Method for de-icing aerodynamic wall of aircraft

Abstract

The invention relates to a method for de-icing an aerodynamic wall (32) of an aircraft, said method comprising a step of activating a thermal de-icing system (70) configured to cause at least local heating at an outer face (F32) of the aerodynamic wall (32), and a step of activating a further de-icing system (68) by means of elastic deformation configured to cause at least local heating at the outer face (F32) of the aerodynamic wall (32). The de-icing system (68) is configured to deform the aerodynamic wall (32). When the aerodynamic wall (32) has an ice mass, activation of the thermal de-icing system (70) allows the ice mass to be weakened by splitting the ice block into a plurality of small pieces, which helps to promote the efficiency of the de-icing system by means of elastic deformation. The invention also relates to an aircraft equipped with a deicing device allowing the implementation of the above method.

Description

Technical Field

[0001] The present application relates to a method for deicing a wall using at least two different deicing systems, and also to an aircraft equipped with a deicing device allowing the implementation of said method. Background Art

[0002] according to Figure 1 In the illustrated embodiment, the aircraft 10 includes a fuselage 12, wings 14 positioned on both sides of the fuselage 12, and propulsion assemblies 16 positioned below the wings 14 and connected to the wings via pylons 18. Each propulsion assembly 16 includes a motor 20 and a pod 22 positioned around the motor 20.

[0003] Depending on the weather conditions and the flight phase, ice masses 24 may form on certain aerodynamic walls 26 of the aircraft, such as the leading edge. In order to optimize flight conditions, these possible ice masses should be removed or their formation should be limited or prevented. To this end, Figure 2 As shown, aerodynamic walls 26 that are susceptible to frost or ice formation may include a de-icing system 28 .

[0004] In this application, "ice" may refer to frost or ice to varying degrees, and "de-icing" refers to the process of limiting or preventing the formation of frost and / or ice and / or clearing frost and / or ice from a surface.

[0005] According to one embodiment, the deicing system is of electrical type and comprises at least one resistance mat, which is laid against the inner face of the aerodynamic wall to be treated and covers the entire area to be treated, and a current generator connected to the resistance mat. Heat is generated by the Joule effect through the current in the resistance mat, which prevents or limits the formation of ice. In the presence of ice masses, the electrical deicing system allows the ice masses to be peeled off from the aerodynamic wall.

[0006] Electrical deicing systems comprising resistor mats consume electrical energy, which is a function of the surface area covered by the resistor mat, the activation (power-on) time and the power per unit area of ​​the resistor mat. Since the resistor mat extends continuously over the entire surface area of ​​the area to be treated and the activation time is relatively long, the disadvantage of the resistor mat is the high energy consumption.

[0007] according to Figure 2 In another embodiment, which can be seen in FIG. 2 , the de-icing system 28 is of the mechanical type and comprises at least one vibration generator 30 , for example a piezoelectric element, which is positioned against the inner face of the aerodynamic wall 26 . Figure 2 As shown in part (A) of FIG. 1 , when ice mass 24 is formed on the outer surface of aerodynamic wall 26, vibration generator 30 is activated, as shown in FIG. Figure 2As shown in part (B) of FIG. 1 . When the vibration generator 30 is activated, the vibration caused by the vibration generator 30 is transmitted (transmitted) to the ice mass 24 through the aerodynamic wall 26, and the ice mass (pack) is cracked and detached. Depending on the characteristics of the ice mass 24 and in particular its thickness, it is necessary to arrange the vibration generators 30 closer to each other and thus increase their number, which will lead to an increase in the onboard mass (weight) and energy consumption. Summary of the invention

[0008] The object of the present invention is to remedy all or part of the disadvantages of the prior art.

[0009] To this end, the invention relates to a method for de-icing an aerodynamic wall of an aircraft, the aerodynamic wall having an outer wall against which an air flow can flow in a longitudinal direction and an inner wall opposite the outer wall.

[0010] According to the invention, the method uses at least one first de-icing system and at least one second de-icing system different from the first de-icing system, the first de-icing system being configured to be able to be in an activated state in which it causes at least one elastic deformation of the aerodynamic wall, the second de-icing system being configured to be able to be in an activated state in which it causes at least local heating at the outer side of the aerodynamic wall, the de-icing method comprising a step of activating the second de-icing system and a step of activating the first de-icing system.

