Fan blade with improved leading edge profile

Abstract

The invention relates to a blade (4) for a turbojet engine fan (2), comprising a root (16), a tip (18), a leading edge (20) and a trailing edge (22), the leading edge (20) of the blade having a variable sweep angle (α) between the root (16) and the tip (20), such that a curve (C) representing changes in the sweep angle (α), defining the variations in the sweep angle (α) depending on positions along the leading edge (20), from the root (16) to the tip (18) of the blade (4), comprises a concave portion passing through a positive maximum value (Fmax) of the sweep angle (α) located in a region closer to the root (16) than to the tip (18), and / or comprises a convex portion passing through a negative minimum value (Fmin) of the sweep angle (α) located in a region closer to the tip (18) than to the root (16).

Description

Blower blade with improved leading edge profile Description Technical Field

[0001] This presentation concerns the field of turbojet engines, more specifically turbojet fan blades, and in particular their architecture. Previous technique

[0002] Climate change is a major concern for many legislative and regulatory bodies worldwide. Indeed, various restrictions on carbon emissions have been, are being, or will be adopted by different countries. In particular, an ambitious standard applies to both new types of aircraft and those already in service, requiring the implementation of technological solutions to bring them into compliance with current regulations. Civil aviation has been actively working for several years now to contribute to the fight against climate change.

[0003] Technological research efforts have already led to very significant improvements in the environmental performance of aircraft. The Applicant takes into account the factors impacting all phases of design and development in order to obtain less energy-intensive and more environmentally friendly aeronautical components and products, whose integration and use in civil aviation have moderate environmental consequences, with the aim of improving the energy efficiency of aircraft.

[0004] Consequently, the Applicant is constantly working to reduce its negative climate impact by using methods and operating virtuous development and manufacturing processes that minimize greenhouse gas emissions to the minimum possible in order to reduce the environmental footprint of its activity.

[0005] This sustained research and development work focuses on new generations of aircraft engines, the weight reduction of aircraft, particularly through the materials used and lighter on-board equipment, the development of the use of electrical technologies to provide propulsion, and, as essential complements to technological progress, aviation biofuels.

[0006] In this context, the design of a turbofan fan blade must meet various multidisciplinary criteria. Indeed, a fan blade is designed to optimize its efficiency and thrust (particularly its flow capacity at both high and low engine speeds) while ensuring good mechanical strength, especially at high rotational speeds where the mechanical stresses on the blade are most severe. The design of a fan blade must also comply with the noise targets defined at various operating points as described in current standards.

[0007] Numerous fan blade geometries have been proposed. They are generally characterized by a leading edge stacking law and the evolution of its sweep angle, with the aim of improving the aerodynamic performance of the blade and reducing fan noise. For example, publications EP 2669475 B1 and EP 1 452741 A1 describe specific swept blade geometries for turbofan or turbojet compressor fans. US 9359064 B2 discloses another example of a fan blade geometry as defined in the preamble of claim 1.

[0008] However, in certain configurations, these blades can experience operability issues at low speeds. This is the case, for example, with shrouded fan blades located at the inlet of the low-pressure compressor, upstream of the flow separation nozzle. Typically, under normal operating conditions, the compression ratio of these shrouded fans is between 1.3 and 1.6, with a blade tip velocity between 400 and 500 m / s.

[0009] In this context, the clearance flow, which is the turbulent flow created in the gap between the blade tips and the casing surrounding the fan, can be blocked. This means that the clearance flows of adjacent blades interfere with each other, causing a loss of operability and reducing the fan's efficiency. Flow separation, originating at the leading edge at approximately 50% of the blade height, can also occur on the upper surface and contribute to the loss of operability by degrading the blade's performance, especially when this separation propagates to the blade tip and interacts with the blocked clearance flow. These phenomena negatively impact the aerodynamic performance of the blades and, consequently, the fan's efficiency.

