Patent Publication Number: US-9896192-B2

Title: Minimally intrusive wingtip vortex wake mitigation using microvane arrays

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND 
     Field 
     Mitigation of wake turbulence through the weakening of wake vortices generated at outboard edges of aircraft wings, or outboard and/or inboard edges of ailerons, flaps, or other airfoils. 
     Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     The persistence of a vortex trailing behind a lifting airfoil such as a wing, is dependent not only upon vortex strength, but also upon how well a core of the vortex is defined. Vortex strength is governed by lift, which depends upon area and pressure differences between upper and lower airfoil surfaces. The core is well-defined if the airfoil is thin and if there is a high pressure difference between the upper and lower surfaces near a tip of the airfoil. This allows a streamline to roll around the outboard edge or tip of a wing or the inboard and/or outboard edges or tips of a flap, aileron, or other airfoil, from the high pressure region to the low pressure region along a very short distance. This results in a highly curved, fast moving streamline in a well-defined vortex core, which can persist for a long time downstream of the airfoil. The formation of a well-defined vortex core may be impeded by lengthening the distance traveled by the streamlines rolling from the high pressure region feature such as extra thickness, a non-lifting wing-tip extension, or a device such as a winglet. Such devices are designed to have lift characteristics that compensate for their own weight. They also tend to add drag, although they are designed to add as little as possible. The purpose of such features is to prevent the lifting vortex from forming a well-defined core, thereby allowing the vortex to dissipate quickly. 
     SUMMARY 
     An airfoil tip vortex mitigation arrangement comprising one or more flow directors configured and positioned to re-direct freestream air over a low pressure surface of an airfoil in such a way as to displace and weaken a main tip vortex generated at a tip of the airfoil. 
    
    
     
