Wing/winglet configurations and methods for aircraft

Abstract: The specification discloses a wing/winglet configuration, which basically comprises: (a) a wing (6) having upper and lower wing surfaces, a wing leading edge (22), a wing trailing edge (24), and a wing tip (12); and (b) a winglet (16) having upper (26) and lower (28) winglet surfaces, a winglet leading edge (30), a winglet trailing edge (32), and a winglet root (34), where the winglet is mounted to the wing tip at winglet/wing tip intersection and extends upwardly from the wing tip. Aft portions of the upper wing surface and upper winglet surface are flattened near the winglet/wing tip intersection to substantially reduce flow separation near wing/winglet intersection.

Description: Technical Field This invention relates to a wing/winglet configuration, comprising: a wing having upper and lower wing surfaces, a wing leading edge, a wing trailing edge, and a wing tip; a wing reference plane being defined as a plane which is parallel to a longitudinal axis of an aircraft body to which the wing and winglet may be attached and which extends through the wing; a wing local chord being defined for any given portion of the wing as a line between the wing leading edge and wing trailing edge, where the wing local chord lies in a wing chord plane which is parallel to the longitudinal axis and perpendicular to the wing reference plane; and for any given wing chord plane which intersects the wing upper surface, a wing reference point being defined as a point on the wing upper surface that is furthest from the wing reference plane; and a winglet having upper and lower winglet surfaces, a winglet leading edge, a winglet trailing edge, and a winglet root, where the winglet root is attached to the wing tip at a wing/winglet intersection and the winglet extends upwardly from said wing tip; a winglet reference plane being defined as a plane which is parallel to the longitudinal axis and which extends through the winglet; a winglet local chord being defined for any given portion of the winglet as a line between the winglet leading edge and winglet trailing edge, where the winglet local chord lies in a winglet chord plane which is parallel to the longitudinal axis and perpendicular to the winglet reference plane; and for any giving winglet chord plane which intersects the winglet upper surface, a winglet reference point being defined as a point on the winglet upper surface that is furthest from the winglet reference plane; and a corner flow region being defined in which flow separation is likely to occur. Such a wing/winglet configuration is known from e.g. US-A-4 245 804. The invention further relates to an aircraft equipped with this wing/winglet configuration. Background of the Invention It has long been known that providing an endplate on the wing tip of an aircraft airfoil reduces induced drag at the wing tip. Induced drag is caused by the difference in fluid flow pressure on the upper and lower surfaces of the airfoil. At the wing tip, this pressure differential produces fluid flow that is transverse to the stream of lift-producing fluid; that is, because the fluid pressure is higher below the wing than above, fluid flows from below the wing to above the wing around the wing tip in a direction transverse to that of the lift-producing fluid. The endplate on the wing tip impedes this transverse fluid flow and thus reduces the induced drag on the airfoil. These endplates are relatively thin fins or shields that extend streamwise along the wing and project upwardly to impede transverse fluid flow along the top of the wing. Non-planar structures, such as tubular members with circular or elliptical cross-sections, have also been mounted on wing tips to reduce induced drag by impeding transverse fluid flow. The above-described endplates and other non-planar structures, while generally effective at reducing induced drag in low speed aircraft, were inappropriate for use with modern high speed aircraft because on high speed aircraft they tended to increase profile drag, interference drag, compressibility drag, flow separation at the wing tip/endplate intersection, and/or the bending moment at the root of the wing. Therefore, endplates and other non-planar structures have not been successfully employed on modern high speed aircraft. For these and other reasons, designers began to apply the same aerodynamic principles that govern wing airfoil design to the design of endplates. Such aerodynamically efficient endplates will be referred to herein as winglets. These winglets are able to reduce induced drag while lowering profile drag, interference drag, compressibility drag, and/or the bending moment at the root of the wing. However, it is well-known that these winglets are still susceptible to flow separation at the wing/winglet intersection. More particularly, the flow separation at the wing/winglet intersection is caused by interference between the fluid flowing over the upper surface of the wing and the fluid flowing over the upper surface of the winglet. The fluid streams flowing over these two upper surfaces diverge as they pass over the wing and winglet upper surfaces between the thickest portions and the trailing edges of the wing and winglet. These diverging streams of fluid will be discussed in more detail below with reference to Figures 3 and 4. Flow separation occurs in the areas on the upper surfaces between these divergent streams of fluid. This flow separation is particularly acute at low speed, high lifting conditions. Prior Art Figures 2-6 depict a conventional wing/winglet configuration as is generally known in the art. The main features of this configuration will be discussed below to the intent that they are relevant to the present invention. Before discussing the prior art wing/winglet configuration depicted in Figures 2-6, however, it is first necessary to describe in general terms the aircraft on which any such wing/winglet configuration will be mounted. An example of such an aircraft is depicted in Figure 1 and is generally indicated at reference character 2. In the following description, the terms "fore, front" denote the direction towards the lower left of Figure 1, the terms "aft, rear" denote the direction towards the upper right, the terms "left, port" denote the direction towards the lower right, and the terms "right, starboard" denote the direction towards the upper left. The terms "up, top" and "down, bottom" refer to the corresponding directions shown in Figure 1. The terms "inner" and "outer" refer to directions closer to and farther from, respectively, the aircraft 2. The aircraft 2 basically comprises an aircraft body 4, left and right wings 6, and a tail section 8. A prior art wing/winglet combination 10 may be mounted on the outer end of each wing 6. A coordinate system is defined for the aircraft 2 in the following manner. A longitudinal axis x is defined to intend through the center of the aircraft body 4 in the fore and aft directions. Further, a vertical axis z is defined in the up and down directions, while a transverse axis y is defined in the left and right directions. The longitudinal axis x, vertical axis z, and transverse axis y are orthogonal to each other and meet at an origin located at the forwardmost point of the aircraft 2. For each wing 6 of the aircraft 2, a wing reference plane is defined in the following manner. The wing reference planes are parallel to the longitudinal axis of the aircraft 2. Further, each wing reference plane