Abstract:
A moveable control surface mounting rib assembly ( 110 ) comprises a rib ( 144 ) which is attached to one side of a spar member ( 112 ) and a stiffener ( 160 ) attached to another side of the spar member. The stiffener abuts the upper and lower flanges of the spar member in order to react the loads. The invention is applicable to the mounting of any movable control surface.

Description:
RELATED APPLICATION 
     This application claims priority to UK Patent Application No. 1120992.1 filed 7 Dec. 2011, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention is concerned with an aircraft rib assembly. More particularly, the present invention is concerned with an aircraft rib assembly suitable for attachment of an aircraft control surface at any of a number of positions along the spar of an aircraft wing, vertical stabiliser or horizontal stabiliser. 
     It is desirable to mount movable control surfaces such as inter alia flaps, slats, ailerons and rudders to fixed aerodynamic structures such as wings and stabilisers (e.g. vertical or horizontal stabilisers). This is usually achieved using a fixed rib which extends from a structural component of the fixed structure (such as a fore or aft rib) to the mounting point of the relevant control surface. 
     In one example, spoilers are aerodynamic devices which are attached to the trailing edge of aircraft wings in order to disrupt the boundary layer flowing across the wing during flight. They are used to reduce the aerodynamic effect of the wing in order to decrease lift and increase drag to slow the aircraft. 
     Known spoilers are mounted using a spoiler rib which projects rearwardly from the rear spar of the wing. The rib comprises a spoiler attachment point lug at the rear, a first limb and a second limb both projecting from the attachment point at an angle to each other, forming a “V”. The rib is attached to the wing at two points. The first limb is attached at an upper point where the rear spar meets the upper wing skin. The second limb is attached at a lower point where the spar meets the lower wing skin. At both points the rib is mechanically fastened to both the spar and the skin. 
     Fastening at the top and bottom of the spar provides the largest moment arm to react the forces experienced by the spoiler in flight. 
     Although optimised for load reaction, a problem with this design is that because the rib needs to be attached at the extreme top and bottom of the rear spar, each rib must be individually manufactured for its position along the wingspan because the depth of the spar reduces from fuselage to wing tip. 
     A further disadvantage of the prior art is that the geometry of the wing skin is somewhat variable (particularly for in-service aircraft) and, as such, some shimming or fettling of the rib is required in order to securely fix it in place. 
     Because of wing bending in flight, the upper and lower wing skins are subject to high strains. This necessitates bolts of a high diameter between the wing skin overhang and the ribs, and also thick skin material at the rib attachment positions. It is generally not practical to make the wing skin locally thicker at these points and, as such, the thickness is carried throughout the wingspan which adds significant weight to the aircraft. The additional weight in order to support this structural requirement may be as much as 10 kg per meter of wingspan. 
     Finally, struts supporting the lower shroud panel have to be fastened onto the ribs by means of bolt-on support brackets which constitute additional parts and require more expensive assembly. 
     Similar problems arise with other control surfaces mounted to fixed aerodynamic structures, for example ailerons mounted to the trailing edge of wings and horizontal stabilisers, and rudders mounted to the trailing edge of vertical stabilisers. 
     SUMMARY OF INVENTION 
     It is an aim of the present invention to overcome, or at least mitigate, the above problems. 
     According to a first aspect of the invention, there is provided a fixed aerodynamic structure assembly comprising:
         a fixed aerodynamic structure spar having an interior facing side and an exterior facing side,   a first movable control surface rib attached to, and extending from the exterior facing side of the wing spar,   a first stiffener positioned on the interior facing side of the fixed aerodynamic structure spar and connected to the first movable control surface rib through the wing spar.       

