Abstract:
A multilayer construction for aerodynamic riblets includes a first layer composed of a material with protuberances, the first layer material exhibiting a first characteristic having long-term durability and a second layer composed of a material, exhibiting a second characteristic with capability for adherence to a surface.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is copending with U.S. patent application Ser. No. 12/361,882 filed on Jan. 29, 2009 entitled Shaped Memory Riblets and U.S. patent application Ser. No. 12/361,918 filed on Jan. 29, 2009 entitled Amorphous Metal Riblets the disclosures of which are incorporated herein by reference. 
     BACKGROUND INFORMATION 
     1. Field 
     Embodiments of the disclosure relate generally to the field of surface geometries for aerodynamic improvements to aircraft or surfaces having a flow interface and more particularly to embodiments and fabrication methods for rigid riblets having improved damage resistance. 
     2. Background 
     Increasing fuel efficiency in modern aircraft is being accomplished through improvement in aerodynamic performance and reduction of structural weight. Recent advances in the use of microstructures such as riblets on aerodynamic surfaces have shown significant promise in reducing drag to assist in reducing fuel usage. Riblets have various forms but advantageous embodiments may be ridge-like structures that minimize drag on the surface of an aircraft. Riblets may be used in areas of a surface of an aircraft where turbulent regions may be present. Riblets may limit circulation causing a breakup of large scale vortices in these turbulent regions near the surface in the boundary layer to reduce drag. 
     In certain tested applications, riblets have been pyramidal or inverted V shaped ridges spaced on the aerodynamic surface to extend along the surface in the direction of fluid flow. Riblet structures have typically employed polymeric materials, typically thermoplastics. However in service use such as on an aircraft aerodynamic surface, polymers are relatively soft and thus reducing the durability of the surface. Existing solutions with polymeric tips may readily deform hundreds of percent with fingernail pressure and may be unrecoverable. Such structures may be undesirable in normal service use on an aircraft or other vehicle. Additionally aircraft surfaces are typically required to withstand interactions with various chemicals including Skydrol®, a hydraulic fluid produced by Solutia, Inc. In certain applications elastomers that resist or recover from severe deformation created at the tip may be employed to form the riblets. However, many elastomers and other polymers may not be compatible with Skydrol® or other aircraft fluids or solvents. 
     The practicality of riblets for commercial aircraft use would therefore be significantly enhanced with a riblet structure providing increased durability and aircraft fluids compatibility. 
     SUMMARY 
     Exemplary embodiments provide a multilayer construction having a first layer composed of a material with riblets, the first layer material exhibiting a first characteristic of having long term durability and a second layer composed of a material exhibiting a second characteristic with capability for adherence to a surface. The multilayer construction is employed in exemplary embodiments wherein the riblets are implemented on a vehicle, the riblets having long-term durability due to the rigidity of the first layer. 
     In various embodiments, the multilayer construction for an array of aerodynamic riblets is created by a plurality of rigid tips with a layer supporting the rigid tips in predetermined spaced relation and adhering the rigid tips to a vehicle surface. In exemplary embodiments, the rigid tips are formed from material selected from the set of nickel, chromium, metal alloy, glass, ceramic, silicon carbide and silicon nitride. Additionally, the supporting layer may be continuously cast with the tips as a surface layer. Alternatively, a polymer support layer is deposited on the surface layer opposite the tips. An adhesive layer deposited on the polymer support layer forms a multilayer appliqué, and provides the capability for adhering the appliqué to the vehicle surface. 
     In another exemplary embodiment, the supporting layer is an elastomeric layer engaging the tips and a metal foil and a polymer layer are provided intermediate the elastomeric layer and the adhesive layer. The metal foil, polymer layer and adhesive layer may be provided as a preformed appliqué. For exemplary embodiments using the elastomeric layer, the tips each incorporate a base and each base may be embedded in the elastomeric layer. 
     For greater flexibility in certain applications, each tip is longitudinally segmented. 
     An aircraft structure may be created by an array of aerodynamic riblets having a plurality of rigid tips formed from material selected from the set of nickel, chromium, metal alloy, glass, ceramic, silicon carbide and silicon nitride and segmented longitudinally at predetermined locations. An elastomeric layer engages bases extending from the rigid tips and a polymer support layer is deposited on the elastomeric layer opposite the tips. An adhesive layer deposited on the polymer support layer to forms a multilayer appliqué. The adhesive layer adheres to a surface of the aircraft. 
