Abstract:
The invention relates to a window replacement for filling a window frame in an aircraft, in particular a passenger aircraft, the window replacement being fixable by means of a retainer in the window frame in place of the window assembly when the window assembly is removed. According to the invention, the window replacement comprises a panel-shaped element with at least a single curvature, the panel-shaped element, being made of a synthetic material. In one configurational variant, the window replacement according to the invention is formed by a monolithic panel-shaped element. In a second configurational variant, the panel-shaped element of the window replacement is formed by a sandwich panel. Both embodiments allow a weight reduction in comparison with a window replacement of a metallic material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of German Patent Application No. 10 2006 044 093.5 filed on Sep. 20, 2006, the complete disclosure of which is herein incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a window replacement for filling a window frame in an aircraft, in particular a passenger aircraft, the window replacement being fixable by means of a retainer in the window frame in place of the window assembly when the window assembly is removed. 
       BACKGROUND OF THE INVENTION 
       [0003]    On account of the increases in the air freight market, a trebling of the volume of freight is expected over the next 20 years, which in turn will create a demand for approximately 3000 further freight aircraft. Since only few freight aircraft companies buy new freight aircraft from the manufacturers, by far the majority of all freight aircraft are converted passenger aircraft. During the conversion, the core is taken out of the passenger aircraft and all fixtures and fittings necessary for passenger service, such as the galley and panellings, are removed. A large cutout for the freight door is made in the fuselage, the bottom of the cabin is reinforced and the windows are closed by metal covers. 
         [0004]    During the conversion to the freight version, the first step taken in the area of the windows of the passenger aircraft is to remove the window retainers and then take out the transparent interior and exterior window panes together with the window seal, known as the window assembly. The actual window frame, which serves for reinforcing the fuselage structure in the area of the window frame and takes up all the structural loads, remains in place. 
         [0005]    The replacement of the windows by metal covers means additional weight, causing losses in payload of several tonnes in comparison with freight aircraft that are already supplied ex works without windows. This results in considerable extra costs over the remaining lifetime of the converted aircraft, which can be at least partly compensated by means of the window replacement according to the invention. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the invention is to avoid the disadvantages described above of the known closure cover for window frames of passenger aircraft. 
         [0007]    This object is achieved by a window replacement with the features of Patent Claim  1 . 
         [0008]    The fact that the window replacement is formed by a panel-shaped element with at least a single curvature, with the panel-shaped element being formed from a synthetic material, allows the existing losses in payload in the conversion of passenger aircraft into freight aircraft to be reduced. One reason for the weight advantage in this case is the much lower density of the composite material of the novel window replacement in comparison with metal, combined with its outstanding specific strength and rigidity values. 
         [0009]    In a first configurational variant (“monolithic window replacement”), the window replacement is formed by a panel-shaped element of a carbon fiber reinforced curable epoxy resin that is curved at least once. The outer contour of the panel-shaped element corresponds substantially to the outer contour of the window assembly fitted as standard in the window frame. The panel-shaped element may be formed with a single or double curvature (spherical), in order to achieve optimum adaptation to the local curvature of the aircraft fuselage in the window area. In comparison with the thickness of the window assembly, the window replacement in this configurational variant may have a lower thickness, so that it may be necessary for the cross-sectional geometry of a peripheral seal and/or the shaping of the retainer to be modified slightly in comparison with the sealing system of the window assembly and of the retainer used for fixing the window assembly to allow the window replacement to be used. 
         [0010]    In the first configurational variant of the window replacement, production is performed with preference by means of a continuous production process, for example with a preimpregnated web material (“CRP prepreg material”) of a curable carbon fiber reinforced epoxy resin. The web material already has the required layer structure, the suitable layer orientation and the correct material thickness. Before the final curing process, first the panel-shaped element is removed from the endless web material, for example by cutting out or punching out two-dimensional pieces of an appropriate geometry. At least the edge contour of the panel-shaped elements corresponds approximately to the window assemblies to be replaced. The required further geometrical formation of the monolithic window replacement, in particular the adaptation to different local curvatures of the aircraft fuselage, can be performed before the curing process or during the curing process. It may be that thickenings are also effected in the area of the edge contour of the window replacement before the curing process, to provide edge reinforcement and thickness adaptation of the panel-shaped elements. After the completion of the curing process, the peripheral seal is pulled onto the panel-shaped element to form the finished monolithic window replacement. 
         [0011]    Alternatively, fiber reinforced thermoplastic materials may also be used for producing the monolithic window replacement. 
         [0012]    By contrast with the first configurational variant, in a second configurational variant the panel-shaped element of the window replacement is formed as a sandwich panel, which likewise has at least a single curvature, in order to permit adaptation to the curved outer skin of the fuselage or to the window frame. The sandwich panel has the usual construction with a core structure, preferably a core structure capable of allowing drainage, arranged between two outer layers. 