     By “fixed aerodynamic structure” we mean a surface extending from e.g. the fuselage which is primarily intended to influence the air flow around the aircraft, for example a wing, horizontal stabiliser or vertical stabiliser. 
     The rib may be configured for the attachment of a movable control surface. 
     By “movable control surface” we mean an aerodynamic surface mounted to be movable relative to the fixed aerodynamic structure, for example to affect the aerodynamic properties of the surface. This may be, inter alia, a flap, slat, spoiler, aileron or rudder. 
     The invention allows the load from the movable control surface rib to be reacted by the stiffener. For example with a wing spoiler mounting system, by providing a stiffener, torque about a spanwise axis can be reacted. The direction of the incident forces will tend to pull the reinforcement rearward, and its position nested within and abutting the spar and wing skins provides a continuous surface for the load to be reacted over without the need for a large number of fasteners attached to the wing skin. 
     The need for the rib itself to be secured to both wing skins is also eliminated and, as such, the rib can have a single limb and can be secured to the rear spa at an intermediate point between the skins. This removes the necessity for the rib to be individually manufactured for each position along the wing. Therefore replacement of a rib is made easier. Of course the stiffener will need to be sized to the position along the wing spar, but its position in front of the spar means that it is less susceptible to damage and far less likely to require replacement. 
     A further advantage is that the present invention is lighter and, therefore, less costly in terms of manufacturing costs and aircraft efficiency. Another advantage is the lack of part variability in spoiler ribs and the cost savings associated therewith. 
     The single attachment point for each rib will negate the significant effects of thermal strains in use, as seen in the prior art. 
     Preferably the first stiffener and the rib are connected via at least one mechanical fastener passing through the wing spar. Preferably the fastener exerts a clamping force on the wing spar sandwiched between the stiffener and the rib. 
     Preferably the first stiffener comprises:
         a first surface in abutment with the interior facing side of the fixed aerodynamic structure spar,   a second surface extending from a first end of the first surface towards the interior of the fixed aerodynamic structure in use,   in which the third surface is in abutment with a first corresponding surface of the aircraft fixed aerodynamic structure.       

     Preferably the first corresponding surface of the aircraft fixed aerodynamic structure is a first flange of a spar extending towards the interior of the fixed aerodynamic structure. 
     Preferably the first stiffener comprises:
         a third surface extending from a second, opposite, end of the first surface to the second surface towards the interior of the fixed aerodynamic structure,   in which the second surface is in abutment with a second corresponding surface of the aircraft fixed aerodynamic structure.       

     Preferably the second corresponding surface of the aircraft fixed aerodynamic structure is a second flange of the spar extending towards the interior of the fixed aerodynamic structure. 
     Preferably the spar defines a concavity facing the interior of the fixed aerodynamic structure, and in which the first stiffener is nested in the concavity of the spar. The spar and stiffener may typically be substantially “C” shaped in cross-section. 
     Preferably the assembly comprises:
         an first skin extending over a first end of the fixed aerodynamic structure spar; and,   a second skin extending over a second end of the fixed aerodynamic structure spar,   in which the movable control surface rib extends from a mounting position on the spar between, and spaced from, the first and second skins.       

     Preferably the mounting position is substantially midway between the first and second skins. 
     Preferably the assembly comprises:
         a second movable control surface rib attached to, and extending from the exterior facing side of the fixed aerodynamic structure spar,   a second stiffener positioned on the interior facing side of the fixed aerodynamic structure spar and connected to the second movable control surface rib through the wing spar,   in which the first and second movable control surface ribs are proximate so as to define a common mounting point for a movable control surface.       