     The embodiments disclosed are fabricated in an exemplary method by forming a master tool having protuberances corresponding to a desired riblet array and forming a complementary tool from the master tool. A plurality of rigid tips is then cast in the master tool using electroforming, casting or other desirable deposition technique. The cast rigid tips are then removed from the complementary tool and adhered to an aerodynamic surface. 
     In exemplary aspects of the method, resist is applied to the complementary tool for a segregating the rigid tips and removed subsequent to casting the rigid tips. An elastomeric layer is then cast engaging the rigid tips and a multilayer appliqué is applied to the elastomeric layer to form a riblet array appliqué. 
     In exemplary embodiments of the method, the multilayer appliqué comprises a metal foil, a polymer support layer and an adhesive layer. An adhesive liner covering the adhesive layer and masking covering the riblets may be employed for handling. The riblet array may then be adhered to the aerodynamic service by removing the adhesive liner and applying the riblet array appliqué to the aerodynamic surface and removing the masking. 
     In an alternative method, casting the plurality of rigid tips includes casting of the plurality of tips and an intermediate surface layer as a cladding. An elastomeric layer is then cast to the cladding. 
     A method for fabricating an array of aerodynamic riblets for an aircraft surface may be accomplished by diamond machining a form and curing an acrylate film on the form. The acrylate film is then stripped from the form and applied to a roller to form a master tool having protuberances corresponding to a desired riblet array. A silicon complementary web tool is created by impression on the master tool. A metal coating is then sputtered on the complimentary web tool and a plurality of rigid tips is then electroformed in the complimentary web tool. A multilayer appliqué having a metal foil, a polymer support layer and an adhesive layer to the elastomeric layer is applied to form a riblet array appliqué. The rigid tips are then adhered to an aerodynamic surface using the adhesive layer of the applique and the silicone complementary web tool is then stripped from the rigid tips. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of embodiments disclosed herein will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is an isometric view of a portion of an aerodynamic surface such as a wing or fuselage skin showing exemplary riblets extending in the flow direction; 
         FIG. 2A  is a lateral section view perpendicular to the flow direction of a first embodiment for rigid tipped riblets; 
         FIG. 2B  is a lateral section view of a modification of the embodiment of  FIG. 2A  with an additional support layer; 
         FIG. 2C  is a lateral section view of a modification of the embodiment of  FIG. 2A  with rigid cladding over an elastomer core; 
         FIG. 2D  is a lateral section view of a modification of the embodiment of  FIG. 2A  without an adhesive layer for direct thermoplastic boding; 
         FIG. 3  is a lateral section view of a second embodiment for rigid tipped riblets with lateral structural separation of the riblets; 
         FIG. 4  is a lateral section view of a third embodiment for rigid tipped riblets with reduced cross-section and with lateral separation; 
         FIG. 5A  is a top view of a portion of an aerodynamic surface employing riblets of the first embodiment as shown in  FIG. 2B ; 
         FIG. 5B  is a section view comparable to  FIG. 2B  for reference with the features of  FIG. 5A ; 
         FIG. 6A  is a top view of a portion of an aerodynamic surface employing riblets of the second embodiment shown in  FIG. 2B  with additional longitudinal separation of riblet sections; 
         FIG. 6B  is a section view comparable to  FIG. 4  for reference with the features of  FIG. 6A ; 
         FIG. 7A  is a flow diagram of processing steps for a first exemplary method of fabrication of rigid tipped riblets of the first embodiment; 
         FIG. 7B  is a flow diagram of processing steps for a second exemplary method of fabrication of rigid tipped riblets of the first embodiment; 
         FIG. 7C  is a flow diagram of roll-to-roll processing for the method shown in  FIG. 7B   
         FIG. 8  is a flow diagram of processing steps for an exemplary method of fabrication of rigid tipped riblets of the second embodiment; 
         FIG. 9  is a flow diagram of processing steps for an exemplary method of fabrication of rigid tipped riblets of a third embodiment; 
         FIG. 10  is a flow diagram describing use of the rigid tipped riblets embodiments disclosed herein in the context of an aircraft manufacturing and service method; and 
