Abstract:
An aircraft flow body includes a flow surface exposed to an airstream during flight, at least one structural component attached to an interior of the flow surface, at least one perforated area having a plurality of openings extending through the flow surface, a manifold positioned interior to the flow surface in fluid communication with the openings and upstream of the at least one structural component, and at least one suction duct having a first end in communication with the manifold and a second end placed exterior to the flow surface downstream of the structural component. The at least one suction duct includes a suction opening facing away from the first end and adapted for inducing a suction force into the at least one suction duct when the flow surface is exposed to the airstream during flight, thereby inducing a flow of air from through the openings to the suction opening.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the passive boundary layer suction on a flow body of an aircraft and in particular to a flow body of an aircraft comprising a flow surface exposed to an airstream during flight of the aircraft, at least one perforated area comprising a plurality of openings extending through the flow surface as well as a suction duct for suction of air through the at least one perforated area. 
       BACKGROUND OF THE INVENTION 
       [0002]    Influencing the boundary layer on a flow body, which is exposed to an airflow, supports improving the aerodynamic characteristics of the flow body and, more particularly, supports the reduction of its aerodynamic drag. Generally, the boundary layer control is conducted either through suction, hence enabling extended laminar boundary layers, or through re-energizing the boundary layer by blowing. Besides active systems, which actively move air into or out of the boundary layer by means of pumps or similar devices, also passive systems are known. Both concepts require a suitable perforation in a dedicated area of an exterior flow surface to allow air to pass through the flow surface. 
         [0003]    U.S. 2013/0270390 A1 shows an aircraft with a partially perforated outer surface as well as a suction profile body, with which a suction force is generated. By applying the suction force to the perforated surface, air is moved through the perforation and thus removed from the boundary layer. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    For providing a passive boundary layer control through suction of air from a flow boundary layer, a pressure differential between the location of the perforation and a second location at or near the flow body may be used. Such a passive suction system needs to fulfil several requirements, which include ensuring the functionality, i.e. providing a sufficient mass flow driven by the pressure differential, minimizing parasitic drag, respecting space constraints interior to the flow surface of the flow body and space allocation requirements, minimizing additional weight and allowing for a maximum suction area and hence laminar flow area. An aspect of the invention may provide an alternate solution for a flow body of an aircraft, which is adapted for conducting a passive suction of air from the boundary layer of the flow body. 
         [0005]    A flow body of an aircraft is proposed, the flow body comprising a flow surface exposed to an airflow during flight of the aircraft, at least one structural component attached to an interior of the flow surface, at least one perforated area comprising a plurality of openings extending through the flow surface, a manifold positioned interior to the flow surface in fluid communication with the openings and upstream of the at least one structural component, and at least one suction duct having a first end and a second end, the first end being in fluid communication with the manifold and the second end placed exterior to the flow surface downstream of the at least one structural component. The at least one suction duct comprises a suction opening facing away from the first end, wherein the suction opening is adapted for inducing a suction force into the at least one suction duct when the flow surface is exposed to an airstream during flight, thereby inducing a flow of air from through the openings to the suction opening. 
         [0006]    The flow body of an aircraft is a component that comprises a flow surface, which is exposed to an airflow during flight of the aircraft. In the context of the invention the flow body is an exterior part that influences the aerodynamic characteristics of the aircraft. Inter alia, the flow body may be a wing, a part or a section of a wing, be it in a spanwise or chordwise direction, a part or a section of a tailplane, such as a vertical tailplane, a horizontal tailplane or components thereof. Also, the flow body may be an add-on body, which protrudes from a fuselage of the aircraft, a fairing, a wing, or a part of the tailplane, be it the horizontal or the vertical tailplane, into the surrounding of the aircraft. Depending on the overall design of the aircraft, still further different flow bodies may be considered and the following description should not be construed as limiting the flow body to any kind mentioned in the following. 