[0010] There is therefore a real need for a blower blade that is free, at least in part, from the disadvantages inherent in the aforementioned known configuration. Description of the invention

[0011] The present disclosure relates to a turbine fan blade for a turbojet engine as defined in claim 1, comprising a foot, a top, a leading edge and a trailing edge, the leading edge of the blade having a variable sweep angle between the foot and the top, such that a sweep angle evolution curve, defining the variations of the sweep angle as a function of positions along the leading edge, from the foot to the top of the blade, includes a concave portion passing through a positive maximum sweep angle value located in a region closer to the foot than to the top, and / or includes a convex portion passing through a negative minimum sweep angle value located in a region closer to the top than to the foot.

[0012] In the present presentation, it is understood that the terms "concave" and "Convex" refers to the concave portions of the curve of evolution of the angle of sag, in which the curve passes through an optimum, which can be a minimum or a maximum.

[0013] By convention, we therefore define a "concave portion" corresponding to a portion of the curve passing through a maximum "pointing" towards increasing arrow angles, and a "convex portion" corresponding to a portion of the curve passing through a minimum "pointing" towards decreasing arrow angles, the troughs of the concave and convex portions being consequently opposite each other.

[0014] When the maximum deflection angle value is positive (this is referred to as the "snout" of the deflection angle evolution curve) and located in a region closer to the toe than to the apex, this implies a profiling towards the rear (in other words downstream according to the direction of the main airflow in the engine) of the leading edge of the blade, in a lower part of the blade.

[0015] Similarly, when the minimum value of the sweep angle is negative (this is referred to as the "front antinode" of the sweep angle evolution curve) and located in a region closer to the top than to the bottom, this implies a forward profiling (in other words upstream according to the direction of the main airflow in the engine) of the leading edge of the blade, in an upper part of the blade.

[0016] Such a configuration (aft snout near the blade root and / or forward snout near the blade tip) reduces flow blockage and limits flow separation at the leading edge and its propagation. This improves the aerodynamic performance of the blade and increases the fan's operability at low speeds. These effects are even more pronounced when the sweep angle curve exhibits both a rear snout near the blade root and a forward snout near the blade tip.

[0017] In some embodiments, when the evolution curve of the deflection angle includes the concave portion, the evolution curve is first decreasing from the foot to a local minimum, then increasing from the local minimum to the maximum value of the deflection angle, then decreasing from the maximum value.

[0018] In some embodiments, when the evolution curve of the deflection angle includes the concave portion, the evolution curve is increasing from the foot up to the maximum deflection angle value, and then decreasing from the maximum value.

[0019] In some embodiments, a reference value Fr of deflection angle is a value of the deflection angle at a radial height of 5% of a total blade height measured from its base to its apex, the reference value Fr being between 18° and 35°, and the maximum value being between Fr +10° and Fr + 25°.

[0020] In some embodiments, when the evolution curve of the deflection angle includes the concave portion, the evolution curve is initially constant and equal to the maximum value from the foot up to an intermediate height, and then decreasing from the intermediate height.

[0021] In some embodiments, a reference value Fr of deflection angle is a value of the deflection angle at a radial height of 5% of a total blade height measured from its base to its apex, the reference value Fr being equal to the maximum value and between 25° and 40°.

[0022] In the present exposition, the evolution curve is decreasing at least from a zero value of the arrow angle to the minimum value of the arrow angle, then increasing from the minimum value to the peak, the minimum value of the arrow angle being between -15° and -25°.

[0023] In some embodiments, when the evolution curve of the deflection angle includes both the concave portion and the convex portion, the evolution curve of the deflection angle includes, between the maximum value and the minimum value of the deflection angle, a transition zone in which the evolution curve is monotonic and decreasing.

[0024] In some embodiments, a starting point of the transition zone is located at a radial height between 35% and 70%, preferably between 55% and 65%, of a total blade height measured from its base to its top.

[0025] In some embodiments, the starting point of the transition zone is defined as a point on the evolution curve at which the value of the deflection angle passes through a value equal to the value of the deflection angle at the foot of the blade.

[0026] In some embodiments, an average slope of the transition zone between the start point and an end point of the transition zone is between -0.002 deg -1 and -0.009 deg -1 .

