UNDUCTED AIRCRAFT PROPELLER

Abstract

The unducted aeronautical thruster comprises a propeller extending around a main axis and comprising at least one variable-pitch rotor blade around a rotor pitch axis, each variable-pitch rotor blade. At least one variable-pitch rotor blade has an activity factor of between 145 and 230, preferably between 155 and 205, and its maximum relative thickness position (x°_EPmax) is maximum at a relative height (h°) of less than 40%, preferably less than 30%, more preferably less than 20%. Figure for abstract: Fig. 5

Description

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

[0001] The present invention relates to an unducted aeronautical propeller of an aircraft. Technological background

[0002] The blades of a propeller (also called a "propeller", "open fan" or "open rotor" in English) of an unducted aeronautical propeller may be subjected to an upstream flow which is not uniform and parallel to the engine axis, this is a so-called incidence flight, occurring in particular during the takeoff or landing phases of the aircraft and / or with a crosswind. For example, when the aeronautical propeller is installed under the wing, the presence of the wing may induce an upward flow at the propeller which increases the incidence of the flow perceived 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" in English) and may be dimensioning from a mechanical point of view.

[0003] In incidence, the propeller does not only provide a tractive force, it also provides vertical and lateral forces and moments which can in certain cases reach the same value as the traction. These forces are unsteady and depend on the azimuthal position around the main axis of the blade considered. When the aeronautical propeller is installed under the wing, the descending propeller blade (blade at 3 o'clock in the case of a propeller rotating clockwise seen from the front) sees an increase in incidence and is therefore subjected to increased forces unlike the ascending blade (at 9 o'clock in the case of a propeller rotating clockwise seen from the front) on which reduced forces are exerted. Consequently, over an engine revolution, the same propeller blade is subjected to variable forces dependent on its azimuthal position.

[0004] The aeronautical propeller may also have a stator (or a 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 envisage a USF (upstream propeller - downstream stator) or CROR (upstream propeller - downstream propeller) application case. The case of a fast propeller (without downstream row of blades) can also be compatible with the invention. By fast propeller, we mean a propeller capable of flying at a speed greater than M=0.7 in CR.

[0006] The interest of a downstream stator or downstream propeller is to recover the gyration of the flow induced by the upstream propeller, which improves the efficiency of the aircraft propeller. It should also be noted that the upstream incidence (aircraft incidence or induced by the installation - wings, mast, etc.) is not completely filtered by the upstream propeller. In addition to the variation in IP forces, the downstream stator or the downstream propeller may be subjected to different incidences (due to the aircraft incidence) depending on their azimuthal position.

[0007] However, at high incidence, there is a risk of airflow separation, i.e. a risk that the airflow will separate from the upper surface or the lower surface. For example, these flow separations can occur on the upper surface and between mid-span and the free end of a propeller blade with a high aerodynamic load during takeoff and / or a flight phase at high incidence or high thrust. Flow separations at the propeller and / or stator blades can degrade the performance of the aeronautical propeller as well as that of the aircraft (for example, if the wing or a lifting surface of the aircraft perceives a separated flow coming from the blades of the aeronautical propeller).

[0008] It may thus be desired to control the resistance to incidence of the propeller blades (and / or the stator), from a mechanical, aerodynamic and acoustic point of view. Summary of the invention

[0009] There is therefore proposed an unducted aeronautical propeller comprising a propeller extending around a main axis and comprising at least one variable-pitch rotor blade around a rotor pitch axis, each variable-pitch rotor blade having: - a leading edge; - a trailing edge; - an internal radius at the leading edge relative to the main axis; - an external radius at the leading edge relative to the main axis; - an internal radius at the trailing edge relative to the main axis; - an external radius at the trailing edge relative to the main axis; - at each relative height with respect to the main axis from the internal radius to the external radius, the leading edge and / or the trailing edge: • a chord line extending in a section 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 maximum relative thickness position; characterized in that at least one variable-pitch rotor blade has an activity factor of 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%, more preferably less than 20%.

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

[0011] Optionally, 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%.