         [0007]    The at least one structural component may preferably be a part of an interior main structure of the flow body, such as a spar or a stringer, for providing dimensional stability. Common design principles of aircraft and flow bodies thereof are based on the combination of a skin as a shape determining part and a support structure as an interior part. The support structure often comprises a set of structural components, which are arranged in a predetermined geometrical relationship and allow to create a lightweight and stiff three-dimensional body. 
         [0008]    As stated above, the flow body comprises at least one perforated area having openings extending through the flow surface. Depending on the desired intensity of a boundary layer control measure, also a multitude of separate perforated areas may be considered. Each of the at least one perforated area comprises a set of openings, which may be arranged in a regular or irregular pattern or a combination thereof on the flow surface. Also, different patterns may be generated in a single perforated area. The openings may comprise a diameter that depends on the airflow conditions, the material of the flow body and the desired porosity and may extend through the flow surface perpendicularly or at an angle thereto that differs from a perpendicular extension. 
         [0009]    The manifold acts as an interface or as a collection device for receiving the air flow that passes the openings into the interior of the flow body and for routing it into the at least one suction duct. The at least one suction duct, which is coupled with the manifold, induces a suction force on all openings at the same time. While the manifold may be or lead into a plenum chamber, it may also be realized in the form of a line section or, in other words, a feeder duct. In case the manifold is intended to be a plenum chamber, it may be created by simply providing a wall at a distance to the at least one perforated area in order to enclose a defined space in fluid communication with the at least one suction duct and the exterior of the openings. In this regard, providing a wall may be realized through using a part of a main structure, such as the at least one structural component of the flow body, or by installing a separate wall. 
         [0010]    Locating the manifold upstream of the at least one structural component and placing the suction opening downstream of the at least one structural component requires the at least one suction duct extending over the at least one structural component along the flow surface to a region with a suitable pressure and flow condition for inducing a suction force. Hence, modifications of the at least one structural component are not required, since the at least one suction duct does not need to be routed through the at least one structural component. Resultantly, the flow body according to the invention is perfectly suitable for retrofit solutions to already existing aircraft, since a part of a flow body intended for an addition of a boundary layer control may be replaced by the flow body according to the invention without any further modifications. 
         [0011]    The suction opening, which faces away from the first end of the at least one suction duct is placed in a low pressure area to generate a suitable pressure gradient between the porous surface and the suction opening, to primarily ensure the required mass flow and secondly by compensating also the pressure losses in the suction duct. The suction opening is flown over by with the airflow around or along the flow body. Depending on the shape and the orientation of the suction opening, the suction force, which is given as the low pressure at the second end multiplied with the cross-sectional area of the suction opening, may be influenced to match the desired airflow through the openings. Since the at least one suction duct is placed exterior to the flow surface, it is preferable to let it extend from the flow surface as little as possible to minimize an unavoidable parasitic drag. However, by optimizing the shape of the at least one suction duct the positive effect of the boundary layer control clearly overweighs its parasitic drag. 
         [0012]    In an advantageous embodiment, the flow body further comprises a leading edge, which faces into the airflow during flight, wherein the at least one perforated area is created as a section of the flow surface extending over the leading edge. Exemplarily, a vertical or horizontal tail plane may be equipped with a partially perforated leading edge region, from which at least one suction duct extends in a downstream direction on the flow surface. Consequently, the front spar as a structural component of the flow body, which is further upstream, does not need to be modified as the at least one suction duct does not need to be routed through it. 
         [0013]    In another advantageous embodiment, the at least one suction duct comprises a bifurcated arrangement of two suction ducts. Resultantly, the individual (effective) diameters of both suction ducts can be reduced, as two separate suction ducts commonly provide the suction force. Also, the parasitic drag induced by the two suction ducts occur symmetrically in relation to a flight direction and do not lead to a (very marginal) asymmetric forces on the aircraft. 
         [0014]    Another advantage of the use of a bifurcated arrangement of two suction ducts lies in the capability to always provide a sufficient suction effect in the manifold even when a distinct side slip of the aircraft occurs, in which case a suction duct on one lateral side of the aircraft produces a much smaller suction force than the other one. 
