Patent Publication Number: US-2010127129-A1

Title: High performance airfoil with co-flow jet flow control

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and claims priority to U.S. Provisional Patent Application Ser. No. 60/603,212, filed Aug. 20, 2004, entitled HIGH PERFORMANCE AIRFOIL WITH CO-FLOW JET FLOW CONTROL, the entirety of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made by an agency of the United States government or under a contract with an agency of the United States Government. The name of the U.S. Government agency and the Government contract number are NASA-Contract No. NNL04AA39C. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to airfoils and flow control. 
     BACKGROUND OF THE INVENTION 
     Selected airframe, wing and control surface configurations; propulsion, control and guidance systems; and material properties combine to allow an aircraft to take flight and directly affect how the aircraft interacts with and moves through its atmospheric environment. As the aircraft moves through the atmosphere, the wings, fuselage, engines and engine nacelles, control surfaces, pylons, and antennae create and encounter a wide range of airflow patterns and pressures. Control of the airflow over, under, around and through the above aircraft structures has been the subject of constant study and refinement since the earliest days of flight. Often, even seemingly small changes in configuration have a dramatic effect on aircraft performance. 
     Various schemes for controlling airflow with respect to the wings have been developed in an attempt to enhance lift and reduce drag. Exemplary schemes include provision of a rotating cylinder at the leading and trailing edge of the wing, circulation control using tangential blowing at the leading and trailing edges, multi-element airfoils, pulsed jet separation control and the like. However, the penalty to the propulsion system (power loss) is often significant for some of the prior art flow control methods. For example, injecting or blowing air into the air flowing over a wing usually uses engine compressor bleed air. The mass flow rate of the engine bleed is directly proportional to the reduction of the thrust, i.e. the engine will suffer 1% thrust reduction for 1% blow rate used for wing flow control, and suffer 1-3% fuel consumption increase depending on whether the bleed is from the compressor front stage or back stage. To reduce the mass flow rate penalty due to blowing, pulsed jet or closed loop feed back control have been suggested. However, these methods require complicated actuation and sensor systems which may increase the degree of difficulty to implement the control system and increase the weight of the aircraft. Some flow control technologies also require moving parts, which may introduce complicated mechanical systems and increase weight. 
     It would be desirable to improve upon known structures, systems and techniques for flow control to enhance lift, reduce drag, and increase stall margin, among other flight characteristics, with minimal power loss or increased fuel consumption. 
     SUMMARY OF THE INVENTION 
     The aerodynamic structure of the present invention improves upon known structures, systems and techniques for flow control with respect to an airfoil. In an exemplary embodiment, the aerodynamic structure includes an airfoil having an injection slot on the suction surface of the airfoil near the leading edge, as well as a recovery slot on the suction surface of the airfoil near the trailing edge. Employing a pressurized fluid source, which may include bleed air from an engine, a high-energy fluid jet is then injected near the leading edge tangentially along the suction surface of the airfoil, and substantially the same amount of mass flow is sucked in the recovery slot near the trailing edge, which can then be directed back into the circulation system of the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein like designations refer to like elements, and wherein: 
         FIG. 1  illustrates a prior art, conventional airfoil and references thereto; 
         FIG. 2  shows a cross-section of an aerodynamic structure in accordance with the present invention; 
         FIG. 3  depicts a perspective view of the aerodynamic structure in accordance with the present invention; 
         FIG. 4  depicts an aerodynamic structure and flow system in accordance with the present invention; 
         FIG. 5  illustrates fluid streamline patterns of a prior art aerodynamic structure; and 
         FIG. 6  shows fluid streamline patterns of an aerodynamic structure in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior to setting forth an exemplary embodiment of the present invention, the general characteristics and features of an airfoil will be identified and defined as referred to herein. 
     Now referring to  FIG. 1 , an airfoil generally includes a leading edge, a trailing edge, an “upper surface,” a “lower surface,” and a chord. The leading edge is that which encounters a fluid flow first, i.e., the “front” of the airfoil. The trailing edge is at the rear point of the airfoil, where the fluid flow over the upper surface meets the fluid flow across the lower surface of the airfoil. 
