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TECHNICAL FIELD 
     This document relates to awnings, and more particularly to air flow with respect to awning fabric. 
     BACKGROUND 
     Awnings are commonly used in a variety of situations to provide shelter from sun and/or rain. For example, some awnings are commonly used as overhangs attached to a storefront, window, doorway, deck, or other structure. Awnings are commonly used in conjunction with houses and buildings as well as recreational vehicles. 
     Some awnings include a sheet of awning fabric, such as canvas or plastic, stretched and supported by a frame. The awning fabric can be angled so as to allow rain water to run off in one or more directions. The awning fabric of some awnings can cover a relatively large area, which can allow the awning to provide a relatively large amount of shelter from sun and rain but can also cause the awning fabric to catch wind. Wind can cause the awning fabric to flap. Wind that is relatively strong and directed from certain angles can cause the awning fabric to flap undesirably aggressively, which can damage the awning and can be loud and unpleasant for users under and nearby the awning. 
     SUMMARY 
     This document provides methods and materials related to awnings and awning systems. Some embodiments of an awning airfoil system provided herein can include one or more of the features and functions disclosed herein. For example, an awning airfoil system provided herein can spoil airflow so as to reduce flapping of awning fabric. In some cases, an awning airfoil system provided herein can include airfoils attached above and below an awning to spoil airflow on both sides of the awning. 
     In one aspect, an apparatus includes an awning fastener configured for attaching to an awning. The apparatus further includes a first airfoil connected to the awning fastener. The apparatus further includes a second airfoil connected to the awning fastener and positioned opposite of the first airfoil such that the first and second airfoils are positionable on opposite sides of the awning when the awning fastener is connected to the awning. 
     Implementations can include any, all, or none of the following features. The awning fastener includes first and second clamps configured for clamping to an awning edge, wherein the first airfoil is connected to tops of the first and second clamps, and wherein the second airfoil is connected to bottoms of the first and second clamps. A first leading edge of the first airfoil abuts a first alignment guide positioned on a top surface of the awning fastener and a second leading edge of the second airfoil abuts a second alignment guide positioned on a bottom surface of the awning fastener. The first airfoil has a first leading edge positioned proximate the awning fastener and is angled so as to have a first trailing edge spaced away from the awning fastener. The second airfoil has a second leading edge positioned proximate the awning fastener and is angled so as to have a second trailing edge spaced away from the awning fastener. Each of the first and second airfoils are curved with substantially concave surfaces, have a length of between about 24 inches and about 42 inches, and have a depth of between about 2 inches and about 4 inches. Each of the first and second airfoils are angled with respect to the awning fastener by greater than about 15 degrees, have a length of between about 24 inches and about 60 inches, and have a depth of between about 1 inch and about 6 inches. The awning fastener includes first and second arms and at least one pad pivotably attached to a first end of the first arm and facing toward a second end of the second arm. The awning fastener further includes a second pad rigidly attached to the second end of the second arm and facing toward the first pad, the first arm includes a first rim extending from a third end of the first arm toward a fourth end of the second arm, and the second arm includes a second rim extending from the fourth end toward the first rim. An adjustable connector extends through a first hole near a first leading edge of the first airfoil, through a second hole near a second leading edge of the second airfoil, and through at least one hole of the awning fastener. The adjustable connector is configured to connect the first and second airfoils to the awning fastener and to tighten the awning fastener to the awning. An awning has a first edge, and the awning fastener is connected to the awning proximate the first edge of the awning such that the first airfoil is positioned above a top surface of the awning and such that the second airfoil is positioned below a bottom surface of the awning. An assembly includes an awning having a first edge. The awning fastener is clamped to both an awning fabric and a side rail of the awning such that the awning fastener spans a gap extending between the awning fabric and the side rail. 
     In one aspect, a method includes positioning a first airfoil along a first edge of an awning above the awning. The method further includes positioning a second airfoil along the first edge of the awning below the awning. The method further includes attaching the first and second airfoils to the awning. 