         [0013]    The interior window pane and the exterior window pane of the window assembly are in this case reproduced or replaced by the interior outer layer and the exterior outer layer of the sandwich panel, and the function of the space between the interior window pane and the exterior window pane is undertaken by the core structure arranged between the interior and exterior outer layers of the sandwich panel. 
         [0014]    This configurational variant has the advantage in particular that the window replacement has approximately the same dimensions or the same geometrical shapes as a window assembly that is usually fitted in the window frame. Consequently, the window replacement in accordance with the second configurational variant can be fitted in a simple manner directly in the existing window frame without any further structural modifications, in particular to the retainer and/or to a peripheral seal. 
         [0015]    In addition, there is a further weight saving in comparison with the monolithic embodiment, with the same or even to some extent further improved mechanical properties. 
         [0016]    Since, in a way corresponding to the window assembly, the window replacement according to both configurational variants does not have to take up any additional loads apart from the internal cabin pressure—these loads of the fuselage structure are generally taken up by the window frame—the window replacement in both configurational variants can be of a relatively simple construction, that is to say there is no need to provide any special layer structure and/or fiber orientation that is optimized in terms of stress transmission or distribution in the monolithic window replacement or in the sandwich panel. 
         [0017]    A refinement of the window replacement provides that it is surrounded by a peripheral seal. 
         [0018]    As a result, the sealing effect of the window replacement is enhanced and at the same time a firm fit of the window replacement is ensured. 
         [0019]    An advantageous refinement of the window replacement provides that the panel-shaped element is of a monolithic form and the synthetic material is fiber-reinforced. 
         [0020]    The fiber reinforcement achieves a higher mechanical load-bearing capacity of the window replacement. The monolithic form of the panel-shaped element additionally makes it possible for it to be easily produced, for example from a “CRP prepreg material” in web form of a suitable layer structure, in a continuous and therefore low-cost production process. 
         [0021]    In accordance with a further advantageous refinement, it is provided that the synthetic material is a curable carbon fiber reinforced epoxy resin. This refinement allows a higher mechanical strength of the window replacement in comparison with other fiber composite materials. 
         [0022]    A further development provides that the window replacement is formed by a sandwich panel which has a core structure that is arranged between an interior outer layer and an exterior outer layer. 
         [0023]    As a result, the window replacement does not require any structural modifications to the peripheral seal and/or the retainer of the window frame or the window assembly, so that it is possible for it simply to take the place of a window assembly fitted in the usual way in the window frame. Moreover, the heat insulating capacity of the window replacement in comparison with the monolithic embodiment is improved to such an extent that under some circumstances it matches the heat insulating capacity of a window assembly. 
         [0024]    In accordance with a further advantageous refinement, it is provided that the core structure is capable of drainage. 
         [0025]    As a result, condensed water within the sandwich panel is drained away. This avoids any mechanical impairment of the integrity of the core structure of the sandwich panel, for example by condensate freezing. 
         [0026]    A further advantageous refinement provides that the core structure is a folded honeycomb structure or a slit honeycomb structure. 
         [0027]    This refinement permits an adequately high compressive strength of the core structure arranged between the exterior outer layer and the interior outer layer of the sandwich panel to ensure its spacing function, while at the same time water condensing within the core structure can be drained away to the outside. 
         [0028]    A further advantageous embodiment of the window replacement provides that the core structure is formed from a closed-cell rigid foam. 
         [0029]    This achieves the effect of joining the interior outer layer and the exterior outer layer to the core structure formed from the closed-cell rigid foam by a connection that is simple and at the same time can withstand mechanical loading. The closed-cell structure of the rigid foam prevents, or at least hinders, the ingress of water, for example in the form of condensed water. An open-cell rigid foam on the other hand—by contrast with a core structure with cells of large volume that is capable of drainage—would be undesirably conducive to the ingress of water on account of the capillary forces occurring. 
         [0030]    Further advantageous refinements of the window replacement are set out in the further patent claims. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    In the drawings: 
           [0032]      FIG. 1  shows a schematic inner view of a window in a passenger aircraft, 
           [0033]      FIG. 2  shows a cross section along the sectional line II-II through the window frame in accordance with  FIG. 1 , 
           [0034]      FIG. 3  shows a cross section through the window frame with a fitted first configurational variant of a window replacement according to the invention, and 
           [0035]      FIG. 4  shows a second configurational variant of the window replacement. 
       
    
    
       [0036]    In the drawings, the same structural elements in each case have the same reference numerals. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0037]      FIG. 1  shows a schematic inner view of a window usually fitted in a passenger aircraft, with a window frame and a window assembly. 