generally intends at an angle upwardly to reflect the dihedral normally provided to the wing. In general, the wing reference plane upwardly extends at an angle of between 3° and 8° from the transverse axis y of the aircraft 2. Additionally, a winglet reference plane is defined for each winglet. The winglet reference plane is also parallel to the longitudinal axis of the aircraft. The wing span of the aircraft 2 is defined as the distance between the outermost points on the left and right wings 6 and is indicated at line WS in Figure 1. The term "semi-span" is employed to refer to the distance from the longitudinal axis x to the outermost points on either the left or right wing 6. At any point along the span of the wings 6, a wing local chord may be defined. A local chord is defined for any given portion of an airfoil as a line between the airfoil leading and trailing edges that is in a plane which is parallel to the longitudinal axes of the aircraft. Further, the plane in which the local chord resides and that is perpendicular to the wing or winglet reference plane will be referred to herein as a chord plane. The wing/winglet combination 10 will now be described with reference to Figures 2-6. Indicated at 12 is a tip of the wing 6. At a wing/winglet intersection 14, a winglet 16, which is generally trapezoidal in shape, is joined to the wing tip 12 so that the winglet 16 upwardly extends from the wing 6. A strake is indicated by reference character 16a. The wing 12 has upper and lower wing surfaces 18 and 20, a wing leading edge 22, and a wing trailing edge 24. Similarly, the winglet 16 has upper and lower winglet surfaces 26 and 28, a winglet leading edge 30, a winglet trailing edge 32, and a winglet root 34. Conventionally, the terms "upper" and "lower" used in reference to the winglet 16 generally correspond to the "inner" and "outer" directions, respectively, defined above in relation to the aircraft 2. This convention will be followed herein. The winglet 16 is swept back at an angle from the vertical z-axis at least equal to the sweep angle of the leading edges of the wings at the wingtip 14 relative to the transverse y-axis (Fig. 2). The winglet is also canted at a cant angle β outwardly from a plane parallel to the x and y axes (Fig. 3). The streamwise airfoil sections depicted in Figures 6 and 5 are of the conventional winglet 16 at the winglet root 34 and wing 12 at the wing tip 14, respectively. In this description, the term streamwise refers to the direction from the front to the back of the aircraft 2. It should be noted that the airfoil sections depicted in Figures 5 and 6 are normalized in the direction of the x-axis and expanded in the direction of the z-axis to illustrate more clearly the geometric features of these airfoil sections. The winglet 16 and wing 12 are essentially advanced technology wings having generally similar streamwise airfoil sections. Specifically, the upper wing surface 18 is convex and is smoothly and aerodynamically curved, without any discontinuities thereon, from the leading edge 22 to the trailing edge 24 of the wing 12. The lower wing surface 20 of the wing 12 comprises both a convex portion 36 and a concave portion 38. The convex portion 32 is located forward on the lower surface 20 near the leading edge 22, and the concave portion 38 is located aft on the lower surface 20 near the trailing edge 24 of the wing 16. The lower surface 20 is smoothly and aerodynamically curved from the wing leading edge 22 to the wing trailing edge 24. Similarly, the upper surface 26 of the winglet 16 is convex and smoothly curved from the winglet leading edge 30 to the winglet trailing edge 32. As with the lower wing surface 20, the lower winglet surface 28 comprises both a convex portion 40 and a concave portion 42. The lower wing surface is smoothly curved from the winglet leading edge 30 to the winglet trailing edge 36. The determination of the curvature of these upper surfaces 18 and 26 and lower surfaces 20 and 28 are known in the art and will not be discussed further herein. A primary feature of the airfoils depicted in Figures 5 and 6 is that they control shock waves generated when the fluid passing over the airfoil is decelerated to subsonic speeds. More specifically, the aircraft 2 is designed to cruise at high subsonic speeds of approximately Mach = 0.81. Because the fluid flowing over the upper surface of an airfoil is accelerated to provide lift, the fluid that flows over the upper wing surface 18 and upper winglet surface 26 accelerates to transonic speeds at cruise. To reduce overall drag on the aircraft, the fluid traveling over the wing at transonic speeds should be decelerated to less than Mach = 1.0 as it passes over the upper wing and winglet surfaces. The airfoils depicted in Figures 5 and 6 are designed to decelerate the fluid flowing thereover in predictable fashion so that the shock waves created by the deceleration of the fluid to subsonic speeds do not increase drag on the airfoil. A relevant physical characteristic of an advanced technology airfoil as shown in Figures 5 and 6 and described above is that the aft portion of such an airfoil is relatively severely curved. The amount of curvature of the aft portions of the airfoils depicted in Figures 5 and 6 may be described in either of two ways. A first method of defining the amount of curvature of the aft portions of the airfoils of the wing 12 and winglet 16 is to define this curvature relative as a vertical distance between the uppermost point on the upper surface to the uppermost point trailing edge downstream of this point on the upper surface. One way to calculate this vertical distance is to determine distances relative to the wing or winglet reference plane in terms of a percentage of the length of the wing or winglet local chord. Indicated at 44 and 46 in Figures 5 and 6 are the local chords of the wing 12 at the wing tip 14 and the winglet 16 at the winglet root 34, respectively. These local chords 44 and 46 are each included in a chord plane associated therewith. These chord planes are perpendicular to their associated wing or winglet reference planes and coplanar with the surface of the paper on which Figures 5 and 6 are printed. For the prior art wing/winglet combination 10 shown in Figures 2-6 under no load conditions, the wing leading edge 22 and wing trailing edge 24 lie within a wing reference plane, which is indicated at reference character 48. Similarly, in the prior art winglet 16, the winglet leading edge 30 and trailing edge 32 lie within a winglet reference plane 50 when no loads are applied. It should be noted that in some cases twist in the wing may result in the trailing edge being within ±1% of the length of the chord above or below the wing or winglet reference plane. Reference character 52 on the wing upper surface 18 indicates what will be referred to herein as a "wing reference point". This wing reference point 52 is the point on the wing upper surface 18 that is farthest from the wing reference plane 48 and within the wing chord plane associated with the wing chord 44. In the conventional wing airfoil as depicted in Figure 5, the wing reference point 52 lies a distance approximately equal to 5±0.5% of the wing local chord 44 above the wing reference plane 48. Similarly, reference character 54 on the winglet upper surface 26 indicates a winglet reference point that is farthest from the winglet reference plane 50 and within the winglet chord plane associated with the winglet chord 46. In the