Abstract:
A cross flow instability inhibiting assembly generates periodic aerodynamic disturbances on a swept wing. The cross flow instability inhibiting assembly is dynamic in that it can be selectively turned on and off as needed. The cross flow instability inhibiting assembly is a strip of material separating a set of electrodes from a set of electrodes. When energized, the fields created between the electrodes and electrodes create plasma disturbances around the electrodes. The electric fields and plasma create heating and body force disturbances on the air or surrounding fluid. These plasma generated disturbances disrupt development of unstable voriticity due to cross flow, inhibiting transition to turbulent flow of the wing to which it is attached. The electrodes may be connected to electrical power in series or they may be connected to an alternating configuration. The system allows for various uses based on the design of the wing and the conditions in which the host aircraft is flying.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND ART 
     1. Field of the Invention 
     The invention generally relates to performance features of a wing of an aircraft. More particularly, the invention relates to a structure for inhibiting cross flow instabilities from reducing the performance of a wing or other aerodynamic surfaces of an aircraft. 
     2. Description of the Related Art 
     Over the years, wing design has changed dramatically. Wing leading edges are swept to allow higher speeds without suffering large drag increases. Wings and tails are also swept to control the aerodynamic center and balance of an aircraft. Other issues including integration of sensors also drive the wing designer to sweep the wing leading edge back. 
     An important issue to the design of the swept wing is cross flow instabilities. Cross flow is the flow of air along the wingspan, from the root towards the tip, as opposed to over or under the wing, as it is designed to travel across a wing. Cross flow is not parallel to the primary air flow direction (the direction of travel of an aircraft), but flows outwardly, towards the wing tips when the wing is swept back. Cross flow occurs very close to the wing surface in an area referred to as the boundary layer. The air in the boundary layer is heavily influenced by the effects of viscosity and the ‘no-slip’ condition at the surface of the wing. These effects retard the flow of air over the wing and create a “viscous” drag on the wing. The airflow outside the boundary layer (further from the wing surface) is only minimally influenced by the effects of viscosity. 
     On swept wings, cross flow occurs primarily in the boundary layer, but does not occur to the same magnitude in the inviscid region outside the boundary layer. There is a continuous rapid change in the direction of the flow inside the boundary layer with the maximum cross flow occurring just off the surface and reduced cross flow as the distance from the surface is increased. This change in the direction of the airflow with distance normal to the surface creates vorticity that is amplified downstream and causes the flow in the boundary layer to transition from laminar to turbulent. This transition is marked by a change in the flow character and the drag. The laminar boundary layer is ordered, and minimal mixing occurs between layers (lamina). A turbulent boundary layer is marked by turbulent mixing that disrupts the previous laminar flow. The turbulent mixing causes an increased rate of exchange of momentum between the higher velocity flow further from the surface and the lower velocity flow closer to the surface. This increased exchange of momentum creates larger velocities closer to the surface and this leads to higher ‘friction drag’ at the surface. The friction drag of a laminar boundary layer can be about half of the friction drag of a turbulent boundary layer and for a typical all wing subsonic aircraft, this results in about 25% lower total drag and 25% lower fuel consumption. The benefits are smaller if laminar flow is achieved only on the wings and the aircraft consist of a wing and fuselage. 
     To minimize the occurrences in which cross flow instabilities are amplified and cause transition from laminar to turbulent flow, Distributed Roughness Elements (DRE) have been designed into wings. DREs are physical “bumps” added to or designed into a surface of a wing. The physical bumps create a disturbance in the flow field that prevents cross flow instabilities from growing and causing the transition from laminar flow to turbulent flow boundary layer conditions. The bumps create periodic vorticity at a scale that is well damped downstream. This vorticity inhibits the formation of larger scale vorticity that is not damped and would grow and eventually cause the flow to transition from laminar to turbulent. In the past, these DREs have either been fixed geometric bumps in the wing surface or pneumatically powered flexible bumps. Both of these two solutions have their deficiencies. With regard to the fixed physical DREs, there is no control in the magnitude, spacing or disturbance location as these are fixed in place and made during the manufacturer of the wing or applied as an appliqué before flight. The pneumatically controlled DREs are complex, require a fluid source (air or the like), and offer limited control. The shape of pneumatic bumps is typically far from ideal also as the bumps tend to be smooth while sharper disturbances create more voracity. 