[0027] In some embodiments, the end point of the transition zone is defined as a point on the evolution curve at which the value of the deflection angle passes through a value equal to the value of the deflection angle at the tip of the blade.

[0028] The present presentation also relates to a blower comprising a plurality of blades according to any one of the preceding embodiments.

[0029] This presentation also relates to a turbojet engine comprising a fan as described in this presentation.

[0030] The aforementioned features and advantages, as well as others, will become apparent upon reading the detailed description that follows, along with examples of how to construct a blower blade. This detailed description refers to the attached drawings. Brief description of the drawings

[0031] The attached drawings are schematic and primarily intended to illustrate the principles of the presentation. In these drawings, identical elements (or parts of elements) are identified by the same reference symbols from one figure to another.

[0032] [Fig. 1] Figure 1 is a partial longitudinal cross-sectional view of a turbojet fan equipped with blades according to the invention;

[0033] [Fig. 2] Figure 2 is a schematic illustration of the definition of the arrow angle;

[0034] [Fig. 3] Figure 3 represents a curve of evolution of the leading edge sweep angle of a blade according to the invention, from its base to its apex;

[0035] [Fig. 4] Figure 4 shows alternative examples of profiles of the concave portion of the curve of evolution of the angle of sag;

[0036] [Fig. 5] Figure 5 schematically illustrates the definition of a transition zone of the evolution curve of the arrow angle of Figure 3;

[0037] [Fig. 6] Figure 6 represents the profile, in a meridian plane, of a blade according to the invention. Description of the implementation methods

[0038] To make the explanation more concrete, an example of a blower blade is described in detail below, with reference to the attached drawings. It should be noted that the invention is not limited to this example.

[0039] The invention applies to any shrouded turbojet fan blade such as that illustrated in Figure 1. In this figure is partially represented a turbojet fan 2 comprising a plurality of blades 4 according to the invention which are regularly spaced from each other around the longitudinal axis XX of the turbojet, this axis XX being oriented in the direction of the airflow through the fan.

[0040] Typically, blower 2 is a shrouded blower which, in the case of a cruise regime (called "cruise" regime in English), has a compression ratio between 1.3 and 1.6, with a blade tip speed between 400 and 500 m / s.

[0041] This blower 2 is typically located downstream of a first, larger-diameter blower (not shown), which is not shrouded, and at an upstream end of the engine (called the "booster"), specifically upstream of the low-pressure compressor (not shown). Blower 2 can, in particular, be positioned just upstream of a separation nozzle 11, allowing the main flow 12 to be separated into an internal flow towards the combustion chamber and an external flow surrounding the internal flow.

[0042] Each blade 4 is fixed at its base to a disk (or hub) 8, which rotates around the longitudinal axis XX of the turbojet engine in the direction of the arbitrarily given arrow F, corresponding, for example, to a clockwise direction when the fan 2 is viewed from downstream. Each blade may also include a platform 10 that partially forms the inner wall radially delimiting the internal flow path of the cold air stream 12 passing through the fan. A wall 14 of a casing surrounding the fan 2 forms the outer wall that radially delimits this same flow path externally.

[0043] For the remainder of the description, for each blade 4, a radial axis Z-Z is defined as being perpendicular to the longitudinal axis XX and passing through the centroid of the cross-section resulting from the intersection of the blade with the inner wall of the cold air flow channel. A tangential axis YY (not shown in the figures) forms a right-handed orthonormal trihedron with the axes XX and 7-7.

[0044] The blade 4 illustrated in Figure 1 comprises a plurality of blade sections (not shown) which result from a cutting of the blade by planes at constant altitude and perpendicular to the radial axis 7-7.

[0045] Each blade 4 is further delimited radially between a foot 16 and a top 18, and longitudinally between a leading edge 20 and a trailing edge 22. Typically, the blade is twisted from its foot 16 to its top 18 in order to compress the flow of cold air 12 passing through the blower 2 during its operation.

[0046] For the remainder of the description, we will define the minimum radial height of the blade as 0% as corresponding to the point of intersection of the leading edge 20 of the blade with the inner wall delimiting the flow vein of the cold air flow inside, and the maximum radial height of the blade as 100% as corresponding to the highest radially high point of the leading edge 20 line.