         [0038]    A window frame  1  is fixed in the area of a window opening  2  to an outer skin  3  of the fuselage of a passenger aircraft, for example being riveted or adhesively bonded to it. By means of a retainer  4 , a window assembly  5  is pressed into the window frame  1 , in order to ensure a firm and, in particular, pressure-sealed fit of the window assembly  5  in the window frame  1 . In the exemplary embodiment shown, the connection between the retainer  4  and the window frame  1  is achieved by a total of eight fastening elements  6 . 
         [0039]      FIG. 2  shows a schematic cross section through a window frame that is usually used for passenger aircraft, according to  FIG. 1 , along the sectional line II-II. 
         [0040]    The window assembly  5  is pressed by means of the retainer  4  onto a slightly conical bearing surface  7  of the window frame  1 . As a result of the conicity of the bearing surface  7 , the window assembly  5  centres itself of its own accord in the window frame. The window frame  1  is firmly connected to the outer skin  3  of the fuselage by means of fastening elements (not represented), for example in the form of rivets. The window assembly  5  comprises an interior window pane  8  with an exterior window pane  9  arranged parallel to it with a spacing in between, which are enclosed by a peripheral seal  10 . To receive the interior window pane  8  and the exterior window pane  9 , the seal  10  has two peripheral inner grooves or inwardly directed recesses. Between the interior window pane  8  and the exterior window pane  9  there is an air-filled intermediate space  11 . Air is admitted to the intermediate space  11  from the passenger cabin, in order to prevent icing of any condensate that may be deposited. The fastening element  6  comprises an eye bolt  12  and a nut  13  that can be screwed onto it. A vertical web  14  of the window frame  1  also has a bore for leading through a cross bolt  15 . By tightening the nut  13 , the retainer  4 , the window assembly  5  and the window frame  1  are braced with respect to one another. The configuration of the other seven fastening elements corresponds to the construction described above of the fastening element  6 . 
         [0041]      FIG. 3  shows a cross section through a window frame with a fitted first configurational variant of a window replacement. 
         [0042]    The window frame  1  is firmly connected to the outer skin  3  of the fuselage in the area of the window opening  2 . The web  14  of the window frame  1  and the cross bolt  15  are structurally unchanged in relation to the described embodiment in accordance with  FIG. 2 . 
         [0043]    The window replacement  16  is formed by a monolithic panel-shaped element  17 . The panel-shaped element  17  has at least a single curvature, which corresponds approximately to the respective local curvature of the outer skin  3  of the fuselage. On account of the smaller material thickness of the panel-shaped element  17  in comparison with the overall height of the window assembly  5 , a retainer  18  has a greater height in comparison with the retainer  4  represented in  FIG. 2 . A likewise modified peripheral seal  19  surrounds the panel-shaped element  17 . By contrast with the seal  10  described in  FIG. 2 , the seal  19  has just one peripheral recess or groove for receiving an edge of the panel-shaped-element  17  and a correspondingly lower overall height in comparison with the seal  10 . The fixing of the window replacement  16  or the panel-shaped element  17  in the window frame  1  or on the conical bearing surface  7  is performed in a known way by means of the eye bolt  12  and the nut  13  and by means of the cross bolt  15  arranged in the area of the web  14  of the window frame  1 . An inner surface  20  of the panel-shaped element  17  may also have reinforcing elements (not represented), for example in the form of ribs or beads. 
         [0044]    The panel-shaped element  17  is formed for example from a curable fiber reinforced synthetic material, for example from a carbon fiber reinforced epoxy resin. The production of the window replacement  16  or the panel-shaped element  17  may be performed continuously in large numbers, for example by means of a CRP “prepreg” material in web form of the required material thickness and fiber orientation. Before or during the curing of the panel-shaped element  17  that is cut or punched out from the web material, or removed from the web material in some other way, it may be adapted to the respective local curvature of the outer skin  3  of the fuselage. The panel-shaped element  17  may be curved once or twice (spherical). The material thickness of the monolithic panel-shaped element  17  is preferably up to 5 mm. 
         [0045]      FIG. 4  shows a second configurational variant of the window replacement. 