[0011] When ice masses are present on the aerodynamic wall, activating the second de-icing system allows the ice masses to be weakened by dividing (segmenting or splitting) the ice masses into a plurality of small pieces, which helps to improve the efficiency of the first de-icing system. Since both the first and second de-icing systems operate in an optimized manner, this technical solution helps to reduce the energy consumption of the de-icing device.

[0012] According to another feature, the step of activating the first de-icing system is offset in time relative to the step of activating the second de-icing system by a duration of at least 2 seconds.

[0013] According to another feature, the step of activating the second de-icing system is performed simultaneously with the step of activating the first de-icing system.

[0014] According to another feature, the de-icing method comprises a step of deactivating a second de-icing system simultaneously with a step of activating a first de-icing system.

[0015] The present invention also relates to an aircraft comprising an aerodynamic wall and a deicing device for treating the aerodynamic wall, the aerodynamic wall having an outer surface against which an air flow flows in a longitudinal direction and an inner surface opposite to the outer surface, the deicing device comprising at least one first deicing system configured to cause at least one elastic deformation to the aerodynamic wall.

[0016] According to the invention, the de-icing device comprises at least one second de-icing system, different from the first de-icing system, which is configured to be able to cause (generate) local heating at the outer face of the aerodynamic wall.

[0017] According to another feature, the second de-icing system is discontinuous and offset on the aerodynamic wall relative to the first de-icing system.

[0018] According to another feature, the second de-icing system includes a plurality of elongated heating elements positioned at or near the exterior of the aerodynamic wall.

[0019] According to another feature, the second de-icing system includes a first elongated heating element oriented in at least one first direction substantially perpendicular to the longitudinal direction of the air flow.

[0020] According to another feature, the second de-icing system includes a second elongated heating element oriented in at least one second direction intersecting the first direction, the first and second elongated heating elements intersecting.

[0021] According to another feature, the second direction is substantially perpendicular to the first direction.

[0022] According to another feature, the elongated heating elements each have a given width and are spaced apart from one another by a distance greater than 5 times the given width.

[0023] According to another feature, the first de-icing system includes a plurality of elastic deformation generators, each elastic deformation generator being positioned between at least two elongated heating elements.

[0024] According to another feature, each elastic deformation generator is positioned between two first elongated heating elements oriented in a first direction and between two second elongated heating elements oriented in a second direction perpendicular to the first direction.

[0025] According to another feature, the aircraft includes a leading edge having a ridge line (ridge convex line or "crest line"), the first de-icing system includes a plurality of elastic deformation generators distributed along the ridge line, and the second de-icing system includes at least two first slender heating elements oriented along at least one first direction substantially perpendicular to the longitudinal direction of the air flow and positioned on both sides of the ridge line. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Other features and advantages will become apparent from the following description of the invention, given by way of example and with reference to the accompanying drawings, in which:

[0027] - Figure 1 This is a perspective view of the aircraft.

[0028] - Figure 2is a schematic cross-sectional view of an aircraft wall equipped with a mechanical de-icing system showing an embodiment of the prior art, shown in a deactivated state in part (A) and in an activated state in part (B),

[0029] - Figure 3 is a perspective view showing a portion of the leading edge of an aircraft equipped with a de-icing device according to an embodiment of the present application,

[0030] - Figure 4 is a schematic cross-sectional view of an aircraft wall equipped with a mechanical de-icing system according to an embodiment of the present application, in a deactivated state in part (A) and in an activated state in part (B),

[0031] - Figure 5 is a perspective view showing a propulsion assembly of an aircraft according to an embodiment of the present application,

[0032] - Figure 6 yes Figure 5 A longitudinal cross-section of a portion of the propulsion assembly visible in FIG.

[0033] - Figure 7 1 is a longitudinal cross-sectional view showing a sound-absorbing panel equipped with a deicing device according to an embodiment of the present application. DETAILED DESCRIPTION

[0034] according to Figure 3 , Figure 4 and Figure 7 In the embodiment shown, the aerodynamic wall 32 of the aircraft has an outer surface F32 against which the air flow can flow when the aircraft is in flight and on which ice, in particular ice masses 34 ( Figure 4 ), and an inner face F32' opposite to the outer face F32. The air flow flows along the longitudinal direction DL. The transverse plane is perpendicular to the longitudinal direction. The transverse direction is perpendicular to the longitudinal direction and parallel to the outer face F32.

