Abstract:
A cover skin for a variable-shape aerodynamic area, such as a wing structure, tail unit structure, control surface structure or flap structure is described. A cover skin is deformable in one direction without exhibiting substantial deformation in a transverse direction. A deformable framework structure is embedded in a layer of an elastic material, such as rubber or polymer. The framework structure may be comprised of non-deformable elements joined pivotably in auxetic and non-auxetic assemblies of elements that are capable of substantially eliminating transversal contraction, when the cover skin is longitudinal stretched, and substantially eliminating transversal expansion, when the cover skin is longitudinally contracted.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of the filing date of German Patent Application No. 10 2004 056 649.6 filed Nov. 24, 2004, the disclosure of which is hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The field relates to a variable-shaped aerodynamic areas, such as a wing structure, tail unit structure, control surface structure or flap structure. 
     TECHNOLOGICAL BACKGROUND 
     In order to improve the aerodynamics of wings, tail units, control surfaces or flaps of aircraft and other flight equipment, increasingly so-called variable-shape wings are used. The aim is to change the profile curvature without there being any discontinuities such as flap gaps or kinks in the wing contour. One example of such a variable-shape wing structure is described in DE 100 55 961 A1 “Mechanismus zur zumindest bereichsweisen Verstellung der Wölbung von Tragflügeln”. A further example is provided by the test program “Advanced Fighter Technology Integration (AFTI)/F-11 Mission Adaptive Wing (MAW)” implemented in the USA. In this test program an aircraft was equipped with a variable-shape wing and was tested in flight. 
     Many such variable-shape wing concepts are associated with a common problem of finding a cover skin for the variable-shape wing, which cover skin may not only tolerate the deformations resulting from the variability in shape, but may also withstand the air loads that occur. 
     Within the context of the above-mentioned “Mission-Adaptive Wing” program, this problem was solved in that the top of the aerodynamic profile comprises a layer made of glass fiber reinforced polymer, and is elastically bent, while the bottom comprises several overlapping metal sheets which may slide one on top of the other during deformation of the wing. A further approach was presented in the lecture entitled “Formveränderung von Flügelstrukturen mittels integrierter Shape Memory Alloy Aktuatoren” within the context of the 1994 annual conference of the DGLR, where fiber reinforcement with an extremely anisotropic layer construction in an elastic matrix was discussed. Another approach was proposed by Boeing. Their “Flex Skin” concept comprises short straight strips of carbon fiber-reinforced polymer, where strips are embedded in an elastic matrix such as they are insulated from each other. This hybrid structure may be bent, but due to its strip geometry is rigid in a longitudinal direction. 
     SUMMARY OF THE INVENTION 
     A variable-shape aerodynamic area comprises a deformable framework structure that is embedded in a layer made of an elastic material, such as rubber or polymer. In one example, the variable-shape aerodynamic areas includes an improved cover skin. In contrast to known devices, examples of the present invention are adapted to a variable-shape aerodynamic area. A cover skin in the desired direction of deformation may be flexible and/or elastic, and in the directions transverse to it may be rigid. Furthermore, a defined rigidity to bending strain may be selected. 
     A cover skin is capable of being deformed by flexing and/or stretching in one direction without exhibiting substantial deformation in a transverse direction. A deformable framework structure is embedded in a layer of an elastic material, such as a rubber or a polymer. The framework structure may be comprised of non-deformable elements joined pivotably in auxetic and non-auxetic assemblies of elements that are capable of substantially eliminating transversal contraction, when the cover skin is longitudinal stretched, and substantially eliminating transversal expansion, when the cover skin is longitudinally contracted. 