conventional winglet airfoil depicted in Figure 6, this winglet reference point 54 lies a distance approximately equal to 4.5±0.5% of the winglet local chord 46 above the winglet reference plane 50. Because the wing and winglet trailing edges lie within their respective reference planes, the vertical distances between the wing and winglet reference points and the uppermost points on the wing and winglet trailing edges, which are indicated by reference characters 56 and 58 in Figures 5 and 6, are 5±0.5% and 4.5±0.5%, respectively. These vertical distances are relatively large and indicate the rather severe curvature of the aft portions of the upper surfaces 18 and 26. Alternatively, the curvature of these wing and winglet upper surfaces 18 and 26 may be defined in terms of the slope of a line connecting each reference point to the uppermost portion of the trailing edge lying in the chord plane associated with these reference points. To determine these slopes, it is necessary to: (a) determine the location of a given reference point and the uppermost point on the trailing edge in the chord plane associated with the given reference point in terms of an axis x′ and an axis y′, where the x′ and y′ axes define a coordinate system in that chord plane; and (b) calculate the slope from the following slope equation: m = (z′ 1 -z′ 2 )/(x′ 1 -x′ 2 ) where m is the slope of a line connecting a given reference point and the uppermost point on the trailing edge in the chord plane associated with that given reference point, z′₁ and x′₁ are the coordinates of the given wing reference point, and z′₂ and x′₂ are the coordinates of the uppermost point on the trailing edge in the chord plane associated with the given reference point. The sign of the slope m is negative for the right wing and winglet and positive for the left wing and winglet. The slopes of the right wings will be calculated herein, and the symbol "±" will be placed in front of the calculated slope to indicate that these numbers are applicable to both left and right wings and winglets. In the prior art wing/winglet configuration 10, the wing reference point 52 is located a distance approximately equal to 45±5% of the wing local chord 44 aft of the wing leading edge 22. The winglet reference point is located a distance approximately equal to 45±5% of the winglet local chord 46 aft of the winglet leading edge 30. Therefore, the slope of the line between the wing reference point 52 and the trailing edge 24 is within the range of (5±0.5-0)/(45±5-100), or between approximately ±0.07 and ±0.11. This line is indicated by reference character 60 in Figure 5. Similarly, in terms of the winglet reference plane 50 and the length of the winglet local chord 46, the slope of the line between the winglet reference point 54 and the winglet trailing edge 32 is within the range of (5±0.5-0)/(45±0.5-100), or between approximately ±0.06 and ±0.10. This line is indicated by reference character 62 in Figure 6. As described above, the airfoils depicted in Figures 5 and 6 generally conform to well-known and accepted airfoil design principles. Under most conditions, wings conforming to airfoils designed within the ranges set forth above provide maximum lift to and minimum drag on the aircraft to which they are attached. However, when these aerodynamically efficient airfoil designs are employed at the wing/winglet intersection, flow separation occurs in a corner flow region on the upper surfaces of the wing 12 and winglet 16 at the wing/winglet intersection. This flow separation is basically shown by arrows 64 and 66 in Figure 3 and arrows 66 and 68 in Figure 4. Arrows 66 and 68 in Figure 4 generally depict the path of the fluid flowing over the forward portions of the upper wing and winglet surfaces 18 and 26, respectively. As the fluid flows towards the thickest portions of the wing 6 and winglet 16 at the wing/winglet intersection 14, the fluid 66 flowing over the wing upper surface 18 interferes with the fluid 64 flowing over the winglet upper surface 26 at the wing/winglet intersection 14. Subsequently, as the fluid flows past the thickest portions of the wing and winglet, the fluid divides into two diverging streams, which are indicated by the arrows 64 and 66 in Figure 3. More particularly, as it flows over these upper surfaces past this furthest point from the wing and winglet reference planes, the fluid so adheres to the wing upper surface 18 and winglet upper surface 26 that these paths 64 and 66 have components in the direction of freestream air and components which are generally perpendicular to the wing and winglet reference planes 48 and 50, respectively. Since these reference planes 48 and 50 are not parallel to each other, the components of the direction of fluid flow perpendicular thereto are in diverging directions. The area between these divergent fluid flow paths 64 and 66, which is indicated by reference character 70, is defined as a corner flow region in which flow separation occurs. The dimensions of this area 70 vary according to the actual flight conditions, such as air speed and angle of attack of the aircraft 2, but can generally be defined as follows with reference to the wing and winglet semi-span. Specifically, the corner flow region for wing/winglet configuration 10 as mounted on wing 6 is an area on the wing and winglet upper surfaces 18 and 26 that is generally bounded by lines connecting: (a) a point on the wing trailing edge 24 at approximately 97.5±1% of the wing semi-span; (b) a point on the winglet trailing edge 32 at approximately 5±2% of the winglet semi-span; and (c) the point along the wing/winglet intersection 14 which is approximately 40±10% aft of the point where the wing leading edge 22 meets the winglet leading edge 30. For any given wing/winglet configuration, the corner flow region may be determined by wind tunnel testing, computer simulation, and/or flight testing. Many prior attempts to solve or reduce the problem of flow separation near the intersection of the wing tip and winglet have been made. Certain of these attempts are embodied in the winglet/wing tip combination described above. Specifically, the cant angle β is provided to reduce the interference between the paths of fluid flow, which subsequently causes flow separation, at the intersection of the winglet 16 and wing tip 14. In other words, as the angle between the wing and winglet reference planes becomes less acute, the interference and consequently the divergence of the fluid flow paths becomes less. However, the increased cant angle β increases the wing root bending moment contribution due to the winglet and renders the winglet sensitive to wing angle of attack variations. The strake 16a establishes a vortical flow pattern to promote attachment of the boundary layer under high speed conditions. In addition, the length 1 and/or surface area A of the winglet 16 may be reduced to reduce flow interference at the configuration 10. However, reducing the size of the winglet adversely affects the efficiency with which the winglet causes the endplate effect on the wing and thus reduces induced drag. These prior attempts at eliminating flow separation at the winglet/wing tip intersection have therefore been unsatisfactory because they have resulted in less than optimal winglet configurations over the entire flight envelope, generally satisfying only one flight condition. Therefore, an overall decrease in drag can be achieved by providing the winglet 16 on the wing tip 14 as described above because of