Abstract:
A mult-rib box-shaped aeronautical structure comprising upper and lower skins ( 19, 21 ) stiffened by span wise stringers ( 25, 25′, 25 ″), span wise front and rear spars ( 11, 13 ) and chord wise ribs ( 27, 27′, 27 ″), where at least one panel ( 31, 33 ) of any of said skins ( 19, 21 ) is non delimited by said ribs ( 27, 27′, 27 ″) and said stringers ( 25, 25′, 25 ″) and comprise an stiffening element ( 41, 43 ) arranged as a panel breaker to avoiding the need of increasing the panel thickness to withstand buckling loads.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention refers to the skin of an aircraft lifting surface skin and, more in particular, to an stiffening arrangement for those skin panels which can not be stiffened by stringers. 
       BACKGROUND OF THE INVENTION 
       [0002]    The main structure for aircraft lifting surfaces mainly consists of a leading edge, a torsion box, a trailing edge, a root joint and a wing tip. The torsion box in turn consists of several structural elements: upper and lower skins stiffened by stringers on one side; spars and ribs on the other side. Those stringers, spars and ribs create a grid pattern that subdivides the upper and the lower skins in structural panels limited now by those elements, discretizing that way the bulking loads in the skins. Typically, the structural elements forming the torsion box are manufactured separately and are joined with the aid of complicated tooling to achieve the necessary tolerances, which are given by the aerodynamic, assembly and structural requirements. 
         [0003]    As is well known, weight is a fundamental aspect in the aeronautic industry and therefore there is a current trend to use composite material instead of metallic even for primary structures. 
         [0004]    The composite materials that are most used in the aeronautical industry consist of fibers or fiber bundles embedded in a matrix of thermosetting or thermoplastic resin, in the form of a preimpregnated or “prepreg” material. Its main advantages refer to:
       Their high specific strength with respect to metallic materials. It is the strength/weight equation.   Their excellent behavior before fatigue loads.   The possibilities of structural optimization hidden in the anisotropy of the material and the possibility of combining fibers with different orientations, allowing the design of the elements with different mechanical properties adjusted to the different needs in terms of applied loads.       
 
         [0008]    The design and manufacture of large composite skins of aircraft lifting surfaces such as the skins of aircrafts wings involves several problems. One of them is the stabilization of those skin panels which are not stiffened with stringers due to interferences with, particularly, spars or ribs. In the prior art, this stabilization is achieved increasing the panel thickness, increasing, thus, the skin weight which is an important drawback for an aeronautical structure. 
         [0009]    This invention is focused on the solution of this problem. 
       SUMMARY OF THE INVENTION 
       [0010]    One object of the present invention is to provide a mult-rib box-shaped structure of an aircraft lifting surface such as a wing or an horizontal tail plane having skins optimized in weight even in those panels which are not stiffened by span wise stringers due to interferences with another structural elements. 
         [0011]    Another object of the present invention is to provide a mult-rib box-shaped structure of an aircraft lifting surface such as a wing or an horizontal tail plane having skins arranged for reducing buckling risks even in those panels which are not stiffened by span wise stringers due to interferences with another structural elements. 
         [0012]    These and other objects are met by a mult-rib box-shaped aeronautical structure comprising upper and lower skins stiffened by span wise stringers, span wise front and rear spars and chord wise ribs, where at least one panel of any of said skins is non delimited by said ribs and said stringers and comprise an stiffening element arranged as a panel breaker to avoid the need of increasing the panel thickness to withstand the buckling loads. 
         [0013]    In a preferred embodiment the stiffening element is an isolated stiffening element in the panel that need reinforcement to withstand the buckling loads. Hereby it is achieved an arrangement that facilitates its installation in said panels. 
         [0014]    In another preferred embodiment the stiffening element is joined to another structural element, particularly a rib or a stringer ending in an adjacent panel. Hereby it is achieved an arrangement that improves the load distribution in said panels. 
         [0015]    In a preferred embodiment the stiffening element follow a linear trace between the end of the closer stringer and a final point in said panel at a suitable distance of the structural element that delimits said panel instead of an stringer. Hereby it is achieved an optimized division of said panel for weight reduction purposes. 
         [0016]    In a preferred embodiment, the stringers and the stiffening elements have the same transversal shape, preferably a T or a L shape. Hereby it is achieved an arrangement that facilitates the skin manufacturing process. 
         [0017]    Other characteristics and advantages of the present invention will be clear from the following detailed description of embodiments illustrative of its object in relation to the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1   a  is a perspective view of a known multi-rib torsion box of an aircraft wing and  FIG. 1   b  is a cross-section view of  FIG. 1   a  along plane A-A. 
           [0019]      FIG. 2  is an internal plan view of an area of a skin belonging to the torsion box of an aircraft wing according to the prior art. 
           [0020]      FIG. 3  is an internal plan view of an area of a skin belonging to the torsion box of an aircraft wing according to a first embodiment of the present invention. 