[0035] according to Figure 3 In the illustrated application, the aerodynamic wall 32 forms at least a portion of a leading edge 36 of a wing of an aircraft.

[0036] according to Figure 5 and Figure 6In the embodiment shown, the propulsion assembly 38 includes a motorized device 40 and a pod 42 positioned around the motorized device (engine) 40. The pod 42 includes an air inlet 44 at the front, which is configured to direct the air flow in the direction of the motorized device 40, and in particular, the fan 40.1 of the motorized device 40. The air inlet 44 includes: a lip 46 forming a rounded leading edge; an outer wall 48 extending the lip 46 toward the outside of the pod 42; and an inner duct 50 extending the lip 46 toward the inside of the pod 42. The inner duct 50 includes a rear edge 50.1, which is connected to a fan housing 52 positioned around the fan 40.1 of the motorized device 40.

[0037] According to one application, the aerodynamic wall 32 forms at least a portion of an air inlet 44 of a nacelle of an aircraft.

[0038] Of course, the invention is not limited to this application.Thus, the aerodynamic wall may be positioned at different locations of the aircraft.

[0039] According to one embodiment, the aerodynamic wall 32 is in the form of a continuous and solid sheet or plate between the outer face F32 and the inner face F32 ′.

[0040] According to another embodiment, the aerodynamic wall 32 is in the form of an acoustic attenuation panel 54, which is positioned at the lip 46 and / or the inner duct 50 of the air inlet 44 of the pod 42 of the aircraft. This acoustic attenuation panel 54 comprises (in order) from the outer face F32 towards the inner face F32': an acoustic resistance structure 56, one surface of which forms the outer face F32; at least one honeycomb structure 58; and a reflective layer 60, one surface of which forms the inner face F32'.

[0041] The reflective layer 60 includes at least one thin plate which is metallic or made of a composite material and through which sound waves cannot penetrate. The honeycomb structure 58 may be metallic or made of a composite material.

[0042] according to Figure 7 In the embodiment shown in FIG. 5 , the acoustic resistance structure 56 comprises at least one porous layer 62 made of a composite material, which is traversed by orifices and is configured according to the acoustic characteristics required by the acoustic attenuation panel 54. In addition, the acoustic resistance structure 56 may also include at least one coil 64 made of a composite material, which is positioned between the honeycomb structure 58 and the porous layer 62. The acoustic resistance structure 56 may also include a lightning protection layer in the form of a wire mesh or a metal mesh.

[0043] According to another embodiment, the acoustic resistance structure 56 comprises at least one porous metal layer traversed by orifices and configured according to the acoustic characteristics required of the acoustic attenuation panel.

[0044] Of course, the invention is not limited to these embodiments of the aerodynamic wall 32 .

[0045] According to one embodiment, the aircraft comprises an aerodynamic wall 32 and a deicing device 66 for treating said aerodynamic wall 32 , comprising a first deicing system 68 for elastic deformation of the aerodynamic wall 32 and a second deicing system 70 configured to act locally on the outer face F32 of the aerodynamic wall 32 in order to locally weaken the ice mass 34 .

[0046] The first de-icing system 68 is constructed to be able to cause elastic deformation to the aerodynamic wall 32 from a local and small point (like) area and includes at least one elastic deformation generator 68.1, which is constructed to be able to generate shocks, waves, vibrations or small displacements and transfer (transmit) them to the aerodynamic wall 32 in a local manner.

[0047] According to an embodiment, the first de-icing system 68 is of mechanical type and comprises at least one vibration generator 68.1 coupled to the aerodynamic wall 32 and configured to convert electrical energy into vibration energy, and at least one power supply device configured to provide electrical energy to the vibration generator 68.1. Each vibration generator 68.1 is a point (shaped) element that covers a small area of ​​the aerodynamic wall 32. As an example, each vibration generator 68.1 is a piezoelectric element.

[0048] According to one configuration, the first de-icing system 68 comprises a plurality of vibration generators 68.1 distributed on the aerodynamic wall 32. According to one arrangement, each vibration generator 68 is positioned against the inner face F32'. Of course, the invention is not limited to this arrangement. In the case of an acoustic attenuation panel 54, one or more vibration generators 68.1 may be integrated into the acoustic attenuation panel 54, in particular into its honeycomb structure 58.