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

[0013] Also optionally, 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%, more preferably greater than 30%.

[0014] Also optionally, 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%, more preferably between 26% and 32%.

[0015] Also optionally, for each variable-pitch rotor blade, the maximum thickness relative position varies less between the closest relative height to 0% and the first relative height, than between the first relative height and the second relative height.

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

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

[0018] Also optionally, for each variable-pitch rotor blade, the relative position of maximum thickness has at least one local maximum and / or minimum over the relative height.

[0019] Also optionally, each variable-pitch rotor blade 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 relative height closest to 0% up to the relative height of 60%, preferably up to the relative height of 70%.

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

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

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

[0023] Also optionally, for each variable-pitch rotor blade, the normalized maximum thickness is strictly decreasing from the closest relative height to 0% to the closest relative height to 100%.

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

[0025] Also optionally, for each variable-pitch rotor blade, the maximum thickness normalized to the relative height closest to 100% is less than or equal to three times, preferably two and a half times, more preferably two times, the maximum thickness normalized to the relative height of 70%.

[0026] Also optionally, the aeronautical propeller further comprises an external casing and a fixed, non-ducted rectifier mounted on the external casing.

[0027] An aircraft comprising an unducted aeronautical propeller 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 with reference to the appended drawings in which: - [Fig.l] is a cross-sectional view of an aeronautical propeller in which the invention can be implemented, - [Fig.2] is a side view of a rotor blade of a propeller of the aeronautical propeller that [Fig.l], - [Fig.3] is a sectional view of the rotor blade of [Fig.2], - [Fig.4] is another sectional view of the rotor blade of [Fig.2], coupled to a graph illustrating the evolution of the thickness of a 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, - [Fig.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 sectional view of a rotor blade with maximum thickness away from the leading edge, and - [Fig.8] is a sectional view of a rotor blade with maximum thickness close to the leading edge. Detailed description of the invention

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

[0030] The aeronautical propeller 100 is unducted (for example of the “Unducted Single Fan” type in English, also designated by the acronym USF) and is designed to participate in the propulsion of an aircraft. At this stage, it is immediately specified that the solution of the invention applies to any aeronautical propeller (turbomachine, turbojet, turboprop with fast propellers, etc.) that is unducted 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 propeller.

[0031] The aeronautical propeller 100 firstly comprises an external casing 102 and a hub 104 pivotally mounted 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 propeller 100 along the main axis X in a direction of flow of an air flow PHI from the front to the rear of the aeronautical propeller 100 and / or the aircraft.

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

[0034] The aeronautical propeller 100 further comprises a propeller 106 which is propulsive, unducted and mounted on the hub 104 so as to be pivotable relative to the outer 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. The propeller 106 comprises for this purpose rotor blades 108 (for example between 3 and 25, preferably between 10 and 16) organized for example in a single annular row around the main axis X. The rotor blades 108 may for example all be identical and angularly spaced evenly around the principal X axis. Alternatively, non-regular angular spacing could be considered.

[0035] The rotor blades 108 may be variable pitch, each around a respective radial rotor pitch axis Y. The solution presented may cover the cases where: - the rotor pitch axis Y is perpendicular to the main axis X, i.e. it may make a right angle with the main axis X; As illustrated in [Fig.l] where the rotor pitch axis Y and the main axis X are projected onto a plane or section of the aeronautical propeller 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 (in the direction of the main axis X) and a component perpendicular to the main axis X.

[0036] The aeronautical propeller 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 in 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 comprises, for example, a turbomachine, and / or (at least) a turbojet, and / or (at least) a turbofan, and / or (at least) a turbo-engine, comprising, 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 comprise (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 thruster 100 further comprises a fixed rectifier 112 which is not shrouded and mounted on the external casing 102, downstream of the propeller 106. The fixed rectifier 112 forms a stator fixed to the external casing 102 extending around the main axis X, but which cannot rotate around the latter. The fixed rectifier 112 comprises stator blades 114 organized 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 aeronautical propeller 100. In particular, the number of rotor blades 108 is greater than the number of stator blades 114.Indeed, in the case where the number of rotor blades 108 and the number of stator blades 114 were equal, the set of wakes of the propeller 106 could interact simultaneously with the stator blades 114, which would increase the sound levels. The stator blades 114 can be. for example all identical and angularly spaced regularly around the main axis X. Alternatively, the stator blades could be different and / or spaced irregularly in the azimuthal direction around the main axis X.