         [0015]    Preferably, the manifold is located between the leading edge of the flow body and a front spar as the at least one structural component of the flow body due to the above-mentioned reasons. 
         [0016]    Depending on the type of flow body, a certain pressure field on the exterior surface is produced, which pressure field comprises at least one section having a local pressure lower than upstream of the at least one structural component, wherein the suction opening is located directly upstream of this section of low pressure. Preferably, a section having a minimum pressure is created on the exterior surface. By placing the suction opening into this location of low pressure, the suction force is further supported. 
         [0017]    In another advantageous embodiment, the manifold extends substantially parallel to the front spar and the two suction ducts each comprise a bent section between the first end and a section, which is fully placed on the exterior side of the flow surface. This bent section allows to reach from the exterior side of the flow surface into the interior, directly upstream the at least one structural component. 
         [0018]    In a preferred embodiment, the cross-sectional surface area of the bent section exceeds the cross-sectional surface area of the suction opening. Exemplarily, the cross-sectional surface area at the bent section may be at least 50% higher than that at the suction opening. It may also be particularly larger, such as at least twice the cross-sectional surface area at the suction opening. The pressure losses, which generally depend on the square of the flow velocity and certain other factors, in curved or bent ducts exceeds the pressure loss in a straight duct. The pressure loss may significantly be reduced by reducing the flow velocity, which is accomplished through increasing the cross-sectional surface area. 
         [0019]    Increasing the cross-sectional surface area may be accomplished by widening the suction duct, which means that an extension of the suction duct away from the flow surface is substantially prevented, while a suction duct determining dimension substantially parallel to the flow surface is increased. Hence, the cross-sectional surface of the bent section extends on a wider dimension on the flow surface than the cross-sectional surface of the suction opening This further leads to reducing the parasitic drag induced by the outer dimensions and shape of the suction duct. 
         [0020]    In a particularly advantageous embodiment the at least one suction duct comprises a cross section, which is at least partially crescent-shaped. 
         [0021]    The at least one suction duct continuously tapers of from a cutout in the flow body, through which the at least one suction duct extends from an interior to an exterior side, to the suction opening. In a region including the cutout the at least one suction duct may be bent, i.e. it comprises a bent section. 
         [0022]    In a still further advantageous embodiment, the flow body comprises a flow control means, which is adapted for reducing the flow between the manifold and the at least one suction duct selectively. Reduction or elimination of a flow between the manifold and the at least one suction duct leads to switching on, reducing or switching off the boundary layer control. This may be helpful in flight phases other than cruise phases, thereby, reducing the danger of contamination, i.e. clogging of the openings. The flow control means may be placed at or in the manifold and may comprise an aperture, an orifice, a movable flap or other common devices for influencing an air flow through a duct. 
         [0023]    As stated above, a space between the perforated area and the at least one structural component may at least partially be closed interior of the flow body so as to form a plenum chamber in which the at least one manifold may be placed, which is connected to the manifold or which constitutes the manifold. 
         [0024]    Overall, the flow body according to the invention allows to induce a suction force onto at least one suction duct in fluid communication with openings of at least one perforated area, without the necessity to modify a primary structure in the flow body. Routing the at least one suction duct on the exterior flow surface allows to tap a point of lowest pressure, which supports the generation of a suction force. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Further characteristics, advantages and application options of the present invention are disclosed in the following description of the exemplary embodiments in the figures. All the described and/or illustrated characteristics per se and in any combination form the subject of the invention, even irrespective of their composition in the individual claims or their interrelationships. Furthermore, identical or similar components in the figures have the same reference characters. 
           [0026]      FIG. 1  shows a flow body in the form of a vertical tail plane having a suction area. 
           [0027]      FIG. 2  shows the flow body with isobars indicating an exemplary pressure distribution field. 
           [0028]      FIG. 3  shows the same flow body with a manifold and a suction duct attached thereto. 
           [0029]      FIG. 4  shows an aircraft having a flow body according to the invention. 
