuthal position.

[0005] It is also possible to consider a USF (upstream propeller - downstream stator) or CROR (upstream propeller - downstream propeller) application. The case of a high-speed propeller (without a downstream blade row) may also be compatible with the invention. By high-speed propeller, we mean a propeller capable of flying at a speed greater than M=0.7 in CR.

[0006] The advantage of a downstream stator or downstream propeller is to recover the gyration of the flow Induced by the upstream propeller, this improves the efficiency of the aircraft propulsion system. It should also be noted that the upstream angle of attack (aircraft angle of attack or induced by the installation – wing, mast, etc.) is not completely filtered by the upstream propeller. In addition to variations in the upstream angle of attack, the downstream stator or propeller may be subjected to different angles of attack (due to the aircraft's angle of attack) depending on their azimuth position.

[0007] However, at high angles of attack, there is a risk of airflow separation, that is, a risk that the airflow separates from the upper or lower surface of the wing. For example, this flow separation can occur on the upper surface and between mid-span and the free tip of a propeller blade with a high aerodynamic load during takeoff and / or a flight phase at high angle of attack or high thrust. Flow separation at the propeller and / or stator blades can degrade the performance of the aircraft propulsion system as well as that of the aircraft (for example, if the wing or a lifting surface of the aircraft perceives a separated flow originating from the propeller blades).

[0008] It may therefore be desirable to control the angle of attack of the propeller blades (and / or the stator), from a mechanical, aerodynamic, and acoustic point of view. Summary of the invention

[0009] It is therefore proposed an unfaired aeronautical propulsion system comprising a propeller extending around a main axis and comprising at least one rotor blade with variable pitch around a rotor pitch axis, each rotor blade with variable pitch having: - a leading edge; - a trailing edge; - an internal radius at the leading edge relative to the principal axis; - an external radius at the leading edge relative to the main axis; - an internal radius at the trailing edge with respect to the principal axis; - an external radius at the trailing edge with respect to the principal axis; - at each relative height with respect to the principal axis from the inner radius to the outer radius, at the leading edge and / or the trailing edge: • a chord line extending in a cutting plane perpendicular to the rotor pitch axis or perpendicular to the radial component of the pitch axis and passing through the relative height considered and connecting the leading edge to the trailing edge, and • a thickness in the cutting plane varying according to a relative position along the chord line from the leading edge to the trailing edge, this thickness having a maximum at a position relative called relative position of maximum thickness; characterized in that, for each variable pitch rotor blade, the relative position of maximum thickness decreases from a first relative height of between 20% and 40% to a second relative height of between 55% and 100% where the relative position of maximum thickness is between 15% and 35%, preferably between 20% and 30%.

[0010] The invention may further include one or more of the following optional features, according to any technically possible combination.

[0011] Optionally, a variable pitch rotor blade has an activity factor between 145 and 230, preferably between 155 and 205, and its maximum relative thickness position is maximum at a relative height of less than 40%, preferably less than 30%, preferably even less than 20%.

[0012] Optionally also, for each variable pitch rotor blade, the maximum relative thickness position is maximum at the closest relative height to 0%.

[0013] Optionally also, for each variable pitch rotor blade, there is at least one relative height having a relative position of maximum thickness greater than 20%, preferably greater than 25%, preferably even greater than 30%.

[0014] Optionally also, for each variable pitch rotor blade, the relative position of maximum thickness at the first relative height is between 20% and 40%, preferably between 25% and 35%, preferably still between 26% and 32%.

[0015] Optionally also, for each variable pitch rotor blade, the relative position of maximum thickness varies less between the closest relative height at 0% and the first relative height, than between the first relative height and the second relative height.

[0016] Optionally also, for each variable pitch rotor blade, the relative position of maximum thickness remains between 10% and 40%, preferably between 15% and 35%, preferably again between 20% and 34%, for all relative heights.

[0017] Optionally also, for each variable pitch rotor blade, the maximum relative thickness position is strictly decreasing from the relative height of 0% to the relative height of 100%.

[0018] Optionally also, for each variable pitch rotor blade, the relative position of maximum thickness has at least a local maximum and / or minimum on the relative height.

[0019] Optionally, each variable-pitch rotor blade also has, at each relative height, on the one hand, a chord defined as the distance separating the leading edge from the trailing edge along the chord line and, on the other hand, a normalized maximum thickness defined as the maximum thickness divided by the chord at the relative height considered, this normalized maximum thickness being strictly decreasing from the nearest relative height to 0% up to the relative height of 60%, preferably up to the relative height of 70%.