     Both the “upper” and “lower” surfaces are usually curved, with the “upper” surface having a larger curvature, and thus a larger surface length spanning from the leading edge to the trailing edge of the airfoil. Because the of the greater length across the “upper” surface, according to Bernoulli&#39;s theorem, the fluid flowing over the “upper” surface of the airfoil has a higher velocity than the fluid flowing across the “lower” surface of the airfoil. As a result of the increased velocity across the “upper” surface, a lower pressure is created than that experienced on the “lower” surface of the airfoil. This reduced pressure creates suction on the “upper” surface, which constitutes a portion of the lift created by the airfoil. As such, the “upper” surface is referred to herein as the suction surface of the airfoil. As an airfoil may be mounted in an inverted position, i.e., as a spoiler on a race car or the like, the suction side refers to the side experiencing a lower pressure when exposed to a fluid flow, and does not necessarily correlate to the “top” or “upper” surface of an airfoil or aerodynamic structure. Consequently, the “lower” surface as referred to herein will indicate the surface opposite the suction surface. 
     The chord of an airfoil is the straight line drawn through the airfoil from its leading edge to its trailing edge. Further, the chord length is the distance between the leading edge and trailing edge as traversed along the chord. Additionally, a fluid source or fluid flow as used herein can include both liquid as well as gaseous compositions of matter. 
     Now referring to  FIGS. 2 and 3 , the present invention provides an aerodynamic structure  10  having a chord length, a leading edge  14 , and a trailing edge  16 . As discussed previously, the leading edge  14  is the portion of the aerodynamic structure  10  which interacts with fluid first, i.e., the “front” of the structure  10 , with the trailing edge  16  located at the rear point of the aerodynamic structure  10 . The aerodynamic structure  10  further includes a first airfoil surface  18  that generally defines a surface extending from the leading edge  14  to the trailing edge  16 . A second airfoil surface  20 , which is opposite the first airfoil surface  18 , also generally defines a surface extending from the leading edge  14  to the trailing edge  16 . The first airfoil surface  18  corresponds to the suction side of the aerodynamic structure  10 , i.e., the first airfoil surface  18  experiences a pressure lower than that experienced across the second airfoil surface  20  when the aerodynamic structure  10  is subjected to a fluid flow. 
     The first airfoil surface  18  also defines an injection opening  22  located between the leading edge  14  and the trailing edge  16 , and further defines a recovery opening  24  located in between the injection opening  22  and the trailing edge  16 . In an exemplary embodiment, the injection opening  22  is located less than 25% of the chord length form the leading edge  14  of the airfoil. However, the benefits of the present invention may be realized with the injection opening located within 80% of the chord length from the leading edge  14 . Moreover, the recovery opening  24  is preferably located less than 25% of the chord length from the trailing edge  16  of the aerodynamic structure. Similarly to the injection opening placement, however, the benefits of the present invention may be realized with the recovery opening  24  located within 80% of the chord length from the trailing edge  16 . The injection opening  22  defines an injection opening height  26 , which has a value that is generally less than 5% of the chord length. The recovery opening  24  defines a similar recovery opening height  28 , which has a value generally less than 5% of the chord length. While the injection and recovery openings illustrated have a fixed size, an alternative embodiment can include openings capable of having their height varied through the use of mechanical means in which at least a portion of the first airfoil surface  18  is moveable, thereby changing the height of either the injection opening  22  or the recovery opening  24 . 
     Still referring to  FIGS. 2 and 3 , the aerodynamic structure  10  can further define a first cavity  30  that is in fluid communication with the injection opening  22 . Optionally, the first cavity may further contain a baffle material  32 . The baffle material  32  can include a foam-like substance that provides a uniform flow distribution of fluid flowing through it and further ensures a highly uniform fluid jet downstream of the baffle material  32 . In addition to the first cavity  32 , the aerodynamic structure  10  can also define a second cavity  34  coupled to the recovery opening  24 . 
     Now referring to  FIG. 4 , the present invention provides an aerodynamic system  36  that includes the aerodynamic structure  10  as previously described, as well as a pressurized fluid source  38  and a vacuum source  40 . The vacuum source  40  provides a pressure lower than an ambient pressure. The pressurized fluid source  38  is in fluid communication with the injection opening  22  (see  FIG. 2 ), and can include a pump or other means of pressurizing a fluid, and may further include bleed air from an engine  50 . The vacuum source  40  is in fluid communication with the recovery opening  24  (see  FIG. 2 ), and may also include a pumping apparatus or, alternatively, may be coupled to an engine. 
     Referring to  FIGS. 2-4 , An exemplary use of the aerodynamic system  36  provides a method for reducing the boundary layer separation of an aerodynamic structure. The aerodynamic system  36  is provided, which includes aerodynamic structure  10 . A first mass  42  of fluid is routed from the pressurized fluid source  38  towards the injection opening  22 . The first mass  42  may be routed by any means of conducting a fluid, i.e., a conduit, tubing, or the like. If the aerodynamic structure  10  includes the first cavity  30  coupled to the injection opening  22 , then the fluid flow path will route the first mass  42  from the pressurized fluid source  38  and into the first cavity  30 , where the first cavity acts as a plenum enclosing pressurized fluid at or near the injection opening  22 . Additionally, the baffle material  32  provides a uniform flow distribution normal to the downstream surface of the baffle material  32  and insures a highly uniform jet of the first mass  42  of fluid as it heads towards the injection opening  22 . The first mass  42  is then dispersed out of the injection opening  22  and directed substantially tangent to the exterior surface of the aerodynamic structure  10  and towards the recovery opening  24 . 