       DRAWING DESCRIPTIONS 
       These and other features and advantages will become apparent to those skilled in the art in connection with the following detailed description and drawings of one or more embodiments of the invention, in which: 
         FIG. 1  is a partial top perspective view of an airfoil tip vortex mitigation arrangement installed adjacent a tip of an airfoil with arrows showing freestream airflow over the low pressure upper surface of the airfoil and higher pressure airflow rolling up from under the airfoil to form a tip vortex; 
         FIG. 2  is a magnified view of region  2  of  FIG. 1  with arrows showing freestream airflow redirected to form a co-rotating array of vortices, which diffuses the effective core of the main airfoil vortex; and 
         FIG. 3  is a front view of the portion of the airfoil and the tip vortex mitigation arrangement shown in region  2  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     An arrangement for weakening a wake vortex being generated at an outboard edge or tip of a wing, or at an inboard and/or outboard edge or tip of an aileron, flap, or other airfoil, is generally shown at  10  in  FIGS. 1-3 . The arrangement  10  may include one or more flow directors  12  configured and positioned to re-direct freestream air flowing over a low pressure surface  14  of an airfoil  16  such as a wing, strake, flap, canard, flaperon, or elevon. 
     In this document the term “longitudinal” is used to refer to a direction parallel to the motion of freestream air relative to a subject aircraft. The term “inboard” is used to refer to a direction generally toward a longitudinal fuselage centerline of an aircraft from a point spaced laterally from that fuselage centerline, and the term “outboard” is used to refer to a direction away from the longitudinal centerline of the aircraft. The term “inward”, in reference to an airfoil of a subject aircraft, is used to refer to a direction toward a longitudinal centerline of the airfoil from a point spaced laterally from that airfoil centerline and the word “outward” is used to refer to a direction away from that longitudinal airfoil centerline. 
     The flow directors  12  may be arranged to direct freestream air over a low pressure surface  14  in such a way as to displace and weaken a main tip vortex  18  of the airfoil  16 , i.e., a vortex formed at an inboard or outboard tip  20  of the airfoil  16  by relatively high pressure air flowing generally spanwise from under the airfoil  16  and rolling helically upward, inward, and aft around an inboard or outboard edge or tip  20  of the airfoil  16 . The outboard edge or tip of an airfoil  16  may comprise a swept leading edge of the airfoil  16  where the airfoil is, for example, a strake or delta wing variant, and where main vortices  18  may therefore run the length of the leading edge of the airfoil  16 . The flow directors  12  may preferably be designed to be minimally intrusive in an aerodynamic sense. 
     Freestream air, re-directed by the flow directors  12 , displaces and weakens the main tip vortex  18 , hastening dissipation of the main tip vortex  18  so that, among other things, trailing aircraft are less likely to encounter wake turbulence sufficient to compromise operators&#39; ability to control such trailing aircraft. The flow directors  12  may also be arranged to displace and weaken a core  17  of the main tip vortex  18  to hasten the dissipation of the main tip vortex  18  without significantly diminishing local lift effects of the main tip vortex  18 . 
     As best shown in  FIGS. 2 and 3 , the flow directors  12  may comprise one or more arrays of small-scale (height relative to airfoil chord between approximately ¼% and 1%) vortex generators positioned adjacent and along the inboard and/or outboard edge or tip  20  of the airfoil  16  on a low pressure, e.g., upper, surface  14  of the airfoil  16  and oriented to direct freestream air outboard such that one or more co-rotating vane vortices  24  are formed on the low pressure surface of the airfoil  16 . The vane vortices  24  may be formed in respective positions where they will impede the circulation of the relatively high pressure air that&#39;s rolling upward around the inboard or outboard tip  20  and helically inboard and aft from a high pressure region  22 —e.g., an under-surface—of the airfoil  16  to form the radially inner core  17  of the main tip vortex  18 . The vortex generators  12  may be positioned and oriented to cause the vane vortices  24  to co-rotate, i.e., to turn in the same helical sense or direction as the main tip vortex  18 . This may have the effect of reinforcing the main tip vortex  18  adjacent its origin at the airfoil tip  20  so that local lift effects of the main tip vortex  18  are enhanced while diffusing the main vortex core. 
     The flow directors  12  may comprise a plurality of microvanes extending from the low pressure upper surface  14  of the airfoil  16  in respective orientations generally normal to the surface  14  from which they extend. The plurality of microvanes may be disposed in a serial microvane array, as is generally indicated at  26  in  FIGS. 1 and 2 , along the outboard tip  20  of the airfoil  16  in a row generally parallel to an airfoil tip  20 , and where the airfoil  16  is a wing, along its outboard edge or wingtip. The microvanes  12  of the microvane array  26  may be oriented to form a serial array of vane vortices, as are generally indicated at  28  in  FIG. 2 , on the low pressure surface  14  of the airfoil  16  adjacent and along an airfoil tip  20 . The serial array of vane vortices or vortex array  28  may be formed between the microvane array  26  and an airfoil tip  20  so that the serial array of vane vortices  28  blocks or otherwise inhibits the flow of high pressure air rolling helically upward, inward, and aft around the airfoil tip  20 , disrupting the core  17  of the main tip vortex  18 . In other words, the vortex array  28  causes the airfoil tip  20  to act as if it were thicker, thereby impeding the formation of a well-defined vortex core. 
     The microvanes of the microvane array  26  may also be oriented and positioned to form the vortex array  28  such that respective centers of rotation  29  of the vortices of the vortex array  28  are spaced inboard from a center of rotation  19  of the airfoil tip vortex  18  as is best shown in  FIG. 3 . This arrangement of the vane vortex array  28  provides an opposition region, best shown at  30  in  FIG. 3 , in which induced velocities of the vane vortex array  18  and the airfoil tip vortex  18  oppose each other. This causes a combined effective vortex core to be distributed between the vane vortex array  28  and the airfoil tip vortex core  17 , which provides a rapid far field vortex dissipation effect. However, because the vortices of the vane vortex array  28  are rotating in the same sense as the airfoil tip vortex  18 , the induced velocities of the vane vortex array  28  and the airfoil tip vortex  18  reinforce each other, precluding or reducing lift degradation or induced drag penalty. 
     Each microvane of the microvane array  26  may be oriented at an angle of approximately (within plus or minus 15 degrees of) 45 degrees as measured relative to the direction of freestream airflow  32  about an axis generally normal to a portion of the low pressure surface  14 . Microvanes of the microvane array  26  may be supported such that trailing edges of the microvanes  12  are angled outward relative their respective leading edges as shown in  FIGS. 1 and 2 . However, the microvanes  12  could be oriented at any appropriate angle that will allow them to be effective in dissipating wake turbulence within a desired range of airspeeds. Microvane angle may be optimized for final approach speeds and other speeds at which high angles of attack and lift enhancing aircraft configurations such as flap extension tend to create or greatly enhance the strength of tip vortices  18 . 
     Each microvane  12  of the microvane array  26  may have a height approximately 0.4% of a tip chord length of the airfoil, e.g., approximately ¼ inch on a 5 foot chord (between approximately ⅛ inch and ½ inch, or ¼% and 1% of the chord), so as to minimize drag while providing sufficient free stream flow diversion to disrupt the core  17  of the main tip vortex  18  enough to provide a desired reduction in main tip vortex longevity. Each vane  12  has a length to height ratio of approximately 8 (2 inches long if height is ¼ inch), or any other suitable length. Preferably, however, the ratio of length to height may be between approximately 2 and 10 to, again, provide a desired tradeoff between parasite drag and early wake vortex dissipation. 
     The microvane array  26  may extend aft along the airfoil tip  20  to a point where beneficial effects no longer outweigh the associated additional drag penalty. Generally, that point is where the main tip vortex  18  detaches from the airfoil. 
     An arrangement such as is disclosed above may improve an aircraft&#39;s performance, especially under conditions such as low airspeeds or high angles of attack, by increasing the lift generated by airfoils and reducing the danger to following aircraft by causing early dissipation of airfoil tip vortices. 
     This description, rather than describing limitations of an invention, only illustrates an embodiment of the invention recited in the claims. The language of this description is therefore exclusively descriptive and is non-limiting. Obviously, it&#39;s possible to modify this invention from what the description teaches. Within the scope of the claims, one may practice the invention other than as described above.