Aerodynamic surface structure for an outer skin area, in particular of an aircraft, corresponding method and use

Abstract

The invention relates to an aerodynamic surface structure for an outer skin area (4) affected by a turbulent airflow, in particular for an outer skin area (4) of an aircraft (100), the aerodynamic surface structure (2) comprising a micro-structured layer (6) configured to reduce a frictional resistance of the outer skin area (4) to which the aerodynamic surface structure (2) is attached in a turbulent airflow, wherein the micro-structured layer (6) comprises a forward edge (8) facing towards the direction of the airflow (dF). According to the invention a leading edge portion (10) arranged adjacent to the forward edge (8) is proposed, wherein the leading edge portion (10) comprises a flow guiding surface (12) facing towards the direction of the airflow (dF), wherein the flow guiding surface (12) is configured to guide the airflow above the forward edge (8) and towards the micro-structured layer (6). The micro-structured layer (6) may comprise a riblet structure (14).

Description

[0001] Aerodynamic surface structure for an outer skin area, in particular of an aircraft, corresponding method and use

[0002] The invention relates to an aerodynamic surface structure for an outer skin area affected by a turbulent air flow, in particular for an outer skin area of an aircraft, the aerodynamic surface structure comprising a micro-structured layer configured to reduce a frictional resistance of the outer skin area to which the aerodynamic surface structure is attached in a turbulent air flow, wherein the micro-structured layer comprises a forward edge facing towards the direction of the airflow.

[0003] Aerodynamic surface structures can be used in different embodiments and fields of application. By utilizing micro-structured layers, wall sheer stresses of an outer skin area around which an air flow passes can be reduced. Said micro-structured layers are inspired for example by the skin of sharks and particularly reduce frictional resistance of turbulent air flows.

[0004] When applied to an outer skin area, such as an outer skin area of an aircraft, microstructured layers often comprise a forward edge facing towards the direction of the air flow. This forward edge generates aerodynamic pressure drag and negatively influences the airflow around the particular outer skin area. In particular, the airflow is deflected by the edge. This creates detachment bubbles in front of and behind the edge. Accompanied by the deceleration of the flow, the pressure in front of the edge increases, which leads to the increased aerodynamic pressure drag.

[0005] In the prior art, attempts have been made to improve the airflow around such forward edges as described for example in EP 3 323 514 B1 . The document proposes an aircraft structure comprising an edge line having a ramp-shaped cross section. However, this solution is optimized for laminar flows and is therefore unsuitable for micro-structured layers that are configured to reduce a frictional resistance of aerodynamic surface structures affected by non-laminar, turbulent airflow.

[0006] Furthermore, in WO 2018 / 184635 A1 a self-adhesive plastic film is proposed at which at least some sections of the border of the plastic film are in the form of an edge that is waveshaped in the plane of the film. This wave-shaped edge is utilized to avoid erosion of the micro-structured layer. It is not the aim of this structure to improve the aerodynamic flow around a forward edge of a micro-structured layer.

[0007] WO 2014 / 179405 A1 provides methods and systems for applying aerodynamically functional coatings to a surface. The proposed method, however, is not particularly suitable for micro-structured layers that have to remain uncoated in order to ensure that the intended aerodynamic functions can be provided.

[0008] In the prior art, it is also known to protect technical markings or advertising films on an aircraft by utilizing a so called edge sealer, which only protects the edges of such technical markings or advertising films, or by applying clear lacquer on the entire surface of the markings or films. However, it has been found that edge sealer does not provide the required durability to be utilized together with micro-structured layers. Further, clear lacquer cannot be used to entirely coat a micro-structured layer, because the beneficial aerodynamic effects of such layer would be inhibited.

[0009] As such micro-structured layer structures typically have a much longer lifetime than advertising films or have a much greater surface area than technical markings, such that the known solutions cannot be utilized.

[0010] In view of the above, there remains a need for an effective means to reduce the aerodynamic pressure drag associated with the forward edge of the micro-structured layer. Thus, it was an object of the invention to decrease the pressure drag associated with the forward edge of the micro-structured layers and to improve the aerodynamic properties of the aerodynamic surface structure, when subjected to a turbulent airflow.

[0011] The invention attains the aforementioned object by suggesting a leading edge portion arranged adjacent of the forward edge, wherein the leading edge portion comprises a flow guiding surface facing towards the direction of the airflow, wherein the flow guiding surface is configured to guide the airflow above the forward edge and towards the micro-structured layer.

[0012] By utilizing a leading edge portion comprising a flow guiding surface that guides the airflow above the forward edge and towards the micro-structured layer, the aerodynamic properties of the aerodynamic surface structure can be improved and thus the pressure drag associated with the forward edge can be reduced. Furthermore, it has been found that by applying a leading edge portion having a flow guiding surface not only the aerodynamic properties can be improved but also the durability of the micro-structured layer. Furthermore, it has been found that the flow guiding surfaces is also useful in providing UV-protection increasing the lifespan of the micro-structured layer and thus the aerodynamic surface structure as such.

[0013] Moreover, the leading edge portion having the flow guiding surface has also been found to be beneficial in terms of erosion protection. On the one hand, ballistic particles are prevented from impacting the forward edge of the micro-structured layer but are captured by the leading edge portion. In addition, by improving the aerodynamic properties, also particles that are contained in the airflow, such as dust or ice, can - at least partially - be deflected together with the turbulent airflow around the leading edge portion so that they do not impact with either the forward edge or the leading edge. As a further advantage, it has been found that the leading edge portion comprising the flow guiding surface also provides a chemical protection to the micro-structured layer, for example with regard to deicing liquids and hydraulic fluids that might otherwise directly affect the micro-structured layer or an area between the outer skin area of an aircraft and the micro-structured layer.

[0014] Compared to solutions that are based on an overall coating of an aerodynamic surface structure, the advantage of the proposed solution is that the micro-structured layer itself is not covered or only partially covered. Thus, the beneficial aerodynamic properties of the micro-structured layer are preserved by the proposed solution. In this description of the invention, the term aerodynamic surface structure is directed towards a structure that can be applied to an outer skin area affected by a turbulent airflow. For example, the outer skin area may be an outer skin area of an aircraft. The aerodynamic surface structure may have a variable size and may comprise only a few square centimeters or 1 square meter or more. The aerodynamic surface structure may be a layer on the outer skin, in particular, of an aircraft.