     SUMMARY OF THE INVENTION 
     A cross flow instability inhibiting assembly generates periodic disturbances on a swept wing. The cross flow instability inhibiting assembly includes a base of dielectric material having an inner surface and an outer surface. A plurality of electrodes is fixedly secured to the inner surface. The plurality of electrodes is electrically connected to a source of electrical current. The cross flow instability inhibiting assembly also includes a plurality of electrodes fixedly secured to the outer surface. The plurality of electrodes generates a plasma and an aerodynamic disturbance area disposed above or adjacent to each of the plurality of electrodes. The plasma generated disturbances, arranged periodically on the surface, prevent and inhibit cross flow instabilities across the swept wing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is an exploded perspective view of one embodiment of the invention fixedly secured to a swept wing of an aircraft; 
         FIG. 2  is a bottom view of one device incorporating the invention as it is attached to a sheet of devices shown partially cutaway; 
         FIG. 3  is an end view of  FIG. 2 ; 
         FIG. 4  is a top view of the device of  FIG. 2  incorporating the invention as it is attached to a sheet of devices shown partially cutaway; 
         FIG. 5  is a side view of  FIG. 4 ; 
         FIG. 6  is a bottom view of a device having an alternative embodiment of the invention as it is attached to a sheet of devices shown partially cutaway; 
         FIG. 7  is a collection of exemplary cross sections of the electrodes; 
         FIG. 8  is a collection of plasmas created by variously shaped electrodes under various levels of voltage application; 
         FIG. 9  is a first alternate embodiment wherein the ‘divots’ of the prior embodiment are filled with dielectric and a ‘ring’ of conductor(s) line the divot and provide a ‘ring’ upper electrode; and 
         FIG. 10  is a second alternate embodiment wherein the divot is filled as in  FIG. 9  and multiple layers of dielectric are used to provide alternate conduction paths to the lower electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , an aircraft, generally shown at  10 , includes a wing  12 ,  14 . The aircraft  10  also includes a propulsion device  16 , which is typically a jet engine. Because the aircraft  10  is designed to be unmanned, the aircraft  10  does not have a cockpit or a fuselage. It should be appreciated by those skilled in the art that the aircraft  10  can be a manned aircraft. Further, the means of propulsion need not be provided by a jet engine as a propeller driven by another type of engine (e.g., internal combustion engine), or a glider could incorporate the invention. 
     The wing  12 ,  14  defines a leading edge  18 ,  20 . The leading edge  18 ,  20  divides the air through which the aircraft  10  is traveling so that a portion of the air goes over the swept wing  12 ,  14 , and a portion of the air goes underneath the swept wing  12 ,  14 . The wing  12 ,  14  is swept back from perpendicular to the airflow. Leading edge  18  is swept back through angle  19  from an unswept wing configuration. For purposes of simplicity, one embodiment of the invention, a cross flow instability inhibiting assembly, is generally indicated at  22  along the leading edge  20  of the swept wing  14 . It should be appreciated by those skilled in the art that the invention  22  would also extend along the leading edge  18  of swept wing  12 . While the invention  22  extends along almost the entire leading edge  20 , the cross flow instability inhibiting assembly  22  may only extend along a portion or multiple portions of the leading edge  20 . Determining the location(s) of where the cross flow instability inhibiting assembly  22  will be located is determined by the wing  12 ,  14  design and the portions of the wing  12 ,  14  that are designed to maintain laminar flow. 
     The cross flow instability inhibiting assembly  22  is shown in  FIG. 1  to include a base  24  that is fixedly secured to an upper side  26  of the swept wing  14 . It should be appreciated that the cross flow instability inhibiting assembly  22  may also be fixedly secured in an equal or differing configuration on the bottom side (not shown) of the swept wing  14  or on other aerodynamic surfaces. The cross flow instability inhibiting assembly  22  is creating a plurality of plasma squares  28 , best shown on the far right side of  FIG. 1 . These plasma squares  28  have a height (shown best in  FIG. 5 ) allowing the plasma square  28  to act as solid structures on the surface of the wing  14  such that it creates vorticity at a scale and spacing that is well damped and will dissipate downstream. This vorticity prevents the formation and growth of larger and more widely spaced vorticity that is unstable and leads to transition from laminar to turbulent flow. If the cross flow instability inhibiting assembly  22  is turned off, the plasma squares  28  will disappear and the base  24  will have an aerodynamically smooth surface. As such, the cross flow instability inhibiting assembly  22  would not have an effect on the air flow moving over the wings  12 ,  14  because the plasma would not be created by the cross flow instability inhibiting assembly  22 . This would lead to streamwise vorticity growth and transition from laminar to turbulent conditions. It should be appreciated by those skilled in the art that although  FIG. 1  shows a single array of plasma squares  28 , these plasma squares  28  could be replaced by plasmas in any number of shape configurations, as will be discussed in greater detail below. Further, the array of plasma squares  28  could be replaced with a two-dimensional array of multiple rows in columns as is deemed appropriate based on the design of the swept wing  12 ,  14 . In addition, the two dimensional arrays that may be created may have the plasma squares  28  offset from each other from row to row depending on the design parameters of the swept wing  12 ,  14 . These rows could be powered individually or in groups depending on the location of conducting paths and switches on the lower surface and the desired disturbance spacing and location for a given flight condition. 
