Abstract:
A method for the production of a riblet film, having at least one surface that reduces airflow resistance involves the provision of a metal film that is structured using a mechanical treatment.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
       [0001]    Exemplary embodiments of the present invention relate to a method for the production of a surface of a component with reduced airflow resistance. In particular, the component in this case is a structural component, such as is found in aircraft, helicopters, or flying bodies. After the method is carried out, the surface of the component has a reduced airflow resistance. 
         [0002]    The reduction of fuel consumption and the reduction of emissions of carbon dioxide/nitrogen oxide (CO2/NOx) are important goals, particularly for the airplane industry or the automobile industry. In addition to lightweight construction for the purpose of reducing weight, and improving the degree of efficiency of engines, the reduction of airflow resistance is an essential component in the attempt to reach this goal. 
         [0003]    Methods for the production of surfaces that reduce the airflow resistance thereof are known. It has been known for a long time, and proven in oil channel experiments, that the resistance of liquid or gaseous media flowing over solid surfaces can be reduced by a suitable structuring of the surfaces. The structuring of the solid surface in this case has different shapes and sizes. The article “Experiments on drag-reducing surfaces and their optimization with adjustable geometry” from Bechert, D. W. et al in the Journal of Fluid Mechanics, Vol. 338, pp. 59-97, 1997 discloses gluing a structured plastic film to a body. 
         [0004]    Based on this, airplane companies such as Airbus and Boeing have carried out experiments in which plastic films with a corresponding structuring, from the 3M Company, were glued onto the outer surface of an aircraft. In flight tests with a large airplane, 70% of the surface of which carried such plastic films glued thereon, it was possible to demonstrate a reduction in resistance of 2% (Szodruch J., Dziomba, B., Aircraft Drag Reduction Technologies, 29th Aerospace Science Meeting, Reno, Nevada, 07-10 Jan. 1991). This led to a savings of fuel of 1.5% (Roberts J. P., Drag Reduction: An Industrial Challenge, Special Course on Skin Friction Drag Reduction, AGARD Report 786, 1992.) Calculated up to the average life of an aircraft, this is a fuel savings of approx. 10,000 tons of kerosene. 
         [0005]    The structured plastic films used to date, however, not only are deficient in their mechanical resistance, but also demonstrate degradation of the plastic film and the adhesive with which the plastic films are adhered to the aircraft. A result of the deficient mechanical resistance is, by way of example, the rounding of the tip radius of sawtooth-shaped structures, which can lead to a mitigation of the friction-reducing effect. (Hage, W., Zur Widerstandsverminderung von dreidimensionalen Ribletstrukturen and anderer Oberflächen, Dissertation TU Berlin 2004). The degradation can also lead to the plastic films becoming detached during operation. 
         [0006]    It is also known to produce solid metallic bodies with a surface structuring of a suitable size. By way of example, German patent document DE 10314373A1 disclose a fine-casting method by means of which turbine blades made of titanium aluminides (TiAl) are produced with a structured surface. By way of example, the structuring of metallic surfaces using laser treatment or grinding is known from Oehlert et. al., Exploratory Experiments on Machined Riblets for 2-D Compressor Blades, Proceeding of IMECE2007, 2007 ASME International Mechanical Engineering Congress and Exposition, Nov. 11-15, 2007 Seattle, Wash., USA. 
         [0007]    Such a direct structuring of aircraft structures by means of casting processes or machining methods (structuring by laser, or micromachining, etc.) is, however, neither technically nor economically possible at the surface sizes required for aircraft, helicopters, or other flying devices. 
         [0008]    Moreover, aircraft or helicopters made of fiber-reinforced plastic (FRP) must be specially protected again lightning strikes. For this purpose, current-conducting metal meshes made of copper or bronze are used, laminated into the FRP structure. Metal inserts that are also laminated into the structure—so-called “expanded foils”—are also used for this purpose. It is known that damage caused by lightning is greater with greater thickness of the organic, non-conducting layer of matrix resin and paint over the embedded metal insert. 