[0015] The term “micro-structured layer” relates to such a layer that comprises a micro structure having a specific aerodynamic effect, namely such a micro-structured layer that is configured to reduce a frictional resistance of an aerodynamic surface structure in a turbulent airflow. In this regard, the micro-structured layer may comprise ribs or other surface elements.

[0016] The term “forward edge” is directed towards an edge which faces towards the direction of the airflow. In terms of an aircraft, the forward edge will normally face to the front of the aircraft.

[0017] The leading edge portion is arranged adjacent to the forward edge. This means that the leading edge portion either directly contacts the forward edge or there may also be a certain gap between the leading edge portion and the forward edge. The leading edge portion interacts with the airflow as the first part of the aerodynamic surface structure. In other words, the leading edge portion faces in the direction of the airflow. Preferably, the optional gap between the leading edge portion and the forward edge has a width of 10 mm or smaller. In one embodiment, the gap has a width larger than 10 mm.

[0018] In one embodiment there might be a gap between the forward edge of the micro-structured layer and the flow guiding surface. It has been found that although such gap is optionally present, the beneficial aerodynamic properties of the aerodynamic surface structure can be reached at least partially.

[0019] In one embodiment, the micro-structured layer comprises or consist of a foil layer. By providing the micro-structured layer in the form of a foil layer, the aerodynamic surface structure may conveniently be applied to an outer skin area of an aircraft or on other outer skin areas affected by a turbulent airflow. Furthermore, it is possible to provide such aerodynamic surface structure as a retrofit solution to an aircraft and other vehicles. In one embodiment, the foil layer comprises or consists of hot embossed foils and extruded foils.

[0020] According to one embodiment, the foil layer may comprise a functional lacquer layer, and wherein the functional lacquer layer comprises a micro-structured surface. This solution has been found to be conveniently to manufacture and on the other hand provides a limited weight per surface area so that such a foil may also be used for the outer skin of aircraft. The functional lacquer layer may be a UV-crosslinkable lacquer layer.

[0021] Preferably, the foil layer comprises a backing layer, and wherein the backing layer is attached to the functional lacquer layer. In one embodiment the backing layer is made of a thermoplastic material. The thermoplastic material may be selected from the group consisting of polymethyl (meth)acrylates, polybutyl (meth)acrylates, polyethylene terephthalates (PET), polybutylene terephthalates (PBT), polyvinylidene fluorides, polyvinyl chlorides, polyesters, including polycarbonates and polyvinyl acetate, preferably polyesters such as PBT and PET, polyamides, polyolefins such as polyethylene, polypropylene, polystyrene, and also polybutadiene, polyacrylonitrile, polyacetal, polyacrylonitrileethylene- propylene-diene-styrene copolymers (A-EPDM), polyimide (PI), polyetherimides (PEI), cellulose triacetate (TAC), phenolic resins, urea resins, melamine resins, alkyd resins, epoxy resins, polyurethanes, including thermoplastic polyurethane (TPU), polyether ketones, polyphenylene sulfides, polyethers, polyvinyl alcohols, and mixtures thereof. Particularly preferred substrates or layers on the surface thereof are polyolefins such as, for example, PP (polypropylene), which may alternatively be isotactic, syndiotactic or atactic and may alternatively be unoriented or oriented through mono- or biaxial drawing, SAN (styrene-acrylonitrile copolymers), PC (polycarbonates), PMMA (polymethyl methacrylates), PBT (poly(butylene terephthalate)s), PA (polyamides), ASA (acrylonitrile- styrene-acrylic ester copolymers) and ABS (acrylonitrile-butadiene-styrene copolymers), and also their physical mixtures (blends). Particularly preferred are PP, SAN, ABS, ASA and also blends of ABS or ASA with PA or PBT or PC. Especially preferred are PET, PBT, PP, PE, and polymethyl methacrylate (PMMA) or impact-modified PMMA.

[0022] According to one embodiment, the micro-structured surface comprises a riblet structure having ribs oriented along the direction of the airflow. Such a riblet structure is inspired by the skin of sharks and consists of ribs oriented along the direction of the airflow. This structure reduces the frictional resistance of turbulent flow by typically up to 10 %. This reduction is mainly achieved by separating the vortices, which are characteristic for turbulent flows, from the surface. This separation prevents crossflow, which generates losses, and reduces the interaction between wall and flow. To enable the riblets functionality, the flow has to be attached to the surface, whereas separated flow, prevents the functionality of the riblets. Therefore, the aforementioned separation bubble behind the step not only generates pressure losses, it also reduces the functionality of the present micro-structured layer.

[0023] In one embodiment, the ribs extend all over the micro-structured layer. In an alternative embodiment, the ribs extend only over parts of the micro-structured layer. In particular, areas comprising ribs may alternate with areas not comprising ribs. Since it is known that riblet structures increase the frictional resistance for laminar flows, preferably such areas are not equipped with a riblet structure.

[0024] In one embodiment, an adhesive layer is arranged between the functional lacquer layer and the backing layer. Preferably, the backing layer is attached to the outer skin area by means of an adhesive. According to an alternative embodiment, the backing layer may comprise the micro-structured layer. In one embodiment, the foil layer comprises an adhesive layer configured to attach the foil layer to the outer skin area. Preferably, the adhesive layer is attached to the backing layer.

[0025] According to one embodiment, the foil layer comprises a height from a lower side of the adhesive layer to an upper side of the functional lacquer layer and wherein the height is 40 - 500 micrometers, preferably 45-400 micrometers, more preferably 50-300 micrometers. In one embodiment, the height may be 55 - 250 micrometers or 60 - 200 micrometers. The mentioned range has been found to be beneficial to provide the required surface properties of the micro-structured layer, a durable aerodynamic surface structure and a beneficial weight per area, so that the aerodynamic surface structure may be utilized for example for an aircraft outer skin area.