           [0030]      FIG. 5  shows a top view on a cross-section of a flow body having a bifurcated arrangement of two suction ducts. 
           [0031]      FIG. 6  shows a three-dimensional view on a detail of the bifurcated arrangement of two suction ducts of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    In  FIG. 1 , a flow body  2  in the form of a vertical tailplane is depicted. The flow body  2  comprises a base  4 , which adjoins a tail section  6  of a fuselage  8  of an aircraft. The flow body  2  comprises a leading edge  10  that faces the airflow, which is indicated with arrows “f”. Starting from the leading edge  10 , a perforated area  12  extends to both sides of the flow body  2  in a downstream direction. For the sake of simplification, the perforated area  12  may be considered a curved polygon, e.g. a rectangle or slightly tapered surface covering the leading edge  10  having a multitude of openings  14 . 
         [0033]    The openings  14  may be arranged in a regular and/or irregular pattern. Also, the perforated area  12  may be segmented into a multitude of separate perforated areas, depending on the intended volume flow at various locations on the flow body  2 . The openings  14  allow air to pass through a flow surface  20 , which is the outer surface of the flow body  2 , to be drawn into an interior of the flow body  2  out of a boundary layer created on the flow surface  20 . 
         [0034]    Indicated with a dashed line, a front spar  16  extends from the base  4  to a tip portion  18  of the flow body  2 . Further spars may be arranged inside the flow body  2 , which are not depicted for simplification. A skilled person will be readily able to provide a sufficiently stiff support structure having one or more structural components for using the flow body  2  in the desired conditions. The front spar  16  is a structural component, which is located at a most forward or upstream position in the flow body  2 . Further upstream, the perforated area  12  is placed, which substantially extends over a leading edge area covering the region between the front spar  16  and the leading edge  10 . By removal of air through the openings  14 , the flow in the boundary layer on this leading edge region is at least partially laminar. 
         [0035]    Depending on the intended flight parameters of the aircraft, e.g. the speed and altitude, the influences of other flow bodies further upstream, and other factors, a certain pressure distribution is generated on the flow surface  20 .  FIG. 2  exemplarily shows the flow body  2  of  FIG. 1  with a schematic isobar plot of a pressure field, which is generated during flight. Here, several exemplary isobar lines each show an equal or constant pressure on the flow surface  20 . In the pressure field there may be a region of lowest pressure  22 , which is located downstream the front spar  16 . A suction duct extending between this region of lowest pressure  22  and a location interior of the perforated area  12  having a higher pressure, a suction force is initiated. This will be explained with following  FIG. 3 . 
         [0036]      FIG. 3  again shows the same flow body  2  as in  FIGS. 1 and 2  and shows a partially sectional view with a suction duct  24 , which extends from a manifold  26 , which is placed between the front spar  16  and the leading edge  10 , and a location  28  just upstream the region of lowest pressure  22 . The suction duct runs on an exterior side of the flow surface  20  and thereby passes the front spar  16  without having to reach through it. This means that no installation space inside the flow body is required for including the suction duct  24  and its extension to the region of lowest pressure  22 . Since only one side of the flow body  2  is depicted the other (hidden) side of the flow body  2  may also comprise such a suction duct  24  extending from the manifold  26  in a symmetrical manner. However, this is not necessary and a skilled person may only provide a single suction duct  24  if desired. 
         [0037]    The suction duct comprises a first end  23  and a second end  25 . While the first end  23  is connected to the manifold  26 , the second end  25  comprises a suction opening  28 , which faces away from the first end  23  and the leading edge  10 , respectively. The manifold  26 , which is arranged substantially parallel to the leading edge  10 , here resembles an interface between the suction duct  24  and two inlets  30  and  32 , which feed air that passes the perforated area  12  into the manifold  26 . Both inlets  30  and  32  are designed as line sections, which are exemplarily extending parallel to the leading edge  10  and which each are open to a side facing away from the manifold  26 . The space enclosed by the perforated area  12  and limited by the front spar  16  may be closed in order to create a plenum chamber, which receives air that passes through the openings  14 , which air is then drawn into the suction duct  24 . The whole arrangement allows to provide a fluid coupling between the openings  14  in the leading edge region and the suction duct  24 . 