the reduction in induced drag. However, most current winglet designs still operate at reduced efficiency due to flow separation at the intersection of the winglet and wing tip. A search of the patent literature has revealed a number of U.S. patents. These patents are noted below. The above-mentioned U.S. Patent No. 4,245,804, Ishimitsu et al., discloses a wing/winglet configuration having all the features of the pre-characterizing portion of claim 1. In this document, the winglet is canted upwardly and outwardly from a wing tip. The leading edge of the winglet is also "toed in" towards the wing. Such a canted, toed in winglet is sensitive to wing angle of attack variations, increases the wing bending moment caused by the winglet, and thus does not provide an optimal winglet configuration over the entire flight envelope. U.S. Patent No. 4,714,215, Jupp et al., discloses a winglet attached to a wing tip. The winglet is highly swept back and of low aspect ratio. This winglet/wingtip intersection provides a less than optimal configuration because the reduced size of the winglet decreases the winglets effectiveness at reducing induced drag. U.S. Patent No. 4,205,810, Ishimitsu et al., discloses a winglet that is twisted relative to the direction of freestream fluid. This twisted winglet is relatively radically curved at its aft, inner surface. Ishimitsu et al. 's ('810) twisted winglet is relatively radically curved at its aft, inner surface. Ishimitsu et al.'s ('810) twisted winglet may increase flow separation under low speed, high lift conditions. U.S. Patent No. 2,074,201, Bechereau, discloses providing a bulge or bulges on an airfoil at a place where eddying may occur. The Bechereau patent thus does not teach a winglet, but rather a non-planar endplate of the type discussed above. The Bechereau patent thus would not be appropriate for modern high speed aircraft. A report entitled "Design and Analysis of Winglets for Military aircraft," Boeing Commercial Airplane Company Technical Report AFFDL-TR-76-6, discusses the effects of different chordwise locations, sweep angles, taper ratios, areas, lengths, and cant angles of a twisted, cambered winglet. The different configurations studied in this test do not substantially reduce the effects of flow separation at the winglet/wingtip intersection under all flight conditions. The following references discovered in the search appear to be no more relevant, and quite likely less relevant, than the patents noted above. Accordingly, these are listed here without discussion. U.S. Patent No. 4190210 issued 26 Feb. 1980 to Hackett; U.S. Patent No. 4,017,041 issued 12 April 1977 to Nelson; U.S. Patent No. 4,247,063 issued 27 Jan. 1981 to Jenkins; U.S. Patent No. 4,240,597 issued 23 December 1980 to Ellis et al.; and U.S. Patent No. 3,845,918 issued 5 Nov. 1974 to White, Jr. Objects of the Invention From the foregoing, it should be apparent that one important objective of the invention is to provide a novel, improved winglet/wing tip intersection configuration which improves the performance of the wing and winglet. Another important object of the present invention is to provide winglet/wing tip intersection configuration that achieve a desirable balance among the following factors: reducing or eliminating flow separation at the wing tip; eliminating diverging flow surfaces at the intersection of the wing tip and winglet; reducing overall drag on the aircraft over the entire aircraft flight envelope; controlling rapid leading edge flow accelerations while still maintaining advanced airfoil technology characteristics at cruise conditions; allowing the application of wing and winglet twist and thickness distributions to satisfy desirable spanload distributions with the goal of minimizing overall induced drag; and allowing decreased airplane approach speed Summary of the Invention In accordance with the present invention, the above objects are achieved with a wing/winglet configuration of the type described in pre-characterizing portion of claim 1, in that an aft portion of at least one of the upper wing surface and upper winglet surface in said corner flow region is so flattened near the wing/winglet intersection that flow separation near the wing/winglet intersection is substantially reduced, a vertical distance between the wing reference point in the corner flow region and an uppermost point on the wing trailing edge furthest from the wing reference plane and in the given chord plane being less than 4.5% of the length of the wing chord associated with the given wing chord plane; and/or a vertical distance between the winglet reference point in the corner flow region and an uppermost point on the winglet trailing edge furthest from the winglet reference plane and in the given winglet chord plane being less than 4.0% of the length of the winglet chord associated with the given winglet chord plane. In a preferred embodiment of the wing/winglet configuration of the invention at least one of the leading edges of the wing and winglet is so drooped near the wing/winglet intersection relative to the leading edge distal from the wing/winglet intersection that premature shock wave development is substantially reduced. In a further preferred embodiment, the winglet (116) so extends from the wing tip that the winglet is substantially parallel to a vertical axis of the aircraft body to which the wing and winglet may be attached. This arrangment of the winglet minimizes the wing root bending moment, and renders the winglet less sensitive to wing angle of attack variations. Brief Description of the Drawing Figure 1 is a perspective of an aircraft on which the wing/winglet configurations of the prior art and present invention may be mounted; Figure 2 is a side, cut-away view of a wing/winglet configuration as is known in the art; Figure 3 is a rear plan view of the prior art wing/winglet configuration; Figure 4 is a front plan view of the prior art wing/winglet configuration; Figure 5 is a streamwise sectional view of the prior art wing taken at the wing tip; Figure 6 is a streamwise sectional view of the prior art winglet taken at the winglet root; Figure 7 is a top plan view of the wing on which the wing/winglet configurations of the present invention are mounted; Figure 8 is a top plan view of the wing tip and winglet showing the corner flow region in which flow separation is expected and the flattened area on the wing and winglet upper surface; Figure 9 is a perspective view of the wing/winglet configuration of the present invention; Figure 10 is a rear plan view of the wing/winglet configuration of the present invention; Figure 11 is a front plan view of the wing/winglet configuration of the present invention; Figure 12 is a streamwise sectional view of the wing of the present invention taken at the wing tip; Figure 13 is a streamwise sectional view of the winglet of the present invention taken at the winglet root; Figure 14 is a geometric comparison of streamwise airfoil sections taken at 95±2% of the wing semi-span and 100% of the wing semi-span-span for the wing of the present invention; and Figure 15 is a geometric comparison of streamwise airfoil sections taken at 25±3% of the winglet semi-span and 0% of the winglet semi-span for the winglet of the present invention. Detailed Description of the Preferred Embodiment Depicted in Figures 7-9 is a wing/winglet configuration 110 constructed in accordance with, and embodying, the principles of the present invention. This configuration 110 is mounted