[0026] According to one embodiment, the leading edge portion comprises a coating and wherein said coating forms the flow guiding surface. It has been surprisingly found that by utilizing a coating an aerodynamically effective flow guiding surface can be generated. This is beneficial, since such a coating can be easily applied to the micro-structured layer. Furthermore, it has been found that the coating is also effective in protecting the forward edge of the micro-structured layer from erosion and for deflecting particles together with the turbulent airflow around the forward edge. In addition, the coating also provides protection with regard to UV-radiation and chemical influences from deicing fluids or hydraulic fluids. Preferably, the coating partially covers the micro-structured layer. In this way, an improved adhesion can be achieved between the coating and the micro-structured layer, since the coating flows into the valleys of said micro-structured layer. On the other hand, the percentage of the micro-structured layer that is covered by the coating is very limited so that the aerodynamic properties of the micro-structured layer are not significantly influenced.

[0027] In one embodiment, the coating has a minimum width which corresponds to the height of the forward edge facing towards the direction of the airflow. In one embodiment, the coating has a maximum width of 250mm or 200mm or 150mm or 100mm in the direction of the airflow. In this way, a durable adhesion is achieved between the coating and the microstructured layer while on the other hand, the influence of the coating or on the aerodynamic properties of the micro-structured layer is very limited.

[0028] Preferably, the coating is applied by at least one of the following: rolling, especially with an application roller, spray coating, paint stick application, application with a cartridge.

[0029] According to an embodiment the coating comprises or consist of a coating composition selected from radiation-curable coating compositions, physically curing coating compositions, chemically curing coating compositions, melts and mixtures thereof.

[0030] The coating composition is preferably a liquid or a highly viscous composition. This allows to effectively fill the recessions and protrusions of the structured surface, thus reducing undesired inclusion of air.

[0031] If the coating composition is a radiation-curable coating composition, a physically curing coating composition or a chemically curing coating composition, it preferably has a viscosity at 23 °C of 1 to 500,000 mPa*s, more preferably 25 to 100,000 mPa*s, even more preferably 50 to 50,000 mPa*s, particularly 80 to 9,000 mPa*s, particularly preferably 90 to 3,000 mPa*s, especially 100 to 2,000 mPa*s, as determined according to DIN 53019-1 :2008-09, DIN 53019-2:2001-02, DIN 53019-3:2008-09 and DIN 53019-4:2016-10.

[0032] If the coating composition is a melt, for example a TPU melt, it preferably has a glass transition temperature of 20 to -60°C, more preferably of 0 to -45°C, determined by dynamic scanning calorimetry (DSC) according to DIN EN ISO 11357-2:2014-07 with a heating rate of 10 K / min. In one embodiment, the coating composition comprises an aircraft lacquer. Preferably, an aircraft lacquer is utilized that has already been certified for the respective aircraft type or series. By utilizing a lacquer that has already been certified for an aircraft, complex recertification can be avoided. Furthermore, it is ensured that the lacquers are chemically compatible with each other.

[0033] According to one embodiment, the leading edge portion further comprises a film, in particular wherein the film is at least partially attached to the outer skin area and wherein the coating partially covers the film and the micro-structured layer. The film can be used to provide the flow guiding surface of the leading edge portion. Alternatively, the film and the coating together may form the flow guiding surface. It has been found that the application of the film has beneficial aerodynamic properties by guiding the airflow above the forward edge and towards the micro-structured layer by way of establishing a flow guiding surface.

[0034] According to one embodiment, the film is a first film and wherein the flow guiding surface comprises a second film that is arranged on the first film, and wherein the first film and the second film are arranged in a staggered manner towards the direction of the airflow. By providing the films in a staggered manner, a flow guiding surface is established that guides the airflow above the forward edge and towards the micro-structured layer.

[0035] Preferably, the first film and the second film overlap with the micro-structured layer. In this way, it can also be achieved that the film is durably attached to the micro-structured layer. In one embodiment, an adhesion promoter is applied to an overlapping region at which the first film or the second film overlaps with the micro-structured layer. In one embodiment, the adhesion promoter penetrates the valleys between the ribs of the micro-structured layer, such as the riblet surface, in order to improve the adhesion with the micro-structured layer.

[0036] The flow guiding surface and the micro-structured layer each comprise a corresponding height perpendicular to the outer skin area. In particular, the height is measured from the outer skin area and may involve a backing or an adhesion layer. In one embodiment, the height of the micro-structured layer exceeds the height of the flow guiding surface, in particular, by 500 micrometers, preferably 200 micrometers, more preferably 150 micrometers, even more preferably 120 micrometers or 100 micrometers or 75 micrometers or less. According to an alternative embodiment, the height of the flow guiding surface exceeds the height of the micro-structured layer, in particular by 300 micrometers, preferably 200 micrometers, more preferably 100 or 75 micrometers or less.

[0037] According to one embodiment, the height of the flow guiding surface may be substantially equal to the height of the micro-structured layer. In one embodiment, the height of the flow guiding surface may be smaller than the height of the micro-structured layer but exceed the height of a riblet lacquer base layer, in particular by 10 micrometers or by 20 micrometers or by 30 micrometers or by 40 micrometers or by 60 micrometers. Alternatively, the micro-structured layer may exceed the height of a riblet lacquer base layer by 10 % of the total height of the riblet or by 25 % of the total height of the riblet or by 50 % of the total height of the riblet or by 75 % of the total height of the riblet or by 90 % of the total height of the riblet.

[0038] It has been surprisingly found that also in case the height of the flow guiding surface differs from the height from the micro-structured layer in the mentioned range, beneficial aerodynamic properties can be achieved. In other words, the turbulent airflow is capable of overcoming these differences without negatively affecting the flow properties in a significant manner.

[0039] Until now, the invention has been described with respect to an aerodynamic surface structure. In another aspect, the invention relates to an aircraft comprising an outer skin area and an aerodynamic surface structure attached to the outer skin area. The aircraft attains the object of the invention in that the aerodynamic surface structure is configured according to any one of the preceding embodiments. The aircraft takes advantage of the same benefits and preferred embodiments as the aerodynamic surface structure and vice versa. In this regard and in order to avoid unnecessary repetitions, reference is made to the above explanations.