         FIG. 11  is a block diagram representing an aircraft employing the rigid tipped riblets with embodiments as disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of rigid tipped riblets having a structure as will be described in greater detail subsequently is shown as a portion of an aerodynamic surface for an aircraft is shown in  FIG. 1 . “Rigid” as used herein generally refers to a high modulus of elasticity and/or a high load to failure. Many of these materials may have a small strain elastic region. Exemplary embodiments herein employ rigid materials which may have moduli of elasticity up to and larger than about 25×10 6  lbs/in 2  with deformation response essentially all elastic. The aircraft  110  employs a structure with a surface  111 , shown enlarged, having multiple substantially parallel riblets  112  arranged parallel to the flow direction as represented by arrow  114 . For the exemplary embodiment shown, the dimension  116  perpendicular to the surface  111  (as shown in  FIGS. 2A and 2B  for example) is approximately 0.002 inch while the tip-to-tip spacing  118  between the riblets is approximately 0.003 inch. Spacing may vary depending on the fluid dynamic properties of the air, water or other fluid for which the application of riblets is employed. The aerodynamic surface is typically curved and may be, without limitation, a portion of a wing, an engine nacelle, a control surface, a fuselage or other suitable surface. Therefore flexibility and conformability of the riblets and any structure supporting and affixing the riblets to the surface may be required. While described herein with respect to an aircraft aerodynamic surface the embodiments disclosed herein are equally applicable for drag reduction on surfaces of other aerospace vehicles such as, without limitation, missiles or rockets and other vehicles such as cars, trucks, buses and trains moving in a gaseous fluid, commonly air, or on boats, submarines, hydrofoils, fluid flow conduits or other surfaces exposed to liquid fluid flow. 
     The embodiments disclosed herein recognize and provide the capability for riblets that may resist various impacts and/or other forces that may reduce riblet durability. Further, certain of the different advantageous embodiments provide a multi-layer structure that may have a support layer and a plurality of riblet tips located on or extending from the support layer. The tips which form the riblets may be fabricated from stiff metals such as nickel (used for the embodiments described herein) or alternative rigid materials such as chromium, other metal alloys, glass, ceramics, Silicon Carbide or Silicon Nitride. The materials of the multilayer structure are flexible and may be formed as an appliqué separately or in combination with the riblets for fastening, bonding, coupling or otherwise attaching to a surface to improve aerodynamics of a vehicle such as an aircraft. 
     A first embodiment for rigid tipped riblets is shown in  FIG. 2A  as a multilayer construction. Individual tips  202  of the riblets protrude from a surface layer  204  to provide a first layer  201  of the multilayer construction. The protruding riblets and continuous surface layer are formed by casting or deposition, as will be described in greater detail subsequently, of the rigid material desired for providing a first characteristic of durability. In an exemplary embodiment, nickel is employed. For the embodiment shown in  FIG. 2A  a second layer  203  created by an adhesive layer  206  is deposited on a bottom  204   a  of the surface layer  204 . Exemplary adhesives for use in various embodiment may include, without limitation, acrylic pressure sensitive adhesive, sylilated polyurethane pressure sensitive adhesive; thermoplastic adhesive; heat-reactive adhesive or epoxy adhesive. In alternative embodiments, a supporting polymer layer  207  engages the surface layer  204  intermediate the surface layer and adhesive layer as shown in  FIG. 2B  as a portion of the second layer. The polymer layer  207  may be, without limitation, a polymer film or other suitable material. In certain embodiments polyetheretherketone (PEEK) is employed as the film. Additionally, a foil or metallic layer  310  as will be described with respect to the embodiment of  FIG. 3  may be employed for lightning strike protection, particularly where the riblet tips  202  and surface layer  204  are non-metallic. The polymer, adhesive and/or other elements in the second layer provide a second characteristic of resilience and the ability to adhere to the surface. 
       FIG. 2C  is an additional alternative embodiment wherein the nickel or alternative rigid material is employed as a contoured surface cladding  208  forming the tips  202 ′ and surface layer  204 ′ as the first layer of the multilayer construction. As the second layer, a polymer layer  210  is employed. The polymer layer  210  in certain embodiments as described herein may be an elastomer and may be cast into the cladding  208  or conversely the cladding  208  cast over the polymer layer  210 . The polymer layer  210  provides both a support layer  206 ′ and light weight cores  212  for the tips  202 ′ to maintain the predetermined spaced relation of the tips  202 ′. Exemplary elastomers used in exemplary embodiments may be polyurethane elastomers, polysulfide elastomers, epoxy-based elastomers, silicones, fluoroelastomers, fluorosilicone elastomers, EPDM elastomers, or other polymers with lower strain to yield, for example thermoplastic polyurethanes, PEEK, PEKK or polyamide. This alternative embodiment may allow weight reduction and flexibility of the structure may be further enhanced. The polymer layer  210  may then be adhered to a surface using an adhesive layer  206  or directly as described with respect to  FIG. 2D . 