     Concurrently, the vacuum source  40  creates a pressure at the recovery opening  24  lower than that of the environment external to the recovery opening  24 , resulting in a second mass  44  of fluid being drawn into the recovery opening  24 . The second mass can either be drawn into the recovery opening  24  and into the second cavity  34  coupled to the recovery opening, or, in the absence of the second cavity  34 , the second mass of fluid can be drawn directly from the recovery opening towards the vacuum source. Further, while a single injection opening and recovery opening may extend along the span of the aerodynamic structure, alternatively, fluid may be dispensed from multiple injection openings along the span of the wing and recovered by numerous recovery openings also positioned along the span of the aerodynamic structure. Moreover, the injection and recovery openings may only span a portion of the aerodynamic structure, rather than the entire length. 
     Although the injection and recovery of fluid along the aerodynamic structure can be realized by separate and independent injection and recovery resources, the fluid flow can also be recirculated by a pump system or by an aircraft engine system. In jet aircraft, the high-pressure fluid in the rear stages of the engine compressor can be used for the fluid dispersion out of the injection opening  22 . The second mass  44  can then be drawn into the recovery opening  24  and directed to the front stage of the compressor or the inlet where the pressure is low. The fluid-flow is hence recirculated to save energy expenditure. In non-jet or reciprocating engine powered craft, the fluid to the injection opening  22  can be provided by a pump or compressor driven by the engine. Further, the fluid can be provided by a compressed air supply, such as a pressurized tank. 
       FIG. 5  illustrates the fluid streamlines as they pass over a generic airfoil structure, with the separation of flow in the boundary layer towards the trailing edge clearly evident.  FIG. 6  shows an aerodynamic structure in accordance with the invention. The first mass  42  forms a high-energy jet as it is injected tangentially along the structure and substantially the same amount of mass fluid flow is recovered near the trailing edge. The turbulent shear layer between the main flow and the high-energy jet formed by the dispersion of the first mass  42  of fluid causes strong turbulence diffusion and mixing; thereby enhancing the lateral transport of energy from the jet to the main flow, thereby allowing the main flow to overcome the severe adverse pressure gradient experienced towards the trailing edge of the aerodynamic structure. This diffusion allows the main flow to stay attached at high angle of attack (AOA), resulting in the removal of boundary layer separation. At a certain AOA, the aerodynamic structure of the present invention can achieve a significantly higher lift due to the augmented circulation. The operating range of AOA, and hence the stall margin, is significantly increased. Moreover, the energized main flow will fill the wake deficit and dramatically reduce the airfoil drag, or even generate thrust (negative drag). The filled wake will also reduce noise due to the weak wake mixing. In addition, the aerodynamic structure does not need a high lift flap system, further reducing noise. The method and systems described can be applied to any type of airfoil, including high-speed thin airfoils as well as low-speed, thicker airfoils. 
     In addition, since the aerodynamic system of the present invention disperses and recovers substantially the same amount of mass fluid flow, the high-energy fluid flow can be recirculated through the propulsion system and has a smaller energy expenditure to the overall airframe-propulsion system when compared to a method where only injection or dispersion of a mass of fluid is involved. Moreover, the lift can be controlled by adjusting the pressure at which the first mass  42  is injected along the surface of the aerodynamic structure  10 , resulting in the absence of a need for moving parts. 
     In summary, the aerodynamic structure provides numerous advantages including both lift enhancement and separation suppression. The present invention tremendously reduces the drag, can achieve very high C L /C D  (infinity when C D =0) at low AOA (cruise), and high lift and drag at high AOA (take off and landing). Moreover, these advantages significantly increase the AOA operating range and stall margin, and further minimize the penalty to the propulsion system. The present invention can also be integrated into virtually any airfoil, whether thick or thin, in conventional, sweep wing configurations, and can be applied to helicopter rotor blades as well. 
     In addition, the above advantages of the aerodynamic structure of the present invention may derive superior aircraft performance for either a portion of or the entirety of a mission, which include increased fuel efficiency and shortened take-off and landing distances, and the integration of the systems of the present invention is simplified as moving parts are not necessary. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.