[0020] Optionally also, for each variable pitch rotor blade, the maximum normalized thickness at the closest relative height to 0% is between 0.1 and 0.25, preferably between 0.13 and 0.2.

[0021] Optionally also, for each variable pitch rotor blade, the maximum normalized thickness at the closest relative height to 100% is between 0.02 and 0.048, preferably between 0.03 and 0.06.

[0022] Optionally also, for each variable pitch rotor blade, the maximum normalized thickness decreases from the nearest relative height to 0% until reaching, at a relative height between 25% and 50%, one quarter of the maximum normalized thickness at the nearest relative height to 0%.

[0023] Optionally also, for each variable pitch rotor blade, the maximum normalized thickness is strictly decreasing from the nearest relative height at 0% to the nearest relative height at 100%.

[0024] Optionally also, for each variable pitch rotor blade, the maximum normalized thickness increases by the relative height of 70%, preferably by the relative height of 80%, up to the relative height of 100%.

[0025] Optionally also, for each variable pitch rotor blade, the maximum normalized thickness at the closest relative height to 100% is less than or equal to three times, preferably two and a half times, preferably again twice, the maximum normalized thickness at the relative height of 70%.

[0026] Optionally also, the aeronautical propulsion system further comprises an external casing and a fixed, uncased rectifier mounted on the external casing.

[0027] An aircraft comprising an unfaired aeronautical propulsion system according to the invention is also proposed. Brief description of the figures

[0028] The invention will be better understood with the aid of the following description, given solely by way of example and made with reference to the accompanying drawings in which: - [Fig. 1] is a cross-sectional view of an aeronautical propulsion system in which the invention can be implemented, - [Fig.2] is a side view of a rotor blade of an aeronautical propeller that [Fig.1], - [Fig.3] is a cross-sectional view of the rotor blade of [Fig.2], - [Fig.4] is another cross-sectional view of the rotor blade of [Fig.2], coupled to a graph illustrating the evolution of the thickness of a cross-section of the rotor blade from a leading edge to a trailing edge of the latter, - [Fig. 5] is a graph illustrating the evolution of a relative position of maximum thickness as a function of a relative height along the rotor blade, - Figure 6 is a graph illustrating the evolution of a normalized maximum thickness as a function of a relative height along the rotor blade. - [Fig. 7] is a cross-sectional view of a rotor blade with its maximum thickness located furthest from the leading edge, and - [Fig.8] is a cross-sectional view of a rotor blade with maximum thickness close to the leading edge. Detailed description of the invention

[0029] With reference to [Fig.1], an aeronautical propulsion system 100 in which the invention is implemented will now be described.

[0030] The aeronautical propulsion unit 100 is unducted (for example, of the "Unducted Single Fan" type, also designated by the acronym USF) and is designed to contribute to the propulsion of an aircraft. At this stage, it is specified from the outset that the solution of the invention applies to any unducted aeronautical propulsion unit (turbomachine, turbojet, high-speed propeller turboprop, etc.) and / or of the "Contra Rotating Open Rotor" (also designated by the acronym CROR) or "Open Fan" type, since part of the aforementioned problem is not necessarily specific to the aforementioned type of aeronautical propulsion unit.

[0031] The aeronautical propulsion unit 100 first of all comprises an external casing 102 and a hub 104 mounted pivoting relative to the external casing 102 around a main axis X.

[0032] Subsequently, the terms "upstream" and "downstream" will be used to specify the relative position of the elements of the aeronautical propulsion 100 along the main axis X in a flow direction of an airflow PHI from the front to the rear of the aeronautical propulsion 100 and / or the aircraft.

[0033] The hub 104 is thus, for example, located upstream of the external casing 102.

[0034] The aeronautical propulsion unit 100 further comprises a propeller 106 which is propulsive, unfaired and mounted on the hub 104 so as to pivot relative to the external casing 102 around the main axis X. The propeller 106 is designed to drive the airflow PHI downstream to propel the aircraft forward in flight. For this purpose, the propeller 106 has rotor blades 108 (for example, between 3 and 25, preferably between 10 and 16) arranged, for example, in a single annular row around the main axis X. The rotor blades 108 may, for example, all be identical and spaced angularly at regular intervals around the principal X axis. Alternatively, a non-regular angular spacing could be considered.