     In the form shown in  FIG. 2A ,  2 B or  2 C, the embodiment may fabricated as a multilayer appliqué  209 , as shown in  FIG. 2B , including the tips  202 , surface layer  204 , polymer layer  207  and adhesive layer  206  which can then be adhered to the aerodynamic surface  111  using the adhesive layer  206 . 
     In alternative embodiments, the surface layer  204  may be directly adhered to or deposited on an aircraft surface  111 .  FIG. 2D  demonstrates an embodiment similar to that described with respect to  FIG. 2C  however, no adhesive layer is employed. Elastomeric layer  210 ′ is a thermoplastic cast into the nickel cladding  208  which allows direct bonding to the aircraft surface  111  with application of heat. 
     Another embodiment for rigid tipped riblets is shown in  FIG. 3 . With complex or multiple curved surfaces, it may be desirable in the first layer  301  for the individual riblet tips  302  to be separated from each other perpendicular to the flow direction for greater lateral flexibility. For the embodiment shown individual tips  302  protrude from an elastomeric layer  304 . Tips  302  have an internal angle  303  of approximately 30° for the exemplary embodiment. A base  306  expands from each tip. In certain embodiments the elastomeric layer  304  surrounds the base  306  to provide greater structural continuity. In alternative embodiments a bottom face  308  of the base adheres directly to the exposed surface  304   a  of the elastomeric layer  304 . 
     The second layer  303  is created by a multilayer structure incorporating a screen and/or foil metallic layer  310  such as aluminum, a polymer layer  312  such as PEEK and an adhesive layer  314  supports the elastomeric layer  304 . The polymer layer  312  and adhesive layer  314  may be supplied as a portion of the preformed appliqué as described with respect to  FIG. 9  below or directly deposited on the elastomeric layer  304 . The metallic layer  310  provides a conducting material for lightning strike protection in an exemplary aircraft usage of the embodiment. The metallic layer, polymer and adhesive multilayer structure may be comparable to a current lightning strike appliqué (LSA) employed for composite aircraft structural surfaces. 
     The elastomer layer  304  supporting the riblet tips  302  may provide elastic sideways deformation and recovery for the tips  302  when lateral forces are applied thereby further enhancing the durability of the rigid riblet tips. Additionally, the elastomeric layer  304  flexibility may allow greater ability to conform to complex contour shapes. 
       FIG. 4  demonstrates a third embodiment for the rigid tipped riblets  112  in  FIG. 1  which takes advantage of the structural capability provided by the material from which the riblets  112  are formed to allow a sharper profile of tips  402 . For the embodiment shown in each of the tips  402  extends from a base  406  supported in an elastomer layer  404 . As with the embodiment described with respect to  FIG. 3  the base  406  of each tip  402  is surrounded by the elastomer to structurally retain the base  406  within the elastomer layer  404 . In alternative embodiments, the extended bottom surface  408  of the base  406  may be adhered to the surface  404   a  of the elastomer layer  404 . The embodiment of  FIG. 4  also employs riblet tips  402  separated perpendicular to the flow direction  114  as in the embodiment of  FIG. 3 . However, in alternative embodiments a continuous surface layer  204  from which the tips  202  extend as disclosed for the embodiment described with respect to  FIG. 2A  may be employed. 
     As also disclosed in  FIG. 4  the embodiment employs a supporting polymer layer  410  on which the elastomer layer  404  is adhered or deposited. An adhesive layer  412  extends from the polymer layer opposite the elastomer layer  410  forming a multilayer appliqué  414 . 
