Abstract:
A riblet forming system incorporates a gantry which supports a laser and a paint applicator is positioned and moved over an aerodynamic surface. A computer control system is connected for control of the gantry on a predetermined path over the aerodynamic surface and further connected to the laser to control cutting a riblet topography along the predetermined path in a predeposited paint layer.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of application Ser. No. 14/450,089 filed on 098/01/2014 entitled DRAG REDUCTION RIBLETS INTEGRATED IN A PAINT LAYER and having a common assignee with the present application, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND INFORMATION 
       [0002]    Field 
         [0003]    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 fabrication methods and apparatus for aerodynamic riblets formed in a paint layer. 
         [0004]    Background 
         [0005]    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. 
         [0006]    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 thermoplastic in the form of appliques to create the desired topology. These appliques are bonded to the aircraft paint with adhesives. Polymers are relatively soft however, and 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 for they add weight and potentially dissolve when exposed to solvents. The strength of adhesives to bond the appliques to the vehicle surface may also create issues. Aircraft surfaces are typically required to withstand interactions with various chemicals including Skydrol®, a hydraulic fluid produced by Solutia, Inc. 
         [0007]    The practicality of riblets for commercial aircraft use would therefore be significantly enhanced with a riblet structure which is easily applied to aerodynamic surfaces, which does not add weight, is not easily compromised by exposure to the commercial air transport environment, and easily repaired or replaced when damaged. 
       SUMMARY 
       [0008]    Exemplary embodiments provide a riblet forming system having a print head with an array of print jets. A gantry supports the print head and is positioned and moved over an aerodynamic surface. A computer control system is connected for control of the gantry on a predetermined path over the aerodynamic surface and further connected to the print head to control the print jets to sequentially apply print dots from the array of print jets forming a riblet profile along the predetermined path. 
         [0009]    In another embodiment, a riblet impression system incorporates an impression mold element and the gantry supports the impression mold element for positioning and motion over the aerodynamic surface. The computer control system is connected to and controls the gantry on a predetermined path over the aerodynamic surface and further controls the gantry vertically with respect to the aerodynamic surface to sequentially impress the impression mold element in a malleable paint layer on the aerodynamic surface forming a riblet profile along the predetermined path. 
         [0010]    In yet another embodiment, a riblet construction system employs a gantry adapted for motion over an aerodynamic surface on a predetermined path with a laser mounted to the gantry. A control system is connected to the gantry to move the gantry on the predetermined path and is connected to the laser to control the laser cutting a riblet topology in a predeposited paint layer. 
         [0011]    In a further embodiment, a riblet stereo lithography system includes an agent applicator applying a stereo lithographic agent on an aerodynamic surface. An activator trails the agent applicator on a predetermined path over the aerodynamic surface and activates the agent to create a riblet structure. A spray nozzle trails the activator and flushes away non-activated material leaving the riblet structure in place on the surface. 
         [0012]    The embodiments provide a method for creation of riblets on an aerodynamic surface wherein a paint layer is applied to an aerodynamic surface. An impression mold element having a negative profile mold of a riblet array is positioned prior to drying of the paint layer and lowered into contact with the paint layer with sufficient pressure to imprint a riblet array in the paint layer. The impression mold element is sequentially transitioned over the paint layer on a predetermined path to create a riblet array. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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: 
           [0014]      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; 
           [0015]      FIG. 2A  is a section view of an array of riblets in a paint layer as described for the embodiments herein; 
           [0016]      FIG. 2B  is a section view of a generalized case wherein the riblets are separated by an extent of flat surface 
           [0017]      FIG. 2C  is a section view of an array of riblets created with paint on an aerodynamic surface as described for the embodiments herein; 
           [0018]      FIG. 3  is a top view of the array of riblets of  FIG. 2A ; 
           [0019]      FIG. 4A  is a top view of the array of riblets of  FIG. 2B  with additional features; 
           [0020]      FIG. 4B  is a top view of an array of riblets with a predetermined coordinated pattern; 
           [0021]      FIG. 5  is a block diagram of a paint print head for application of riblets to an aerodynamic surface; 
           [0022]      FIG. 6A  is a block diagram of an impression roller system for creating riblets in a malleable paint layer; 
           [0023]      FIG. 6B  is a flow diagram of a method for creating riblets in a malleable paint layer with the impression roller system of  FIG. 6A ; 
           [0024]      FIG. 6C  is a block diagram of an imprint block system for creating riblets in a malleable paint layer; 
           [0025]      FIG. 6D  is a flow diagram of a method for creating riblets in a malleable paint layer with the imprint block system of  FIG. 6C ; 
           [0026]      FIG. 7A  is a section schematic view of preparation of a master tool for creation of an impression roller or an imprint block for creating riblets in a malleable paint layer; 
           [0027]      FIG. 7B-7D  provide a flow diagram of an alternative for preparation of a riblet negative for an impression roller or an imprint block for creating riblets in a malleable paint layer; 