[0049] The first de-icing system 68 is configured to be able to be in a deactivated state in which no elastic deformation is caused and in an activated state in which at least one elastic deformation of the aerodynamic wall 32 is caused. In the case of a mechanical first de-icing system 68, it is configured to be able to be: in a deactivated state in which one or more vibration generators 68.1 do not generate vibrations; and in an activated state in which one or more vibration generators generate vibrations. To this end, the first de-icing system 68 includes at least one first controller, which is configured to be able to control the state switching (switch) between the aforementioned activated state and the deactivated state. Alternatively, the first de-icing system 68 can also be permanently in an activated state.

[0050] According to one embodiment, the second de-icing system 70 is of the thermal type and is configured to generate local heating at the outer face F32 of the aerodynamic wall 32 , which allows the ice mass 34 to be locally weakened by locally reducing its thickness and / or locally limiting its adhesion on the aerodynamic wall 32 .

[0051] According to a first variant, the second de-icing system 70 is of the electrical type and comprises a plurality of strips 70.1 to 70.4 of electrically conductive material (strip-shaped electrically conductive material) positioned at or close to the outer face F32 of the aerodynamic wall 32. Each of these strips 70.1 to 70.4 of electrically conductive material is configured to generate heat when an electric current passes through it (energized).

[0052] According to one configuration, the strips 70.1 to 70.4 of electrically conductive material are of the resistive type. Furthermore, the de-icing system 70 of the electrical type also comprises at least one power source directly connected to the strips 70.1 to 70.4 of electrically conductive material.

[0053] According to another configuration, the strips of conductive material 70.1 to 70.4 are of the inductive type. Furthermore, the de-icing system 70 of the electrical type also comprises at least one electromagnetic field generator 72 (for example a coil), which is positioned at the inner face F32' and is powered by at least one power source, the strips of conductive material 70.1 to 70.4 being positioned in the electromagnetic field generated by the electromagnetic field generator 72 and being configured to be able to generate heat by the phenomenon of (electromagnetic) induction.

[0054] According to a second variant, the second de-icing system 70 is of the fluid type and comprises a plurality of ducts configured to be able to convey a hot fluid, for example hot air coming from a maneuvering device of the aircraft, positioned at the aerodynamic wall 32 .

[0055] Whatever the variant, the second de-icing system 70 comprises elongated heating elements 70 . 1 to 70 . 4 , ie strips of electrically conductive material or ducts of a hot fluid, positioned at or close to the outer face F32 of the aerodynamic wall 32 .

[0056] The second de-icing system 70 is configured to be able to be in a deactivated state, in which it does not generate any heating, and in an activated state, in which it generates at least local heating. In the case where the second de-icing system 70 is of electrical type, it is configured to be able to be in a non-activated state (deactivated) in which the strips of electrically conductive material 70.1 to 70.4 do not cause heating, and in an activated state in which the strips of electrically conductive material 70.1 to 70.4 cause heating. To this end, the de-icing system 70 of the electrical type comprises at least one second controller, which is configured to be able to manipulate the state change between the activated state and the deactivated state. As a variant, the second de-icing system 70 can be permanently in the activated state.

[0057] According to an embodiment, the elongated heating elements 70.1, 70.2 are oriented in at least one first direction substantially parallel to the transverse direction (perpendicular to the longitudinal direction of the air flow).

[0058] According to a first configuration, the leading edge 36 equipped with the de-icing device 66 comprises a ridge line C, which corresponds to a line located most forwardly of the aerodynamic surface 32 (the concept of forward / rearward refers to the flow direction of the air flow flowing from the front to the rear). In this case, the first de-icing system 68 comprises a plurality of elastic deformation generators 68.1 distributed on the ridge line C and at least two elongated heating elements 70.1, 70.2 located along the first direction and on both sides of the ridge line C. Therefore, each first de-icing system 68 is located between two elongated heating elements 70.1, 70.2 oriented along the first direction.