[0038] The fixed rectifier 112 is designed to straighten at least a portion of the airflow PHI passing through the propeller 106, in order to improve the performance of the aeronautical thruster 100. More specifically, the purpose of the fixed rectifier 112 is to take up the gyration induced by the propeller 106 in order to improve the performance of the unducted configuration. Nevertheless, its presence induces a dominant noise source resulting from the interaction with the wake of the propeller 106 (and the blade tip vortex when the truncation of the stator blades 114 is not sufficient). It is therefore appropriate to reduce the noise generated by the fixed rectifier 114 and its interaction with the wake of the propeller 106 while preserving good aerodynamic performance, because the reduction of noise emissions and consumption is a major issue for unducted engine architectures.

[0039] At least one stator blade 114 is variable pitch, each about a respective radial axis Y'. Preferably, all the stator blades 114 are variable pitch. This makes it possible to adapt the triangulation of the flow upstream of the stator blades 114 according to the operating point, which makes it possible to improve the aerodynamic performance and to reduce the noise emitted by the interaction between the wakes of the upstream propeller 106 and the fixed rectifier. Indeed, the variation in the pitch of the stator blades 114 makes it possible to modify the directivity of the sound and to reduce the tonal interaction noise.

[0040] The aeronautical propeller 100 further comprises, 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 firstly comprises a leading edge BA where the air flow PHI arrives and a trailing edge BF from which the air flow PHI departs. The leading edge BA extends from a root BA_P which is the point of the leading edge BA closest to the main axis X to a head BA_T which is the point of the leading edge BA furthest from the main axis X. Between the root BA_P and the head BA_T, the leading edge BA may have a curvature of constant direction, i.e. without an inflection point, and furthermore 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 main axis X to a head BF_T which is the point of the trailing edge BF furthest from the main axis X. Between the foot BF_P and the head BF_T, the trailing edge BF may have a curvature of constant direction, i.e. without an inflection point, and furthermore regular, i.e. without discontinuity.

[0043] Each rotor blade 108 may further be truncated in its free end, as in the example illustrated, that is to say that there is a truncated section 202, for example straight, connecting the heads BA_T, BF_T. In this case, there is a discontinuity of curvature at the level of the head BA_T between the leading edge BA and the truncated section 202 and another discontinuity of curvature at the level of the head BF_T between the truncated section 202 and the trailing edge BF. Alternatively, the rotor blade 108 could be non-truncated, in which case the heads BA_T, BF_T would be merged.

[0044] Each rotor blade 108 has an internal radius Ri_BA at the leading edge BA equal by definition to the distance between the main axis X and the root BA_P, and an internal radius Ri_BF at the trailing edge Ri equal by definition 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 equal by definition to the distance between the main axis X and the head BA_T an external radius Re_BF at the trailing edge BF equal by definition to the distance between the main axis X and the head 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 propeller 100, that is to say when the pitch angle of the rotor blades 108 would make it possible to move the air flow PHI from the front to the rear of the aeronautical propeller 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 optimum pitch angle for their operation in cruise mode, i.e. at a flight Mach greater than 0.7 for a medium-haul aircraft.