           [0028]      FIG. 7E  shows attachment of the riblet negative to a roller; 
           [0029]      FIG. 8  is a system block diagram for a multiple element system for dispensing paint, rolling the malleable paint with an impression roller and hardening the paint with a catalyzing agent; 
           [0030]      FIG. 9A  is a block diagram of a laser system for cutting riblet topology into paint; 
           [0031]      FIG. 9B  is a block diagram of a catalyzing system for stereo lithographic building of riblet structure; 
           [0032]      FIG. 10  is a flow diagram describing use of the riblet embodiments disclosed herein in the context of an aircraft manufacturing and service method; and 
           [0033]      FIG. 11  is a block diagram representing an aircraft employing the riblets with embodiments as disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    An exemplary embodiment of 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 . 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 .  FIG. 2A  shows a cross section of immediately adjacent riblets as may be applicable for high speed aerodynamic applications. In a generalized case, as shown in  FIG. 2B , the riblets may be spaced laterally on the surface  111 . For the exemplary embodiment shown, dimension  116  perpendicular to the surface  111  as shown in  FIGS. 2A and 2B  and tip-to-tip spacing  118  between the riblets as shown in  FIG. 3  are determined by the dynamic pressure, Q, exerted on the surface and may vary from location to location on the vehicle. Larger dimensions for the riblets and/or spacing may be present for hydrodynamic or low speed aerodynamic applications while extremely small dimension may be employed for high speed aerodynamic applications. The riblets  112  shown in  FIGS. 2A and 2B  are integral to a complete paint layer  208 . Alternatively, riblets may be formed from paint strips or stripes  210  separately applied to the aerodynamic surface  111  as shown in  FIGS. 2B and 4A . Spacing of the riblets 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 on the surface may be required. To assist in providing necessary flexibility of the riblet structure, the riblets may be discontinuous as shown in  FIG. 4A  with gaps  402  spaced at longitudinal distances  406  along the length of riblets  112 . Separation  404  between paint strips forming riblets in the embodiment of  FIG. 2B  may be employed to eliminate lateral bending stresses which might deform riblets in the array. 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. While shown in the drawings previously described as substantially parallel, the riblets may be employed in an array of any coordinated pattern and may diverge, converge or intersect as shown in  FIG. 4B . 
         [0035]    The embodiments disclosed herein provide multiple structures and apparatus for creating or applying riblets to a surface such as printing heads, which are computer controlled, and can be programmed to apply aircraft markings with high levels of accuracy. Such heads are employed to apply topcoat or clearcoat riblets. In other embodiments riblets may be created by an impression mold element in a malleable paint layer which has not yet dried. Computer controlled rollers encompassing the impression mold element can be passed over the airplane surfaces during painting of the airplane while the paint is still malleable, thereby mechanically embedding the desired topology into the final surface film. Alternatively, a textured surface encompassing the impression mold element is pressed against the paint while the paint is still malleable. Such a surface could be made of flexible material to eliminate a requirement for contour matching if a rigid material were used. In other embodiments, riblets may be formed on or in a paint surface. In one example, computer controlled lasers are used to cut the required topology into the paint. In another embodiment, lasers are used to pass over surfaces that are wetted with a paint or other agent activated by laser light or other catalyzing system for stereo lithographic building of riblet structure. The non-activated material can then be flushed away leaving the consolidated riblet structure in place. 
         [0036]    As shown in  FIG. 5 , a print head  502  having an array of print jets  504  is supported by a controllable gantry  506 . The array of print jets may be two dimensional to allow printing of multiple layers  508 ,  510  of paint along an axis of movement created by the gantry as well as painting a swath lateral to the axis of motion. The print heads  504  are shown in exaggerated size for clarity in the drawings but would be suitably sized for very small scale printing to allow sequential accumulation of multiple print “dots” to create the desired riblet size and shape. A computer control system  512  controls the gantry  506  for motion of the print head and also controls the array of print jets  504  to print the desired riblet profile and length. The two dimensional array of print jets  504  allows simultaneous printing of parallel riblets  112  and may also allow simultaneous printing of paint intermediate the riblets for a complete paint layer. 
         [0037]    An alternative to creating paint buildup to produce the riblet shape, riblets may be created in a malleable paint layer. As shown in  FIG. 6A , an aerodynamic surface  111  with an applied layer of paint  602  which has not yet hardened and remains in a malleable condition is engaged by a roller  604  suspended from a controlled motion gantry  606 . Roller  604  has a molded surface  605  having a negative impression of the desired riblet shape as will be described in greater detail subsequently. The riblet sizing on the roller is exaggerated in the figure for clarity. The gantry  606  is operated for motion along the aerodynamic surface  111  by a computer control system  608  which controls vertical motion of the gantry as well as lateral motion to roll the roller  604  over the paint layer. A sensor  610  on the gantry or roller provides force feedback to the computer control system  608  to allow appropriate pressure to be exerted by the gantry  606  through the roller  604  to form the riblets  112  in the malleable paint layer. Additional sensors for positioning and control of the roller on the aerodynamic surface by the computer control system  608  as well as input to the computer control system from a three dimensional CAD model may be employed as is known in the art. 
