Abstract:
An aircraft control surface mounting structure including a cantilever rib mounted to a spar, the rib includes a first arm and a second arm both orientated to be substantially parallel to the line of flight of the aircraft.

Description:
This application claims priority to British Application No. 0815020.3 filed 19 Aug. 2008, the entire contents of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a structure for mounting aircraft control surfaces. More specifically, the present invention relates to a cantilever rib for mounting a flap from a rear spar of a wing. 
     BACKGROUND OF THE INVENTION 
     Control surfaces are used on aircraft to influence the passage of fluid over various flight surfaces such as wings. By “control surfaces” we mean movable components that produce an aerodynamic effect for example flaps, slats, ailerons and spoilers. Flaps, for example, are a type of control surface which are mounted to the rear of a wing and can be rotated with respect to the wing trailing edge to change the shape of the aerofoil profile of the wing assembly as a whole. This helps prevent wing stall at low speeds (e.g. during landing) when the flaps are fully deployed, and increases efficiency of the wing at high speeds (e.g. during cruise) when the flaps are stowed. 
     Control surfaces such as flaps need to be securely mounted to the aircraft. Flaps can be mounted to the wings of the aircraft in a variety of ways, for example via an underslung beam attached to the underside of the aircraft wing, or via a cantilever rib mounted directly to, and projecting perpendicularly from, a rear spar of the wing. 
     Both of these methods require that the beam or rib project from the outer mean line (OML) of the wing due to the fact that the desired pivot axis of the flap is below the OML. As such, the beam or rib will necessarily project into the fluid stream across the flight/control surfaces. This increases drag and hence reduces the efficiency of the aircraft which is undesirable. 
     In addition such beams or ribs need to be large enough to provide a load path from the flap to the wing as well as house actuators for deployment of the flap. As such they are often large. 
     The problem of control surface structure drag is particularly prevalent in large fixed wing aircraft in which the wings are swept to increase aerodynamic efficiency. The trailing edge of the wing, is non-perpendicular to the line of flight, and hence the direction of flap deployment is also non-perpendicular to the line of flight (rather it is parallel to the trailing edge of the wing). The beam or rib is mounted perpendicularly to both the trailing edge of the wing (i.e. the rear spar) and the leading edge of the flap. Specifically, it spans the shortest distance between them. 
     This is problematic as the beam or rib is non-parallel to the line of flight in swept wings. As such the “footprint” of the beam or rib viewed in the direction of the line of flight is relatively large due to its orientation. This further increases the effects of drag on the beam or rib. 
     It is known to cover these components in an aerodynamic fairing to reduce the drag produced, however this cannot completely eliminate it. 
     It is an object of the present invention to provide an improved mounting structure. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided an aircraft control surface mounting structure comprising a body, the body having a main axis, a first attachment means defined at a first end of the body for attachment to a fixed structure of an aircraft and a second attachment means defined at a second end of the body for attachment to an aircraft control surface, the second attachment means defining a control surface pivot axis, wherein the control surface pivot axis is at a non-perpendicular angle to the main axis of the body. 
     According to a second aspect or the invention there is provided an aircraft control surface mounting structure comprising a body, the body having a main axis, a first attachment means defined at a first end of the body for attachment to a fixed structure of an aircraft and a second attachment means defined at a second end of the body for attachment to an aircraft control surface, the first attachment means being arranged to mount the body to the fixed structure of the aircraft such that the main axis of the body is at a non-perpendicular angle thereto. 
     Advantageously, by moving the axes away from a perpendicular relationship, the main axis can be adjusted to be more aligned, and even parallel with the line of flight of the aircraft. As such, the “footprint” of the body (such as a beam or rib) is reduced, and the drag is also reduced. 
     It will be understood that by “main axis” we mean an axis normal to the shortest two dimensions of the body. 