         [0009]    Exemplary embodiments of the present invention are directed to a component having a reduced airflow resistance when a fluid flows over the same, the component overcoming the disadvantages of the prior art and being able to be utilized in a cost-effective manner, with low technical complexity. 
         [0010]    Exemplary embodiments of the present invention are directed to a metal film structured such that a metallic riblet film results. Riblets are microscopically small grooves on the surface of the metal film. The riblet film is applied to a component in order to reduce the airflow resistance thereof. In particular, if the riblet film is used on the component, the riblets are oriented substantially parallel to the local flow. The use of the riblet film creates a mechanically resistant and long-term durable surface of the component. 
         [0011]    In other words, the invention provides the outer surface of the component, regardless of the size shape or, properties thereof, with properties such that the airflow resistance is reduced, and the mechanical resistance of the outer surface is increased by the riblets. By applying the riblet film to the component, the airflow resistance is reduced. In this case, it is possible that the riblet film is applied both to metal (alloy) components and to fiber composite components. Both the thickness of the riblet film and the structure thereof can be adapted to the component, and the size and shape of the riblets, as well as the thickness of the riblet film, are variable. It is advantageous that the thickness of the riblet film is less than 500 μm, and advantageously between 50-400 μm, particularly at least 200 μm. The thickness of the riblet film ensures a simple adaptation to the respective component shape with the least possible weight. 
         [0012]    Exemplary embodiments of the present invention also provide a method of producing a riblet film, wherein at least one surface of the riblet film reduces the airflow resistance. A metal film of any size and shape is subjected to a mechanical treatment, wherein a structuring of the metal film is carried out such that a riblet film is produced. The riblet film has riblets on at least one of its surfaces. The riblets are arranged in such a manner that they significantly reduce the airflow resistance when a fluid flows over the same. The riblets are preferably arranged with an orientation along the direction of flow of the fluid. At least one surface of the riblet film is subjected to a surface treatment such that a multi-functional layer is produced on the surface. 
         [0013]    The metal film is made of a material having at least all of the light metals in the main groups or transition groups of the periodic table, particularly aluminum or titanium. Of course, the metals also include metal alloys containing at least one of the named metals, including lithium, beryllium, scandium, or yttrium, and particularly also steel alloys. 
         [0014]    Due to the inherent electrical conductivity of the metal riblet films, the structured riblet films applied externally to the component assume the function of lightning protection in addition to reducing the airflow resistance. By assuming the function of lightning protection, they obviate the need for metal meshes or “expanded foils” to be laminated into the structure. In addition, a cover paint beneath the riblet film can be dispensed with, such that the weight of the riblet film is compensated for by itself. This means that, by way of example, when an aircraft is equipped with a riblet film the fuel consumption remains constant due to the constant weight of the aircraft, and due to the riblets on the riblet film, the fuel consumption is reduced. The riblet film can reduce airflow resistance up to approx. 10%, which in turn reduces the consumption of fuel. 
         [0015]    All of the metal and metal alloys named above can be used as the material of the metal film, preferably titanium and its alloys, or steel alloys, and particularly aluminum and its alloys. The structuring of the metal film by means of mechanical treatment includes mechanical forming or molding processes such as rolling, grinding, or laser treatment, preferably rolling (including incremental methods such as the so-called “incremental rolling”), or stamping. 
         [0016]    In the process of forming, a structure is transmitted from a roll or roller to the metal film, such that a structure of microscopically small grooves—the riblets—is created on the surface of the metal film. The roll or the roller in this case has the negative shape with respect to the later riblets. The transmission of the structure from the roll or roller to the metal film is realized, by way of example, by rolling down the roll or roller onto the metal film. The rolling method for the production of metallic riblets is described, by way of example, in Economical and Ecological Benefits of Process-integrated Surface Structuring, M. Thome, G. Hirt, Key Engineering Materials Vol. 344, 2007, pp. 939-946. Reference is hereby made to the same. 