     In this case, the first and second movable control surface ribs and first and second stiffeners may be mirror images of each other. 
     Preferably the assembly comprises a first shroud panel extending in an exterior direction from a first end of the fixed aerodynamic spar in use, which first shroud panel is connected to the first movable control surface rib by a first strut. 
     Preferably the first strut is connected to a position on the first movable control surface rib spaced from the fixed aerodynamic structure spar. 
     Preferably the first strut is then connected to a position on the first shroud panel at a position spaced from the fixed aerodynamic structure spar. 
     The assembly may comprise a second shroud panel extending in an exterior direction from a second end of the fixed aerodynamic structure spar in use, which second shroud panel is connected to the first movable control surface rib by a second strut. 
     The second strut may be connected to a position on the first movable control surface rib spaced from the fixed aerodynamic structure spar. 
     Also, the second strut may be connected to a position on the upper shroud panel at a position spaced from the fixed aerodynamic structure spar. 
     The fixed aerodynamic structure may be an aircraft wing, horizontal stabiliser or vertical stabiliser. 
     According to a second aspect of the invention there is provided a method of manufacture of an aircraft control surface assembly comprising the steps of:
         providing a fixed aerodynamic structure spar having an exterior facing side and an interior facing side in use;   providing a first stiffener positioned at the interior facing side of, and in abutment with, the fixed aerodynamic structure spar;   providing a first rib,   attaching the first rib to the first stiffener through the fixed aerodynamic structure spar such that the fixed aerodynamic structure spar is sandwiched therebetween.       

    
    