on the left and right wings 6 of the aircraft 2 as shown in Figure 1. The terminology and coordinate system described above with reference to Figure 1 also applies to the following discussion of Figures 7-15. Indicated at reference character 112 is the tip of the wing 6, and indicated at 114 is a wing/winglet intersection area of the wing/winglet configuration 110. A winglet 116, which is generally trapezoidal in shape, upwardly extends from the wing tip 112 at the wing/winglet intersection 114. The wing tip 112 has upper and lower wing surfaces 118 and 120, a wing leading edge 122, and a wing trailing edge 124. Similarly, the winglet 116 has upper and lower winglet surfaces 126 and 128, a winglet leading edge 130, a winglet trailing edge 132, and a winglet root 134. The winglet 116 is swept back at an angle α at least equal to the sweep angle of the aircraft wings at the leading edge 18 of the wingtip 14 (Fig. 9). The winglet 116 is not canted outwardly in the present invention; in other words, the cant angle β defined above with reference to Figure 3 equals zero for the wing/winglet configuration 110. Generally, the wing/winglet configuration 110 in accordance with the preferred embodiment of the present invention contains three features. First, the aft portion of the upper wing and winglet surfaces 110 and 126 are flattened to prevent flow separation at the wing/winglet intersection 114. Second, the wing and winglet leading edges 122 and 130 are drooped downwardly to prevent premature shock wave development. Third, the winglet is not canted outwardly, so the wing bending moments are not substantially increased by the addition of the winglet 116. Each of these features will be discussed below in further detail. In addition to these features, certain other features of the preferred embodiment of the present invention will then be discussed. I. Flattened Upper Surfaces The streamwise airfoil sections depicted in Figures 12 and 13 and by solid lines in Figures 14 and 15 are of the wing 112 at the wing tip 114 and the winglet 116 at the winglet root 134, respectively. The dashed lines in Figures 14 and 15, on the other hand, depict the streamwise airfoil sections of the wing 112 and winglet 116 at a point distal from the wing/winglet intersection 114. As discussed above, the term "streamwise" refers to the direction from the front to the back of the aircraft 2. It should be noted that the airfoil sections depicted in Figures 12 - 15 are normalized in the direction of the x-axis and expanded in the direction of the z-axis to illustrate more clearly the primary features of the wing 112 and winglet 116. As shown in Figure 12, the upper surface 118 of the wing 6 is convex. This convex upper surface 118 is aerodynamically curved from the leading edge 122 to the trailing edge 124. However, the aft portion of this upper surface 118 is relatively flat when compared with the same portion on the prior art airfoil depicted in Figure 5. The lower surface 120 is also aerodynamically curved from the leading edge 122 to the trailing edge 124. This lower surface 120 is convex in an area indicated by reference character 136 and concave in an area indicated by reference character 138. The exact dimensions of the lower surface 120 are not relevant to the present invention and may be determined as necessary to satisfy the span-wise load distributions of the wing 6. The lower surface 120 will therefore not be discussed further below. As shown in Figure 13, the upper surface 126 of the winglet 116 is also convex. This convex upper surface 126 is aerodynamically curved from the leading edge 130 to the trailing edge 132. The aft portion of this upper surface 126 is relatively flat when compared with the same portion on the prior art winglet airfoil depicted in Figure 6. The lower surface 128 is also aerodynamically curved from the leading edge 130 to the trailing edge 132. This lower surface 128 is convex in an area indicated by reference character 140 and concave in an area indicated by reference character 142. As with the lower surface 120 of the wing 6, the exact dimensions of the lower surface 128 are not relevant to the present invention and may be determined as necessary to satisfy the span-wise load distributions of the winglet 116. The lower surface 128 will therefore not be discussed in further detail. The flattened upper surfaces 118 and 126 are perhaps best shown by a comparison of the solid and dashed lines in Figures 14 and 15. According to the present invention, the upper wing surface 118 and/or the upper winglet surface 126 are flattened near the wing/winglet intersection 114 and the wing and winglet trailing edges 124 and 132 (solid lines) relative to the upper wing surface 118 and upper winglet surface 126 distal from the wing/winglet intersection 114 (dotted lines). This flattening substantially reduces separation of fluid flow from the upper wing surface 118 and the upper winglet surface 126 near the wing/winglet intersection 114. The relative flatness of the aft portions of these upper wing and winglet surfaces 118 and 126 may be represented in at least two ways. First, this flatness may be measured in terms of the distance between the uppermost point on the upper surface and the uppermost point on the trailing edge down stream of that upper most point in terms of a percentage of the length of the local chord of that chord plane. Specifically, a wing reference point 152 is defined as a point that is farthest from the wing chord 144 and within the chord plane associated with the wing chord 144. When no loads are applied to the wing/winglet configuration 110, the wing reference point 152 at the wing tip 112 is located a distance of approximately 5.0% of the wing local chord 144 above the wing reference plane 148. In addition, the uppermost point of the wing trailing edge 124 is located approximately 4.4% of the wing local chord 144 above the wing reference plane 148. Thus, the vertical distance, which is indicated by line 156 in Figure 12, between the wing reference point 152 and wing trailing edge 124 is approximately 0.6% of the wing local chord. In any event, this vertical distance 156 should be greater than 0.2% and less than 1.0%, and preferably between 0.3% and 0.7%, of the wing local chord 144 at the wing tip 114. Similarly, a winglet reference point 154 is defined as a point that is farthest from the winglet chord 146 and within the chord plane associated with the winglet chord 146. Under no load conditions, the winglet reference point 154 at the winglet root 134 is located a distance of approximately 3.8% of the winglet local chord 146 above the winglet reference plane 150. In addition, the uppermost point of the winglet trailing edge 132 is located approximately 3.2% of the winglet local chord 146 above the winglet reference plane 150. Thus, the vertical distance, which is indicated at reference character 158 in Figure 13, between the winglet reference point 154 and winglet trailing edge 132 is approximately 0.6% of the wing local chord. In most cases, this vertical distance 158 is greater than 0.2% and less than 1.0%, and preferably between 0.3% and 0.7%, of the winglet local chord 146 at the winglet root 134. These vertical distances 156 and 158 are much smaller than those distances 56 and 58 described above for the prior art wing/winglet configuration 10, indicating that the aft portions of the upper wing surface 118 and upper winglet surface 126 are significantly flatter than those of the prior art upper surfaces 