[0035] The rotor blades 108 can have variable pitch, each about a respective radial rotor pitch axis Y. The presented solution can cover cases where: - the rotor pitch axis Y is perpendicular to the main axis X, i.e., it can form a right angle with the main axis X; As illustrated in [Fig. 1] where the rotor pitch axis Y and the main axis X are projected onto a plane or section of the aeronautical propulsion unit containing the main axis X, - the rotor pitch axis Y is not perpendicular to the main axis X, i.e. it is inclined; For example, if the rotor pitch change axis Y has a longitudinal component (along the direction of the main axis X) and a component perpendicular to the main axis X.

[0036] The aeronautical propulsion unit 100 further comprises a motor 110 for driving the hub 104, and therefore the propeller 106 via the hub 104. The motor 110 extends, for example, into the external casing 102. The motor 110 is, for example, located downstream of the propeller 106. Such an arrangement is known as a "tractor" (from the English "puller"). The engine 110 includes, for example, a turbomachine, and / or (at least) a turbojet, and / or (at least) a turbofan, and / or (at least) a turbomotor, including, for example, a gas generator conventionally comprising, from upstream to downstream, at least one compressor, at least one combustion chamber and at least one power turbine intended to drive the propeller 106. Alternatively, the engine 110 may include (at least) an electric motor, and / or (at least) a hydrogen engine, and / or (at least) a hybrid engine: thermal and / or electric and / or hydrogen.

[0037] The aeronautical propulsion unit 100 further comprises a fixed, unshrouded rectifier 112 mounted on the outer casing 102, downstream of the propeller 106. The fixed rectifier 112 forms a stator fixed to the outer casing 102 extending around the main axis X, but not able to rotate about it. The fixed rectifier 112 comprises stator blades 114 arranged, for example, in a single annular row around the main axis X. For example, between 3 and 25 stator blades 114, preferably between 8 and 14 stator blades 114, are provided. Preferably, the number of stator blades 114 is different from the number of rotor blades 108, in order to reduce the noise of the aircraft propulsion 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 were equal, the entire wake of the propeller 106 could interact simultaneously with the stator blades 114, thus increasing noise levels. The stator blades 114 can be... for example all identical and regularly spaced angularly around the main X axis. Alternatively, the stator blades could be different and / or non-regularly spaced in the azimuthal direction around the main X axis.

[0038] The fixed rectifier 112 is designed to rectify at least a portion of the airflow PHI passing through the propeller 106, in order to improve the performance of the aircraft propulsion unit 100. More specifically, the fixed rectifier 112 aims to counteract the gyration induced by the propeller 106 in order to improve the performance of the unfaired configuration. However, its presence introduces a dominant noise source resulting from interaction with the wake of the propeller 106 (and the blade tip vortex when the truncation of the stator blades 114 is insufficient). It is therefore necessary to reduce the noise generated by the fixed rectifier 114 and its interaction with the wake of the propeller 106 while maintaining good aerodynamic performance, as reducing noise emissions and fuel consumption is a major challenge for unfaired engine architectures.

[0039] At least one stator blade 114 has variable pitch, each about a respective radial axis Y'. Preferably, all the stator blades 114 have variable pitch. This allows the flow triangulation upstream of the stator blades 114 to be adapted according to the operating point, thereby improving aerodynamic performance and reducing noise emitted by the interaction between the wakes of the upstream propeller 106 and the fixed stator. Indeed, varying the pitch of the stator blades 114 allows the directivity of the sound to be modified and reduces the tonal interaction noise.

[0040] The aeronautical propulsion unit 100 further includes, for example, an air inlet 116 for supplying primary flow to the engine 110. This air inlet 116 is, for example, provided between the propeller 106 and the fixed rectifier 112.

[0041] With reference to [Fig. 2], each rotor blade 108 first comprises a leading edge BA where the airflow PHI arrives and a trailing edge BF from which the airflow PHI departs. The leading edge BA extends from a root BA_P, which is the point of the leading edge BA closest to the principal axis X, to a tip BA_T, which is the point of the leading edge BA furthest from the principal axis X. Between the root BA_P and the tip BA_T, the leading edge BA may have a curvature in a constant direction, i.e., without a point of inflection, and moreover, regular, i.e., without discontinuity.

[0042] Similarly, the trailing edge BF extends from a foot BF_P, which is the point of the trailing edge BF closest to the principal axis X, to a head BF_T, which is the point of the trailing edge BF furthest from the principal axis X. Between the foot BF_P and the head BF_T, the trailing edge BF can exhibit a curvature of constant direction, i.e. without a point of inflection, and moreover regular, i.e. without discontinuity.