     A cover skin for a variable-shape aerodynamic area is capable of being adapted for use in a wing structure, tail unit structure, control surface structure or flap structure. Embodiments of the cover skin comprise a deformable framework (“Fachwerk”) structure that is embedded in a layer made of an elastic material such as rubber or polymer. According to one embodiment of the cover skin according to the invention the framework construction comprises elements which are movably interconnected by respective joints. The elements of the framework construction may be interconnected such as an elastic layer. The framework construction, including the elements and the joints, may be made in a single piece from an elastic or flexible material at least in some regions. The single-piece framework construction may be cast from a polymeric material or a metal. Alternatively or in addition, the single-piece framework construction may be made from a fiber-reinforced polymer. 
     A single-piece framework construction may be cut from a prefabricated plate. The prefabricated plate from which the single-piece framework construction is cut may be made from materials such as metal, polymer or a fiber reinforced polymer. According to one embodiment of the invention, the single-piece framework construction comprises the regions that form the elements and the regions that form the joints, wherein the regions that form the elements are less flexible or elastic than the regions that form the joints. 
     The elements of the framework construction may be intrinsically elastic. The cross section of the regions that form the joints may be smaller than the cross section of the regions that form the elements. One embodiment of the invention allows for regions that form the elements to be made from a fiber reinforced polymer, and the regions that form the joints to be made from a polymer without fiber reinforcement or with a fiber reinforcement that is weaker such as better flexibility or elasticity is allowed. 
     In an alternative embodiment of the invention, the framework structure may be attached to an elastic carrier fleece. In this arrangement, the framework structure may attached onto the elastic carrier fleece by methods such as sewing or riveting. An improvement allows for the elastic layer to be an elastically arranged fiber reinforcement, such as milled fibres or from a fleece/nonwoven fabric. According to another embodiment of the invention, the elements may be of elongated cross section in transverse direction and may be embedded on edge in the elastic layer. For example, the cover skin in a first direction may be flexible and/or elastic, and in a second direction, which extends transversely to the first direction, to be rigid. 
     Another embodiment of the invention allows for the framework structure to comprise auxetic elementary cells that are formed by the elements. In this arrangement, the framework structure may contain auxetic elementary cells formed by the elements, and conventional elementary cells, and comprises a predefined transversal contraction ratio. According to a particular embodiment of the invention, the transversal contraction ratio is zero. 
     According to one embodiment of the cover skin according to the invention, on two opposite sides in relation to a direction in which the cover skin is flexible and/or elastic, elongated carrier elements that are used for attachment are provided. For example, the carrier elements may be embedded in the elastic layer. The carrier elements may be plate-shaped. 
     According to one embodiment, the cover skin is installed with initial tension (for example biased) such as bulging during contraction is prevented. Other functional materials, such as carbon black, metal powder or conductive fleece fibers may be admixed to the material that forms the elastic layer. For example, a metallic web or a metallic fleece is embedded in one example of the cover skin. The cover skin may comprise an elastic paint. The cover skin may be temperature-resistant at temperatures greater than 180° C. and at temperatures less than −55° C., if materials are selected that remain operably flexible and/or elastic at such temperatures. In some examples, the materials selected need only accommodate a range of temperature-resistance for a low temperature of at least −55° C. to a high temperature of at least +120° C., depending on the temperatures experienced by the structures during deformation of the cover skin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are described in more detail in the detailed description. 
         FIG. 1  illustrates a cross-sectional view of an aerodynamic surface with a variable-shape region in the form of a wing structure, tail unit structure, control surface structure or flap structure. 
         FIGS. 2 to 4  depict enlarged cross-sectional views of the variable-shape region of the aerodynamic area shown in  FIG. 1 . 
         FIGS. 5 and 6  further show an enlarged cross-sectional views showing a section of the cover skin in the variable-shape region of  FIGS. 2 to 4 . 
         FIG. 7  ( a ) depicts a further enlarged sectional view of the cover skin of the variable-shape aerodynamic area that shows the internal structure of the cover skin. 
         FIG. 7  ( b ) illustrates a sectional top view of the variable-shape aerodynamic area of  FIG. 7   a ) along the line A-B at a smaller scale. 