[0040] In one embodiment, the aerodynamic surface structure is attached to an outer skin area of the aircraft affected by turbulent airflow. In particular, the outer skin area may comprise a surface that widens perpendicular to the direction of airflow. Beside the aerodynamic challenges associated with such outer skin areas, it has been found that surfaces that widens perpendicular to the direction of the airflow are affected by erosion, in particular high erosion. By utilizing an aerodynamic surface structure according to the invention, not only the aerodynamic properties can be improved, but also the effects of erosion on the forward edge of a micro-structures surface can be reduced. Preferably, the aerodynamic surface structure is attached to an underside of the fuselage and / or a forward fuselage and / or wing and / or stabilizers and / or nacelle of an aircraft. For example, the aerodynamic surface structure may be attached to the center wing fairing, also called belly fairing. Preferably, the aerodynamic surface structure is attached to an outer skin area affected by high wall shear stresses. It has been found that even though the aerodynamic surface structure is affected by erosion, a high durability of the microstructured layer can be achieved by utilizing the aerodynamic surface structure according to the invention.

[0041] In a further aspect, the invention relates to a method for applying an aerodynamic surface structure according to one of the preceding embodiments to an outer skin area. The method comprises the steps: Attaching a micro-structured layer configured to reduce a frictional resistance of the outer skin area to which the aerodynamic surface structure is attached in a turbulent airflow to an outer skin area of an aircraft, wherein the micro-structured layer comprises a forward edge facing towards the direction of the airflow, attaching a leading edge portion adjacent to the forward edge to the outer skin area, wherein the leading edge portion comprises a flow guiding surface facing towards the direction of the airflow, wherein the flow guiding surface is configured to guide the airflow above the forward edge and towards the micro-structured layer.

[0042] According to one embodiment, the leading edge portion comprises a coating and wherein said coating forms the flow guiding surface. Preferably, the coating is applied by at least one of the following: rolling, especially with an application roller, spray coating, paint stick application, application with a cartridge.

[0043] The method takes advantage of the same benefits and preferred embodiments as the aerodynamic surface structure and the aircraft according to the invention and vice versa. In this regard and in orderto avoid unnecessary repetitions, reference is made to the above explanations.

[0044] In another aspect, the invention relates to a use of an aerodynamic surface structure according to any one of the preceding embodiments for at least one of the following: aircraft, turbines, such as gas, water or wind turbines, in particular the rotor of said turbines, land vehicle, in particular car, truck or train, marine vehicle, in particular ship or vessel, pipeline, such as water, gas or oil pipeline. The aerodynamic surface structure according to the invention has been found to be beneficial for the mentioned uses. They have in common that they comprise an outer skin area that is affected by a turbulent airflow. Thus, the aerodynamic surface structure according to the invention can be beneficially used to reduce a frictional resistance of the aerodynamic surface structure in a turbulent airflow while on the other hand also the aerodynamic flow conditions around the forward edge of the micro-structured layer are improved.

[0045] The use of the aerodynamic surface structure takes advantage of the same benefits and preferred embodiments as the aerodynamic surface structure and vice versa. In this regard an in order to avoid unnecessary repetitions, reference is made to the above explanations.

[0046] For a more complete understanding of the invention, the invention will now be described in detail with reference to the accompanying drawings. The detailed description will illustrate and describe what is considered as a preferred embodiment of the invention. It should of course be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention may not be limited to the exact form and detail shown and described herein, nor to anything less than the whole of the invention disclosed herein and as claimed hereinafter. Further, the features described in the description, the drawings and the claims disclosing the invention may be relevant for the invention considered alone or in combination. In particular, any reference signs in the claims shall not be construed as limiting the scope of the invention. The wording “comprising” does not exclude other elements or steps. The wording “a” or “an” does not exclude a plurality.

[0047] This invention will now be described with reference to the accompanying drawings, which illustrate, by way of example and not by way of limitation, one of several possible embodiments of the device as proposed herein, and wherein:

[0048] Fig. 1 a-c show embodiments of aerodynamic surface structures according to the invention in a sectional views;

[0049] Fig. 2 shows an alternative embodiments of an aerodynamic surface structure according to the invention in a sectional view;

[0050] Fig. 3-6 show alternative embodiments of an aerodynamic surface structure according to the invention in sectional views; Fig. 7a shows an embodiment of a micro-structured layer comprising a riblet structure in a perspective view;

[0051] 7b-d show embodiments of micro-structured layers comprising riblet structures and flow guiding surfaces in perspective views;

[0052] Fig. 8 shows an embodiment of an aircraft comprising an aerodynamic surface structure according to the invention in a side view;

[0053] Fig. 9 shows streamlines around a step-formed forward edge obtained from a simulation;

[0054] Fig. 10 shows geometries of simulated leading edge portions arranged adjacent to the forward edge of an aerodynamic surface structure;

[0055] Fig. 11-17 show simulation results regarding the pressure drag for different leading edge geometries and heights;

[0056] Fig. 18 shows an aerodynamic surface structure comprising a micro-structured layer without a leading edge portion;

[0057] Fig. 19, 20 show an aerodynamic surface structure according to the invention comprising a leading edge portion having a flow guiding surface.

[0058] Fig. 1 a-c each show an embodiment of an aerodynamic surface structure 2. The aerodynamic surface structure 2 is attached to an outer skin area 4 affected by a turbulent airflow. The outer skin area 4 may be the outer skin of an aircraft 100 as shown in Fig. 8. The aerodynamic surface structure 2 comprises a micro-structured layer 6 configured to reduce a frictional resistance of the aerodynamic surface structure 2 in a turbulent airflow. The micro-structured layer 6 comprises a forward edge 8 which faces towards a direction of the airflow dp.