[0047] According to the invention, the leading edge 20 of the blade 4 has a variable sweep angle from the foot 16 to the tip 18. It should be noted that the blade 4 illustrated in Figure 1 is schematic and allows the description of the different elements constituting it, but does not necessarily represent its real profile, in particular the law of evolution of the sweep angle of the leading edge according to the invention.

[0048] As shown in Figure 2, the sweep angle is the angle α formed at a point on the leading edge 20 of the blade 4 between the tangent T to the leading edge and the line W perpendicular to the relative velocity vector Vr. This line W lies in the plane P, which contains both the tangent T and the relative velocity vector Vr. This angle is measured in the plane P. When the angle α formed between the vector T (tangent oriented towards increasing radii) and the vector W (oriented towards increasing radii) is positive (as shown in Figure 2), the leading edge is said to have a trailing sweep. Conversely, when the angle α is negative, the leading edge is said to have a leading sweep.

[0049] This definition is consistent with and identical to that given more precisely in the publication by Leroy H. Smith and Hsuan Yeh entitled "Sweep and Dihedral Effects in Axial-Flow Turbomachinery" (published in the Journal of Basic Engineering, September 1963, p. 401). All values ​​indicated in this application are calculated according to the formulas detailed in that publication.

[0050] Line C illustrated in Figure 3 represents an example of the evolution curve of the deflection angle a in degrees (°) of the leading edge 20 of a blade 4 according to the invention as a function of the radial height of the blade 4 as a percentage (%) of its total height from its foot 16 to its top 18.

[0051] Note that in the following description, the description of curve C is considered from foot 16 to vertex 18. Thus, a decreasing evolution of curve C means that the values ​​of the angle of deflection a decrease as one moves away from foot 16 and towards vertex 18. Conversely, an increasing evolution of curve C means that the values ​​of the angle of deflection a increase as one moves away from foot 16 and towards vertex 18.

[0052] In this example of embodiment, the leading edge 20 of the blade 4 thus has a backsweep (i.e. a positive sweep angle) on a lower part of the blade 4, closer to the foot 16, and a frontsweep (i.e. a negative sweep angle) on an upper part of the blade 4, closer to the apex 18.

[0053] In the example illustrated in Figure 3, the transition between the trailing and leading edges—that is, the point along the leading edge 20 at which the leading edge passes through a zero angle—is located at mid-height of the blade 4, i.e., 50% of its total height. However, this is not a limiting factor; this transition could be located closer to the foot 16 or closer to the tip 18, provided that the evolution curve C exhibits a low antinode implying a positive maximum deflection, and / or a high antinode implying a negative minimum deflection.

[0054] By "lower antinode" we understand a concave portion of the curve C located closer to the foot 16 than to the vertex 18, and by "higher antinode" we understand a convex portion of the curve C located closer to the vertex 18 than to the foot 16.

[0055] By convention, a "concave portion" corresponds to a portion of the curve passing through a maximum "pointing" towards the angles of increasing arrows (which corresponds to a left-to-right direction in Figure 3), and a "convex portion" corresponds to a portion of the curve passing through a minimum "pointing" towards the angles of decreasing arrows (which corresponds to a right-to-left direction in Figure 3), the troughs of the concave and convex portions being consequently opposite each other.

[0056] Thus, the concave portion of the curve C passes through a maximum value Fmax positive of the angle of sag a located in a region of the leading edge 20 closer to the foot 16 than to the apex 18, and the convex portion of the curve C passes through a minimum value Fmin negative of the angle of sag a located in a region of the leading edge 20 closer to the apex 18 than to the foot 16.

[0057] We speak of the depth P1 of the lower antinode, the maximum depth of the concave portion of the curve C, corresponding to the difference between 0° and Fmax. Similarly, we speak of the depth P2 of the upper antinode, the maximum depth of the convex portion of the curve C, corresponding to the difference between 0° and Fmin.