[0043] Each rotor blade 108 can also be truncated at its free end, as in the illustrated example, i.e., there is a truncated section 202, for example straight, connecting the tips BA_T, BF_T. In this case, there is a discontinuity of curvature at the tip BA_T between the leading edge BA and the truncated section 202, and another discontinuity of curvature at the tip BF_T between the truncated section 202 and the trailing edge BF. Alternatively, the rotor blade 108 could be untruncated, in which case the tips BA_T, BF_T would coincide.

[0044] Each rotor blade 108 has an internal radius Ri_BA at the leading edge BA, which is by definition equal to the distance between the main axis X and the root BA_P, and an internal radius Ri_BF at the trailing edge Ri, which is by definition equal to the distance between the main axis X and the root BF_P. Similarly, each rotor blade 108 has an external radius Re_BA at the leading edge BA, which is by definition equal to the distance between the main axis X and the tip BA_T, and an external radius Re_BF at the trailing edge BF, which is by definition equal to the distance between the main axis X and the tip BF_T. The internal radii Ri_BA, Ri_BF and the external radii Re_BA, Re_BF can be measured when the rotor blade 108 has any pitch angle allowing the nominal operation of the aeronautical propulsion unit 100, that is to say when the pitch angle of the rotor blades 108 would allow the airflow PHI to be moved from the front to the rear of the aeronautical propulsion unit 100 and / or the aircraft.For example, the internal radii Ri_BA, Ri_BF and the external radii Re_BA, Re_BF could be measured when the rotor blades 108 have the optimal pitch angle for their operation in cruise regime, i.e. at a flight Mach greater than 0.7 for a medium-haul aircraft.

[0045] To locate a height h_BA between the inner radius Ri_BA and the outer radius Re_BA, it is possible to define a relative height h°_BA using the formula h°_BA = h-BA, which can be expressed as a percentage. The relative height h°_BA therefore varies between 0% at the inner radius Ri_BA and 100% at the outer radius Re_BA. To identify a height h_BF between the inner radius Ri_BF and the outer radius Re_BF, a relative height h°_BF can be defined using the formula h°_BF = h0p - BF. This can be expressed as a percentage. The relative height h°_BF therefore varies between 0% at the inner radius Ri_BF and 100% at the outer radius Re_BF. With the relative heights h°_BA and h°BF, it is possible that there may be no leading edge or trailing edge for relative heights close to 0% or close to 100%, due to a difference in height between the feet BA_P, BF_P and / or between the tops BA_T, BF_T.

[0046] Thus, to avoid this problem, it is also possible to define a relative height called complete h°_C between the highest of the two feet BA_P, BF_P(BF_P in (the example in Figure 2) and the lower of the two heads BA_T, BF_T (BF_T in the example in Figure 2) by the formula h 0 r —______________h__ 1 - BF_P)-min(BA_T, BF_T) Thus, with this definition of complete relative height, at each relative height h°_C between 0% and 100% there exists a leading edge and a trailing edge, which allows us to define at any relative height h°_C a complete section (i.e. including the leading edge and the trailing edge) as will be explained later with reference to [Fig.3],

[0047] Subsequently, h° can represent h°_BA and / or h°_BF and / or the complete relative height h°_C, since the invention can be implemented according to a relative height measured at the level of the leading edge and / or the trailing edge.

[0048] Figure 3 is a cross-section or aerodynamic profile of the rotor blade 108 along a cutting plane. This cutting plane can be defined in several ways: - when the Y alignment axis is perpendicular to the main X axis, the cutting plane can be perpendicular to the Y alignment axis and passing at a certain relative height h°, When the Y-axis is inclined relative to the X-axis, the cutting plane can be perpendicular to the radial component of the Y-axis. and passing at a certain relative height h°.

[0049] As can be seen, the rotor blade 108 has an intrados face 302 and a The upper surface (extrados) 304, respectively concave and convex, are connected to each other by the leading edge (BA) and the trailing edge (BF). The latter may, for example, be trunculated to simplify the manufacturing process, as illustrated in the figure on the second enlargement.

[0050] The leading edge BA and the trailing edge BF are connected by a chord line 306 extending in the cutting plane and separated, on this chord line 306, by a distance called chord c. The orientation of the 306 chord line and the c chord can vary depending on the relative height h° considered. The angle between the line of the chord 306 and the axis P perpendicular to the main axis X, allows us to define a calibration angle V in the cutting plane.

