Abstract:
An airfoil having a fore airfoil element, an aft airfoil element, and a slot region in between them. These elements induce laminar flow over substantially all of the fore airfoil element and also provide for laminar flow in at least a portion of the slot region.  
     The method of the invention is one for inducing natural laminar flow over an airfoil. In the method, a fore airfoil element, having a leading and trailing edge, and an aft airfoil element define a slot region. Natural laminar flow is induced over substantially all of the fore airfoil element, by inducing the pressures on both surfaces of the fore airfoil element to decrease to a location proximate the trailing edge of the fore airfoil element using pressures created by the aft airfoil element.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to airfoils, and, more particularly, to slotted airfoils.  
         BACKGROUND OF THE INVENTION  
         [0002]    While airfoils can be used in a number of applications other than for the wings or control surfaces of aircraft, most applications for airfoils involve their use in aircraft wings in engine-driven aircraft. In such applications, the efficiency of the aircraft depends in large part on the drag forces impeding the passage of the aircraft through the air. Aircraft drag comprises many components including: wing profile drag, induced drag, fuselage drag, empennage drag, cooling drag, nacelle drag, trim drag, and interference drag. Generally, the wing profile drag and the induced drag represent the largest contributions to the total aircraft drag. For almost all aircraft, cruise occurs at speeds for which the profile drag dominates. Thus, wing profile drag represents the largest contribution to the total aircraft drag at cruise conditions. For transport aircraft (airliners), the wing profile drag represents about one third of the total aircraft drag at cruise conditions. As the aircraft size decreases from transport through commuter to general-aviation aircraft, including business jets, and finally unmanned aerial vehicles (UAV&#39;s), the proportion of the total aircraft drag due to the wing profile drag increases, primarily because the ratio of the wing area to the total aircraft wetted area increases. For a sailplane, which has small fuselage wetted area and no powerplant-installation drag, over half the total aircraft drag in cruise is wing profile drag. Thus, there has been a longstanding search for airfoil shapes that provide acceptable lift but exhibit low profile drag.  
           [0003]    Traditionally, three approaches have been taken to reduce wing profile drag. All three approaches involve only the cross-sectional shape (airfoil) of the wing. One approach is to employ a high-lift system (leading-edge slat plus double- or triple-slotted trailing-edge flap) to achieve a higher maximum lift coefficient, which allows the wing area, and therefore the aircraft wetted area, to be reduced. This approach has several disadvantages. Low profile drag is not possible because the slat introduces disturbances that cause the laminar flow to transition to turbulent flow near the leading edge of the wing. The maximum lift coefficient is limited to about 4, which limits the reduction in wing area. High-lift systems are complex, both mechanically and structurally, resulting in higher weight and cost. This approach can provide a maximum wing profile-drag reduction of about 50 percent compared to a conventional, turbulent-flow wing with no high-lift system and has been adopted for all current transport aircraft.  
           [0004]    A second approach is to employ a natural-laminar-flow (NLF) airfoil. By appropriate airfoil shaping, extensive (≧30-percent chord) laminar flow can be achieved on both the upper and lower wing surfaces. The extent of laminar flow is limited to about 70-percent chord by the pressure-recovery gradient along the aft portion of the airfoil upper surface. The pressure-recovery gradient becomes steeper as the extent of the favorable gradient along the forward portion of the airfoil becomes greater, eventually reaching a limit beyond which trailing-edge separation occurs, resulting in an unacceptably low maximum lift coefficient. The extent of laminar flow is also limited by wing leading-edge sweep, which also constrains the achievement of laminar flow because it introduces cross-flow instabilities into the laminar boundary layer that cause premature transition to turbulent flow. This approach can also provide a wing profile-drag reduction of about 50 percent compared to a conventional, turbulent-flow wing and has been adopted for most current general-aviation aircraft (e.g., “business jets” and “sailplanes”).  
           [0005]    A third approach is to employ a laminar-flow-control (LFC) airfoil. By incorporating suction through porous or slotted wing skins, 100-percent-chord laminar-flow can be achieved on both the upper and lower wing surfaces. Surface cooling can also increase the extent of laminar flow although it is not currently considered practical. In the case of LFC by suction, the use of the term “slots” in the wing skins refers to the many, very narrow slits in the surface through which air is sucked into the wing, and not a slot between the main and flap elements of a high-lift system. LFC systems are very complex, mechanically, structurally, and operationally, resulting in high weight and cost. This approach can provide a maximum wing profile-drag reduction of about 75 percent compared to a conventional, turbulent-flow wing but has not been adopted for any production aircraft.  