       FIG. 5  shows a top view of the embodiment as disclosed in  FIG. 2B . The riblets formed by the tips  202  extend longitudinally along surface layer  204  in the flow direction  114 . The thin surface layer  204  provides for flexibility in adhering to curvature having tangents substantially perpendicular to the riblets. However as previously described, the surface  111  on which the riblets  112  may be employed may have multiple complex curvatures requiring greater flexibility. The embodiments previously described may therefore be adapted as shown in  FIG. 6A  wherein the individual tips  402  as described with respect to  FIG. 4  are laterally separated by spacing  118  substantially perpendicular to the flow direction  114  with bases  406  attached to or captured within an elastomer layer  404 . This provides even greater flexibility for adhering to surfaces with curvatures having tangents (generally shown as represented by arrow  604 ) substantially perpendicular to the riblets  112 . The scale of the drawings herein based on the small riblet dimensions makes the surfaces appear flat even though they may be curved in larger scale. An aluminum foil layer  407  has been added to the embodiment of  FIG. 6B  for demonstration of an embodiment for lightning strike protection with tips  402  which may be non-metallic. Additionally the individual riblets incorporate longitudinal separation in the flow direction using gaps  602  to segment the riblet to provide greater flexibility for adhering to surfaces having curvatures with tangents substantially parallel to the riblets  112  in the flow direction  114 . For the embodiment shown gaps  602  may be evenly spaced in the riblets  112  at substantially equal longitudinal distances  606 . In alternative embodiments spacing on individual riblets  112  and between riblets  112  may be uneven and chosen in a predetermined manner to accommodate surface curvature as required. 
       FIG. 7A  is a flow diagram showing a first exemplary manufacturing process for a riblet structure as defined in the embodiment described with respect to  FIG. 2A . In step  701  a master tool  712  is created using, as an example without limitation, diamond machining of a copper form or other suitable material on which an acrylate film is cured then stripped to define spaced protuberances  714  corresponding to the desired riblet dimensions. The tool  712  as shown in  FIG. 7A  may be a section of a flat tool, or a roller employed for roll-to-roll web processing. Exemplary details of a web processing format are shown in  FIG. 7C . For the embodiment shown in  FIG. 7A  nickel is employed for the rigid tips  202 . A complimentary tool  716  is created in step  702  by impression, casting or electroforming on the master tool  712  which provides grooves  718  corresponding to the riblet shape. Spacing between the grooves  718  provides a substantially flat intermediate surface  720  corresponding to the dimension  118  desired between the tips  202 . In step  703 , rigid tips  202  and surface layer  204  may be deposited by electroforming onto the complimentary tool  716 . In certain embodiments, a release compound is applied to the surfaces on the complimentary tool to assist in removal of the cast riblets and surface layer from the tool. Adhesive layer  206  is then applied, in step  704 , to the surface layer  204  opposite the rigid tips  202 . The adhesive layer  206  may be combined with a polymer layer, such as support layer  207  as shown in  FIG. 2B  and supplied as a preformed appliqué which is then joined with the electroformed surface layer  204 . A removable adhesive liner  722  for handling of the completed appliqué is added as also shown in step  704 . The appliqué, created by surface layer  204  and adhesive layer  206 , is removed from the complimentary tool  716  and a masking layer  724  is applied for handling as shown in step  705 . For exemplary embodiments, the masking employed may be, without limitation, static masking films, masking films with low tack pressure sensitive adhesive, or castable films of silicone. Application to the aircraft surface  726  is accomplished by removal of the adhesive liner  722  followed by attachment of the adhesive layer  206  of the appliqué to aircraft surface  726 . Removal of the masking layer  724  completes the riblet appliqué processing. 
     The complimentary tool  716  may be a “web tool” which may be silicone or polymeric film. Roll-to-roll processing for the steps described subsequently may then be employed as shown in  FIG. 7C  and the web tool  716  may be left in place as the masking that is removed after installation of the array of riblets  112  on the aircraft surface  726 . As shown in  FIG. 7B  for a method employing the web tool approach, a master tool  712  is created in step  731  define spaced protuberances  714  corresponding to the desired riblet dimensions. The tool  712  as shown in  FIG. 7B  may be a section of a flat tool, or a roller employed for roll-to-roll web processing. A complimentary web tool  746  is created in step  732  by roll processing silicone on the master tool  712  which provides grooves  718  corresponding to the riblet shape. Spacing between the grooves provides a flat intermediate surface  720  corresponding to the dimension  118  desired between the rigid tips  202 . A conductive layer, shown as the dashed line designated as element  747 , is then sputtered onto the silicon web tool, in step  733 , providing a conductive surface on the web tool. In step  734 , rigid tips  202  and surface layer  204  are deposited by electroforming onto the web tool. Adhesive layer  206  is then applied in step  735  to the surface layer  204  opposite the rigid tips  202 . The adhesive layer  206  may be combined with a polymer layer  207 , as shown for the embodiment in  FIG. 2B , and supplied as a preformed appliqué  723  which is then joined with the electroformed surface layer  204 . A removable adhesive liner  722  for handling of the completed appliqué  723  is added as also shown in step  735 . Application to the aircraft surface  724  is accomplished by removal of the adhesive liner  722  shown in step  736  followed by attachment of the adhesive layer  206  of the appliqué to aircraft surface  724  in step  737 . Stripping of the silicone web tool  746  exposes the rigid tips  202  of the riblets and completes the riblet appliqué processing. 