[0051] A parameter that allows for a first estimate of the distribution 5 of chord along the span or radial direction of a rotor blade 108 is its Activity Factor FA, which is defined as follows: FA = 100,000 16 where Ri RURe corresponds either to the internal radius Ri_BA of the rotor blade 108 at the leading edge BA, or to the internal radius Ri_BF of the rotor blade 108 at the trailing edge BF; Re corresponds either to the external radius Re_BA of the rotor blade 108 at the leading edge BA, or to the external radius Re_BF of the rotor blade 108 at the trailing edge BF (see the figure 2); £ represents a radial distance from the principal axis X, divided by the outer radius Re, and c(f) represents a chord between the leading edge BA and the trailing edge BF of an aerodynamic section or profile at said radial distance f.

[0052] The activity factor FA of the rotor blade 108 is preferably between 145 and 230, more preferably between 155 and 205. With such values, this makes it possible to ensure a chord distribution c on the rotor blade 108 sufficient to ensure the balancing and mechanical strength of the blade in combination with an adequate and optimized thickness distribution for the resistance of the forces on the rotor blades 108 in the flight phases at angle of attack, as proposed in this invention.

[0053] Furthermore, it is possible to define the (average) camber line or skeleton 308 of a cross-section of a rotor blade 108. For a cross-section of a blade, the camber line or skeleton 308 is defined as the line midway between the lower and upper surfaces. For example, this camber line or skeleton 308 can be obtained by drawing inscribed circles in the cross-section, such that the positions of the centers of these inscribed circles (see the dashed circles) define the camber line or skeleton 308.

[0054] With reference to [Fig. 4], the rotor blade 108 has a thickness EP in the cutting plane varying from the leading edge BA to the trailing edge BF. For example, the thickness EP is defined as the distance between the lower surface 302 and the upper surface 304 of the rotor blade 108, perpendicular to the camber line or skeleton 308.

[0055] It is possible to index the thickness EP according to a relative position x° along the chord line 306 from the leading edge BA to the trailing edge BF.

[0056] The thickness EP of a blade cross-section varies according to a relative position x° along the chord line 306 from the leading edge BA to the trailing edge BF. The relative position x° is defined by X0 = X / C, where x is the distance along the chord line 306 from the leading edge. Thus, the relative position x° is 0% at the leading edge BA and 100% at the trailing edge BF. The thickness EP has a maximum EPmax at a relative position called the maximum thickness relative position x°_EPmax.

[0057] With reference to [Fig.5] and [Fig.6], four examples 502, 504, 506 of dimensioning of the rotor blade 108 will be described.

[0058] It should be noted that the curves in Figures 5 and 6 may not start perfectly at h°=0% and end at h°=100% when the relative height h° is the relative height at the leading edge or the relative height at the trailing edge. Thus, when we subsequently refer to the relative height h° "closest to 0%" or "closest to 100%", this will mean the relative height where a complete section still exists. In the example of [Fig. 2], the relative height h°_BA closest to 0% is therefore the relative height h°_BA at the level of the foot BF_P and the relative height The closest h°_BA to 100% is therefore the relative height h°_BA at the level of the BF_T head. For the case where the relative height h° is the complete relative height h°_C, the expression "closest to 0%" will mean "from 0%" and "closest to 100%" will mean "from 100%".

[0059] With particular reference to [Fig. 5], preferably, as is the case for the four examples 502, 504, 506, and 508, the maximum relative thickness position x°_EPmax is maximized at a relative height h° less than 40%, preferably less than 30%, and preferably even less than 20%. In particular, for the three examples 502, 504, and 506, the maximum relative thickness position x°_EPmax is maximized at the relative height h° closest to 0%, i.e., at the level of the blade root section. This contributes to the blade's mechanical resistance to IP stresses during attack phases of flight, as well as to limiting the flow acceleration and / or the intensity of shocks that can form during cruise near the hub 104 or the housing 102, which could increase aerodynamic losses. For example, the relative position of maximum thickness x°_EPmax is greater than 20%, preferably greater than 25%, and preferably even greater than 30%.This allows control of velocity gradients on the root section where the passage area between two adjacent blades in the azimuthal direction is minimal, particularly when the number of rotor blades 108 is large (for example, greater than or equal to 10, preferably greater than or equal to 12).

[0060] Preferably, as is the case for the four examples 502, 504, 506, 508, the relative position of maximum thickness x°_EPmax at a first relative height RI between 20% and 40% is between 20% and 40%, preferably between 24% and 35%, preferably still between 26% and 32%. This ensures that the position of the maximum thickness at the bottom of the rotor blade 108 is located relatively downstream (and close to the center of gravity of the rotor blade 108), particularly in relation to the position of the maximum thickness at the top of the rotor blade 108, thus ensuring good balance and mechanical strength of the rotor blade 108. This would also allow for increasing the thickness of the cuts near the root of the rotor blade 108 and at the spar and / or other structural elements inside the rotor blade 108, enabling the transmission of forces under the hub 104.