     Implementations can include any, all, or none of the following features. The method including attaching the first and second airfoils to an awning fabric of the awning. The method including attaching the first and second airfoils to a side rail of the awning. The awning is a retractable awning attached to a side of a recreational vehicle and the side rail is a telescoping side rail of the awning. The method including attaching the first and second airfoils via a clamp that attaches to both an awning fabric and a side rail of the awning such that the clamp spans a gap extending between the awning fabric and the side rail. The method including attaching the first and second airfoils aligned substantially parallel to a side edge of the awning. The method including positioning a third airfoil along a second edge of the awning above the awning, positioning a fourth airfoil along the second edge of the awning below the awning, and attaching the first and second airfoils to the awning. The second edge is an edge of the awning opposite the first edge of the awning. The method includes attaching the first and second airfoils to a side rail of the awning such that first and second leading edges of the airfoils face toward the side rail and first and second trailing edges curve away from awning fabric of the awning. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic perspective view of a vehicle, awning, and awning airfoil system. 
         FIG. 2  is a schematic perspective view of the awning airfoil system and a portion of the awning of  FIG. 1 , viewed from a top, windward direction. 
         FIG. 3  is a schematic perspective view of the awning airfoil system and a portion of the awning of  FIG. 1 , viewed from a top, leeward direction. 
         FIG. 4  is a schematic perspective view of the awning airfoil system and a portion of the awning of  FIG. 1 , viewed from a bottom, windward direction. 
         FIG. 5  is a schematic end view of the awning airfoil system and a portion of the awning of  FIG. 1 . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic perspective view of vehicle  10 , awning  12 , and awning airfoil system  14 . In some embodiments, awning airfoil system  14  can attach to awning  12  to spoil aerodynamic flow of air across awning  12  to reduce flapping by awning  12  during windy conditions. 
     Vehicle  10  can be a recreational vehicle (RV) having wheels  16  supporting vehicle body  18 . In some embodiments, vehicle  10  can be a towable RV with trailer hitch  20 . In other embodiments, vehicle  10  can be a motorized RV. Vehicle  10  includes awning  12  attached to a side of vehicle body  18 . 
     Awning  12  includes awning frame  22  and awning fabric  24 . Awning frame  22  can include side rails  26  and  28 , end rail  30 , and support rails  32  and  34 . Awning frame  22  can stretch and support awning fabric  24 . Awning fabric  24  can extend from the side of the vehicle body  18  to the end rail  30 . Side rails  26  and  28  can extend substantially parallel to awning fabric  24  and can be connected to opposing ends of end rail  30  to support end rail  30  at a distance from vehicle body  18 . Support rails  32  and  34  extend diagonally from vehicle body  18  to connect to distal ends of support rails  32  and  34 , respectively, so as to support awning frame  22  and awning fabric  24  with an angled orientation. In some embodiments, awning  12  can be a retractable awning and awning frame  22  can be a retractable frame. For example, some or all of side rails  26  and  28  and end rails  32  and  34  can be telescoping rails that allow for awning frame to extend and retract. 
     In some embodiments, awning  12  can be positioned and configured differently than as illustrated. For example, awning  12  can be a non-retractable awning. In some embodiments, awning  12  can be attached to a structure other than vehicle  10 , such as a patio, house, or building. In some embodiments, awning  12  can be a portable awning. 
     Awning airfoil system  14  can be connected to awning  12 . As illustrated in  FIG. 1 , awning airfoil system  14  is attached to awning  12  and positioned along an edge of awning  12  near side rail  28 . One or more additional awning airfoil systems (not shown) can also be attached to awning  12 , for example along the edge of awning  12  near side rail  28  positioned next to awning airfoil system  14  and/or along an edge of awning  12  near side rail  26  positioned opposite awning airfoil system  14 . 
     Awning airfoil system  14  can attach airfoils  36  and  38  to the awning  12 . Airfoil  36  can be positioned along an edge of awning  12  above awning  12  and airfoil  38  can be positioned along the edge of awning  12  below awning  12 . Airfoils  36  and  38  can spoil aerodynamic flow of air across awning  12  to reduce flapping by awning  12  during windy conditions. 
       FIG. 2  is a schematic perspective view of awning airfoil system  14  and a portion of awning  12 , viewed from a top, windward direction. In an example illustrated in  FIG. 2 , wind is blowing in prevailing wind direction W toward side rail  28  and across awning  12 . Awning airfoil system  14  includes awning fastener  40  connected to airfoils  36  and  38  and can fasten awning airfoil system  14  to awning  12 . In some embodiments, awning fastener  40  can be a clamp system including clamps  42  and  44 , each including lower and upper clamp arms  46 ,  48 ,  50 , and  52 . Clamps  42  and  44  can attach airfoils  36  and  38  to side rail  28 . 