     Referring to  FIG. 2 , a sheet of cross flow instability inhibiting assemblies  22  is generally shown at  30 . The cross flow instability inhibiting assemblies  22  are formed in a single sheet and then severed from each other along etching lines  32  (example shown in  FIG. 2 ).  FIG. 2  represents an underside of the sheet  30  wherein the cross flow instability inhibiting assemblies  22  are viewed from their inner surfaces. The inner surface  34  of the cross flow instability inhibiting assembly  22  abuts the wing  14  when secured thereto. The base  24  of the cross flow instability inhibiting assembly  22  is made of a dielectric material having an inner surface  34  (shown in  FIG. 2 ) and an outer surface  36  (best seen in  FIGS. 3 and 4 ). As is typical with a substrate, the inner  34  and outer  36  surfaces are parallel to each other. The dielectric material is typically flexible and therefore can conform to the wing shape. 
     Returning attention to  FIG. 2 , the lower surface consists of a series of shaped electrodes, in this case circles  38 , connected by conducting paths  40 ,  42 . One polarity of the AC power supply is attached to one end of the series of circles  38  creating one line of electrically connected electrodes  38 . A second channel from the power supply is connected to the electrically separate, second series of electrodes  42 . The lower surface lines  40 ,  42  can be either the alternating potential (voltage) or the connection to ground, but there are several advantages of using the bottom lines  40 ,  42  as the alternating voltage. By having the voltage source connected to the bottom line  40 ,  42 , the top electrode  44  is held at ground. This increases safety and reduces the likelihood of the upper surface voltage potential arcing to nearby conducting material. The circles  50 , shown in  FIG. 4 , are regions where the conducting material of the top electrode  44  has been etched away. These circles  50  are directly above the conductor circles  38  on the lower surface  34  shown in  FIG. 2 . While not shown in these Figures, the conductor circles  38  and the circles  50  may not have equal areas. In fact, they may not have the same shapes (e.g., one may be an oval and the other may be a circle). It is, however, important that at least a portion of each of the conductors  38  be coaxial with each of the etched areas  50 . 
     The use of alternating electrodes  38  being serially connected is an example of how the plurality of electrodes  38  may be subdivided and controlled independently of each other in various configurations through switches, graphically represented at  45 ,  47 . While only one voltage source  43  and set of switches  45 ,  47  are shown to be electrically connected to the lead lines  40 ,  42 , it should be appreciated that these all lead lines  40 ,  42  will be connected to similar voltage sources and switching mechanisms when installed on a wing. 
     As an alternative to the embodiment shown in  FIG. 2 , wherein like prime numerals represent similar structures, the plurality of electrodes  38 ′ are connected together by a single, continuous lead line  40 ′ only in  FIG. 6 . In this particular example, the plurality of electrodes  38 ′ are not designed to operate independently of any of the other plurality of electrodes  38 ′ and all will be turned on and off together in unison. 
       FIG. 4  represents the outer surface  36  of the cross flow instability inhibiting assembly  22 . In a preferred embodiment, a ground layer  44  covers the entire outer surface  36 . The ground layer  44  is a material suitable for conducting a current. In the preferred embodiment, the ground layer  44  is a layer of copper sufficiently thick to easily carry the necessary currents. Nickel and gold plating are preferred to prevent or minimize oxidation of the copper. The ground layer  44  is the entire upper surface except for the holes  50  (or other shapes) where the conductor is etched away only down to the upper surface of the dielectric and not through the dielectric. A hole  48  that extends completely through assembly  22  provides a means to electrically connect the upper surface  44  to a ground pad on the bottom side. This allows a completely smooth upper surface. A hole  48  is lined with copper, nickel and gold in the preferred embodiment and, therefore, electrically connects the entire upper surface with the pad  46  on the lower surface, best shown in  FIG. 5 . As an alternative to utilizing the hole  48 , an electrically conducting strap (not shown) may be used to provide the conducting path around the cross flow instability inhibiting assembly  22 . 