18 and 26. A second method of quantifying the flattening of the upper surfaces 118 and 126 is by calculating the slope of the line connecting the wing and winglet reference points 152 and 154 with the uppermost portions of the wing and winglet trailing edges 124 and 132 that lie in the chord plane associated with the local chords 144 and 146, respectively. The uppermost portion of the wing or winglet trailing edge in the chord plane associated with these chords may also be referred to as the uppermost point on the wing trailing edge downstream, or to the aft, of the corresponding reference point. These slopes as defined above generally correspond to the amount of vertical drop of these upper surfaces; that is, the smaller the slope, the flatter the upper surface. In the preferred embodiment, the wing reference point 152 is located aft of the wing leading edge 122 a distance approximately equal to 52.5% of the wing local chord 144. Accordingly, employing the slope equation (1) set forth above, the slope of the line connecting the wing reference point 152 with the uppermost point of the wing trailing edge 124 at the wing tip 112 is (5.0-4.4)/(52.5-100), or approximately ±0.012. This line is indicated at reference character 160 in Figure 12. However, a slope between ±0.007 and ±0.027 at the wing tip 112 is within the scope of the present invention. The winglet reference point 154 is located aft of the winglet leading edge 130 a distance approximately equal to 40% of the winglet local chord 146. The slope of the line connecting the winglet reference point 154 with the uppermost point of the winglet trailing edge 132 at the winglet root 134 is therefore (3.8-3.2)/(40-100), or approximately ±0.010. This line is indicated at 162 in Figure 13. A slope between ±0.005 and ±0.015 at the winglet root 134 is within the scope of the present invention, however. The slopes of these lines 160 and 162 are much smaller than the slopes of the comparable lines 60 and 62 calculated for the prior art wing/winglet configuration 10, illustrating that the upper surfaces 118 and 126 of the present invention are quite flat in comparison to the upper surfaces 18 and 26 of the prior art configuration 10. The flattening of the wing and winglet upper surfaces 118 and 120 is most severe at the wing tip 112 and winglet root 134. However, these upper surfaces of the wing 6 and winglet 116 are gradually flattened towards the wing/winglet intersection 114. More particularly, as shown in Figures 7 and 8, the wing 6 may be referenced to the semi-span of the aircraft 2. According to this reference system, the wing 6 is attached to the aircraft at approximately 11% of the aircraft semi-span. Further, in the preferred embodiment, starting at approximately 75% semi-span the chord of the wing 6 is intended to obtain a favorable maximum wing thickness to wing chord ratio. This chord extension will be explained in further detail below in the discussion of the secondary features of the present invention. Referring now to Figure 8, a flattened area in which the wing upper surface 118 and winglet upper surface 126 are flattened is indicated by lines parallel to the y-axis on the wing and the z-axis on the winglet. As shown, this flattened area includes the area on the wing and winglet upper surfaces 118 and 126 that is generally bounded by lines connecting: (a) a point on the wing trailing edge 24 at approximately 95±2% of the wing semi-span; (b) a point on the winglet trailing edge 32 at approximately 25±3% of the winglet semi-span; and (c) the point along the wing/winglet intersection which is approximately 40±10% aft of the point where the wing leading edge 22 meets the winglet leading edge 30. The corner flow region in which flow separation is likely to occur is indicated by lines parallel to the x-axis. This corner flow region is determined for a particular wing/winglet configuration by wind tunnel testing, flight testing, and/or computer simulation. For the preferred embodiment, this region is the same as the corner flow region defined above for prior art apparatus 10. Specifically, the corner flow region is the area on the wing and winglet upper surfaces 118 and 126 that is generally bounded by lines connecting: (a) a point on the wing trailing edge 24 at approximately 97.5±1% of the wing semi-span; (b) a point on the winglet trailing edge 32 at approximately 5±2% of the winglet semi-span; and (c) the point along the wing/winglet intersection which is approximately 40±10% aft of the point where the wing leading edge 22 meets the winglet leading edge 30. As can be seen from Figure 8, the wing and winglet upper surfaces 118 and 126 are flattened at least inside of the corner flow region. In the preferred embodiment, the upper surfaces 118 and 126 are also flattened outside the corner flow region to allow the upper surfaces 118 and 126 to be gradually flattened from points outside the corner flow region to the wing tip 114 and winglet root 134 where the flattening is the greatest. This gradual flattening provides for an aerodynamically smooth transition to the point of maximum flattening without any discontinuities on the upper surfaces 118 and 126. This gradual flattening may be described in terms of the vertical distance between a given wing reference point and the uppermost point on the wing trailing edge associated with that given wing reference point. This vertical distance is approximately 5.3% of the wing local chord at 95±2% semi-span and gradually decreases to 0.6% of the wing local chord at the wing tip 114, or at 100% semi-span. Similarly, the gradual flattening of the winglet upper surface 126 may be described in terms of the vertical distance between a given winglet reference point and the uppermost point on the winglet trailing edge associated therewith. For the preferred embodiment, this vertical distance is approximately 4.6% of the winglet local chord at 25±3% semi-span and gradually decreases to 0.6% of the winglet local chord at the winglet root 134, or at 0% semi-span. In the Figures 14 and 15, the suffix "a" indicates the wing or winglet at 95±2% wing semi-span and 25±3% winglet semi-span, respectively. In general, for any chord plane which intersects the wing upper surface 118 within the corner flow region, this vertical distance should be at least less than 5.0% of the wing local chord associated with that chord plane. Similarly, for any winglet chord plane which intersects the winglet upper surface 126 within the corner flow region, the vertical distance should be at least less than 4.0% of the winglet local chord associated with that winglet chord plane. Alternatively, this gradual flattening may be described in terms of the slope of the line connecting a wing reference point to the closest point on the wing trailing edge in the chord plane associated therewith. In the wing/winglet configuration 110 of the preferred embodiment, this slope gradually decreases from approximately ±0.1 at 95±2% of the wing semi-span to approximately ±0.012 at 100% of the wing semi-span. For the winglet 116, the slope gradually decreases from approximately ±0.7 at 25±3% of the winglet semi-span to approximately ±0.010 at 0% of the winglet semi-span. Generally, then, for any chord plane which intersects the wing upper surface 118 within the corner flow region, the slope should be less than approximately ±0.12. Similarly, for any winglet chord plane which intersects the winglet upper surface 126 within the corner flow region, the slope should be less than