[0059] The aerodynamic surface structure 2 furthermore comprises a leading edge portion 10. The leading edge portion 10 is arranged adjacent to the forward edge 8. The leading edge portion 10 comprises a flow guiding surface 12. The flow guiding surface 12 faces towards the direction of the airflow dp. The flow guiding surface 12 is configured to guide the airflow above the forward edge 8 and towards the micro-structured layer 6. The micro-structured layer 6 comprises a micro-structured surface 14 in the form of a riblet structure 14 having ribs 16 that are oriented along the direction of the airflow dp as also shown in Fig. 7.

[0060] The ribs 16 may extend all over the micro-structured layer 6 as shown in Fig. 1 a-c. Alternatively, the ribs may only extend over parts of the micro-structured layer 6.

[0061] The micro-structured layer 6 comprises or consists of a foil layer 18. The foil layer 18 comprise a functional lacquer layer 38 and wherein the functional lacquer layer 38 comprises the micro-structured surface 14. The micro-structured surface 14 comprises the riblet structure 14 having ribs 16 oriented along the direction of the airflow dp.

[0062] Furthermore, the foil layer 18 comprises a backing layer 30. The backing layer 30 is attached to the functional lacquer layer 38. An adhesive layer 34 is attached to the backing layer 30, and wherein the foil layer 18 comprises a height H from a lower side of the adhesive layer 34 to an upper side of the functional lacquer layer 38.

[0063] As shown in Figs. 1 a-c, the leading edge portion 10 comprises a coating 20. The coating 20 forms the flow guiding surface 12. The coating 20 partially covers the micro-structured layer 6. In this regard, the coating has a maximum width w in the direction of the airflow dp. The coating 20 may be employed by rolling, especially with an application roller, spray coating, paint stick application or maybe applied with a cartridge. In the embodiment of Fig. 1 , the flow guiding surface 12 is solely provided by the coating 20.

[0064] The flow guiding surface 12 and thus the coating 20 comprises a corresponding height Hf. In the embodiment of FIG. 1 a, the height Hf exceeds the height H of the foil layer 18. In the embodiment of Fig. 1 b, the height Htof the flow guiding surface 12 is equal or substantially equal to the height H of the foil layer 18. In the embodiment of Fig. 1 c, the height Htof the flow guiding surface 12 is smaller than the height H of the foil layer 18 but exceeds the height of the adhesive layer 34, the backing layer 30 and a riblet lacquer base layer 32.

[0065] In the embodiment of Fig. 2, the leading edge portion 10 further comprises a film 22. The film 22 provides the flow guiding surface 12. The film is at least partially attached to the outer skin area 4. In the embodiment of Fig. 2, the film 22 is attached adjacent to the forward edge 8. Apart from these differences, the aerodynamic surface structure 2 shown in Fig. 2 is similar to the one shown in Fig. 1 a-c. In the embodiment of Fig. 3, the leading edge portion 10 comprises a film 22. The film 22 provides, together with the coating 20, the flow guiding surface 12. The film is at least partially attached to the outer skin area 4. In the embodiment of Fig. 3, the film 22 is attached adjacent to the forward edge 8. The transition area between the film 22 and the forward edge 8 is covered by the coating 20. Apart from these differences, the aerodynamic surface structure 2 shown in Fig. 3 is similar to the one shown in Fig. 1 a-c. The film 22 may comprise a coating (not shown) on its forward edge.

[0066] Fig. 4 shows an alternative embodiment of the aerodynamic surface structure 2. In the embodiment of Fig. 4, no coating is provided. Rather, the film 22 is a first film 22, and wherein the flow guiding surface 12 comprises a second film 24. The second film 24 is arranged on the first film 22. The first film 22 and the second film 24 are arranged in a staggered manner 26 towards the direction of the airflow thereby providing the flow guiding surface 12. As can be seen in Fig. 4, the flow guiding surface 12 comprises a corresponding height Hf. The micro-structured layer 6 comprises a height H starting perpendicular to the outer skin area 4. Both heights Hf, H are measured from the outer skin area 4 to the upper limit of the corresponding structure.

[0067] Fig. 5 shows another embodiment of the aerodynamic surface structure 2. In this embodiment, only one film 22 is provided. The film 22 overlaps with the micro-structured Iayer 6. Further, an adhesion promoter 28 is applied to an overlapping region 36 at which the film 22 overlaps with the micro-structured layer 6. In the proximity of the forward edge and underneath the film 22, a coating 20 may be provided.

[0068] As shown in Fig. 6, a first film 22 and a second film 24 may be utilized, wherein the films are at least partially arranged upon one another. In this embodiment, the second film 24 overlaps with the micro-structured layer 6. An adhesion promoter 28 is applied to an overlapping region 36, at which the second film 24 overlaps with the micro-structured layer 6. In the proximity of the forward edge and between the first film 22 and the second film 24 a coating 20 may be provided.

[0069] Fig. 7a shows an embodiment of the micro-structured layer 6. The micro-structured layer 6 comprises a micro-structured surface 14 in the form of riblet structure 14. The riblet structure 14 comprises ribs 16 that are oriented along the direction of the airflow dp. Furthermore, it can be seen that the micro-structured layer 6 comprises a forward edge 8 which faces towards the direction of the airflow dp. Fig. 7b shows an alternative embodiment of the micro-structured layer 6. The microstructured layer 6 comprises a micro-structured surface 14 in the form of riblet structure 14. The riblet structure 14 comprises ribs 16 that are oriented along the direction of the flow dp. The ribs 16 are arranged on a lacquer base layer 32. Furthermore, it can be seen that the micro-structured layer 6 comprises a forward edge 8 which faces towards the direction of the flow dp. Further, a leading edge portion 10 is shown. The leading edge portion 10 is arranged adjacent to the forward edge 8. The leading edge portion 10 comprises a flow guiding surface 12. The flow guiding surface 12 faces towards the direction of the flow dp. The flow guiding surface 12 is configured to guide the flow above the forward edge 8 and towards the micro-structured layer 6. The leading edge portion 10 comprises a coating 20. The coating 20 forms the flow guiding surface 12. The flow guiding surface 12 and thus the coating 20 comprises a corresponding height Hf. In the embodiment of FIG. 7b, the height Hf of the flow guiding surface 12 exceeds a height H of the ribs 16.