     Disposed within the ground layer  44  is a plurality of etched away regions  50 . The shapes of the regions  50  are exposed dielectric material and are aligned with the lower surface electrodes  38 . More specifically, the regions  50  and electrodes  38  are paired in a manner such that each pair is coaxial. This is best seen in  FIG. 5 . The plurality of exposed dielectric regions  50  and the plurality of electrodes  38  are separated physically by the base  24  of dielectric material. The base  24  may be pliable or rigid. In one embodiment, the base is fabricated from Teflon. 
     Referring to  FIG. 7 , cross sections of three different embodiments of a region  50  are shown with similar reference numerals multiply primed. Depending on the design of the wing  14  and the characteristics of the air flowing thereby, a particular shape for a region  50 ,  50 ′,  50 ″,  50 ′″ may be determined to be more effective at inhibiting cross flow than another. In that situation, a particular region cross section would be selected. It is conceivable that a single cross flow instability inhibiting assembly  22  may include regions  50  having multiple cross sectional configurations or shapes, wherein these regions  50  in varying shapes would be alternated or segregated in particular segments based on the requirements of the wing hosting the cross flow instability inhibiting assembly  22 . It should be appreciated by those skilled in the art that the set of example region cross sections  50 ′,  50 ″,  50 ′″ are not intended to be limiting as other cross-sectional shapes could be employed as well. The upper surface shaped dielectric regions  50  are expected to be matched by the shape of the lower surface electrodes  38  though, as stated above, this is not a requirement. In addition, there may be advantages to increasing or decreasing the size of the lower surface electrode relative to the upper surface dielectric region to create larger or smaller disturbances on the upper surface of the wing. Also, offsets to the coaxial alignment of the upper surface exposed dielectric material and the lower surface electrode may be desirable to optimize the voracity disturbance created by the device. 
     Referring to  FIG. 8 , multiple examples of cross flow instability inhibiting assemblies are shown. In the upper left hand corner, the regions  50  are squares. In the lower left hand corner, the regions are triangles. The dielectric layer  24  on the left hand side is 10 mils (0.010″). The right side of  FIG. 8  represents plasmas created by regions that are circles. The upper right hand corner represents a dielectric layer  24  having a thickness of 5 mils (0.005″), whereas the lower right hand corner shows a dielectric layer  24  having a thickness of 10 mils (0.010″). Three examples for each of the various cross flow instability inhibiting assemblies  22  are shown representing three different levels of voltage being applied to the plurality of electrodes  38 .  FIG. 8  represents the fact that the plasmas  62  generated by the cross flow instability inhibiting assembly  22  is enhanced with increased voltage. It is also determined that the plasmas  62  may be created through different thicknesses of the dielectric by managing the voltage applied across the electrodes  38 . As is shown in the right side of  FIG. 8 , the same plasmas  62  may be generated over a large range of thicknesses of the dielectric layer  24  by varying the voltage between the top and bottom surfaces. 
     It has been determined through study that the wing  12 ,  14  works best when the cross flow instability inhibiting assembly  22  is otherwise smooth. Therefore, each of the plurality of regions  50  is filled with a non-conductive material  60 . The non-conductive material  60  does not affect the electrical fields created in the regions  50 , but merely prevents air flow disturbances from being created in the space defined thereby. The etching away of the conductor to create this region  50  leaves a ‘divot’ typically between 5 and 50 microns. For many applications, this divot is large enough to create voracity that could cause premature transition to turbulent flow. The turbulence created by the divot may mimic or offset the effect of the plasma being created at the same location. In the preferred embodiment, the divots are filled after the device is constructed. Various dielectric materials can be used and light sanding of the device outer surface ensures a smooth final surface. 