[0045] To locate a height h_BA between the internal radius Ri_BA and the external radius Re_BA, it is possible to define a relative height h°_BA by the formula h ° BA = h-BA which can be expressed as a percentage. The relative height h°_BA therefore varies between 0% at the internal radius Ri_BA and 100% at the external radius Re_BA. To locate a height h_BF between the internal radius Ri_BF and the external radius Re_BF, it is possible to define a relative height h°_BF by the formula h 0 pp — BF C'U' Can be expressed as a percentage. The relative height h°_BF therefore varies between 0% at the internal radius Ri_BF and 100% at the external radius Re_BF. With the relative heights h°_BA and h°BF, it is possible that there is 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 heads 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 of figure 2) and the lower of the two heads BA_T, BF_T (BF_T in the example of 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, there exists at each relative height h°_C between 0% and 100% a leading edge and a trailing edge, which makes it possible to define at any relative height h°_C a complete section (i.e. comprising the leading edge and the trailing edge) as will be explained later with reference to [Fig.3],

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

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

[0049] As can be seen, the rotor blade 108 has a lower surface face 302 and an upper surface face 304, respectively concave and convex, connected to each other by the leading edge BA and the trailing edge BF. The latter may for example have a truncation 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 chord line 306 and the chord c can vary depending on the relative height h° considered. The angle between the chord line 306 and the axis P perpendicular to the main axis X makes it possible to define a setting angle V in the cutting plane.

[0051] A parameter which makes it possible to give 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: l , where Ri FA=» ] 0 Ri / Re" 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 main 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 a section or aerodynamic 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 blading in combination with an adequate and optimized thickness distribution for withstanding the forces on the rotor blades 108 in the phases of flight at incidence, as proposed in this invention.

[0053] Furthermore, it is possible to define the (average) camber line or skeleton 308 of a section of a rotor blade 108. For a section of a blade, the camber line or skeleton 308 is defined as the line halfway between the intrados and extrados faces. For example, this camber line or skeleton 308 can be obtained by passing inscribed circles through the section, so that the positions of the centers of these inscribed circles (see the dotted circles) define the camber line or skeleton 308.

[0054] With reference to [Fig. 4], the rotor blade 108 has a thickness EP in the section plane varying from the leading edge BA to the trailing edge BF. For example, the thickness EP is defined as the distance between the intrados 302 and the extrados 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 section of a blade thus 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 X 0 = 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 relative position of maximum thickness 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 might not start perfectly at h°=0% and end at h°=100% when the relative height h° is the height relative to the leading edge or the height relative to the trailing edge. Thus, when we subsequently speak of relative height h° “closest to 0%” or “closest to 100%”, this will mean the relative height where there is still a complete cross-section. In the example in [Fig.2], the relative height h°_BA closest to 0% is therefore the relative height h°_BA at the level of the root BF_P and the relative height h°_BA closest to 100% is therefore the relative height h°_BA at the head BF_T. For the case where the relative height h° is the complete relative height h°_C, the expression "closest to 0%" will mean "to 0%" and "closest to 100%" will mean "to 100%".

[0059] With particular reference to [Fig. 5], preferably, as is the case for the four examples 502, 504, 506, 508, the relative position of maximum thickness x°_EPmax is maximum at the relative height h° less than 40%, preferably less than 30%, more preferably less than 20%. In particular, for the three examples 502, 504, 506, the relative position of maximum thickness x°_EPmax is maximum at the relative height h° closest to 0%, i.e. at the level of the blade root cut. This contributes to the mechanical resistance of the blade to IP forces during phases of flight at incidence, as well as to limiting the acceleration of the flow and / or the intensity of the shocks which may form during cruising near the hub 104 or the casing 102, which could increase aerodynamic losses. For example, the maximum thickness relative position x°_EPmax is greater than 20%, preferably greater than 25%, more preferably greater than 30%.This makes it possible to control the speed gradients on the root section where the passage section between two adjacent blades in the azimuthal direction is minimal, in particular 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%, more preferably between 26% and 32%. This ensures that the position of the maximum thickness in the lower part of the rotor blade 108 is located relatively downstream (and close to the center of gravity of the rotor blade 108), in particular relative to the position of the maximum thickness in the upper part of the rotor blade 108, which ensures good balancing and good mechanical strength of the rotor blade 108. This would also make it possible to increase the thickness of the sections close to the root of the rotor blade 108 and at the level of the spar and / or other structural elements inside the rotor blade 108 making it possible to transmit the forces under the hub 104.