       FIG. 3  is a schematic perspective view of awning airfoil system  14  and a portion of awning  12 , viewed from a top, leeward direction with respect to prevailing wind direction W. A portion of awning fabric  24  is cut away in  FIG. 3  to show pads  54  and  56  attached at ends of clamp arms  46  and  48 , respectively. Awning fastener  40  can also include pads  58  and  60  (with pad  58  obscured by awning fabric  24 ) attached at ends of clamp arms  50  and  52 , respectively. Pads  54 ,  56 ,  58  and  60  can be relatively pliable and resilient, as suitable for the application, for attaching awning airfoil system  14  to awning fabric  24 . In some embodiments, clamp arms  46 ,  48 ,  50 , and  52  can be relatively rigid and pads  54 ,  56 ,  58 , and  60  can be relatively pliable and resilient. 
       FIG. 4  is a schematic perspective view of awning airfoil system  14  and a portion of awning  12 , viewed from a bottom, windward direction with respect to prevailing wind direction W.  FIG. 4  shows connector  62  connecting clamp arm  46  to clamp arm  48  and connector  64  connecting clamp arm  50  to clamp arm  52 . Connector  62  includes bolt  66  extending through holes in clamp arms  46  and  48  and includes knob  68  threaded on an end of bolt  66 . Knob  68  can be tightened to tighten clamp arms  46  and  48  on side rail  28  and on awning fabric  24 . Connector  64  includes bolt  70  extending through holes in clamp arms  50  and  52  and includes knob  72  threaded on an end of bolt  70 . Knob  72  can be tightened to tighten clamp arms  50  and  52  on side rail  28  and on awning fabric  24 . 
     In some embodiments, connector  62  can also include lock nut  67  and washer  69  located below clamp arm  48  that rigidly connects clamp arm  48  to airfoil  36  via bolt  66 . Connector  64  can also include lock nut  71  and washer  73  located below clamp arm  48  that rigidly connects clamp arm  48  to airfoil  36  via bolt  70 . Lock nuts  67  and  71  and washers  69  and  73  can help facilitate connecting and disconnecting awning airfoil system  14  to and from awning  12 . In other embodiments, some or all of lock nuts  67  and  71  and washers  69  and  73  can be omitted. 
       FIG. 5  is a schematic end view of awning airfoil system  14  and a portion of awning  12 . As shown in  FIG. 5 , a gap is defined between edge  74  of awning fabric  24  and side rail  28 . Clamp  42  can extend over the gap so as to clamp upon both awning fabric  24  and side rail  28 . Clamp  42  includes rims  76  and  78  to hold clamp  42  to side rail  28 . Rim  76  can extend upward from a windward end of clamp arm  46  and rim  78  can extend downward from a windward end of clamp arm  48  such that both rims  76  and  78  extend inward toward side rail  28 . In some embodiments, rims  76  and  78  can keep awning airfoil system  14  from sliding leeward when blown by wind. In some embodiments, rims  76  and  78  can resist twisting by awning airfoil system  14  in windy conditions. 
     Clamp  42  includes pads  54  and  56  to hold clamp  42  to awning fabric  24 . Pad  54  can extend upward from a leeward end of clamp arm  46  and pad  56  can extend downward from a leeward end of clamp arm  48  such that both pads  54  and  56  extend inward toward awning fabric  24 . In some embodiments, pad  56  can be mounted rigidly to clamp arm  48  and pad  54  can be mounted hingedly to clamp arm  46 . For example, in some embodiments, pad  54  can be mounted to foot  80 , which can be pivotably connected to pivot mount  82  at the leeward end of clamp arm  46 . Foot  80  can pivot about an axis that is perpendicular to a longitudinal length of clamp arm  46 . This pivoting movement of foot  80  and pad  54  can allow for pad  54  to rock and adjust to different relative angles as appropriate for use with a given awning  12 . This can allow awning airfoil system  14  to be used with various awnings  12  having relatively thick or thin side rails  28 , and still allow pads  54  and  56  to meet at a substantially flat interface even when clamp arms  46  and  48  are non-parallel. Knob  68  on connector  62  can be adjusted to adjust for size variation in a given side rail  28 . 