     Turning attention to  FIG. 9 , the top layer of an alternate embodiment is shown that provides a perfectly smooth outer surface  145  and ‘ring’ electrodes  152 ,  154 ,  156  on the surface. These ring electrodes  152 ,  154 ,  156  form a tube or cylinder having a single side wall circumscribing the region  150 . It should be appreciated by those skilled in the art that any number of ring electrodes  152 ,  154 ,  156  may be designed into the invention to form the tube side wall. Further, with the cross sections of  FIG. 7 , it should be appreciated that the tube may have a non-circular cross section resulting in more than one side wall required to complete the tube (replacing the cylinder). Like elements similar to those discussed in prior embodiments have reference numbers offset from prior embodiments by 100. In  FIG. 9 , a cross section of the dielectric layer  124  is shown. The dielectric layer  124  defines the regions  150  therein. In this embodiment, each region  150  includes three concentric layers  152 ,  154 ,  156 . These layers are fabricated from different conductive materials. A non-exhaustive list of conductive materials that may be used to create the conductive layers  152 ,  154 ,  156  include, but are not limited to, gold, nickel and copper. Any number of conductive layers  152 ,  154 ,  156  may be used. In one embodiment, it is contemplated that a single conductive layer  152  would be all that is necessary. It is the conductive layer  152  (or conductive layers  152 ,  154 ,  156 ) that is affected by the current passing through the pads  160  to create a plasma disposed immediately adjacent to conducting layers  152 ,  154 ,  156 , and above dielectric material  150 . Outer surface  145  is now a dielectric material and the ground potential is only present in the ring electrodes  152 ,  154 ,  156 . This is substantially different than the previous embodiment arrangement in which the top surface  44  is conducting material and divot  50  was constructed by removal of the conducting material. The top layer of this alternate embodiment is constructed to leave a perfectly smooth outer surface. This is accomplished by starting with dielectric material that is coated with a conductor on one side. The lower portion of the upper surface is coated with conductor and is etched to leave conducting paths and pads  160 . Holes are drilled at the location  150 , centered above pads  160 . The center of the hole is plated with conductors  152 ,  154 , and  156 . Finally, a liquid dielectric material is squeezed into the holes  150  and not allowed to escape or run over the upper surface. Once the dielectric material is cured, this top layer is complete and can be assembled with the other layers to be described in  FIG. 10 . An example dielectric material that could be squeezed into hole  150  is ‘pre-preg’. 
       FIG. 10  shows the assembly of the top layer shown in  FIG. 5  with multiple dielectric layers for this alternate embodiment. Each dielectric layer,  125  and  127  provides lower surface electrodes  138  and conducting paths  140 ,  142 , to those electrodes. These dielectric layers are sandwiched together with the top layer described in  FIG. 9 . Each dielectric layer  125  or  127  that is added to the sandwich provides at least two new paths to lower surface electrodes  138  and provides additional control of plasma disturbance spacing. The multiple dielectric layers  125 ,  127  provide a means for separating conducting lines (for example, lines  140  in different layers) physically from each other. While this configuration may be thicker, and in some cases, less flexible, the interference between conducting lines and electrodes  138  is greatly reduced. The electrodes  138  continue to be paired up with the regions  150  on a one-to-one basis. Additionally, the multiple dielectric layers  125 ,  127  allow for more complex control of individual plasma spots at regions  150  by providing additional paths to electrodes  138  and there could be more electrodes individually controlled or controlled in small groups. This provides greater control of the periodicity of the plasma generated disturbances. It is understood that additional layers could be added for additional control. 
     Referring to  FIG. 8 , multiple examples of cross flow instability inhibiting assemblies are shown for purposes of providing examples only. In the upper left hand corner, the regions  50  are squares. In the lower left hand corner, the regions are triangles. The dielectric layer  24  on the left hand side is 10 mils (0.010″). The right side of  FIG. 8  represents plasmas created by regions that are circles. The upper right hand corner represents a dielectric layer  24  having a thickness of 5 mils (0.005″), whereas the lower right hand corner shows a dielectric layer  24  having a thickness of 10 mils (0.010″). Three examples for each of the various cross flow instability inhibiting assemblies  22  are shown representing three different levels of voltage being applied to the plurality of electrodes  38 .  FIG. 8  represents the fact that the plasmas  62  generated by the cross flow instability inhibiting assembly  22  is enhanced with increased voltage. It is also determined that the plasmas  62  may be created through different thicknesses of the dielectric by managing the voltage applied across the electrodes  38 . As is shown in the right side of  FIG. 8 , the same plasmas  62  may be generated over a large range of thicknesses of the dielectric layer  24  by varying the voltage between the top and bottom surfaces. These example embodiments are merely illustrative and are not considered exhaustive. 
     The invention has been described in an illustrative manner. It is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation. 
     Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.