[0058] It should be noted that in this example, and in the rest of the description, the evolution curve C includes both a concave portion (a low antinode) and a convex portion (a high antinode). However, the invention also applies to a configuration in which the evolution curve C includes only a concave portion, or only a convex portion.

[0059] Furthermore, a reference value Fr of the deflection angle is defined, which is a value of the deflection angle a at a radial height of 5% of the total height of the blade 4 measured from its foot 16 to its top 18.

[0060] Preferably, the reference value Fr is between 18° and 35° (approximately 19° in the example in Figure 3), so that the maximum value Fmax is between Fr +10° and Fr + 25° (approximately 30° in the example in Figure 3).

[0061] In the example shown in Figure 3, the curve C representing the evolution of the deflection angle α is initially decreasing from the foot 16 to a local minimum, which could be, for example (but not limited to), the reference value Fr, and therefore located at 5% of the blade height 4. From this local minimum, the curve C increases up to the maximum value Fmax of the deflection angle α, then decreases from the maximum value Fmax. This is referred to as a "convex then concave" profile at the foot.

[0062] This profile is not, however, limiting; different profiles for the concave portion may also be applicable to the invention. Figure 4 shows alternative examples of concave profiles.

[0063] Curve C1 corresponds to a "convex then concave" profile, as illustrated in Figure 3. Curve C2 corresponds to a "purely concave" profile, in which the evolution curve increases from foot 16 to the maximum value Fmax of the deflection angle α, then decreases from the maximum value Fmax. In this case, as with curve C1, the reference value Fr is between 18° and 35°, and the maximum value Fmax is between Fr +10° and Fr +25°.

[0064] Curve C3 corresponds to a "linearized concave" profile, in which the evolution curve is initially constant and equal to the maximum value Fmax from foot 16 up to an intermediate height, for example, approximately equal to 15% of the total height of blade 4, and then decreases from this intermediate height. In other words, in this configuration, the concave portion of the evolution curve does not have an increasing segment, as the maximum depth of the lower section is reached at foot 16 of blade 4. In this case, the reference value Fr is equal to the maximum value Fmax and is between 25° and 40°.

[0065] Regardless of the configuration chosen for the shape of the concave portion (lower belly), the fact that the blade has a positive sweep near the blade foot, in other words a back sweep, improves the aerodynamic performance of the blade.

[0066] More specifically, the depth of the lower belly modifies the flow distribution between the foot 16 and the tip 18, and thus controls the amount of flow sent to the tip (above 80% of the blade height 4), and therefore the flow release. The deeper the lower belly, the more the flow is released. However, an excessive depth of the lower belly (i.e., excessive sweep angle values) could accentuate flow separation at the leading edge.

[0067] The Fr and Fmax values ​​defined above reduce flow blockage, while limiting flow separation at the leading edge and its propagation. This improves the aerodynamic performance of blade 4 and increases operability at low engine speeds.

[0068] Furthermore, in the upper part of the dawn 4, the evolution curve C is first decreasing until the minimum value Fmin of the angle of the arrow a, then increasing from the minimum value Fmin until the peak 18, the minimum value Fmin of the angle of the arrow a being between -15° and -25° (approximately -15° in the example of figure 3).

[0069] Preferably, the value of the deflection angle a is negative at the top 18, i.e. at 100% of the total height of the blade 4. In other words, the minimum value Fmin of deflection angle a is increasing from the minimum value Fmin up to the top 18, while remaining negative.

[0070] The P2 depth of the convex portion (high belly) also controls the amount of flow sent to the top, as well as the size of the leading edge separation 20. The deeper the high belly, especially for values ​​less than or equal to -15°, the more the flow of play is unblocked, which allows a gain in operability.

[0071] However, values ​​strictly below -25° imply significant leading-edge separation, and therefore a loss of operability. The range defined above, between -15° and -25°, is thus a compromise that improves the aerodynamic performance of the blade and increases operability at low engine speeds.

[0072] Furthermore, in the example shown in Figure 3, the presence of a low antinode and a high antinode implies an evolution law (described by curve C) of the leading edge slant angle α in the shape of an "S". This shape, obtained by the combination of a low antinode and a high antinode, further improves the aforementioned effect.