           [0021]      FIG. 4  is an internal plan view of an area of a skin belonging to the torsion box of an aircraft wing according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The invention relates to a multi-rib torsion box structure of composite materials with longitudinal stiffeners, having preferably a T-shaped or a L-shaped cross-section. The composite material can be both carbon fiber and fiberglass with thermosetting or thermoplastic resin. The main field of application is aeronautical lifting surfaces structures, although they can also be applied to other structures with similar features. 
         [0023]    The main structure of an aircraft lifting surface such as a wing consists of a leading edge, a torsion box, a trailing edge, a root joint and a wing tip. A multi-rib torsion box  1  such as the one depicted in  FIGS. 1   a  and  1   b  is structurally based on a span wise front spar  11  and a span wise rear spar  13  (understanding the terms front and rear in relation to the flight direction of the aircraft), a plurality of chord wise ribs  27 ,  27 ′,  27 ″,  27 ′″ and the upper and lower skins  19 ,  21  with a plurality of span wise stringers  25 ,  25 ′,  25 ″. 
         [0024]    The main functions of ribs  27 ,  27 ′,  27 ″,  27 ′″ is to provide torsion rigidity and to limit the skins  19 ,  21  and the stringers  25 ,  25 ′,  25 ″ longitudinally so as to discretize the buckling loads and maintain the shape of the aerodynamic surface. 
         [0025]    The primary function of the skins  19 ,  21  is to provide a continuous surface to give support and distribute the aerodynamic loads and, thus, it is structured as a set of panels delimited by said ribs  27 ,  27 ′,  27 ″,  27 ′″ and said stringers  25 ,  25 ′,  25 ″ as well as the front spar  11  and the rear spar  13 . 
         [0026]      FIG. 2  shows a common case where the panels  31 ,  33  are delimited by the ribs  27 ,  27 ′,  27 ′, the stringer  25 ′ and the front spar  11 . In this case, the stringer  25  cannot be extended to decrease the size of the panels  31 ,  33  because it would interfere with the front spar  11 . 
         [0027]    According to the invention those panels  31 ,  33  lacking one stringer are provided with stiffening elements so that their thickness does not need to be increased to avoid buckling. Those stiffening elements act therefore as panel breakers on the skin decreasing the panels size allowing a thickness decrease and a weight reduction. 
         [0028]    In a preferred embodiment, illustrated in  FIG. 3 , said stiffening elements  41  are isolated stiffening elements on each panel  31 ,  33  that, preferably, follow a lineal trace between the end of the stringer  25  in the adjacent panel  35  and a point close to the rib  27  at a suitable distance of the front spar  11  to comply with the structural requirements. Said stiffening elements  41  “break” the initial panels  31 ,  33  into smaller panels  31 ′,  31 ″;  33 ′,  33 ″. Panels  31 ′,  33 ′ are now limited by the stiffening elements  41 , the stringer  25 ′ and the ribs  27 ,  27 ′,  27 ″, having thus a smaller area than the initial panels  31 ,  33  allowing a thickness decrease and a weight reduction. 
         [0029]    In another preferred embodiment, said stiffening elements are installed on the skins  19 ,  21  joined to another structural element. For instance, as shown in  FIG. 4 , the stiffening element  43  that, preferably, follows a lineal trace between the end of the stringer  25  in the adjacent panel  35  and a point close to the rib  27 ′ at a suitable distance of the front spar  11  to comply with the structural requirements, “breaks” the initial panel  33  into the smaller panels  33 ′,  33 ″. Panel  33 ′ is now limited by the stiffening element  43 , the stringer  25 ′ and the ribs  27 ′,  27 ″, having thus a smaller area than the initial panel  33  allowing a thickness decrease and a weight reduction. On the other hand, the stiffening element  43  is joined to the rib  27 ′ and to the stringer  25  ending in the adjacent panel  35 , i.e. a joint arrangement that provides a better load continuity and distribution. 
         [0030]    The stiffening elements  41 ,  43  of both embodiments can be installed on the skins  19 ,  21  by a co-curing or a co-boding procedure or by a riveted joint. In the second case, the trace shall leave enough space between the stiffening element  43  and the front spar  11  to allow the enlargement of the stiffening element foots needed in the joining areas with stringer  25  and rib  27 ′. 
         [0031]    In a preferred embodiment, said stiffening elements  41 ,  43  shall have the same transversal section than the stringer  25 , i.e. a T-shaped or a L-shaped transversal section. 
         [0032]    Among others, the advantages of the present invention are the following:
       Reduction of the skin thickness and weight of the involved panels in an amount close to the 20%.   Reduction of the buckling risk in the skin panels.       
 
         [0035]    Although the present invention has been fully described in connection with preferred embodiments, it is evident that modifications may be introduced within the scope thereof, not considering this as limited by these embodiments, but by the contents of the following claims.