[0059] according to Figure 3 In the second arrangement, which can be seen in FIG. 1 , the second de-icing system 70 comprises first elongated heating elements 70.1, 70.2 oriented in a first direction and second elongated heating elements 70.3, 70.4 oriented in at least one second direction intersecting the first direction, in particular perpendicular to the first direction, the first and second elongated heating elements 70.1 to 70.4 intersecting. According to this second arrangement, each elastic deformation generator 68.1 of the first de-icing system 68 is positioned between two first elongated heating elements 70.1, 70.2 oriented in the first direction and between two second elongated heating elements 70.3, 70.4.

[0060] According to an embodiment, the elongated heating elements 70.1 to 70.4 each have a given width, approximately 1 to 5 cm, and are spaced apart from each other by a distance greater than 5 times the given width.

[0061] Of course, the invention is not limited to these embodiments of the first de-icing system 68 and the second de-icing system 70. Whatever the embodiment, the second de-icing system 70 is configured to be able to split the ice mass 34 to divide it into small pieces of ice, and the first de-icing system 68 is configured to be able to peel these small pieces of ice from the aerodynamic wall 32. Therefore, the second de-icing system 70 allows to optimize the operation of the first de-icing system 68. According to one configuration, each second de-icing system 70 is discontinuous (and does not cover the entire surface to be treated) and is offset on the aerodynamic wall 32 relative to the first de-icing system 68. According to one configuration, the first de-icing system 68 comprises a plurality of elastic deformation generators 68.1 distributed along at least one line, which is positioned in a plane substantially perpendicular to the air flow flowing on the outer face F32 of the aerodynamic wall 32. In the case of (for) a leading edge, this above-mentioned installation (or arrangement) line of the elastic deformation generators 68.1 corresponds to the ridge line (C). According to one configuration, each elastic deformation generator 68.1 is positioned between at least two elongated heating elements 70.1 to 70.4, being substantially equidistant from said elongated heating elements 70.1 to 70.4.

[0062] The invention also relates to a method for deicing an aerodynamic wall 32 using at least one first deicing system 68 and at least one second deicing system 70, the first deicing system being configured to be able to assume an activated state, in which it causes at least one elastic deformation of the aerodynamic wall 32, and a deactivated state, in which it does not cause any elastic deformation of the aerodynamic wall 32, and a second deicing system being configured, different from the first deicing system 68, to be able to assume an activated state, in which it causes at least local heating at the outer face F32 of the aerodynamic wall, and in which it does not cause any heating. The deicing method comprises an activation step of activating the second deicing system 70 to switch the second deicing system from the deactivated state to the activated state, and an activation step of activating the first deicing system 68 to switch the first deicing system 68 from the deactivated state to the activated state.

[0063] Activating the second de-icing system 70 allows the ice mass 34 to be weakened by breaking it up into smaller pieces, which helps to increase the efficiency of the first defrosting system 68. This solution helps to reduce the energy consumption of the de-icing device, since each of the first and second de-icing systems operates in an optimized manner.

[0064] According to one mode of operation, the step of activating the first de-icing system 68 is offset in time relative to the step of activating the second de-icing system 70 by at least 2 seconds.

[0065] The first de-icing system 68 is activated for a very short duration (duration, less than 2 seconds or even on the order of a fraction of a second).

[0066] The second de-icing system 70 has a longer operating mode (more than 2 seconds). Therefore, the de-icing method comprises a step of activating the first de-icing system 68 while deactivating the second de-icing system 70.

[0067] According to another operating mode, the step of activating the second de-icing system 70 is performed simultaneously with the step of activating the first de-icing system 68 .

[0068] In operation, the second de-icing system 70 is switched to an active state to cause heating at each of the elongated heating elements 70.1 to 70.4, such as Figure 4 Since the elongated heating elements 70.1 to 70.4 do not cover the entire surface to be treated but only at most 20% of the surface in a discontinuous manner, the energy consumption of the second deicing system 70 is much lower than the resistance mats used in the prior art that cover the entire surface to be treated in a continuous manner.

[0069] After a short time, the ice mass 34 is weakened at the alignment with the elongated heating elements 70.1 to 70.4 and includes a reduced thickness area 74 at the alignment of these heating elements. After activating the second de-icing system 70, the ice mass 34 no longer forms a single compact block (ice). The ice mass 34 is divided into a plurality of small ice cubes that are easier to peel off. Then, the first de-icing system 68 is switched to an activated state. Unlike the prior art, since the ice mass 34 has been weakened at the alignment with the elongated heating elements 70.1 to 70.4, the action of the first de-icing system 68 required to peel off the ice mass 34 will consume less energy than the prior art. According to another advantage, when the elongated heating elements 70.1, 70.2 are oriented perpendicular to the air flow, the reduced thickness area 74 can provide wind (action) force to the air flow, which will promote the peeling of ice cubes.