[0073] Furthermore, the evolution curve C includes a transition zone Tr, located between the maximum value Fmax and the minimum value Fmin of the angle of the arrow a. Typically, the evolution curve C is substantially linear in the transition zone Tr, and is monotonic, in particular strictly decreasing.

[0074] Figure 5 shows an example of determining the transition zone T r This example is not exhaustive. A height Tri designates the starting height of the transition zone Tr, and a height -2 designates the ending height of the transition zone Tr.

[0075] Furthermore, the evolution curve C of the angle of deflection enters the transition zone from the point Fi, with coordinates (x a , z a ), where x a is the value of the angle of deflection a at point Fi and z a the height of point Fi along the dawn 4.

[0076] In this example, to determine x a We consider the point at which the curve C passes through a deflection angle value a equal to the deflection angle value at foot 16, that is, at a height of 0% of the total height of blade 4, after having reached the maximum value F ma x. Thus, in the example shown in Figure 5, x a is approximately equal to 20°, and z a is approximately equal to 42% of the total height of the 4th dawn.

[0077] In addition, the evolution curve C of the deflection angle a exits the transition zone from the point F2, with coordinates (xt>, Zb), where Xb is the value of the deflection angle a at the point F2 and Zb the height of the point F2 along the blade 4.

[0078] In this example, to determine Xb, we consider the point at which the curve C passes through a deflection angle equal to the deflection angle at apex 18, that is, at a height of 100% of the total height of blade 4, before reaching the maximum value Fmin. Thus, in the example shown in Figure 5, Xb is approximately equal to -8°, and Zb is approximately equal to 58% of the total height of blade 4.

[0079] We understand therefore that at point F1, the value of the angle of deflection a is positive, and at point F2, the value of the angle of deflection a is negative, the evolution curve C passing through a zero value in the transition zone Tr, between F1 and F2.

[0080] It should be noted that the determination of the transition zone Tr is described above with reference to the case shown in Figure 3, corresponding to the evolution curve C1 of Figure 4. However, different methods for determining the transition zone Tr can be used for this case, and also for the cases corresponding to the evolution curves C2 and C3 of Figure 4. For curve C3, for example, the point F1 from which the evolution curve C3 enters the transition zone can be the point from which said curve begins to decrease.

[0081] For the evolution curves C1 and C2, it is preferable that the starting point Fi of the transition zone be located at a radial height (in other words, the value of z a) between 35% and 70%, preferably between 55% and 65%, of the total height of blade 4. For the evolution curve C3, it is preferable that the starting point Fi of the transition zone be located at a radial height between 10% and 20%, preferably between 13% and 17%, of the total height of blade 4.

[0082] Indeed, the initial height Tri of the transition zone Tr determines the size of the leading edge separation. The higher the initial height Tri of the transition zone, the smaller the leading edge separation (because it originates at the blade fold, described below, caused by the transition), and therefore the more efficiently the blade operates, resulting in improved operability. The initial height Tri of the transition also has a marginal impact on the flow blockage. If the transition is too high or too low, the flow blockage increases.

[0083] The ranges defined above therefore represent a compromise allowing for improved operability at low engine speeds. It is also preferable that the average slope of the evolution curve C, between points Fi and F2, be between -0.002 degrees 1 and -0.009 deg 1 the slope being calculated using the formula Xfj x a

[0084] In the example shown in Figure 5, x a "20°; z a "0.42 and Xb " - 8°; Zb " 0.58. Therefore, the slope is approximately equal to -0.0057 deg -1 .

[0085] This range of values ​​for the slope of the curve C in the transition zone Tr also makes it possible to improve the aerodynamic performance of the blade 4 and its leading edge 20. This transition zone Tr, involving a "rapid" transition between positive and negative values ​​of sweep angles a, implies a relatively marked dip in the profile of the leading edge 20 of the blade 4.

[0086] This profile is schematically represented in Figure 6, depicting a blade 4 in a meridian plane. Each measurement point (visible in Figures 3 to 5) of the deflection angle a, between the foot 16 and the apex 18, implies a position of a given slice 25 of the blade 4.