[0061] Furthermore, for most of the examples in [Fig.5], the relative position of maximum thickness x°_EPmax decreases in a strictly monotonous manner from the first relative height RI to a second relative height R2 of 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 will be visible for the three examples 502, 504, 506 in [Fig.6]. However, with a thin rotor blade 108, a separation on the extrados can more easily appear in the upper part of the blade and at the leading edge, which generates aerodynamic losses and increases the noise. By positioning the maximum thickness EPmax close to the leading edge BA (i.e. with a low relative position of maximum thickness x°_EPmax), this attenuates or avoids the separation of the air flow, in particular in the event of over-incidence as may be the case during the landing and / or takeoff phases of the aircraft (these are flight phases that are particularly important for the certification of aircraft noise levels 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 root 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 root of the blade during the incidence flight phases.

[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%, more preferably between 20% and 34%, for all the 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 head of the rotor blade 108, which makes it possible to limit or avoid separations and therefore to improve efficiency while limiting noise.

[0067] In an alternative embodiment, the relative position of maximum thickness 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 locally move the relative position of maximum thickness x°_EPmax away from the leading edge. This makes it possible to better distribute the aerodynamic load on the sections at the tip of the rotor blade 108 and therefore to reduce the local speed of the flow and / or the intensity of shocks in cruise mode, which helps reduce 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 normalized maximum thickness EPmax° is strictly decreasing from the relative height h° of 0% to at least the relative height h° of 60%, preferably 70%.

[0070] For example, for examples 504, 506, the normalized maximum thickness EPmax° is strictly decreasing from the relative height h° of 0% up to the relative height of 100%. This makes it possible to have thinner profiles in the upper part and therefore to reduce the mass and weight of rotor blade 108.

[0071] On the other hand, for examples 506, 508, the normalized maximum thickness EPmax° increases from the relative height h° by approximately 70%, preferably by 80% up to the relative height h° closest to 100%. In this case, the normalized maximum thickness EPmax° at the relative height h° closest to 100% is preferably less than or equal to three times the normalized maximum thickness EPmax° at the relative height h° of 70%, or preferably less than or equal to 2.5 times the normalized maximum thickness EPmax° at the relative height h° of 70%, or even preferably less than or equal to 2 times the normalized maximum 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 placement of the first bending mode of the blade, which may be necessary to withstand the IP forces.This increase in the maximum thickness near the blade head can also be useful for mechanical strength when increasing the sweep and / or dihedral to reduce noise. From an aero-acoustic point of view, this increase in the thickness of the blade head sections makes it possible to control the incidence and avoid possible separations.

[0072] Preferably, the normalized maximum thickness EPmax° at the closest 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 hold and transmit the aerodynamic and mechanical forces onto the rotor blade 108.

[0073] Preferably, the normalized maximum thickness EPmax° at the relative height h° closest to 100% is between 0.02 and 0.08, preferably between 0.03 and 0.06. Limiting the thickness at the tip makes it possible to reduce the mass near the free end of the rotor blade 108 and therefore to limit the possible moments transmitted to the rotor blade 108. In addition, this is necessary to limit the mass released in the event of loss of a part of the blade during possible bird ingestion.

[0074] Preferably, the normalized maximum thickness EPmax° decreases from the relative height h° closest to 0% until reaching, at a relative height h° between 25% and 50%, a quarter of the normalized maximum thickness EPmax° at the relative height h° closest to 0%. Reducing the thickness towards the free end makes it possible to reduce the mass near the free end of the rotor blade 108 and therefore to limit the possible moments transmitted to the rotor blade 108. In addition, this is necessary to limit the mass released in the event of loss of part of the blade during possible ingestion of a bird.

[0075] With reference to [Fig.7], when the maximum thickness is too far from the leading edge, an air flow reaching the leading edge with a high incidence I has difficulty in catching the extrados, that is to say in remaining at a short distance from the extrados, and moves away from it producing a separation generating turbulence T.

[0076] With reference to [Fig.8], on the other hand, when the maximum thickness EPmax is brought closer to the leading edge BA, the latter is more curved or rounded. This allows better grip of the air flow with a high incidence.