           [0006]    There exists, therefore, a need for an improved laminar-flow airfoil that overcomes the disadvantages of prior airfoils.  
         SUMMARY  
         [0007]    According to one aspect of the invention, there is provided an airfoil, having a fore airfoil element, an aft airfoil element, and a slot region in between them. The configuration of each of the forward and aft airfoil elements, along with the configuration of the slot region therein defined, induces laminar flow over substantially all of the fore airfoil element. This configuration also provides for laminar flow in at least a portion of the slot region.  
           [0008]    According to another aspect of the invention, the fore and/or aft airfoil elements move, rotate, change shape or translate with respect to one another such that the slot region formed therein varies in shape or volume while still inducing laminar flow over substantially all of the fore airfoil element. Optionally, this variable slot region geometry may adjust in response to operational parameters, for example through a computer-controlled feedback system.  
           [0009]    The method embodiment of the invention comprises a method of inducing natural laminar flow over an airfoil. In that method, there is provided a fore airfoil element having a leading and trailing edge. An aft airfoil element is also provided, the fore and aft airfoil elements defining a slot region. Natural laminar flow is induced over substantially all of the fore airfoil element, by inducing the pressures on both surfaces of the fore airfoil element to decrease (i.e., a favorable pressure gradient) to a location proximate the trailing edge of the fore airfoil element using pressures created by the aft airfoil element. Preferably, the method includes the step of inducing natural laminar flow in at least a portion of the slot region, at least partially, using pressures created by the aft airfoil element. 
       
    
    
       [0010]    Other features and advantages will be apparent from the accompanying drawings, and from the detailed description, which follows.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the detailed description, serve to explain the principles of the invention.  
       IN THE DRAWINGS  
       [0012]    [0012]FIG. 1 illustrates a cross-sectional view of an embodiment of a slotted natural laminar-flow airfoil; and  
         [0013]    [0013]FIG. 2 illustrates the pressure distribution created by the embodiment of FIG. 1 under certain specified conditions. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    Reference will now be made to an embodiment of the invention, an example of which is illustrated in the accompanying drawings  
         [0015]    In accordance with the invention, there is provided an airfoil that includes a fore airfoil element. As here embodied, and depicted in FIG. 1, fore airfoil element  100  has an upper surface  106 , a lower surface  107 , a leading edge  105  and a trailing edge  104 . Leading edge  105  provides a first contact between fore airfoil element  100  and the air surrounding the airfoil assembly  10 . While the fore element of the airfoil depicted in FIG. 1 has a specific configuration, it is not the only airfoil configuration operable with the present invention. As will be set out below, rather than the invention being specific airfoil shapes, it is the interaction of the fore and aft elements of the airfoil, and their effect on the flow over and through the combined elements that provides the benefits of the invention. With the flow characteristics and pressure distributions described below, one skilled in the development of airfoil shapes can devise a specific shape for the fore airfoil element that will achieve the benefits of the invention.  
         [0016]    In accordance with the invention, there is provided an aft airfoil element. As here embodied and depicted in FIG. 1, there is an aft element  110  having an upper surface  113 , a lower surface  114 , a leading edge  111 , and a trailing edge  112 . As here embodied, the aft airfoil element is smaller in overall size than fore airfoil element  100 . Aft airfoil element  110  provides additional lift to airfoil assembly  10 . By not allowing the pressure generated around fore airfoil element  100  to recover to free stream at trailing edge  104  of fore airfoil element  100 , aft airfoil element  110  allows the flow to accelerate along both surfaces substantially all the way to trailing edge  104  of fore airfoil element  100 , thereby allowing laminar flow to develop over essentially the entire fore airfoil element  100 . That is, the wake of fore airfoil element  100  does not significantly impinge on the flow around aft airfoil element  110  and/or through the slot region  120 .  