         [0038]    As shown in  FIG. 6B , operation of the embodiment of  FIG. 6A  involves applying a paint layer to an aerodynamic surface, step  650 . A roller having a negative profile mold of a riblet array is positioned with a mobile gantry under control of a computer control system above the aerodynamic surface prior to drying of the paint layer, step  652 . The roller is then lowered into contact with the paint layer with sufficient pressure to imprint a riblet array in the paint layer, step  654 , and rolled over the paint layer by the mobile gantry on a predetermined path to create the riblet array, step  656 . Pressure on the roller is measured by a sensor for vertical control of the mobile gantry, step  658 . Based on input from the sensor, the computer control system operates the mobile gantry to maintain a predetermined pressure on the roller, step  660 . 
         [0039]    As an alternative to a roller, a pressure foot  612  with a negative mold or imprint of the desired riblet profile may be attached to the gantry  606  as shown in  FIG. 6C . The gantry  606  would then be sequentially positioned by the computer control system  608  and vertically pressed into the paint layer  602  to create the riblets and employing pressure sensor  610  for control feedback. Pressure foot  612  may be flexible to match the contour of the surface at the location positioned by the gantry. 
         [0040]    Operation of the pressure foot is similar to the operation of the roller embodiment previously described. As shown in  FIG. 6D , a paint layer is applied to an aerodynamic surface, step  670 . A pressure foot having a negative profile mold of a riblet array is positioned with a mobile gantry under control of a computer control system above the aerodynamic surface prior to drying of the paint layer, step  672 . The pressure foot is then lowered into contact with the paint layer with sufficient pressure to imprint a riblet array in the paint layer, step  674 . The pressure foot is then raised and moved by the mobile gantry to sequential positions on a predetermined path and lowered at each position to create the riblet array, step  676 . Pressure on the pressure foot is measured by a sensor for vertical control of the mobile gantry, step  678 . Based on input from the sensor, the computer control system operates the mobile gantry to create a predetermined pressure on the roller at each sequential position, step  680 . 
         [0041]    Shown in  FIGS. 7A-7E , is an exemplary tool fabrication technique for implementing the embodiments described with respect to  FIGS. 6A and 6C .  FIG. 7A  shows a master tool  712  which provides the necessary shape or profile  714  for the riblets. A moldable material is then applied to the master tool to create a negative mold  716  which may then be used to actually form the riblets in the malleable paint layer. As shown in  FIG. 7B , master tool  712  may be created using, as an example, diamond machining of a copper form  718 . An acrylate film  720 , or similar material, is cured on the master tool  712 , as shown in  FIG. 7C . As shown in  FIG. 7D , the film  720  may then be stripped from the master tool  712  and applied to a roller core  722  to create the roller  604  as shown in  FIG. 6A  or applied to a pressure foot  612  as shown in  FIG. 6C . 
         [0042]    An integrated system may be employed as shown in  FIG. 8  wherein a gantry  606  under control of a computer control system  608  as previously described, carries a paint application unit  802  which applies a paint layer under control of the computer control system  608 . A roller  604  mounted on the gantry trailing the paint application unit  802  imprints the riblet profile into the paint layer as previously described with respect to  FIGS. 6A and 6B . The paint may employ a hardening or activating agent formulation and an activator  804  is mounted to the gantry  606  following the roller  604  to harden the formed paint layer including the riblets. UV or laser activation may be employed or, alternatively, a chemical hardener may be dispensed by the activator  804 . 
         [0043]    In an embodiment shown in  FIG. 9A , a computer controlled laser  902  mounted to the gantry  606  is transitioned over the aerodynamic surface  111  with a paint layer  904  previously applied to the surface. The laser  902  is activated by the computer control system  608  to cut the required topology for the riblets into the paint layer  904 . 
         [0044]    In another embodiment shown in  FIG. 9B , laser  902  under control computer control system  608  is passed by gantry  606  over the aerodynamic surface that has been wetted with an agent  906  by an applicator  908 , wherein the agent is activated by laser light for stero lithographic building of structure for the riblets  112 . The non-activated material is then flushed away by a spray nozzle  910  leaving the riblets  112  in place on the surface. The applicator  908  and spray nozzle  910  may be attached to the gantry  606  to lead and follow the laser  902  similar to the integrated system as described with respect to  FIG. 8 . An alternative activating or other catalyzing system such as UV may be employed instead of the laser. 
         [0045]    The embodiments described herein allow rapid repair of damaged riblets by stripping of the paint layer in the damaged area and reapplication of paint and forming of riblets by any of the methods described. 
         [0046]    Referring more particularly to  FIGS. 10 and 11 , embodiments of the 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 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 riblets embodied in paint as described herein may also be fabricated and applied as a portion of routine maintenance and service. 
         [0047]    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. 
         [0048]    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 paint 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. 
         [0049]    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 . 
         [0050]    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.