     As shown if  FIG. 7C , a roll-to-roll web processing approach may be employed for the methods described. Master tool  712  is created using, as an example, diamond machining of a copper form  742  on which an acrylate film  744  is cured then stripped and applied to a roller  745  to provide the master tool  712  shown in the drawing. Complimentary web tool  746  is then created by impression on master tool  712 . Conductive layer  747  is sputtered onto the web tool  746  using sputtering gun  750  and electroforming of the tips  202  surface layer  204 , as shown for example in  FIG. 7B , onto the web tool  746  is accomplished with deposition tool  752 . The adhesive layer  206  is then deposited on the surface layer  204  with deposition tool  754  and the removable adhesive liner  722  attached by application from roll  756 . The multilayer appliqué  725  is then available for attachment to the aircraft surface  724  as shown, for example, in step  737  of  FIG. 7B . 
       FIG. 8  is a flow diagram showing a manufacturing process for a riblet structure as defined in the embodiment described with respect to  FIG. 3 . In step  801  a web tool  812  is created as previously described with respect to  FIG. 7C  to define spaced protuberances  814  corresponding to the desired riblet dimensions. The tool  812 , as shown in  FIG. 8 , may be a section of a flat tool or a roll tool employed for web processing. For the embodiment shown in  FIG. 8 , nickel is employed for the rigid tips  302 . A complimentary tool  816  is created in step  802  by impression on the web tool  816  which provides grooves  818  corresponding to the riblet shape. Spacing between the grooves provides a substantially flat intermediate surface  820  corresponding to the dimension  118  desired between the riblet tips  302 . In certain embodiments, the complimentary tool  816  may be nickel or a silicon web tool as described with respect to  FIG. 7C . In step  803  resist  822  is applied to the flat surfaces  820  on the nickel tool and rigid tips  302  are deposited by electro-forming onto the tool in step  804 . The resist  822  is then removed in step  806  providing the spaced riblets in the tool. For the embodiment shown the bases  306  are placed into relief extending from the tool  816  by the removal of the resist as shown in step  806 . The elastomer layer  304  is then cast over the bases  306  in step  807 . In alternative embodiments electroforming of the rigid tips  302  provides a base substantially flush with the flat surface for direct adherence to the elastomer surface  305  as previously described with respect to  FIG. 3 . For the exemplary process shown with respect to  FIG. 8  a preformed appliqué  824  comprising the multilayer structure of aluminum foil as a metallic layer  310 , polymer layer  312  and adhesive layer  314  is adhered to the cast elastomer  304  in step  808 . A removable adhesive liner  826  for preservation of the adhesive during further processing is shown as a portion of the preformed appliqué. The multilayer structure is then removed from the complimentary tool  816  creating a multilayer riblet array appliqué  829  and exposing the rigid tips  302 . Masking  828  is applied over the tips  302  and elastomer  304  to assist in handling during additional processing and as also shown in step  808 . The masking  828  in exemplary embodiments may be, without limitation, a solution cast releasable polymer such as silicon or an adhesive film such as Mylar® with a low tack acrylic adhesive applied during roll processing. Alternatively, the complimentary web tool  816  when fabricated from a water/fluid soluble polymer may be employed as masking layer  828  to allow removal of the masking by dissolving with water or other fluid after installation. 
     The completed multilayer riblet array appliqué  829  may then be applied to an airplane surface  830  by removing the adhesive liner  826  and adhering the adhesive layer  314  to the surface  830  as shown in step  809 . The masking  828  is then removed from the tips  302  and elastomer  304 . 