         [0017]    In the process of stamping, the riblet structure is transmitted to the metal film by a stamp or roll. The stamp in this case has the negative shape with respect to the later riblets. The structuring of a metal sheet using a stamp is known, by way of example, from German patent document DE 102008032618 A1, to which reference is hereby made. 
         [0018]    The structuring of the metal film by means of grinding or laser treatment is described, by way of example, in Exploratory Experiments on Machined Riblets for 2-D compressor blades, Oehlert et. al, Proceedings of IMECE2007, 2007 ASME International Mechanical Engineering Congress and Exposition, Nov. 11-15, 2007, Seattle, Wash., USA, to which reference is hereby made. 
         [0019]    In contrast to plastic films, the structuring of the metal film also has the additional advantage that, as a result of the mechanical treatment, the metal is strengthened, and thereby the resistance thereof to mechanical loads (e.g. erosion, rain, hail, etc.) is increased. 
         [0020]    The riblet film can also take on the function of a heat conductor, such that, by way of example, it is possible to prevent the formation of ice. As a result of heating of the metallic riblet film, no ice is able to form, which is particularly advantageous in the case of aircraft or helicopters, because no stalling can occur as a result of the formation of ice. 
         [0021]    The surface treatment advantageously includes an anodizing. In the anodizing process, at least one surface of the riblet film is dipped into an electrolyte, and an oxide layer is formed on the surface of the riblet film once an electric voltage is applied. The thickness of the oxide layer is preferably 0.1 to 20 μm. This results in an increase in hardness and improvement of the erosion resistance of the riblet film. 
         [0022]    The OOL preferably has a porous structure. This topography ensures an inner interlock structure, for example of an adhesive, adhesive primer, or resin, with the riblet film made of metal, and ensures improved adhesion and long-term durability of the riblet film. 
         [0023]    An adhesive primer is advantageously applied to at least the surface of the riblet film which is opposite the riblets. The adhesive primer can improve the adhesive strength of the riblet film on the component, such that the riblet film does not separate from the component during operation of the component. As a result, durable, long-term adhesion between the riblet film and the component is ensured. 
         [0024]    The application of an adhesive primer protects the oxide layer from damage or alteration, for example as a result of scratching during the application of the riblet film to the component. The applied adhesive primer also enables storage or packing of the film between the production and the application thereof to the component. 
         [0025]    The surface treatment of the surface of the riblet film having the riblets advantageously includes an impregnation with a water-repelling or dirt-repelling liquid. This prevents contamination with dirt, the accumulation of ice, or similar deposits. The oxide layer is preferably impregnated on the surface having the riblets. This functionalization can have a temporary or long-term effect. 
         [0026]    Particularly in the case of a temporary surface treatment, the pore structure of the oxide layer is impregnated with the water-repelling or dirt-repelling liquid. The diffusion of the liquid from the pores results in a water-repelling or dirt-repelling effect. After the liquid stored in the pores is used up, the efficacy of the surface of the riblet film can be regenerated by means of a subsequent impregnation. 
         [0027]    A surface treatment with a long-term effect on the surface having the riblets includes an embedding of water-repelling or dirt-repelling solids into the pore structure. In addition, a coating with a thickness of several nanometers has a long-term effect, wherein the coating can be plasma-supported. 
         [0028]    Fluorinated or partially fluorinated polymers, or modified silanes, are advantageously used in the impregnation, embedding, or coating process. 
         [0029]    It is advantageous for the production of the riblet film if the riblet structures have trapezoidal, semi-circular, or sawtooth-shaped structures. These, substantially oriented parallel to the flow, reduce the airflow resistance and therefore increase the uplift, for example of an aircraft, a turbine blade, a helicopter rotor, or another body. The angle between the direction of flow and the riblet is advantageously less than 5°, and preferably approximately 0°. 