Claims

1. A method for deicing an aerodynamic wall (32) of an aircraft, the aerodynamic wall (32) having an outer face (F32) and an inner face (F32') opposite the outer face (F32), an air flow being able to flow along the longitudinal direction against the outer face (F32), characterized in that: The method uses at least one first de-icing system (68) and at least one second de-icing system (70), the at least one first de-icing system (68) being configured to be capable of being in an activation state in which the first de-icing system (68) causes at least one elastic deformation of the aerodynamic wall (32), and the at least one second de-icing system being different from the first de-icing system (68) and being configured to be capable of being in an activation state in which the second de-icing system (70) causes at least local heating at the outer surface (F32) of the aerodynamic wall (32), the de-icing method comprising the steps of activating the second de-icing system (70) and activating the first de-icing system (68).

2. The deicing method according to claim 1, characterized in that: The step of activating the first de-icing system (68) is offset in time relative to the step of activating the second de-icing system (70) by a duration of at least 2 seconds.

3. The deicing method according to claim 1, characterized in that: The step of activating the second de-icing system (70) is performed simultaneously with the step of activating the first de-icing system (68).

4. A de-icing method according to any one of the preceding claims, characterised in that The de-icing method includes the step of activating the first de-icing system (68) while simultaneously deactivating the second de-icing system (70).

5. An aircraft, comprising an aerodynamic wall (32) and a deicing device (66) for treating the aerodynamic wall (32), the aerodynamic wall having an outer face (F32) against which an air flow can flow in a longitudinal direction and an inner face (F32') opposite the outer face (F32), the deicing device (66) comprising at least one first deicing system (68), the first deicing system (68) being configured to cause at least one elastic deformation of the aerodynamic wall (32), characterized in that The de-icing device (66) further comprises at least one second de-icing system (70) which is different from the first de-icing system (68) and is configured to cause at least local heating at the outer face (F32) of the aerodynamic wall (32).

6. The aircraft according to claim 5, characterized in that The second de-icing system (70) is discontinuous and offset on the aerodynamic wall (32) relative to the first de-icing system (68).

7. The aircraft according to claim 6, characterized in that The second de-icing system (70) includes a plurality of elongated heating elements (70.1 to 70.4) positioned at or near the outer face (F32) of the aerodynamic wall (32).

8. The aircraft according to claim 7, characterized in that The second de-icing system (70) comprises a first elongated heating element (70.1, 70.2) oriented in at least one first direction substantially perpendicular to the longitudinal direction of the air flow.

9. The aircraft according to claim 8, characterized in that The second de-icing system (70) comprises a second elongated heating element (70.3, 70.4) oriented in at least one second direction intersecting the first direction, the first elongated heating element (70.1, 70.2) and the second elongated heating element (70.3, 70.4) intersecting.

10. The aircraft according to claim 9, characterized in that The second direction is substantially perpendicular to the first direction.

11. An aircraft according to any one of claims 7 to 10, characterized in that Each elongated heating element (70.1 to 70.4) has a respective given width and is spaced apart from one another by a distance greater than 5 times the given width.

12. An aircraft according to any one of claims 7 to 11, characterized in that The first de-icing system (68) comprises a plurality of elastic deformation generators (68.1), each elastic deformation generator (68.1) being positioned between at least two elongated heating elements (70.1 to 70.4).

13. The aircraft according to claim 12, characterized in that Each elastic deformation generator (68.1) is positioned between two first elongated heating elements (70.1, 70.2) oriented in a first direction and between two second elongated heating elements (70.3, 70.4) oriented in a second direction intersecting the first direction.

14. An aircraft according to any one of claims 7 to 13, characterised in that The aircraft comprises at least one leading edge having a ridge line (C), wherein the first de-icing system (68) comprises a plurality of elastic deformation generators (68.1) distributed along the ridge line (C), and the second de-icing system (70) comprises at least two first elongated heating elements (70.1, 70.2) oriented in at least a first direction substantially perpendicular to the longitudinal direction of the air flow and positioned on both sides of the ridge line (C).

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