[0087] Thus, in Figure 6, we observe that in the lower part of the blade 4, near the foot 16, the leading edge 20 tends to curve backwards ("backward sweep", i.e., positive values ​​of the sweep angle a), up to a fold 26, corresponding to the transition zone Tr in which the evolution curve C of the deflection angle a goes from positive values ​​to negative values. From this fold 26, the leading edge 20 tends to go forward (“forward deflection”, i.e., negative values ​​of the deflection angle a).

[0088] Although the present invention has been described with reference to specific embodiments, it is evident that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various embodiments illustrated / mentioned can be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

Demands

1. A fan blade (4) for a turbojet engine (2), comprising a root (16), a tip (18), a leading edge (20), and a trailing edge (22), the leading edge (20) of the blade having a variable sweep angle (a) between the root (16) and the tip (20), such that a sweep angle (a) evolution curve (C), defining the variations of the sweep angle (a) as a function of positions along the leading edge (20), from the root (16) to the tip (18) of the blade (4), comprises a concave portion passing through a positive maximum value (Fmax) of the sweep angle (a) located in a region closer to the root (16) than to the tip (18), and comprises a convex portion passing through a negative minimum value (Fmin) of the sweep angle (a) located in a region closer to the tip (18) than to the root (16). (16),characterized in that the evolution curve (C) is decreasing at least from a zero value of the arrow angle to the minimum value (Fmin) of the arrow angle (a), then increasing from the minimum value (Fmin) to the peak (18), the minimum value (Fmin) of the arrow angle (a) being between -15° and -25°.

2. Blade (4) according to claim 1, wherein, when the evolution curve (C) of the deflection angle (a) includes the concave portion, the evolution curve (C) is first decreasing from the foot (16) to a local minimum, then increasing from the local minimum to the maximum value (Fmax) of deflection angle, then decreasing from the maximum value (Fmax).

3. Blade (4) according to claim 1, wherein, when the evolution curve (C) of the deflection angle (a) includes the concave portion, the evolution curve (C) is increasing from the foot (16) up to the maximum value (Fmax) of deflection angle, and then decreasing from the maximum value (Fmax).

4. Blade (4) according to claim 2 or 3, wherein a reference value Fr of deflection angle (a) is a value of the deflection angle at a radial height of 5% of a total height of the blade (4) measured from its foot (16) to its apex (18), the reference value Fr being between 18° and 35°, and the maximum value (Fmax) being between Fr +10° and Fr + 25°.

5. Blade (4) according to claim 1, wherein, when the evolution curve (C) of the deflection angle (a) includes the concave portion, the evolution curve (C) is first constant and equal to the maximum value (Fmax) from the foot (16) up to an intermediate height, then decreasing from the intermediate height.

6. Blade (4) according to claim 5, wherein a reference value Fr of deflection angle (a) is a value of the deflection angle at a radial height of 5% of a total height of the blade (4) measured from its foot (16) to its top (18), the reference value Fr being equal to the maximum value (Fmax) and between 25° and 40°.

7. Blade (4) according to any one of claims 1 to 6, wherein, when the evolution curve (C) of the deflection angle (a) includes both the concave portion and the convex portion, the evolution curve (C) of the deflection angle (a) includes, between the maximum value (Fmax) and the minimum value (Fmin) of deflection angle (a), a transition zone (Tr) in which the evolution curve (C) is monotonic and decreasing.

8. Blade (4) according to claim 7, wherein a starting point (Fi) of the transition zone (Tr) is located at a radial height (z a ) between 35% and 70%, preferably between 55% and 65%, of a total height of the dawn (4) measured from its base (16) to its top (18).

9. Blade (4) according to claim 8, wherein an average slope of the transition zone (Tr) between the start point (Fi) and an end point (F2) of the transition zone (Tr) is between -0.002 deg -1 and -0.009 deg -1 .

10. Blower (2) comprising a plurality of blades (4) according to any one of the preceding claims.

11. Turbojet comprising a fan (2) according to claim 10.

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