     Clamp  42  can connect airfoils  36  and  38  to opposite sides of awning  12  along an edge of awning  12 . In some embodiments, airfoil  36  can be connected to a top side of clamp arm  48 , and airfoil  38  can be connected to a bottom side of clamp arm  46 . Bolt  66  can extend through holes in airfoils  36  and  38  as well as clamp arms  46  and  48  so as to form a connected awning airfoil system  14 . 
     In some embodiments, awning airfoil system  14  can include alignment guides  84  and  86  for aligning airfoils  36  and  38 . For example, alignment guides  84  and  86  can be ridges extending from clamp arms  46  and  48 . Alignment guide  86  can extend from a bottom side of clamp arm  46  proximate leading edge  88  of airfoil  38 . Alignment guide  84  can extend from a top side of clamp arm  48  proximate leading edge  90  of airfoil  36 . Alignment guides  84  and  86  can help retain airfoils  36  and  38  to be oriented perpendicular to clamp  42  during operation of awning airfoil system  14 . Clamp  44  (not shown in  FIG. 5 ) can have similar alignment guides which cooperate with alignment guides  84  and  86  to retain airfoils  36  and  38  oriented perpendicular to clamps  42  and  44  during operation of awning airfoil system  14 . 
     While clamp  44  (not shown in  FIG. 5 ) is obscured by clamp  42  in  FIG. 5 , it should be understood that structure and operation of clamp  44  can be similar to that shown and described with respect to clamp  42 . Clamps  42  and  44  can be used together for attaching awning airfoil system  14  to awning  12 . In other embodiments, awning airfoil system  14  can be used with more or fewer than two clamps  42  and  44  as suitable for the application. For example, shape of clamp  42  could be modified such that a single clamp  42  is suitable for retaining awning airfoil system  14  to awning  12 . 
     In some embodiments, awning airfoil system  14  can be configured with one or both of the airfoils  36  and  38  angled away from clamp  42 . For example, airfoil  36  is illustrated in  FIG. 5  with leading edge  90  of airfoil  36  positioned proximate clamp  42  and trailing edge  92  spaced away from clamp  42 . Airfoil  36  can be curved from leading edge  90  to trailing edge  92  such that airfoil  36  curves away from clamp  42 , and consequently, curves away from awning  12 . Wind blowing toward awning airfoil system  14  generally in windward direction W can be spoiled by airfoil  36 . Wind can blow across leading edge  90  and be turned by airfoil  36  to be directed away from awning  12  as that wind passes from leading edge  90  toward and past trailing edge  92 . Airfoil  36  can disrupt airflow across a top surface of awning  12  to reduce flapping by awning fabric  24 . In some cases, airfoil  36  can increase air turbulence above the top of awning fabric  24 , thus reducing laminar air flow, reducing air flow speed, and/or reducing propensity to cause awning fabric  24  to flap aggressively. 
     Airfoil  38  is illustrated in  FIG. 5  with leading edge  88  of airfoil  38  positioned proximate clamp  42  and trailing edge  94  spaced away from clamp  42 . In a manner similar to that of airfoil  36 , airfoil  38  can be curved from leading edge  88  to trailing edge  94  such that airfoil  38  curves away from clamp  42 , and consequently, curves away from awning  12 . Wind blowing toward awning airfoil system  14  generally in windward direction W can be spoiled by airfoil  38 . Wind can blow across leading edge  88  and be turned by airfoil  38  to be directed away from awning  12  as that wind passes from leading edge  88  toward and past trailing edge  94 . Airfoil  38  can disrupt airflow across a bottom surface of awning  12  to reduce flapping by awning fabric  24 . In some cases, airfoil  36  can increase air turbulence below the bottom of awning fabric  24 , thus reducing laminar air flow, reducing air flow speed, and/or reducing propensity to cause awning fabric  24  to flap aggressively. 
     Together, airfoils  36  and  38  can spoil air flow across both the top and bottom of awning  12  to reduce flapping of awning fabric  24 . Airfoils  36  and  38  can work together to reduce flapping of awning fabric  24 , for example, during weather conditions in which wind direction changes. Spoiling airflow across both the top and bottom of awning via airfoils  36  and  38  can reduce flapping by awning fabric  24  even when there is a change in prevailing wind direction W and/or an instantaneous wind direction. 