         [0030]    The riblet structures have a range of sizes, with a riblet width of approximately 20 μm to 130 μm, preferably 30 μm to 80 μm, and particularly 50 μm to 60 μm. The riblet height is in the range of 50-100% of the riblet width. The riblet height can be the same as the riblet width, or can be a combination of one of the given size ranges. A maximum reduction of the skin friction of the air flow can be achieved if the height of at least one riblet is half as large as the distance to a neighboring riblet, wherein the riblet structure can be designed in such a manner that the individual riblets directly border each other—that is, they are flush with each other—or there can be a distance between the individual riblets. A maximum reduction in the airflow resistance can be achieved if the riblets are oriented local to the direction of airflow. 
         [0031]    If the riblet film is used to reduce the airflow resistance of a component, wherein the riblet film is produced by means of one of the methods described above, the airflow resistance is reduced when the riblet film is applied to the component. 
         [0032]    The application of the riblet film to the component is carried out, by way of example, by adhering the riblet film to the component, or—if the component is entirely or partially manufactured from a fiber composite component—by curing (co-curing) the riblet film with the material of the fiber composite component. The riblet film advantageously has an improved adhesion to the component as a result of the inner interlock structure between the adhesive, the adhesive primer, or the resin with the riblet film on the one hand, and the component on the other, by means of the porous oxide layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0033]    The invention is described below in an exemplary manner with reference to the embodiments illustrated in the figures. To simplify the presentation, no accurate scale has been used in the representation. 
           [0034]      FIG. 1  shows a perspective view of a riblet film; 
           [0035]      FIG. 1   a  shows a cross-section of a riblet film with sawtooth-shaped riblets; 
           [0036]      FIG. 1   b  shows a cross-section of a riblet film with semi-circular riblets; 
           [0037]      FIG. 1   c  shows a cross-section of a riblet film with trapezoidal riblets; 
           [0038]      FIG. 2  shows a riblet film with an upper and a lower oxide layer; 
           [0039]      FIG. 3   a  shows a cutaway view of an oxide layer in  FIG. 2 ; 
           [0040]      FIG. 3   b  shows a top view of an oxide layer in  FIG. 2 ; 
           [0041]      FIG. 4   a  shows an enlargement of detail A in  FIG. 2  prior to the impregnation; 
           [0042]      FIG. 4   b  shows an enlargement of detail A in  FIG. 2  after the impregnation; 
           [0043]      FIG. 5  shows an enlargement of detail B in  FIG. 2  after the application of the riblet film on a component; 
           [0044]      FIG. 6  shows an enlargement of detail B in  FIG. 2  after the application of the riblet film on a fiber composite component; 
           [0045]      FIG. 7  shows a riblet film on the wing of an aircraft; 
           [0046]      FIG. 8  shows the production of a riblet film. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]      FIG. 1  shows a perspective, sketched view of a riblet film  10 . Following a mechanical treatment (for example as described below in connection with  FIG. 8 ), a metal film made of aluminum, which was originally without structure, has a structured surface on one side thereof with riblets. The riblets are microscopic, small grooves on the surface of the aluminum film. In this embodiment, trapezoidal riblets  13  have been molded on the one side of the aluminum film, such that, following the mechanical treatment, a structured surface with trapezoidal riblets  13  is formed on a base  14  of the original aluminum film. The thickness of the base  14  is 200 μm. The trapezoidal riblets  13  extend in parallel over the riblet film  10 . 
         [0048]    Of course, the riblets can also have another shape, for example as described in  FIG. 1   a - c.    
         [0049]      FIG. 1   a  illustrates a cross-section of a riblet film  10  having sawtooth-shaped riblets  11 , which achieve a reduction in the airflow resistance between 5 and 6% when a fluid flows over the same. The height of the riblets  11 , measured starting from the base  14 , is 50 μm. The distance between two tips of the riblets  11  is 100 μm. The radius of the tips is 5% of the width of the riblets  11 , but can also be up to 10%. 