[0061] Moreover, for most of the examples in [Fig.5], the relative position of maximum thickness x°_EPmax decreases in a strictly monotonic way from the first relative height RI to a second relative height R2 between 55% and 100% where the relative position of maximum thickness x°_EPmax is between 15% and 35%, preferably between 22% and 30%.

[0062] Indeed, for high relative heights h°, the thickness EP decreases as follows This will be visible for the three examples 502, 504, and 506 in [Fig. 6]. However, with a thin rotor blade 108, separation on the upper surface can more easily occur at the top of the blade and at the leading edge, generating aerodynamic losses and increasing noise. Positioning the maximum thickness EPmax close to the leading edge BA (i.e., with a low relative maximum thickness x°_EPmax) reduces or prevents airflow separation, particularly in cases of high angles of attack, such as during aircraft landing and / or takeoff (these flight phases are particularly important for aircraft noise level certification according to international standards). This effect will be explained with reference to Figures 7 and 8.

[0063] Preferably, as is the case for the four examples 502, 504, 506, 508, the relative position of maximum thickness x°_EPmax varies less between the relative height h° of 0% and the first relative height RI, than between the first relative height RI and the second relative height R2. In particular, for the three examples 502, 504, 506, max{x°_EPmax}-min{x°_EPmax} when h° varies from 0% to 20% is less than max{x°_EPmax}-min{x°_EPmax} when h° varies between 40% and 55%.

[0064] This ensures that the relative position of maximum thickness x°_EPmax does not vary too much at the foot of the rotor blade 108, which is beneficial to the balancing and mechanical strength of the rotor blade 108 and helps to maintain the IP forces at the blade foot during the flight phases in angle of attack.

[0065] Preferably, as is the case for the four examples 502, 504, 506, 508, the relative position of maximum thickness x°_EPmax remains between 10% and 40%, preferably between 15% and 35%, preferably again between 20% and 34%, for all relative heights h°.

[0066] Preferably, as is the case for the three examples 502, 504, 506, the relative position of maximum thickness x°_EPmax is strictly decreasing from the relative height h° of 0% to the relative height h° of 100%. This implies that the position of maximum thickness moves towards the leading edge BA of the rotor blade 108 between the root and the tip of the rotor blade 108, which makes it possible to limit or avoid separations and thus improve efficiency while limiting noise.

[0067] In an alternative embodiment, the maximum relative thickness position x°_EPmax has at least one local maximum and / or minimum between the relative height h° of 0% and the relative height h° of 100%. In particular, as in Example 508 in [Fig. 5], there is a local minimum in the second relative height R2, preferably when the relative height h° varies between 80% and 100%, which makes it possible to move the maximum relative thickness position x°_EPmax locally away from the leading edge. This allows for a better distribution of the aerodynamic load on the leading edges of the rotor blade 108 and thus reduces the local flow velocity and / or The intensity of the shocks during cruising speed is reduced, which helps to lower the noise perceived by passengers in the cabin. Reducing cabin noise improves passenger comfort during the flight.

[0068] With reference to [Fig.6], the rotor blade 108 has a normalized maximum thickness EPmax° defined as the maximum thickness EPmax at the relative height h° considered divided by the chord c at the relative height h° considered.

[0069] Preferably, the maximum normalized thickness EPmax° is strictly decreasing from the relative height h° from 0% up to at least the relative height h° of 60%, preferably 70%.

[0070] For example, in examples 504 and 506, the maximum normalized thickness EPmax° decreases strictly from the relative height h° from 0% to 100% of the relative height. This allows for thinner profiles in the upper part and therefore reduces the mass and weight of the rotor blade 108.

[0071] In contrast, for examples 506 and 508, the maximum normalized thickness EPmax° increases by approximately 70%, preferably 80%, up to the nearest relative height h° at 100%. In this case, the maximum normalized thickness EPmax° at the nearest relative height h° at 100% is preferably less than or equal to three times the maximum normalized thickness EPmax° at the relative height h° of 70%, or preferably less than or equal to 2.5 times the maximum normalized thickness EPmax° at the relative height h° of 70%, or even more preferably less than or equal to 2 times the maximum normalized thickness EPmax° at the relative height h° of 70%. This increase in the maximum thickness near the blade tip makes it possible to control the frequency distribution of the first bending mode of the blade, which may be necessary for withstanding the IP forces.This increase in maximum thickness near the blade tip can also be useful for mechanical strength when increasing sweep and / or dihedral to reduce noise. From an aeroacoustic perspective, this increased thickness of the tip sections helps control angle of attack and prevent potential separation.