[0070] The embodiments shown in Fig. 7c and 7d are similarto the one shown in Fig. 7b. However, in the embodiment of Fig. 7c, the height Ht of the flow guiding surface 12 is smaller than the height H of the ribs 16 but exceeds the height of the lacquer base layer 32. In the embodiment of Fig. 7d, the height Htof the flow guiding surface 12 is substantially equal to the height of the lacquer base layer 32.

[0071] Fig. 8 shows an example of an aircraft 100. The aircraft 100 comprises an outer skin area 4 and an aerodynamic surface structure 2 attached to the outer skin area 4. The aerodynamic surface structure 2 may be configured as described with respect to Figs. 1 to 5. In particular, the aerodynamic surface structure 2 is attached to an outer skin area 4 affected by turbulent airflow. The outer skin area 4 may further comprise a surface 102 that widens perpendicular to the direction of the airflow dp. The aerodynamic surface structure 2 may be attached to an outer skin area 4 affected by erosion, in particular high erosion. Preferably, the aerodynamic surface structure 2 is attached to an underside of the fuselage of the aircraft 100, such as the center wing fairing. Preferably, the aerodynamic surface structure 2 may also be attached to an outer skin area 4 affected by high wall shear stresses.

[0072] Subsequently, initial simulation and test results that have been conducted to provide a proof-of-concept will be described.

[0073] Figures 9 to 17 illustrate simulation parameters and results that have been conducted in order to analyze the effect of the leading edge portion according to the invention on a microstructured layer comprising a forward edge. For the simulations, the micro-structured layer comprising the forward edge has been simplified as a so called forward facing step. The step is well-known in fluid mechanics and the flow around such step is well researched.

[0074] As shown in Fig. 9 in which streamlines around such a step or forward edge 8 are illustrated, the flow is deflected by this step. This creates two so-called separation bubbles in front and behind the step. Accompanied by the deceleration of the flow, the pressure in front of the edge increases. This leads to an increased pressure on the forward edge which is the reason for the increased pressure drag of the forward edge. This increased pressure drag, in turn, results in an increased fuel consumption of an aircraft.

[0075] In order to reduce the pressure on the leading edge, the invention proposes a leading edge portion 10 arranged adjacent to the forward edge 8, wherein the leading edge portion 10 comprises a flow guiding surface 12 as shown in Fig. 10. During the simulations, different geometries of the flow guiding surface 12 were simulated, namely an S-curve, an exponential curve and a bump. Depending on how the flow guiding surface 12 is realized, for example which application method is used or how much coating material is used and the viscosity of the coating material, these different shapes may result. In the example of Fig. 10, a height of the forward edge 8 of 200 micrometers has been used forthe simulation. The width of the step perpendicular to the flow direction df is 0.01 m.

[0076] For all simulations, the software ANSYS CFX, version 2022 R2 has been used. A block- structured mesh has been utilized and the flow conditions were quasi 2D with a simulation depth of 0.01 m with symmetry boundary conditions. Further, stationary calculations have been conducted. In the simulations absolute values for a step depth of 0.01 m were obtained. While these absolute values can differ depending on which depth exactly is simulated, the relative values comparing an aerodynamic surface structure with and without leading edge portion comprising a flow guiding surface remain quasi constant independent from the simulation parameter step depth.

[0077] First set of simulations

[0078] The first set of simulations utilizes a height of the forward edge 8 of 200 micrometers. Further, an attitude of 30.000 ft and a flow velocity of Ma 0.5 (151 .6 m / s) under standard conditions provided by the International Standard Atmosphere (ISA) have been used. These boundary conditions are representative of local flow conditions in a boundary layer on the aircraft during a typical cruise flight. In Fig. 11 , the pressure drag is shown for four different configurations, namely for the scenario comprising a step without leading edge portion, for a S-curve-shaped leading edge portion, an exponential-shaped leading edge portion and a bump-shaped leading edge portion as shown in Fig. 10.

[0079] It can be seen that compared to the step-geometry, the leading edge portion according to the invention significantly reduces the pressure drag. The amount of this reduction is shown in Fig. 12. The S-curved leading edge portion reduces the pressure drag compared to the step by almost 98 %. For the exponential form of the leading edge portion, a reduction of pressure drag compared to the step of 97.5 % has been obtained. For the bump-form of the leading edge portion a reduction of pressure drag in the range of 98.5 % has been obtained as a result of the simulation.

[0080] It has been assumed within the simulation that the S-curved leading edge portion and the exponential leading edge portion have a width of 10 mm, which means that they start to extend 10 mm before the step. The bump-shaped leading edge portion has a width of 20 mm. It begins 10 mm before the step in the direction of flow and ends 10 mm behind the step in the direction of flow.

[0081] In Fig. 13, simulation results for a scenario named “first film” have been added. This scenario is similar to the embodiment of the invention shown in Fig. 2. For this simulation, it has been assumed that the first film had a height of 75 micrometer and a width of 10 mm. It has been found that the “first film”-scenario leads to a reduction of pressure drag in the range of about 25 % compared to the step-scenario. The pressure drag reductions for the scenarios “S-curve”, “exponential” and “bump” are higher.

[0082] Second set of simulations

[0083] In a second simulation scenario, larger widths of the leading edge portion have been analyzed. The height of the forward edge 8 - and therefore the step -was 200 micrometers. Further, an ISA-altitude of 30000 ft has been simulated considering a flow speed of Ma 0.5.

[0084] As a result, it has been found that the reduction in pressure drag was even higher, as shown in Fig. 14. A reduction of pressure drag for the scenario comprising a leading edge portion having an exponential shape with a width of 250 mm and beginning 250 mm before the step has been in the range of 99 %, compared to the step-scenario. Third set of simulations

[0085] In a third scenario, a lower height of the forward edge has been simulated, namely a height of 40 micrometers. Furthermore, an exponential form of the leading edge portion has been assumed having a total width of 10 mm and beginning 10 mm before the forward edge. All other simulation parameters were unchanged. The results for this simulation run are illustrated in Fig. 15. Comparing the exponential-scenario with the step-scenario, a reduction of pressure drag in the range of 98% was be obtained. This is an even increased pressure drag reduction compared to the 200 micrometer step, showing that the effect is valid for thinner steps as well.