         [0046]    As a difference from the first configurational variant, a window replacement  21  with a sandwich panel  22  is formed as the panel-shaped element  23 . The sandwich panel  22  has a core structure  24 , which is provided on either side with an interior outer layer  25  and an exterior outer layer  26 . Here, the dimensions or the shape of the interior and exterior outer layers  25 ,  26  coincide substantially identically with those of the interior and exterior window panes  8 ,  9  of the window assembly  5 . Therefore, it is easily possible for the window replacement  21  to take the place of the window assembly  5 . The seal  10  described in  FIG. 2  can also (continue to) be used unchanged with the window replacement  21 . To make this possible, the core structure  24  of the sandwich panel  22  has with preference a material thickness that corresponds approximately to a spacing between the interior window pane  8  and the exterior window pane  9  of the conventional window assembly  5  or the spacing of the interior space  11 . In a way corresponding to the window assembly  5 , the interior outer layer  25  and the exterior outer layer  26  are enclosed or bordered by a peripheral seal  27 . The cross-sectional geometry of the seal  27  corresponds substantially to the cross-sectional geometry of the seal  10  of the window assembly  5  (cf.  FIG. 2 ), so that the window assembly  5  can be exchanged for the window replacement  21  without any appreciable structural modifications. In a way corresponding to the window assembly  5 , the window replacement  21  in the fitted state lies against the conical bearing surface  7  of the window frame  1 . In principle, it would be possible to continue to use the old seal  10  of the window assembly  5  for the window replacement  21 , but in practice the window replacement  21  is generally provided with a brand-new seal  27  for safety reasons. 
         [0047]    The interior and exterior outer layers  25 ,  26  are formed from a fiber reinforced, curable synthetic material, in particular with a carbon reinforced epoxy resin. The core structure  24  may be formed, for example, in a known way from Nomex® paper or from aluminium honeycombs. The material thicknesses of the interior and exterior outer layers  25 ,  26  correspond approximately to the corresponding thicknesses of the interior and exterior window panes  8 ,  9  that are usually used in the window assembly  5 . 
         [0048]    Moreover, the core structure  24  is capable of drainage, that is to say the core structure  24  is formed in particular from a folded honeycomb or from a slit honeycomb. As a result, the draining away of any condensate that may form in the area of the core structure  24  is ensured and freezing of the same is prevented. The configuration of the window replacement  21  with the sandwich panel  22  leads to a heat insulating capacity that is comparable with the window assembly  5  usually fitted in a window frame  1  and furthermore leads to reduced weight in comparison with the monolithic embodiment (cf.  FIG. 3 ). An inner surface  28  of the panel-shaped element  23  may also have reinforcing elements (not represented), for example in the form of ribs or beads. 
         [0049]    The second configurational variant of the window replacement  21 , represented in  FIG. 4 , can consequently take the place directly of the window assembly  5  without any structural modifications of the fastening means, in particular of the retainer  4 , the eye bolt  12 , the nut  13 , the web  14  and the cross bolt  15 . 
         [0050]    The first configurational variant has a simpler construction in comparison with the second configurational variant of the window replacement  21 . In addition, the first configurational variant of the window replacement  16  makes it possible for it to be produced easily and at low cost, in particular by means of continuous production processes. However, the window replacement  16  in accordance with the first configurational variant does not have the heat insulating capacity of the window replacement  21  in accordance with, the second configurational variant, which is achieved in particular by the sandwich structure. In addition, the first configurational variant has a lower mass in relation to the second configurational variant of the window replacement  21  with the sandwich panel  22 . 
         [0051]    However, neither of the two configurational variants of, the window replacement  16 ,  21  have to meet increased mechanical requirements, since only the forces caused by the increased cabin pressure have to be taken up. All the structural fuselage loads are borne by the window frame  1  and directed away via the window opening  2 , so that in particular there is no need for a reinforcing fiber orientation based on stress transmission or distribution and/or a specific layer sequence of the reinforcing fabrics that are used. 
         [0052]    Both variants of the window replacement are opaque to visible light and to electromagnetic radiation in the near and far infrared ranges, in order inter alia to make it impossible to see into the space inside the aircraft from the outside. 
       LIST OF REFERENCE NUMERALS 
       [0053]      1  window frame 
         [0054]      2  window opening 
         [0055]      3  outer skin of fuselage 
         [0056]      4  retainer 
         [0057]      5  window assembly 
         [0058]      6  fastening element 
         [0059]      7  conical bearing surface 
         [0060]      8  interior window pane 
         [0061]      9  exterior window pane 
         [0062]      10  seal 
         [0063]      11  intermediate space 
         [0064]      12  eye bolt 
         [0065]      13  nut 
         [0066]      14  web 
         [0067]      15  cross bolt 
         [0068]      16  window replacement 
         [0069]      17  panel-shaped element 
         [0070]      18  retainer 
         [0071]      19  seal 
         [0072]      20  inner surface 
         [0073]      21  window replacement 
         [0074]      22  sandwich panel 
         [0075]      23  panel-shaped element 
         [0076]      24  core structure 
         [0077]      25  interior outer layer 
         [0078]      26  exterior outer layer 
         [0079]      27  seal 
         [0080]      28  inner surface