[0072] Preferably, the maximum normalized thickness EPmax° at the nearest relative height h° to 0% is between 0.1 and 0.25, preferably between 0.13 and 0.20. This ensures that the thickness of the profile near the blade root is sufficient to withstand and transmit the aerodynamic and mechanical forces on the rotor blade 108.

[0073] Preferably, the maximum normalized thickness EPmax° at the nearest relative height h° to 100% is between 0.02 and 0.08, preferably between 0.03 and 0.06. Limiting the thickness at the tip reduces the mass near the free end of the rotor blade 108 and thus limits any moments transmitted to the rotor blade 108. Furthermore, this is necessary to limit the mass released in the event of a loss of a part of the blade in the event of possible bird ingestion.

[0074] Preferably, the maximum normalized thickness EPmax° decreases from the nearest relative height h° to 0% until it reaches, at a relative height h° between 25% and 50%, one-quarter of the maximum normalized thickness EPmax° at the nearest relative height h° to 0%. Reducing the thickness towards the free end reduces the mass near the free end of the rotor blade 108 and thus limits any moments transmitted to the rotor blade 108. Furthermore, this is necessary to limit the mass released in the event of loss of part of the blade during a possible bird strike.

[0075] With reference to [Fig.7], when the maximum thickness is too far from the leading edge, an airflow reaching the leading edge with a large angle of attack I has difficulty catching the extrados, i.e. remaining close to the extrados, and moves away from it producing a separation generating turbulence T.

[0076] With reference to [Fig. 8], however, when the maximum thickness EPmax is brought closer to the leading edge BA, the latter is more convex or rounded. This allows for better airflow capture at a high angle of attack.

[0077] In conclusion, it is clear that an aeronautical propulsion system such as the one described above makes it possible to obtain an optimal operating range for large angles of attack on the leading edge.

[0078] It should also be noted that the invention is not limited to the embodiments described above. It will indeed be apparent to those skilled in the art that various modifications can be made to the embodiments described above, in light of the information just disclosed to them.

[0079] In the detailed presentation of the invention given above, the terms used shall not be interpreted as limiting the invention to the embodiments set forth in this description, but shall be interpreted as including all equivalents which can be foreseen by a person skilled in the art by applying their general knowledge to the implementation of the teaching which has just been disclosed to them.

Claims

Demands

1. An unfaired aeronautical propulsion system (100) comprising a propeller (106) extending about a principal axis (X) and comprising at least one rotor blade (108) with variable pitch about a rotor pitch axis (Y), each variable-pitch rotor blade (108) having: - a leading edge (BA); - a trailing edge (BE); - an internal radius (Ri_BA) at the leading edge (BA) with respect to the principal axis (X); - an external radius (Re_BA) at the leading edge (BA) with respect to the principal axis (X); - an internal radius (Ri_BF) at the trailing edge (BA) with respect to the principal axis (X); - an external ray (Re_BA) at the trailing edge (BA) with respect to the principal axis (X); - at each relative height (h°) with respect to the principal axis (X) from the inner radius (Ri_BA, Ri_BF) to the outer radius (Re_BA, Re_BF) at the leading edge (BA) and / or the trailing edge (BF): • a chord line (306) extending in a cutting plane perpendicular to the rotor pitch axis (Y) or perpendicular to the radial component of the pitch axis (Y) and passing through the relative height (h°) considered and connecting the leading edge (BA) to the trailing edge (BF), and • a thickness (EP) in the cutting plane varying according to a relative position (x°_EP) along the chord line (306) from the leading edge (BA) to the trailing edge (BF), this thickness (EP) having a maximum (EPmax) at a relative position called the relative position of maximum thickness (x°_EPmax); characterized in that, for each rotor blade (108) with variable pitch, the relative position of maximum thickness (x°_EPmax) decreases from a first relative height (RI) between 20% and 40% up to a second relative height (R2) between 55% and 100% where the relative position of maximum thickness (x°_EPmax) is between 15% and 35%, preferably between 20% and 30%.

2. Unfaired aeronautical propulsion (100) according to claim 1, wherein at least one variable pitch rotor blade (108) has an activity factor (AF) between 145 and 230, preferably between 155 and 205, and its maximum relative thickness position (x°_EPmax) is maximum at a relative height (h°) less than 40%, preferably less than 30%, preferably still less than 20%.

3. Unfaired aeronautical propulsion (100) according to claim 1 or 2, wherein, for each variable pitch rotor blade (108), the maximum relative thickness position (x°_EPmax) is maximum at the closest relative height (h°) to 0%.