[0086] Fourth set of simulations

[0087] In a fourth scenario, a larger height (500 micrometers) of the forward edge has been simulated for an exponential-scenario and a bump-scenario. For the bump-scenario a height that exceeds the height of the forward edge by 500 micrometers with a width of 20 mm beginning 10 mm before the step was assumed. In the exponential scenario, the width of the leading edge portion was 10 mm, wherein the leading edge portion began 10 mm before the step. The results of this simulation run are shown in Fig. 16. It can be seen that also for this scenario, a reduction of pressure drag compared to the step-scenario of about 95 % for the exponential scenario and 84 % for the bump-scenario can be obtained.

[0088] Fifth set of simulations

[0089] In a fifth simulation run, a height of the forward edge of 200 micrometers was assumed. However, the flow conditions were adjusted to conditions representing takeoff or landing conditions. Therefore the altitude was reduced to sea level. This means that the altitude according to the International Standard Atmosphere was 0 ft. Furthermore, a speed of the flow of Ma 0.2 (68.1 m / s) was assumed. The step height was 200 micrometers and an exponential form of the leading edge portion was assumed having a width of 10 mm and beginning 10 mm before the step.

[0090] The simulation results are shown in Fig. 17. For the sea level-scenario a reduction of pressure drag compared to the step-scenario in the range of 97.8% was obtained.

[0091] In summary, the simulated scenarios show that by utilizing a leading edge portion arranged adjacent to the forward edge according to the invention, for different forms and variants of the flow guiding surface and different heights of the leading edge portion and the flow guiding surface, a substantial reduction of pressure drag can be achieved. This is true for different heights of the forward edge and under typical aircraft-cruise conditions - as well as during the approach and departure phase at low altitudes.

[0092] Flight tests

[0093] Figures 19 and 20 show another beneficial effect that can be achieved by the aerodynamic surface structure according to the invention, namely a protection of the forward edge of a micro-structured layer from damage due to erosion.

[0094] As a practical proof that such a flow guiding surfaces have an effect, aerodynamic surface structures are investigated in areas on an aircraft where such aerodynamic effects as described above are severe. One practical effect of a successful flow guiding surface is that in areas where there is a tendency for high erosion, the damage to the forward edge is greatly reduced.

[0095] In this regard, aerodynamic surface structures were tested in flight tests. The aerodynamic surface structures used in all examples are commercially available Aeroshark Riblet Film patches, obtained from Lufthansa Technik AG and produced by BASF SE.

[0096] The aerodynamic surface structures were attached to different regions of the aircraft fuselage and engines. Each aerodynamic surface structures had a size of 20x20 cm and a total thickness of 180 micrometers. This results to a forward edge having a height of about 180 micrometers. Forthe leading edge portion, a coating was utilized. The results for these scenarios are shown in Figures 19 and 20. For the scenario of Fig. 18, no such leading edge portion was used.

[0097] A protection film 22 as illustrated in Fig. 3 was used in one example shown in Fig. 20. The film was cut from commercially available graphics foils typically used in aviation with a complete product thickness of about 80 micrometers and a size of 10x20 cm. This means that the forward-facing edge from the film is about 80 micrometers in height.

[0098] The coating used as a component of a flow guiding surface consists of a typical, commercially available clear- or topcoat approved for use on the outside of an aircraft, as detailed in the official Aircraft Maintenance Manual (AMM). In the test, the exact same coating material was used as was present on the surface of the airplane, to circumvent any adhesion or compatibility problems.

[0099] To apply the test patches to an aircraft, the following procedure was followed. First, aerodynamic surface structures in the form of Aeroshark Riblet Film patches were applied on several positions of a cleaned surface of an aircraft with standard foil application methods. Followed by, where mentioned, the application of the protection film patches, again with standard foil application methods.

[0100] As a last step, the coating was applied in the liquid state with typical coating application methods. In this example, a coating roller was used. The coating material was handled and cured as described in the accompanying technical data sheet of the manufacturer.

[0101] The test patches were observed for 980 flight hours. After the end of the test duration, the percentage of damaged forward-facing edge is calculated by dividing the cm of damaged forward edge by the total of forward edge (20 cm) and multiplying by 100 to come to the % of damaged forward edge. The results for three different setups, namely a foil-setup 1 , a setup 2 and a setup 3 are shown based on three test areas on the aircraft in the following table.

[0102] In setup 2, an aerodynamic surface structure according to Fig. 1 a was tested. According to setup 3, the embodiment of Fig. 3 was tested. The protection film had a width of 100 mm. Furthermore, the contact area between the forward edge of the aerodynamic surface structure and the protection film was covered by a coating. The forward edge of the protection film was unprotected. According to the comparative example “Foil - Setup 1 ” an aerodynamic surface structure having a forward edge without the leading edge portion according to the invention was used.

[0103] Based on the provided results shown in the table, the structure suffers from significant erosion in the range of 94% - 100% for Foil - Setup 1 having no flow guiding surface. This means that such an aerodynamic surface structure cannot provide the beneficial aerodynamic properties anymore since large areas of the surface are damaged by erosion after 980 h of flight.

[0104] For the scenarios setup 2 and setup 3 comprising a leading edge portion according to the invention, a significant reduction of the impact of erosion has been found. In particular, it has been found that after 980 h of flight, the forward edge of the micro-structured layer was completely undamaged.

[0105] Thus, it can be concluded that the flow guiding surface according to the invention is also highly effective in protecting the forward edge from erosion thus significantly increasing the exchange intervals of such components. This is most likely because of the avoidance / reduction of the frequency of particle impact by the flow guiding and the reduction of the suction peak (negative pressure) on the edge of the aerodynamic surface structure which reduces the damage propagation when there is a damage.