[0077] In conclusion, it appears clearly that an aeronautical propeller such as that described above makes it possible to obtain an optimal operating range for large incidences on the leading edge.

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

[0079] In the detailed presentation of the invention which is made above, the terms used must not be interpreted as limiting the invention to the embodiments set out in the present description, but must be interpreted to include all equivalents whose prediction is within the reach of those skilled in the art by applying their general knowledge to the implementation of the teaching which has just been disclosed to them.

Claims

Claims

1. Unducted aeronautical propeller (100) comprising a propeller (106) extending around a main axis (X) and comprising at least one rotor blade (108) with variable pitch around a rotor pitch axis (Y), each rotor blade (108) with variable pitch having: - a leading edge (LA); - a trailing edge (BF); - an internal radius (Ri_BA) at the leading edge (BA) relative to the main axis (X); - an external radius (Re_BA) at the leading edge (BA) relative to the main axis (X); - an internal radius (Ri_BF) at the trailing edge (BA) relative to the main axis (X); - an external radius (Re_BA) at the trailing edge (BA) relative to the main axis (X); - at each relative height (h°) with respect to the main axis (X) from the internal radius (Ri_BA, Ri_BF) to the external radius (Re_BA, Re_BF) at the leading edge (BA) and / or the trailing edge (BF): • a chord line (306) extending in a section 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 maximum thickness relative position (x°_EPmax); characterized in that at least one variable-pitch rotor blade (108) has an activity factor (FA) of between 145 and 230, preferably between 155 and 205, and its relative position of maximum thickness (x°_EPmax) is maximum at a relative height (h°) less than 40%, preferably less than 30%, more preferably less than 20%.

2. Unducted aeronautical propeller (100) according to claim 1, in which, for each variable-pitch rotor blade (108), the relative position of maximum thickness (x°_EPmax) decreases from a first relative height (RI) of between 20% and 40% to a second relative height (R2) of between 55% and 100% where the relative position of maximum thickness (x°_EPmax) is between 15% and 35%, preferably between 20% and 30%.

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

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

5. Unducted aeronautical propeller (100) according to any one of claims 1 to 4, in which, 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%, more preferably between 26% and 32%.

6. Unducted aeronautical propeller (100) according to any one of claims 1 to 5, in which, for each variable-pitch rotor blade (108), the maximum thickness relative position (x°_EPmax) varies less between the closest relative height (h°) at 0% and the first relative height (RI), than between the first relative height (RI) and the second relative height (R2).

7. Unducted aeronautical propeller (100) according to any one of claims 1 to 6, in which, 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%, more preferably between 20% and 34%, for all relative heights (h°).

8. Unducted aeronautical propeller (100) according to any one of claims 1 to 7, in which, for each variable-pitch rotor blade (108), 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%.

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

10. Unducted aeronautical propeller (100) according to any one of claims 1 to 9, in which 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 relative height (h°) closest to 0% to the relative height (h°) of 60%, preferably to the relative height (h°) of 70%.

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

2.

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

06.

13. Unducted aeronautical propeller (100) according to any one of claims 10 to 12, in which, for each variable-pitch rotor blade (108), the normalized maximum thickness (EPmax°) decreases from the relative height (h°) closest to 0% until reaching, at a relative height (h°) of between 25% and 50%, a quarter of the normalized maximum thickness (EPmax°) at the relative height (h°) closest to 0%.

14. Unducted aeronautical propeller (100) according to any one of claims 10 to 13, in which, for each variable-pitch rotor blade (108), the normalized maximum thickness (EPmax°) is strictly decreasing from the relative height (h°) closest to 0% to the nearest relative height (h°) to 100%.

15. Unducted aeronautical propeller (100) according to any one of claims 10 to 13, in which, for each variable-pitch rotor blade (108), the normalized maximum thickness (EPmax°) increases from the relative height (h°) of 70%, preferably from the relative height (h°) of 80%, up to the relative height (h°) of 100%.

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

17. An unducted aeronautical thruster (100) according to any one of claims 1 to 16, further comprising an outer casing (102) and an unducted fixed rectifier (112) mounted on the outer casing (102).

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

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