         [0017]    Just as was disclosed with respect to the fore airfoil element  100 , the aft airfoil element  110  is depicted with a specific configuration. That configuration, however, is not the only aft airfoil configuration operable with the present invention. As was noted above, rather than the invention being specific airfoil shapes, it is the interaction of the elements of the airfoil, and their effect on airflow that provides the benefits of the invention. With the flow characteristics and pressure distributions described below, one skilled in the development of airfoil shapes can devise a specific shape for the aft airfoil element that will achieve the benefits of the invention.  
         [0018]    The airfoil assembly  10  can be constructed by conventional manufacturing techniques. This includes, but is not limited to, assembling spars and ribs to create a sub-structure, and overlaying a skin over this sub-structure to provide an aerodynamic surface. State-of-the-art composite fabrication techniques can be used, and those provide the airfoil(s) of the preferred embodiments with the dimensional precision and light weight that maximize the benefits of the present invention. The materials used in the construction of airfoil assembly  10  are similarly those typically used in the relevant industry (e.g., aerospace, automotive, wind turbines, watercraft). This includes, but is not limited to, metals, plastics, and/or composite materials.  
         [0019]    In accordance with the invention, there is provided a slot region defined by surfaces of the fore and aft airfoil elements. As here embodied, the lower aft portion of the fore airfoil  100  is configured to provide a slot  120  between the fore element  100  and the forward upper portion of the aft element  110 . As here embodied, the lower aft portion  108  of the fore airfoil element  100 , and the upper forward portion  109  of the aft element  110 , form a slot  120  therebetween. As depicted in FIG. 1, it is preferred that the slot be defined by generally parallel surfaces of the fore and aft airfoil elements. As here embodied, the slot  120  shown in cross section in FIG. 1 has parallel sides defined by the lower aft portion  108  of the fore airfoil element  100 , and the upper forward portion  109  of the aft element  110 . It is further preferred that the trailing edge of fore airfoil element be proximate the maximum thickness of the lower airfoil element. As here embodied the aft extremity of the trailing edge  104  of fore element  100  is located above, and slightly aft, of the maximum thickness  115  of the aft element  110 .  
         [0020]    In accordance with the invention, the fore airfoil element, the aft airfoil element, and the slot region induce laminar flow over substantially all of the fore airfoil element. Preferably, the laminar flow is natural laminar flow, laminar flow induced solely by the shapes of the airfoil elements. It is the pressure distributions provided by the interaction of the fore and aft airfoil elements, and the slot formed therebetween that provide the advantages of the present invention. Specifically, the airfoil of the present invention provides natural laminar flow over a greater region of a fore airfoil element in a combination of two airfoil elements. Such a combination of airfoil elements provides lower profile-drag coefficients and higher maximum lift coefficients than prior laminar-flow airfoils.  
         [0021]    The operational characteristics of the elements of the embodiment of the airfoil of the invention provide guidance with respect to selecting the specific shapes of the various elements and the slot or slots formed between such elements. Such operational characteristics are best described by reference to the pressure distributions on the elements and their combinations at a specific flow condition.  
         [0022]    The airfoil of the present invention provides a pressure distribution such that there is a net pressure differential between upper surface  106  and lower surface  107  of fore element  100  and between upper surface  113  and lower surface  114  of aft element  110  of airfoil assembly  10 . This pressure differential causes a net force in the positive (generally vertical) direction, i.e., “lift.” The greater the net pressure differential, the greater the lift generated by moving such an airfoil through the air.  
         [0023]    Variations in the pressure distribution (i.e., the pressure gradient) can disturb laminar flow, which can cause turbulence. To provide lower profile-drag coefficients the airfoil shape should induce a pressure gradient that sustains laminar flow over as much of the airfoil cross-sectional length (i.e., chord length) as possible.  
         [0024]    As here embodied, trailing edge  104  of fore airfoil element  100  horizontally overhangs aft airfoil element approximately one-half of the aft airfoil element&#39;s  110  chord length. In addition, upper surface  113  of aft airfoil element  110  exhibits a generally convex exterior, and has a shape disposed to provide aerodynamic lift. The aft section of lower surface  107  of fore airfoil element  100  has a concave exterior that corresponds to the convex exterior of aft airfoil element  110 . This concave exterior is displaced away from the convex exterior to enable the laminar flow to remain undisturbed, resulting in laminar flow along substantially all of fore airfoil element  100  and through at least a portion of slot region  120 .  