     Airfoils  36  and  38  can be positioned proximate an edge of awning  12  to spoil airflow across awning  12 . In some embodiments, airfoils  36  and  38  can be positioned leeward of a windward-most edge of the awning  12 . For example, as illustrated in  FIG. 5 , airfoils  36  and  38  are positioned leeward of side rail  28  with leading edges  88  and  90  of airfoils  36  and  38  facing side rail  28 . Leading edges  88  and  90  of airfoils  36  and  38  can be laterally aligned with the gap between edge  74  of awning fabric  24  and side rail  28 . In other embodiments, airfoils  36  and  38  can be positioned aligned with and/or windward of side rail  28  with leading edges  88  and  90  facing away from side rail  28 . 
     In some embodiments, airfoils  36  and  38  can curve between their leading edges  88 ,  90  and trailing edges  92 ,  94  so as to define a substantially concave surface. For example, in some embodiments, airfoils  36  and  38  can curve substantially continuously between their leading edges  88 ,  90  and trailing edges  92 ,  94 . In one embodiment, airfoils  36  and  38  can be formed as quarter pipes with a substantially constant arc radius such that a tangent of leading edge  90  is substantially perpendicular to a tangent of trailing edge  92  and a tangent of leading edge  88  is perpendicular to a tangent of trailing edge  94 . In other embodiments, airfoils  36  and  38  can have a greater or lesser curvature as suitable for the application. 
     In other embodiments, airfoils  36  and  38  can have one or more curved portions and one or more straight portions between their leading edges  88 ,  90  and trailing edges  92 ,  94 . In other embodiments, airfoils  36  and  38  can extend from clamp  42  with little or no curve. For example, airfoils  36  and  38  can be substantially straight airfoils with leading edges  88  and  90  positioned adjacent clamp  42  and with trailing edges  92  and  94  spaced away from clamp  42  so as to spoil air flow across awning airfoil system  14  and awning  12 . In some embodiments, airfoils  36  and  38  could be angled with respect to clamp  42  and/or awning  12  by greater than about 15 degrees. In some embodiments, airfoils  36  and  38  could be angled with respect to clamp  42  and/or awning  12  by between about 30 to about 45 degrees. 
     Awning airfoil system  14  can be sized to spoil air flow across awning airfoil system  14  and awning  12  and reduce flapping by awning  12 . In some embodiments, airfoils  36  and  38  can be sized with a length of between about 24 inches and about 60 inches. In some airfoils  36  and  38  can be sized with a length of between about 24 inches and about 42 inches. In some airfoils  36  and  38  can be sized with a length of about 30 inches. 
     In some embodiments, airfoils  36  and  38  can be sized with a depth between their leading edges  88 ,  90  and trailing edges  92 ,  94  of between about 1 inch and about 6 inches. In some embodiments, airfoils  36  and  38  can be sized with a depth between their leading edges  88 ,  90  and trailing edges  92 ,  94  of between about 2 inch and about 4 inches. In some embodiments, airfoils  36  and  38  can be sized with a depth between their leading edges  88 ,  90  and trailing edges  92 ,  94  of about 3 inches. 
     Awning airfoil system  14  can be made out of one or more materials suitable for the application such as one or more polymers and or one or more metals. For example, in some embodiments, some or all of airfoils  36  and  38 , clamp arms  46  and  48 , pads  54  and  56 , and foot  80  can all be made of a polymer material, such as polyvinyl chloride (PVC). Bolt  66  can be made of a metal, such as steel. In other embodiments, materials can be varied as suitable for the application. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, some embodiments of awnings  12  need not include side rails  26  and  28 . In such embodiments, awning fastener  40  can be modified to attach to awning  12  without side rails  26  and  28 , for example by clamping to awning fabric  24  alone or in addition to alternative structure. Additionally, specific shape and orientation of various components can be modified and still yield an awning airfoil system suitable for spoiling airflow and reducing flapping of an awning in heavy wind conditions. Accordingly, other embodiments are within the scope of the following claims.

Summary:
This document provides an apparatus including an awning fastener configured for attaching to an awning. The apparatus can further include a first airfoil connected to the awning fastener. The apparatus can further include a second airfoil connected to the awning fastener and positioned opposite of the first airfoil such that the first and second airfoils are positionable on opposite sides of the awning when the awning fastener is connected to the awning.