         [0038]      FIG. 4  shows an aircraft  33 , which more illustratively depicts the arrangement of the flow body  2  in the form of a vertical tailplane at a tail section  6  of the fuselage  8  as well as an exemplary location and dimensional extension of the suction duct  24 . 
         [0039]      FIG. 5  shows a more detailed and less schematic design of a part of a flow body  34  in an exemplary embodiment. Here, a top view on a cross-section of the flow body  34  is shown, which comprises a manifold  36  between a front spar  38  and a leading edge  40 , to which manifold  36  a bifurcated arrangement of two suction ducts  42  and  44  is connected. Both suction ducts  42  and  44  comprise a first end  41  and  45 , respectively, and a second end  43  and  47 , respectively. It is clearly apparent that a flow surface  46  of the flow body  34  requires two cutouts  48  and  50 , through which the suction ducts  42  and  44  extend. From the cutouts  48  and  50 , which are placed just upstream the front spar  38 , the suction ducts  42  and  44  extend further downstream and run alongside the exterior of the flow surface  46 . 
         [0040]    At a location just upstream a point of minimum pressure  52  and  54 , the suction ducts  42  and  44  have a suction opening  56  and  58 , respectively, which faces away from the leading edge  40  and the manifold  36 . Exemplarily, the cross-sectional surfaces of the openings  56  and  58  each enclose an angle α and β, which are less than 90° to the streamline of the surrounding airstream. As indicated with α 1   and β 1 , the angles of the cross-sectional surfaces of the openings  56  and  58  may also be distinctly higher, depending on the optimisation of the overall design of the flow body  34 . The low static pressure due to the air flow around the flow body is the main driver for the pressure difference between the regions of lowest pressure  52  and  54  and the manifold  36  supports the suction. 
         [0041]      FIG. 6  shows a three-dimensional drawing of a part of the flow body  34  of  FIG. 5  to illustrate a possible shape of the local cross-sectional surfaces of the suction ducts  42  and  44 . In order to reduce the parasitic drag, which is unavoidable for any exterior part added to the flow surface  46 , the extension away from the flow surface  46  is minimised through providing a crescent shape. Further, due to conducting a detour or diversion around the front spar  38 , both suction ducts  42  and  44  require a bent section  60  and  62 . In these, the air flow changes its direction about approximately 90 degrees, which would lead to an additional pressure loss, depending on the flow velocity. By widening the cross-sectional surface around the bent sections  60  and  62 , the flow velocities can be minimised, such that the additional pressure loss in the bent sections  60  and  62  can be minimised. This asks for some optimization, since wider ducts interfere more with the external flow, hence create more parasitic drag. 
         [0042]    The manifold  36  may be realized as depicted, i.e. through simply providing a rather short duct section, which is open to both sides, wherein a longitudinal axis  64  of the manifold  36  runs parallel to the front spar  38 . Both suction ducts  42  and  44  are connected to the manifold  36  substantially perpendicular to the longitudinal axis  64 . Furthermore, the local cross-sectional surface areas of both suction ducts  42  and  44  may decrease along the course from the bent area  60  or  62  to the suction opening  56  or  58  continuously, e.g. by narrowing the extension on the flow surface  46 . As stated before, a trade-off between the parasitic drag and the positive effects of the boundary layer control has to be done based on the boundary conditions of the respective aircraft for providing a detailed design of the suction ducts  42  and  44 . 
         [0043]    In addition, it should be pointed out that “comprising” does not exclude other elements or steps, and “a” or “an” does not exclude a plural number. Furthermore, it should be pointed out that characteristics or steps which have been described with reference to one of the above exemplary embodiments can also be used in combination with other characteristics or steps of other exemplary embodiments described above. Reference characters in the claims are not to be interpreted as limitations. 
         [0044]    While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.