         FIGS. 8  ( a ) to ( c ) shows diagrammatic representations corresponding to the top view of  FIG. 7   b ) showing various states of the cover skin, namely a contracted state ( FIG. 8   a )), a neutral state ( FIG. 8   b )) and an expanded state ( FIG. 8   c )). 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 1 , a wing cross-section  1  is illustrated, the rear end of the wing cross-section has a variable-shape area  2 . Although a wing is shown, the aerodynamic structure may be a wing structure, a tail unit structure, a control surface structure or a flap structure. The variable-shape area  2  may assume various positions, such as the three positions which are shown in  FIGS. 2 to 4 .  FIG. 2  shows the variable-shape area  2  in a neutral position or middle position.  FIGS. 3 and 4  show the variable-shape area  2  in two deflected positions, namely in  FIG. 3  set to a top position and in  FIG. 4  set to a bottom position. In the two cases shown in  FIGS. 3 and 4 , the variable-shape area  2  has an aerodynamic effect in two directions that are opposite those shown in the position in  FIG. 2 , such as a control surface or a flap. 
     The front end of the variable-shape area  2  comprises a rigid box-like wing portion  3  which establishes the connection with a non-variable part of the wing cross-section  1 , and a rigid trailing edge  4  which forms the rear end of the variable-shape area  2  and thus also of the wing cross-section. The part of the area  2  that is situated in between is of variable shape. 
     The inner mechanism of the variable-shape area  2  is not essential in the context of the present invention. In the embodiment shown, a row of profiles  5  that extend in span-wise direction is provided, which are interconnected by means of hinge elements  6  such as they are rotatably articulated. 
     The gaps formed between the profiles  5  are covered by a bottom elastic skin  7  that is flexible, and a top elastic skin  8  that is flexible. These cover skins  7 ,  8  form the aerodynamic surface of the wing  1  in the region of the variable-shape area  2 . The flexible elastic cover skins  7 ,  8  are attached not only to the rigid box-like wing portion  3  but also to the rigid trailing edge  4 . In between, the cover skins  7 ,  8  are supported by the profiles  5 . If the variable-shape area  2  is deflected upward, as shown in  FIG. 3 , then the bottom cover skin  7  is elongated while the top cover skin  8  is compressed. In contrast to this, if the variable-shape area  2  is deflected downward, as shown in  FIG. 4 , the bottom cover skin  7  is compressed while the top cover skin  8  is elongated. 
     In  FIG. 5 , the outer cover skins  7 ,  8  are subject to aerodynamic loads, which are shown relating to the top elastic skin  8 . From the outside, an aerodynamic pressure Pa, and from the inside, a pressure Pi act on the cover skin  8 , which results in an outside surface load  17  and an inside surface load  19  on the cover skin  8 . The difference between the pressures Pa and Pi results in a force which tends to cause a bulge in the cover skin  8  in one direction or the other, i.e. to the inside or to the outside, wherein  FIG. 5  shows the case where the outer pressure Pa exceeds the inner pressure Pi and thus, a force is generated which tends to cause a bulge in the cover skin  8  towards the inside, as shown. Furthermore, due to frictional forces caused by the surrounding flow, a thrust load  18  occurs on the outer surface of the cover skin  8 . For aerodynamic reasons, the deformation resulting from the differential pressure on the cover skin  8  should not exceed a predefined permissible magnitude. 
     The differential pressure results in deformation of the cover skin  8 , which deformation for aerodynamic reasons in a concrete application should not exceed a specified permissible magnitude. At the same time, the cover skin  8  should describe a movement of the profiles  5  around their joints  6  or, generally speaking, the cover skin  8  should be in a position to even out any difference in length due to the change in form of the variable-shape area  2  in the form of lengthening or shortening. 