     
       SUMMARY OF DRAWINGS 
       An example aircraft spoiler rib in accordance with the present invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a side view of a prior art spoiler rib assembly; 
         FIG. 2  is a side view of a spoiler rib assembly in accordance with the present invention; 
         FIG. 3  is a sectioned plan view of the spoiler rib assembly along line III-III of  FIG. 2 ; and 
         FIG. 4  is a plan view of the spoiler rib assembly of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Turning to  FIG. 1 , a prior art spoiler rib assembly  10  is shown. The assembly  10  comprises a wing spar  12  (shown schematically) which extends along the length of an aircraft wing. An upper skin  14  and a lower skin  16  are shown either side of the spar  12 . Each of the upper and lower skins  14 ,  16  define an overhang  18 ,  20  respectively which project past the rearmost position of the spar  12 . 
     A spoiler rib  22  is provided which is generally v-shaped having a first limb  24 , and a second limb  26  which join at an apex  28 . A lug  30  is positioned at the apex  28  for rotational attachment of a spoiler. 
     The first limb  24  is I-beam shaped in cross section, comprising a plate  32  having an upper flange  34  and a lower flange  36 . At an end portion  38 , opposite the apex  28 , the first limb  24  has an attachment formation  40  for attachment to the overhang  18  of the upper skin  14  and a second attachment formation  42  for attachment to the region of the spar  12  proximate the upper skin  14 . 
     Similarly, the second limb  26  is I-beam shaped in cross-section, having a plate  44 , an upper flange  46  and a lower flange  48 . At an end portion  50  of the second limb  26 , a third attachment formation  52  is provided for attachment to the overhang  20  of the lower skin  16  and a fourth attachment formation  54  is provided for attachment to the region of the spar  12  where it meets the lower skin  16 . 
     Due to the taper of the aircraft wing, the spar  12  decreases in height as it approaches the wing tip. As a result, the upper and lower skins  14 ,  16  get gradually closer together. Therefore, it will be noted that each rib  22  must be made a different shape and size in order to accommodate the changing profile of the wing. Furthermore, due to variability in the skin geometry, fettling or shimming of the end portions  38 ,  50  is often required to fit the rib  22  to the wing. 
     Turning to  FIGS. 2 to 4 , a spoiler rib assembly  110  is shown in accordance with the present invention. The spoiler rib assembly  110  comprises a trailing edge wing spar  112 . The spar  112  is C-shaped in section having a plate-like member  114 , an upper flange  116  and a lower flange  118 , both projecting forward from the plate  114 , in an interior direction I. The interior direction I is the direction towards the interior of the wing box, which in this case is forward in terms of the whole aircraft coordinate system. The exterior direction E is opposite to the interior direction I and is rearward in the whole aircraft coordinate system. It will be noted that should the spar  112  be a spar at the front of the wing, the interior direction would be rearward, and the exterior direction would be forward. 
     An upper wing skin  120  is attached to the upper flange  116  and defines an overhang  122  which projects rearwardly of the spar  112 . The overhang defines a series of shroud attachment bores  113  regularly spaced in a spanwise direction. 
     A lower wing skin  124  is attached to the lower flange  118  and defines an overhang  126  which projects rearwardly of the spar  112 . 
     An upper shroud  128  is provided which extends from the upper skin overhang  122 . The upper shroud  128  is attached using a series of pivoting butt straps  130 . Each of the butt straps  130  has a first bore  132  and a first end and a second bore  134  at a second end. 
     A central butt strap  136  is also provided in the spanwise centre of the upper shroud  128  and comprises a first row of attachment bores  138  and a second row of attachment bores  140 . 
     A lower shroud  142  is also provided which extends from the overhang  126  of the lower skin  124 . 
     A pair of spoiler ribs  143 ,  144  are provided. The ribs  143 ,  144  are substantially identical, but mirror images of each other and as such only the rib  144  will be described in detail. The rib  144  has a lug  146  for attachment of a spoiler (not shown). The rib  144  comprises a vertically oriented plate  148  having an upper and lower flange  150  and  152  respectively extending therefrom defining a C-section profile. First and second end flanges  154  and  156  are provided extending perpendicular to both plate  148  and upper and lower flanges  150 ,  152 . 
     Beyond the first end flange  154 , a dog-leg section  158  extends upwardly from the plate  148  at an angle to meet the lug  146 . 
     The plate section  148  is significantly longer than the dog-leg section  158 . 
     Two stiffeners  159 ,  160  are provided, for each rib  143 ,  144  respectively. Like the ribs  143 ,  144 , the stiffeners  159 ,  160  are mirror images of each other and as such only the stiffener  160  will be described in detail. The stiffener  160  is concave comprising a central plate  162  bordered on fore and aft sides by flanges  164 ,  166  respectively, and on top and bottom ends by flanges  180 ,  182  respectively. The stiffener  160  extends within the entire height of the spar  112  abutting the plate  114  as well as upper flange  116  and lower flange  118 . It will be noted that the upper and lower flanges  180 ,  182  of the stiffener  160  taper form a tight fit within the spar  112  against the flanges  116 ,  118  respectively. 
     When assembled, the spoiler rib  144  is abutted against the spar  112  with the second end flange  156  in contact with the plate  114  of the spar  112 . Furthermore, the flange  166  of the stiffener  160  is in contact with the opposite side of the spar  112 . As shown in  FIGS. 2 and 3 , the rib  144  and the stiffener  160  are secured together using bolts  168  such that the plate  114  of the spar  112  is sandwiched between them. 
     The upper shroud panel  128  is attached to the wing skin overhang  122  via the pivoting butt straps  130 . Mechanical fasteners join the first bores  132  to the bores  113  on the wing skin, and the second bores  134  are joined to the shroud panel  128 . 
     Use of the pivotable butt straps  130  which contain two pivoting bolt arrangements  176 ,  178  at each end allowing relative movement between the two components. This helps to reduce stresses in both components during in-flight stresses and strains due to mechanical forces and thermal effects. 
     The central butt strap  136  is also attached to the overhang  122  and the shroud panel  128  via respective bores  138 ,  140  and is non-rotatable for stability. 
     Transfer panels  174  are provided to join adjacent shroud panels  128 . 
     Referring to  FIG. 2 , a first tension strut  170  is provided extending from a position on the plate  148  to an aft position on the shroud  128 . A second tension strut  172  is provided extending between a position proximate the flange  154  and the lower shroud  142 . These struts are relatively simple components which keep the various cantilevered structures hanging from the rear of the spar stable. 
     Variations fall within the scope of the present invention. In particular, the spar  112  may be any structural component of a fixed aerodynamic structure. The ribs  143 ,  144  may be used to attach any suitable movable aerodynamic surface, not just a spoiler.