         [0025]    The shape and configuration of slot region  120  can be determined by the shape and position of lower surface  107  of fore airfoil element  100  and the shape and position of upper surface  113  of aft airfoil element  110 . The configuration and shape of fore airfoil element  100 , aft airfoil element  110 , and slot region  120  promote laminar flow through slot region  120 , unlike prior airfoils.  
         [0026]    The extent of laminar flow through slot region  120  contributes to laminar flow over substantially all of fore airfoil element  100 . In another embodiment, slot region  120  geometry can be varied with the goal of maintaining laminar flow over substantially all of fore airfoil element  100  and through at least a portion of slot  120 . The geometry may vary by adjusting the space between fore airfoil element  100  and aft airfoil element  110 , or by adjusting the shape, rotation and/or translation of fore airfoil element  100  or aft airfoil element  110  (or both), or by a combination of these variations. In another embodiment, this variation in geometry of slot region  120  can occur during operation in response to certain design and factual considerations, e.g., through the use of a computer-controlled feedback system.  
         [0027]    [0027]FIG. 2 shows a pressure distribution  200  for the airfoil of FIG. 1. The pressure distribution  200  is plotted on the vertical axis in terms of the pressure coefficient C P . The horizontal axis corresponds to distance along the chord of the airfoil. A pressure coefficient (C P ) of zero corresponds to the static pressure of the free-stream flow far from the airfoil. Negative C P  (above the horizontal axis) represents lower pressure (and higher velocity) than the free stream. Positive C P  (below the horizontal axis) represents higher pressure (and lower velocity) than the free stream.  
         [0028]    [0028]FIG. 2 depicts the pressure distribution on this embodiment along the chord of each airfoil element. The pressure distributions for both fore airfoil element  100  and aft airfoil element  110  are plotted on the same figure to show the pressure distribution for each of the elements of the airfoil assembly  10 . The lower curve  210  of the pressure distribution represents the pressure on upper surface  106  of fore airfoil element  100 . An upper curve  220  represents the pressure on lower surface  107  of fore airfoil element  100 . The vertical distance between the two curves represents the pressure difference between upper surface  106  and lower surface  107  of fore airfoil element  100 . The upper curve  240  of the pressure distribution represents the pressure on upper surface  113  of aft airfoil element  110 . A lower curve  250  represents the pressure on lower surface  114  of aft airfoil element  110 .  
         [0029]    The spike in the positive direction of pressure at leading edge  105  of fore element  100  of airfoil assembly  10  is called the “stagnation point”, the point at which the oncoming flow first attaches to airfoil assembly  10 . Another positive spike in pressure exists just forward of the trailing edge  104  of fore airfoil element  100 , corresponding to the stagnation point of leading edge  111  of aft airfoil element  110 .  
         [0030]    Upper curve  220  and lower curve  210  join together at trailing edge  104  of fore airfoil element  100  to a single value of C P  that is lower than free-stream static pressure. In a typical slotted wing of the prior art, a pressure increase begins well before the trailing edge of the fore airfoil element, particularly on the lower surface.  
         [0031]    In the embodiment shown in FIGS. 1 and 2, almost no pressure increase occurs forward of trailing edge  104  of fore airfoil element  100 . This allows laminar flow to occur over substantially all of fore airfoil element  100 . In addition, laminar flow can occur in at least a portion of slot  120 .  
         [0032]    The airfoil of the present invention provides a pressure distribution such that there exists a net pressure differential between upper surface  106  and lower surface  107  of fore element  100  and between upper surface  113  and lower surface  114  of aft element  110  of airfoil assembly  10 . This pressure differential causes a net force in the positive vertical direction, which, in conventional aircraft applications, provides lift for an airplane. The terms “upper” and “lower” do not, however, indicate the only possible orientation of the airfoil of the present invention. The airfoil of the present invention can also be used in an inverted configuration to that shown in the figures to provide a downward force, which, for example, in automotive applications improves the traction for a vehicle including such an airfoil. The airfoil of the present invention can be used vertically as a “sail” on a watercraft, where the airfoil of the present invention would produce a horizontal force to propel the watercraft in the desired horizontal direction.