4. Unfaired aeronautical propulsion (100) according to any one of claims 1 to 3, wherein, for each variable pitch rotor blade (108), there is at least one relative height (h°) having a maximum relative thickness position (x°_EPmax) greater than 20%, preferably greater than 25%, preferably even greater than 30%.

5. Unfaired aeronautical propulsion system (100) according to any one of claims 1 to 4, wherein, for each variable pitch rotor blade (108), the relative position of maximum thickness (x°_EPmax) at the first relative height (RI) is between 20% and 40%, preferably between 25% and 35%, preferably still between 26% and 32%.

6. Unfaired aeronautical propulsion (100) according to any one of claims 1 to 5, wherein, for each variable pitch rotor blade (108), the maximum thickness relative position (x°_EPmax) varies less between the nearest 0% relative height (h°) and the first relative height (RI), than between the first relative height (RI) and the second relative height (R2).

7. Unfaired aeronautical propulsion (100) according to any one of claims 1 to 6, wherein, for each variable pitch rotor blade (108), the relative position of maximum thickness (x°_EPmax) remains between 10% and 40%, preferably between 15% and 35%, preferably still between 20% and 34%, for all relative heights (h°).

8. Unfaired aeronautical propulsion system (100) according to any one of claims 1 to 7, wherein, for each variable-pitch rotor blade (108), the relative position of maximum thickness (x°_EPmax) is strictly decreasing with respect to the relative height (h°) from 0% to the relative height (h°) of 100%.

9. Unfaired aeronautical propulsion (100) according to any one of claims 1 to 7, wherein, for each variable pitch rotor blade (108), the relative position of maximum thickness (x°_EPmax) has at least one local maximum and / or minimum on the relative height (h°).

10. Unfaired aeronautical propulsion system (100) according to any one of claims 1 to 9, wherein each variable pitch rotor blade (108) has, at each relative height (h°), on the one hand, a chord (c) defined as the distance separating the leading edge (BA) from the trailing edge (BF) along the chord line (306) and, on the other hand, a normalized maximum thickness (EPmax°) defined as the maximum thickness divided by the chord (c) at the relative height (h°) considered, this normalized maximum thickness (EPmax°) being strictly decreasing from the nearest relative height (h°) to 0% up to the relative height (h°) of 60%, preferably up to the relative height (h°) of 70%.

11. Unfaired aeronautical propulsion (100) according to claim 10, wherein, for each variable pitch rotor blade (108), the maximum normalized thickness (EPmax°) at the closest relative height (h°) to 0% is between 0.1 and 0.25, preferably between 0.13 and 0.

2.

12. Unfaired aeronautical propulsion (100) according to claim 10 or 11, wherein, for each variable pitch rotor blade (108), the maximum normalized thickness (EPmax°) at the closest relative height (h°) to 100% is between 0.02 and 0.048, preferably between 0.03 and 0.

06.

13. Unfaired aeronautical propulsion (100) according to any one of claims 10 to 12, wherein, for each variable pitch rotor blade (108), the maximum normalized thickness (EPmax°) decreases from the nearest relative height (h°) to 0% until it reaches, at a relative height (h°) between 25% and 50%, one quarter of the maximum normalized thickness (EPmax°) at the nearest relative height (h°) to 0%.

14. An unfaired aeronautical propulsion system (100) according to any one of claims 10 to 13, wherein, for each variable-pitch rotor blade (108), the maximum normalized thickness (EPmax°) is strictly decreasing from the nearest relative height (h°) to 0% up to the nearest relative height (h°) at 100%.

15. Unfaired aeronautical propulsion (100) according to any one of claims 10 to 13, wherein, for each variable pitch rotor blade (108), the maximum normalized thickness (EPmax°) increases by the relative height (h°) of 70%, preferably by the relative height (h°) of 80%, up to the relative height (h°) of 100%.

16. Unfaired aeronautical propulsion (100) according to claim 15, wherein, for each variable pitch rotor blade (108), the maximum normalized thickness (EPmax°) at the closest relative height (h°) to 100% is less than or equal to three times, preferably two and a half times, preferably again twice, the maximum normalized thickness (EPmax°) at the relative height (h°) of 70%.

17. Unfaired aeronautical propulsion (100) according to any one of claims 1 to 16, further comprising an external casing (102) and an unfaired fixed rectifier (112) mounted on the external casing (102).

18. Aircraft comprising an unfaired aeronautical propulsion unit (100) according to any one of claims 1 to 17.

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