     As shown in  FIG. 6 , the change in shape of the variable-shape surface  2  results in a geometry of the cover skin  8 , which is deformed in relation to the initial position  23 . A point  24  of the cover skin  8 , which is in its initial position  23  and is not deformed, moves to a point  24 ′ as it is displaced by the distance  26  in the direction along the cover skin  8 , and by the distance  25  in the direction substantially perpendicular to the cover skin  8 . For the point  24  to be able to move to point  24 ′, the cover skin  8  has to be flexible in relation to bending, and elastic in relation to elongating. 
     With reference to  FIGS. 7 and 8 , an embodiment of a cover skin, overall designated  8 , is to be described, which embodiment of course is not limited to the top cover skin  8  described in  FIGS. 2 to 6 . Generally speaking the cover skin  8  comprises a deformable framework structure  28  which is embedded in a layer  27  made of an elastic material. This elastic material, which allows for elasticity and flexibility during deformation of the variable-shape aerodynamic area  2 , and may be a rubber material or a polymer material. 
     In the embodiments shown in  FIGS. 7   a - 8   c , the framework structure  28  comprises individual elements  29  (one labelled) that are movably interconnected by respective joints  30  (one labelled). As shown in  FIG. 7   b , the individual elements  29  are of elongated cross section and are embedded on edge in the elastic layer  27 . At the two opposing sides with respect to a direction, in which the cover skin  8  is flexible and/or elastic, elongated carrier elements  34 ,  35  are used to attach the cover skin  8  to the sub-structure, approximately on the rigid box-like wing portion  3  and on the rigid trailing edge  4  or also on the profiles  5 . Attachment may take place by connecting structures such as screws  31 , as shown in cross section in  FIG. 7   a . The carrier elements  34 ,  35  are plate-shaped and embedded in the elastic layer  27 , for example. 
     In the example illustrated in  FIGS. 8   a - 8   c , the cover skin  8  is flexible and/or elastic in a first direction depicted by arrows in  FIGS. 8   a  and  8   c . In a second direction, which extends transversely to the first direction, the cover skin  8  is rigid. The framework structure  28  is designed from two types of elementary cells  39 ,  40 , which are designed such that a transversal contraction ratio of zero results. In other words, the cover skin  8  does not change in length in the transverse direction, if there is a contraction or expansion in the direction of the arrows, as illustrated in  FIGS. 8   a  and  8   c . This is achieved by a combination of so-called auxetic (in German: “auxetisch”) elementary cells  39  and conventional elementary cells  40  in the framework construction  28 . 
     The auxetic elementary cells  39  are elements with a negative transverse contraction, while the conventional elementary cells  40  are elements with a positive transverse contraction, such that the respective effects cancel each other out in the transverse direction. The geometry of the auxetic elementary cells  39  is selected such that an expansion in the desired movement direction also leads to an expansion in transverse direction. Contraction in the desired movement direction also leads to contraction in the transverse direction. Beside each auxetic elementary cell  39  a conventional elementary cell  40  is arranged. It contracts in a transverse direction when it is expanded in the desired movement direction. Conversely, the conventional elementary cell  40  expands during contraction in the desired movement direction. Transverse deformation of the auxetic elementary cells  39  and of the conventional elementary cells  40 , in each instance is mutually cancelled out so that in effect, the resulting transverse deformation of the framework that would be expected of ordinary material subject to stretching or compression is not exhibited during contraction or expansion in the direction of movement for the cover skin  8 . 
     In order to prevent bulging during contraction, the cover skin  8  may be installed with initial tension. As an alternative, a corresponding high degree of bending rigidity of the framework construction  28  may be provided. 
     As an alternative to the embodiment shown, in which the elements  29  of the framework construction  28  are interconnected by joints  30 , the elements  29  also may be indirectly interconnected using the structure of the elastic layer  27 , such that the elements  29  are articulated with regard to one another, as if joined at the joints  30 . 