         [0050]      FIG. 1   b  illustrates a cross-section of another riblet film  10  having sawtooth-shaped riblets  12 , which achieve a reduction in the airflow resistance between 7 and 8% when a fluid flows over the same. The height of the riblets, measured starting from the base  14 , is 30 μm. The distance between two tips of the riblets  12  is 60 μm. The radius of the tips is 2% of the width of the riblets  12 . 
         [0051]      FIG. 1   c  illustrates a cross-section of a riblet film  10  having trapezoidal riblets  13 , which achieve a reduction in the airflow resistance of a fluid which flows over the same between 7 and 8%. The height of the riblets  13 , measured starting from the base  14 , is 25 μm. The distance between two tips of the riblets  13  is 50 μm. 
         [0052]    The maximum reduction of the airflow resistance is achieved when the riblets ( 11 ,  12 ,  13 ) are arranged in such a manner that they are arranged local to the flow. The angle between the riblets ( 11 ,  12 ,  13 ) and the direction of flow in this case is ideally V 
         [0053]    In addition to riblet films  10  made of aluminum, comparable results have been obtained using riblet films made of aluminum alloys, titanium/aluminum alloys, or steel alloys. Because the density of aluminum and its alloys is rather low, this material is particularly suitable for applications of the riblet film  10  in aircraft, helicopters, or other flying bodies. 
         [0054]      FIG. 2  shows a riblet film  10  having an upper oxide layer  15  and a lower oxide layer  16 . The oxide layers  15 ,  16  are the result of a surface treatment. In this embodiment, an oxide layer  15 ,  16 —and more precisely an aluminum oxide layer—is produced on both sides of the riblet film  10  by means of a phosphoric acid/sulfuric acid anodizing. The oxide layer  15 ,  16  increases the hardness of the riblet film  10 . This improves the erosion resistance, by way of example. 
         [0055]    The upper oxide layer  15  has a structure as is shown in  FIG. 3   a, b,  and is additionally impregnated—see  FIG. 4   a, b.  As a result of the impregnation, the configuration prevents contamination with dirt or the accumulation of ice. A riblet film  10  with long-term durability results, which can be used for practically the entire lifetime of an aircraft. 
         [0056]    The lower oxide layer  16  also has a structure as is shown in  FIG. 3   a, b,  following the anodizing, and additionally has an adhesive primer  19 —see  FIG. 5 . Instead of the adhesive primer, an adhesive can also be used. As a result of the inner interlock structure of the adhesive primer  19  with the riblet film  10 , it is possible to ensure a very good adhesion between a component and the riblet film  10  applied thereto. 
         [0057]    The single-side application of the adhesive primer  19  on the side of the riblet film  10  facing the component protects the lower oxide layer  16  from damage. In addition, the riblet film  10  can be more easily stored between its production and the application thereof to the component—for example a turbine blade or a wing of an aircraft, and can be adapted to the component. 
         [0058]      FIG. 3   a  shows a cutaway view of an oxide layer in  FIG. 2 . The oxide layer formed by the anodizing is clearly visible. The topography of the layer shows a pore structure with pores  17  of different sizes and depths. The pore opening is between 10-30 nm, with a pore depth of up to 300 nm. 
         [0059]      FIG. 3   b  shows a top view of an oxide layer in  FIG. 2 . 
         [0060]      FIG. 4   a  shows an enlargement of the detail A in  FIG. 2  prior to the impregnation. An oxide layer  15  has formed on the aluminum material of the riblet film  10  as a result of anodizing. The oxide layer  15  has a pore structure. The pores  17  are such as those described in the context of  FIG. 3   a, b.  For reasons of simplicity, the pores  17  in  FIG. 4   a  are shown with a consistent structure. 