approximately ±0.10. Therefore, the wing and winglet upper surfaces 118 and 126 are gradually flattened from a point outside the corner flow region to the wing/winglet intersection, where the flattening is the greatest. It has been found that, with the upper surfaces 118 and 126 of the wing 6 and the winglet 116 flattened as described above near the wing/winglet intersection 114, flow separation is virtually eliminated over all flight conditions. With the wing/winglet configuration 110 designed according to the present invention, the fluid passing over the upper surfaces of the airfoils does not diverge at as it passes over the thickest point of the airfoil. Accordingly, the present invention prevents flow separation from occurring near the wing/winglet intersection. II. Drooped Leading Edges A second feature of the preferred embodiment of the present invention is that the leading edges 122 and 130 of the wing 6 and winglet 116 are drooped downardly near the wing/winglet intersection to eliminate or at least substantially reduce premature shock wave development. It has been found that the flattened upper surfaces 118 and 126 allow rapid flow accelerations at cruise conditions. These rapid flow accelerations could lead to premature shock wave development, resulting in increased drag and ultimately flow separation on the winglet. More particularly, the leading edges 122 and 130 are drooped near the wing/winglet intersection relative to that leading edge distal from the wing/winglet intersection. The amount that these leading edges 122 and 130 are drooped may be quantified relative to the wing reference plane. As shown by the dotted line in Figure 14, the leading edge 122a of the airfoil at 95±2% semi-span lies within the wing reference plane. In contrast, the leading edge 122 of the airfoil at the wing tip 114 (i.e., 100% semi-span) is drooped below the wing reference plane 148 a distance of between ±1% and ±2%, and preferably ±1.5%, of the wing local chord 144 at the wing tip 114. Similarly, as shown by the dotted line in Figure 15, the leading edge 130a of the winglet airfoil at 25±3% semi-span lies within the winglet reference plane 150. The leading edge 130 of the winglet airfoil at the winglet root 134 (i.e., 0% semi-span), on the other hand, is drooped below the winglet reference plane 150 a distance of between 0.5% and 1.5%, and preferably 1.0%, of the winglet local chord 146 at the winglet root 134. As with the flattening of the wing and winglet upper surfaces 118 and 126, the wing and winglet leading edges 122 and 130 are gradually drooped. More particularly, the drooping of the wing leading edge 122 of the preferred embodiment begins at approximately 95±2% of the wing semi-span and gradually increases to a maximum value at 100% of the wing semi-span. Similarly, the drooping of the winglet leading edge 130 begins at approximately 25±3% of the winglet semi-span and gradually increases to a maximum value at 0% of the winglet semi-span. Drooping the leading edge at the wing/winglet intersection as described above prevents the premature development of shock waves, which would otherwise result in increased drag and ultimately flow separation on the winglet. III. Winglet Not Outwardly Canted As mentioned above, the winglet 116 is not outwardly canted. More specifically, the winglet 116 is vertically aligned such that the winglet reference plane 150 is substantially parallel to the plane formed by the x-axis and the y-axis. Because the flow separation at the wing/winglet intersection 114 is eliminated by the flattening of the aft portions of the upper wing and winglet surfaces 118 and 126, outward canting of the winglet 116 to prevent flow separation is not necessary. The vertically aligned winglet 116 minimizes the bending moment at the root of the wing 6. Further, this vertical winglet 116 is less sensitive to wing angle of attack variations than is an outwardly canted winglet such as the winglet 16 of the prior art wing/winglet configuration 10. IV. Secondary Features Other important but secondary features of the present invention are as follows. First, as shown in Figure 7, the length of the chord of the wing 6 is increased beginning at approximately 75% of the wing semi-span. This lengthening of the wing chord is necessary to decrease the wing thickness to wing chord ratio at the wing trailing edge near the wing/winglet intersection. Due to the flattening of the upper surface 118, the wing trailing edge 124 would otherwise be too thick relative to the length of the wing chord 144 at the wing tip 112. The lengthening of the wing chord near the wing/wingtip intersection 114 yields a wing thickness to chord ratio of just under 12% at the wing tip 112. Second, the winglet 116 is joined to the wing tip 112 so that the juncture between the upper wing surface 118 and upper winglet surface 126 is smooth and continuous at the wing/winglet intersection 114. Similarly, the wing lower surface 120 and winglet lower surface 128 are smoothly and continuously joined by a curved surface 160 at the bottom of the wing/winglet intersection 114. The exact shape of this curved surface 160 is not relevant to the present invention and can be designed as necessary to satisfy load distributions at the wing/winglet intersection. Third, the upper wing and winglet surfaces, lower wing and winglet surfaces, and bottom curved surface at the wing/winglet intersection for any given wing/winglet configuration should be so aerodynamically curved that the pressure distributions on the wing tip 112 and winglet 116 satisfy the requirements of cruise performance. Given the geometric parameters set forth above, a satisfactory design of these surfaces may easily and quickly be determined by one of ordinary skill in the art to obtain the appropriate pressure distributions under cruise conditions for any given wing/winglet configuration. Accordingly, the design of these surfaces will not been discussed herein in further detail. Fourth, in the present invention as shown in Figure 9, the wing leading edge 122 and winglet leading edge 130 are coextensive at the wing/winglet intersection 14. The winglet 116 thus presents a larger surface area for preventing transverse fluid flow over the wing tip 112. V. Summary The wing/winglet configuration 110 embodying the features present invention described above has been found to improve overall performance of the wing to which it is attached. Specifically, during wind tunnel testing, drag polars of a wing/winglet configuration having a geometric substantially as described above were as follows. At Mach = 0.70 and CL's below 0.3, the drag level of the wing was increased. However, at Mach = 0.70, the drag polars clearly showed that at values of CL above 0.3 the winglets became more effective and significantly reduced the drag. At Mach = 0.80 and 0.81, large drag reductions were also shown at CL = 0.55 and higher. In addition to the drag benefits of the winglets, the CLmax capability of the wing was improved. This is a significant result because such winglets usually degrade the CLmax capability of the wing due to flow separation at the wing/winglet intersection. In summary, the results of the wind tunnel testing show that the present invention as embodied in the wing/winglet configuration 110 are very effective at increasing overall wing and winglet performance. The above-described embodiment is to be considered in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and scope of the claims are intended to be embraced therein.