     The rigid materials employed for the tips as described in the embodiments and fabrication processes herein allows very fine tip structure having a dimension  307  of around 15 to 25 microns at the base with a dimension  309  at the extreme end of the tips typically on the order of 100 nanometers (0.1 micron) as shown in  FIG. 3 . Smaller tips may be obtained with tooling and release process refinement. Even thought the tips are very sharp, the very fine spacing of the tips avoids cuts in normal handling by installation personnel. 
       FIG. 9  is a flow diagram showing a manufacturing process for a riblet structure as defined in the embodiment described with respect to  FIG. 2A . In step  901  a master tool  912  is created. The tool  912 , as shown in  FIG. 9 , may be a section of a flat tool or a roller employed for roll-to-roll web processing. For the embodiment shown in  FIG. 9  nickel is employed for the cladding  208  which forms the rigid tips  202 ′ and surface layer  204 ′. A complimentary tool  916  is created in step  902  by impression on the master tool  912  which provides grooves  918  corresponding to the riblet shape. Spacing between the grooves provides a substantially flat intermediate surface  920  corresponding to the dimension  118  desired between the riblets tips  202 ′. In step  903  nickel cladding  208  is deposited by electroforming into the complimentary tool  916  to form rigid tips  202 ′ and surface layer  204 ′ in step  903 . In alternative embodiments, the cladding may be cast or roll formed into the complimentary tool. In certain embodiments, a release compound is applied to the surfaces on the complimentary tool  916  to assist in removal of the tips  202 ′ and surface layer  204 ′ from the tool  916 . Polymer layer  210  is then cast into the cladding  208  to provide both a support layer and light weight cores  212  for the tips in step  904 . As previously described the polymer layer  210  may be an elastomer in certain embodiments. Adhesive layer  206  is then applied in step  905  to the polymer layer  210  opposite the rigid tips  202 ′ to create an appliqué  922 . A removable adhesive liner  924  for handling of the completed appliqué  922  is added, the appliqué  922  with adhesive liner  924  is removed from the nickel tool  916  and masking  926  is applied over the tips  202 ′ and surface layer  204 ′ for handling as also shown in step  905 . Application to the aircraft surface  928  is accomplished as shown in step  906  by removal of the adhesive liner  924  followed by attachment of the adhesive layer  206  of the appliqué  922  to aircraft surface  928 . Removal of the masking  926  completes the process. 
     Referring more particularly to  FIGS. 10 and 11 , embodiments of the rigid riblets disclosed herein and the methods for their fabrication may be described in the context of an aircraft manufacturing and service method  1000  as shown in  FIG. 10  and an aircraft  1102  as shown in  FIG. 11 . During pre-production, exemplary method  1000  may include specification and design  1004  of the aircraft, which may include the riblets, and material procurement  1006 . During production, component and subassembly manufacturing  1008  and system integration  1010  of the aircraft takes place. The riblet appliqués and their manufacturing processes as described herein may be accomplished as a portion of the production, component and subassembly manufacturing step  1008  and/or as a portion of the system integration  1010 . Thereafter, the aircraft may go through certification and delivery  1012  in order to be placed in service  1014 . While in service by a customer, the aircraft  1002  is scheduled for routine maintenance and service  1016  (which may also include modification, reconfiguration, refurbishment, and so on). The riblet appliqués as described herein may also be fabricated and applied as a portion of routine maintenance and service. 
     Each of the processes of method  1000  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of venders, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 11 , the aircraft  1102  produced by exemplary method  1000  may include an airframe  1118  having a surface  111 , as described with respect to  FIG. 1 , and a plurality of systems  1120  and an interior  1122 . Examples of high-level systems  1120  include one or more of a propulsion systems  1124 , an electrical and avionics system  1126 , a hydraulic system  1128 , and an environmental system  1130 . Any number of other systems may be included. The rigid tipped riblets supported by the embodiments disclosed herein may be a portion of the airframe, notably the finishing of skin and exterior surfaces. Although an aerospace example is shown, the principles disclosed by the embodiments herein may be applied to other industries, such as the automotive industry and the marine/ship industry. 
     Apparatus and methods embodied herein may be employed during any one or more of the stages of the production and service method  1000 . For example, components or subassemblies corresponding to production process  1008  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  1102  is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages  1008  and  1010 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  1102 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft  1102  is in service, for example and without limitation, to maintenance and service  1016 . 
     Having now described various embodiments in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present disclosure as defined in the following claims.