[0106] List of references

[0107] 2 aerodynamic surface layer

[0108] 4 outer skin area

[0109] 6 micro-structured layer

[0110] 8 forward edge

[0111] 10 leading edge portion

[0112] 12 flow guiding surface

[0113] 14 micro-structured surface I riblet structure

[0114] 16 ribs

[0115] 18 foil layer

[0116] 20 coating

[0117] 22 first film

[0118] 24 second film

[0119] 26 staggered film arrangement

[0120] 28 adhesion promoter

[0121] 30 backing

[0122] 32 riblet lacquer base layer

[0123] 34 adhesive layer

[0124] 36 overlapping region

[0125] 38 functional lacquer layer

[0126] 100 aircraft

[0127] 102 surface that widens in the direction of flow dp direction of the airflow

[0128] H height of foil layer

[0129] Hf height of the flow guiding surface w coating width

Claims

Claims1. An aerodynamic surface structure (2) for an outer skin area (4) affected by a turbulent airflow, in particular for an outer skin area (4) of an aircraft (100), the aerodynamic surface structure (2) comprising: a micro-structured layer (6) configured to reduce a frictional resistance of the outer skin area (4) to which the aerodynamic surface structure (2) is attached in a turbulent airflow, wherein the micro-structured layer (6) comprises a forward edge (8) facing towards the direction of the airflow (dp), characterized by a leading edge portion (10) arranged adjacent to the forward edge (8), wherein the leading edge portion (10) comprises a flow guiding surface (12) facing towards the direction of the airflow (dp), wherein the flow guiding surface (12) is configured to guide the airflow above the forward edge (8) and towards the micro-structured layer (6).

2. The aerodynamic surface structure (2) according to claim 1 , wherein the micro-structured layer (6) comprises or consist of a foil layer (18).

3. The aerodynamic surface structure (2) according to claim 2, wherein the foil layer (18) comprise a functional lacquer layer (38) and wherein the functional lacquer layer (38) comprises a micro-structured surface (14).

4. The aerodynamic surface structure (2) according to claim 3, wherein the micro-structured surface (14) comprises a riblet structure (14) having ribs (16) oriented along the direction of the airflow (dp).

5. The aerodynamic surface structure (2) according to claim 3 or 4, wherein the foil layer (18) comprises a backing layer (30) and wherein the backing layer (30) is attached to the functional lacquer layer (38).

6. The aerodynamic surface structure (2) according to claim 5, wherein an adhesive layer (34) is attached to the backing layer (30), and wherein the foil layer (18) comprises a height (H) from a lower side of the adhesive layer (34) to an upper side of the functional lacquer layer (38), wherein the height (H) is 40-500 micrometers, preferably 45-400 micrometers, more preferably 50-300 micrometers.

7. The aerodynamic surface structure (2) according to any one of the preceding claims,wherein the leading edge portion (10) comprises a coating (20) and wherein said coating (20) forms the flow guiding surface (12), in particular wherein the coating (20) only partially covers the micro-structured layer (6).

8. The aerodynamic surface structure (2) according to claim 7, wherein the coating (20) comprises or consist of a coating composition selected from radiation-curable coating compositions, physically curing coating compositions, chemically curing coating compositions, melts and mixtures thereof.

9. The aerodynamic surface structure (2) according to any one of claims 7 or 8, wherein the leading edge portion (10) further comprises a film (22), in particular wherein the film (22) is at least partially attached to the outer skin area (4) and wherein the coating (20) partially covers the film (22) and the micro-structured layer (6).

10. The aerodynamic surface structure (2) according to claim 9, wherein the film (22) is a first film (22) and wherein the flow guiding surface (12) comprises a second film (24) that is arranged on top of the first film (22), and wherein the first film (22) and the second film (24) are arranged in a staggered manner (26) towards the direction of the airflow (dp), in particular wherein the first film (22) or the second film (24) overlaps with the microstructured layer (6).11 . The aerodynamic surface structure (2) according to any one of the preceding claims, wherein the flow guiding surface (12) and the micro-structured layer (6) each comprise a corresponding height (Hf, H) perpendicular to the outer skin area (4), wherein the height (H) of the micro-structured layer (6) exceeds the height (Hf) of the flow guiding surface (12), in particular by 200 micrometers, preferably 150 micrometers, more preferably 120 micrometers or 100 micrometers or 75 micrometers, or wherein the height (Hf) of the flow guiding surface (12) exceeds the height (H) of the micro-structured layer (6), in particular by 500 micrometers, preferably 300 micrometers, more preferably 200 micrometers, more preferably 100 or 75 micrometers.

12. An aircraft (100) comprising an outer skin area (4) and an aerodynamic surface structure (2) attached to the outer skin area (4), wherein the aerodynamic surface structure (2) is configured according to any of the preceding claims.

13. Aircraft (100) according to claim 12,wherein the aerodynamic surface structure (2) is attached to an outer skin area (4) affected by turbulent airflow, in particular wherein the outer skin area (4) further comprises a surface (102) that widens perpendicular to the direction of airflow (dp).

14. A method for applying an aerodynamic surface structure (2) according to any one of the preceding claims to an outer skin area (4), the method comprising: attaching a micro-structured layer (6) configured to reduce a frictional resistance of the outer skin area (4) to which the aerodynamic surface structure (2) is attached in a turbulent airflow to an outer skin area (4) of an aircraft (100), wherein the micro-structured layer (6) comprises a forward edge (8) facing towards the direction of the airflow (dp), attaching a leading edge portion (10) adjacent to the forward edge (8) to the outer skin area (4), wherein the leading edge portion (10) comprises a flow guiding surface (12) facing towards the direction of the airflow (dp), wherein the flow guiding surface (12) is configured to guide the airflow above the forward edge (8) and towards the micro-structured layer (6).

15. A use of an aerodynamic surface structure (2) according to any one of the preceding claims for at least one of the following: aircraft (100), turbines, such as gas, water or wind turbines, in particular the rotor of said turbines, land vehicle, in particular car, truck or train, marine vehicle, in particular ship or vessel, pipeline, such as water, gas, oil pipeline.

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