Claims: A wing/winglet configuration (110), comprising: a wing (6) having upper and lower wing surfaces (118,120), a wing leading edge (122), a wing trailing edge (124), and a wing tip (112); a wing reference plane (148) being defined as a plane which is parallel to a longitudinal axis (x) of an aircraft body to which the wing (6) and winglet (116) may be attached and which extends though the wing (6), a wing local chord (144) being defined for any given portion of the wing (6) as a line between the wing leading edge (122) and wing trailing edge (124), where the wing local chord (144) lies in a wing chord plane which is parallel to the longitudinal axis (x) and perpendicular to the wing reference plane (148); and for any given wing chord plane which intersects the wing upper surface (118), a wing reference point (152) being defined as a point on the wing upper surface (118) that is furthest from the wing reference plane (148) and a winglet (116) having upper and lower winglet surfaces (126,128), a winglet leading edge (130), a winglet trailing edge (132), and a winglet root (134), where the winglet root (134) is attached to the wing tip (112) at a wing/winglet intersection (114) and the winglet (116) extends upwardly from said wing tip (112); a winglet reference plane (150) being defined as a plane which is parallel to the longitudinal axis (x) and which extends through the winglet (116); a winglet local chord (146) being defined for any given portion of the winglet (116) as a line between the winglet leading edge (130) and winglet trailing edge (132), where the winglet local chord (146) lies in a winglet chord plane which is parallel to the longitudinal axis (x) and perpendicular to the winglet reference plane (150); and for any given winglet chord plane which intersects the winglet upper surface (126), a winglet reference point (154) being defined as a point on the winglet upper surface (126) that is furthest from the winglet reference plane (150), and a corner flow region being defined in which flow separation is likely to occur, characterized in that an aft portion of at least one of the upper wing surface (118) and upper winglet surface (126) in said corner flow region is so flattened near the wing/winglet intersection (114) that flow separation near the wing/winglet intersection (114) is substantially reduced, a vertical distance (156) between the wing reference point (152) in the corner flow region and an uppermost point on the wing trailing edge (124) furthest from the wing reference plane (148) and in the given chord plane being less than 4.5% of the length of the wing chord (144) associated with the given wing chord plane; and/or a vertical distance (158) between the winglet reference point (154) in the corner flow region and an uppermost point on the winglet trailing edge (132) furthest from the winglet reference plane (150) and in the given winglet chord plane being less than 4.0% of the length of the winglet chord (146) associated with the given winglet chord plane. The wing/winglet configuration (110) as claimed in claim 1, characterized in that the vertical distance (156) between the wing reference point (152) and the uppermost point on the wing trailing edge (124) is greater than 0.2% and less than 1%, and is preferably between 0.3% and 0.7%, of the length of the wing chord associated with the wing chord plane at the wing tip (112). The wing/winglet configuration (110) as claimed in claim 1 or 2, characterized in that the vertical distance (158) between the winglet reference point (154) and the uppermost point on the winglet trailing edge (132) is greater than 0.2% and less than 1%, and is preferably between 0.3% and 0.7%, of the length of the winglet chord associated with the winglet chord plane at the winglet root (134). The wing/winglet configuration (110) as claimed in any one of the preceding claims, characterized in that a. the corner flow region is an area on the wing and winglet upper surfaces (118,126) bounded by lines connecting a point on the wing trailing edge (124) at 97.5±1% of the wing semi-span, a point on the winglet trailing edge (132) at approximately 5±2% of the winglet semi-span, and a point along the wing/winglet intersection (114) located aft of the wing and winglet leading edges (122,130) a distance approximately equal to 40±10% of a wing local chord at the wing/winglet intersection (114); and b. the wing and winglet upper surfaces (118,126) are further flattened in an area on the wing and winglet upper surfaces (118,126) bounded by lines connecting a point on the wing trailing edge (124) at 95±2% of the wing semi-span, a point on the winglet trailing edge (132) at approximately 25±3% of the winglet semi-span, and a point along the wing/winglet intersection (114) located aft of the wing and winglet leading edges (122,130) a distance approximately equal to 40±10% of the wing local chord at the wing/winglet intersection (114). The wing/winglet configuration (110) as claimed in any one of the preceding claims, characterized in that at least one of the leading edges (122,130) of the wing (6) and winglet (116) is so drooped near the wing/winglet intersection (114) relative to the leading edge (122,130) distal from the wing/winglet intersection (114) that premature shock wave development is substantially reduced. The wing/winglet configuration (110) as claimed in claim 6 , characterized in that a. the wing leading edge (122) at the wing tip (112) is drooped between 1% and 2%, and preferably 1.5%, of the length of the wing chord at the wing tip (112) below the wing reference plane (148); and/or b. the winglet leading edge (130) at the winglet root (134) is drooped between 0.5% and 1.5%, and preferably 1.0%, of the length of the winglet chord at the winglet root (134) below the winglet reference plane (150). The wing/winglet configuration (110) as claimed in any one of the preceding claims, characterized in that the winglet (116) so extends from the wing tip (112) that the winglet (116) is substantially parallel to a vertical axis (z) of the aircraft body (4) to which the wing (6) and winglet (116) may be attached. The wing/winglet configuration (110) as claimed in any one of the preceding claims, characterized in that a local chord of the wing (6) is extended near the wing/winglet intersection (114) to obtain a desirable maximum wing thickness to wing chord ratio near the wing/winglet intersection (114). The wing/winglet configuration (110) as claimed in claim 8, characterized in that said maximum wing thickness to wing chord ratio near the wing/winglet intersection (114) is less than 12%. Aircraft (2), comprising a body (4) and a pair of wings (6) arranged on either side of said body (4), each said wing (6) having a wingtip (112) with a winglet (116) joined thereto, said wing (6) and winglet (116) having a configuration as claimed in any one of the preceding claims.

Inventors: GOLDHAMMER MARK I, SCHIPPERS KAREL A, GOLDHAMMER, MARK I., SCHIPPERS, KAREL A.
Applicants: BOEING CO, THE BOEING COMPANY
Application year: 1992
Main classification: B64C5
IPC codes: B64C3, B64C23