     According to yet another alternative, the framework construction  28  including the elements  29  and the joints  30 , at least in some regions, may include an elastic or flexible material. Thus, the single-piece framework construction  28  contains the regions that form the elements  29 , and further contains the regions that form the joints  30 , wherein the regions which form the elements  29  may be less flexible or elastic than the regions which form the joints  30 . As an alternative, the elements  29  of the framework construction may also be intrinsically elastic. 
     The cross-section of the region of the single-piece framework construction, which regions form the joints  30 , may be smaller than the cross-section of the regions that form the elements  29 , or the regions that form the elements  29  may be made from a fiber-reinforced polymer, and the regions that form the joints  30  may be made from a polymer material without fibre reinforcement, or with fibre reinforcement that is correspondingly weaker such that improved flexibility or elasticity may be allowed. In one example, the elements  29  are regions of stiffened material or are a layer of stiffened material. 
     The framework structure  28  may also be attached to an elastic carrier fleece, wherein the framework structure  28  may be connected to the carrier fleece by attachment methods such as sewing or riveting. 
     The single-piece framework construction  28  may be cast from materials such as a polymer material or a metal, or it may include a fiber reinforced polymer. For example, the single-piece framework construction  28  may be cut out from a prefabricated plate, such as by laser cutting or water-jet cutting. This prefabricated plate may include metal, polymer or a fiber-reinforced polymer. 
     The elastic layer  27 , in which the framework structure  28  and if applicable the carrier elements  34 ,  35  are embedded, may comprise an elastic fibre reinforcement, which for example may comprise milled fibres or a fleece, for example. 
     Furthermore, the material that forms the elastic layer  27  may include additional functional materials which are used to serve additional functions, such as absorption of radar emission and attenuating electromagnetic surface waves, for example. For example, such additional functional materials may be carbon black, metal powder or conductive milled fibres. 
     Furthermore, a metallic woven fabric or a metallic nonwoven fabric  33  may be provided on the cover skin  8  or may be embedded in the cover skin  8 , and may serve as protection against lightning. Alternatively, other conductive materials may be coated on the surface of the cover skin  8 , such as an elastic paint  32 , as depicted in  FIG. 7   a.    
     The cover skin  8  is designed such that it resists any temperatures encountered during flight operations. In one example, the materials are selected to be temperature-resistant in a range of temperatures of at least −55° C. for low temperatures and at least +120° C. for high temperatures. More preferably, the materials are selected to increase this range to at least +180° C. for areas experiencing higher temperatures during operation of an aircraft. Even more preferably, in some areas of high speed vehicles or for some high altitude aircraft, materials are selected for even higher or even lower temperatures, respectively. 
     Thus, a cover skin  8  for a variable-shaped aerodynamic area which is aerodynamically smooth and tight, and which has a light weight and defined flexibility and/or elasticity in a desired direction of movement is provided, while, at the same time, a defined rigidity is provided in at least one other spatial direction. 
     It should be noted that the term “comprising” does not exclude other elements or steps and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. 
     It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Wing cross-section 
           2  Variable-shape area 
           3  Rigid box-like wing portion 
           4  Rigid trailing edge 
           5  Profile 
           6  Joint 
           7  Bottom elastic cover skin 
           8  Top elastic cover skin 
           17  External pressure 
           18  Thrust load 
           19  Internal pressure 
           20  Bending line of the top elastic cover skin 
           23  Non-deformed geometry of the top elastic cover skin 
           24 ,  24 ′ Point on the surface of the cover skin 
           25  Vertical deformation of the point  24   
           27  Elastic rubber layer or polymer layer 
           28  Framework structure 
           29  Elements 
           30  Joints 
           31  Attachment screws 
           32  Elastic paint 
           33  Metallic woven fabric or metallic nonwoven fabric 
           34 ,  35  Carrier plate element of the framework structure 
           38  Contact point between the elements of the framework construction 
           39  Auxetic elementary cell of the framework construction 
           40  Conventional elementary cell of the framework construction 
           50  Substructure