         [0061]      FIG. 4   b  shows an enlargement of the detail A in  FIG. 2  following the impregnation. During the impregnation, the oxide layer  15  is filled with a liquid. The liquid diffuses out of the pores  17  and thereby establishes the water-repelling or dirt-repelling effect. The liquid is retained in the pores  17 . As soon as the liquid in the pores  17  is used up, the function of the oxide layer can be re-established by means of a subsequent impregnation. 
         [0062]    Instead of or in addition to the impregnation, water-repelling or dirt-repelling solids can be embedded in the pores  17 ; or a plasma-supported coating, with a size of approx. 50-100 nm, can be used. The embedding or coating provides the advantage of giving the repelling function long life or long-term durability. 
         [0063]    Fluorinated polymers or modified silanes are suitable as the liquid, coating material, or solid material. 
         [0064]      FIG. 5  shows an enlargement of the detail B in  FIG. 2  following the application of the riblet film  10  to the component  20 . The component  20  is a wing of an aircraft, by way of example. 
         [0065]    Following the anodizing, a lower oxide layer  16  is formed on the side of the riblet film  10  facing the component. The oxide layer  16  has a pore structure such as that described in the context of  FIG. 3   a, b.  The adhesive primer  19  diffuses into the pores  17  and covers the surface of the oxide layer  16 . When the riblet film  10  is applied to the component  20 , the adhesive  28  adheres the riblet film  10  to the component on the adhesive primer  19 . An inner interlock structure (positive- and force-fitting connection) is created between the component  20  and the riblet film  10  over the adhesive primer in the pores  17  in the oxide layer  16 . 
         [0066]    The interlock structure between a fiber composite component  21  and a riblet film is shown in  FIG. 6  as an enlargement of the detail B in  FIG. 2 . The riblet film  10  having a lower oxide layer  16  on the side thereof facing the component is applied to the fiber composite component  21  prior to the curing (“co-curing”) of the same. In the production of the composite of the riblet film  10  and the fiber composite component  21 , liquid resin  22  penetrates the pores of the oxide layer  16 . Upon curing, the resin remains in the pores and becomes solid. After the curing, a positive- and force-fitting connection is created between the riblet film  10  and the fiber composite component  21 . 
         [0067]      FIG. 7  shows a riblet film  10  from one of the previous embodiments, on the wing of an aircraft  23 . For reasons of simplicity, an illustration has been chosen which is not to scale. The riblet film  10  is applied to the wing of the aircraft  23  in such a manner that the direction of flow is substantially parallel to the riblets  13 . In a view which is not shown here, the riblets  13  extend from the leading edge to the trailing edge, over the upper and lower sides of the wing. In the illustration shown, the riblet film  10  is only applied on the upper side of the wing. It is equally possible for the riblet film  10  to be applied to the fuselage of the wing or to other parts for which a reduction in the airflow resistance thereof is desired. 
         [0068]      FIG. 8  shows the production of a riblet film  10  in a side view. A metal film  24 , for example made of aluminum or an alloy thereof, titanium or an alloy thereof, or a steel alloy, is provided on a roll  27 , rolled around the same. The metal film  24  is not structured. The length of the metal film  24  can be any arbitrary length. The width of the metal film  24  can also be any arbitrary width, and in this embodiment depends on the width of the roll  27 . 
         [0069]    A riblet structure as shown in  FIG. 1   a - c  is applied to the metal film  24  by means of a stamp  25 . During the stamping, the force F applied by the stamp  25  is received by a counter holder  26 . The stamp  25  has a negative shape with respect to the riblet structure being generated. 
         [0070]    The finished riblet film  10  is rolled onto another roll  27 . 
         [0071]    The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           10  riblet film 
           11  sawtooth-shaped riblets 
           12  semi-circular riblets 
           13  trapezoidal riblets 
           14  base 
           15  upper oxide layer 
           16  lower oxide layer 
           17  pore 
           18  impregnated layer 
           19  adhesive primer 
           20  component 
           21  fiber composite component 
           22  resin 
           23  aircraft 
           24  metal film 
           25  stamp 
           26  counter support 
           27  roll 
           28  adhesive