     According to a third aspect of the invention there is provided an aircraft control surface mounting system comprising a body and a linear actuator, in which the body has a pivot attachment for pivotably mounting an aircraft control surface about a control surface pivot axis, the actuator has an actuation axis, is mounted to the body at a first end of the actuator and has a control attachment at a second end of the actuator for mounting an aircraft control surface thereto, such that actuation of the actuator pivots the control surface about the pivot axis, wherein the pivot axis and the actuation axis are non-perpendicular with respect to each other. 
     Movement of the actuation axis also reduces the “footprint” of the flap control system, thus reducing drag. 
     According to a fourth aspect of the invention there is provided an aircraft defining a main longitudinal axis, the aircraft comprising a wing defining a swept edge, the wing having a control surface proximate the swept edge, the wing further comprising a control surface mounting structure arranged to attach the control surface to the wing, which mounting structure defines a main axis and in which the mounting structure is oriented such that the main axis is substantially parallel to the main longitudinal axis of the aircraft. 
     An example mounting structure will now be described with reference to the accompanying figures in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an aircraft, 
         FIG. 2  is an underside view of a conventional flap deployment mechanism, 
         FIG. 3   a  is an underside view of a cantilever rib, 
         FIG. 3   b  is an underside view of a cantilever rib arrangement, 
         FIG. 4   a  is an underside view of a first cantilever rib in accordance with the present invention, and, 
         FIG. 4   b  is an underside view of a cantilever rib arrangement including the rib of  FIG. 4   a.    
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a passenger aircraft  100  comprises a fuselage  102  and wings  104 . The aircraft  100  defines a main longitudinal axis  101  parallel to the line of flight L. Each wing comprises at least one flap  106  which can be selectively deployed or stowed as known in the art. 
     It should be noted that as is commonplace with modern aircraft, the wings  104  are swept. Each wing comprises a leading edge  108  and a trailing edge  110 , each of which have a different angle (A, B respectively) with respect to the main axis  101 . The sweep angle of each edge is 90-A and 90-B respectively (0 degrees being an unswept wing). The sweep angle is typically 20 to 40 degrees. 
     Referring to  FIG. 2 , the underside of the wing  104  is shown in the region of the flap  106 . The wing  104  comprises a rear spar  112  to which a cantilever rib  114  is mounted at a first end  116 . The cantilever rib  114  projects perpendicularly to the rear spar  112 . 
     The cantilever rib  114  defines a first pivot mount  118  at a second end  120 . The first pivot mount  118  defines a control surface pivot axis X. 
     The flap  106  comprises a flap mount  122 . The flap mount  122  is attached to the flap  106  at a first end  124  and defines a second pivot mount  126  at a second end  128 . 
     The first pivot mount  118  and the second pivot mount  126  engage to form a hinge or joint about the control surface pivot axis X. As such, the flap  106  is rotatable about the control surface pivot axis X relative to the wing  104 . 
     It should be noted that the second end  120  of the cantilever rib  114  and the second end  128  of the flap mount  122  project significantly below the outer mean line (OML) of the aerodynamic surface of the wing  104  and flap  106  when in its stowed position (i.e. when it is substantially in line with the wing  104 ). As such, the mechanism generally projects into the flow of the working fluid under the wing  104 . 
     It will be appreciated that the lateral width of the arrangement comprising the cantilever rib  114  and the flap mount  122  when viewed in the direction of the line of flight (LOF), F 1 , is significant. As such, a fairing of significant width is required to house the arrangement and thus significant drag is created reducing the efficiency of the aircraft. 
     Referring to  FIG. 3   a , an alternative cantilever rib  200  to that of  FIG. 2  is shown. The cantilever rib  200  is attached to a rear spar  112  of the wing  104  (shown cut-away). 
     A first load distribution plate  202  and a second load distribution plate  204  are attached to the rear spar  112  (for example by mechanical fixing means such as bolts) and project rearwardly therefrom. A further pair of load distribution plates are provided on the upper side of the arrangement (not shown). 
     The rib  200  comprises a first rib arm  206  and a second rib arm  208  parallel to, and offset from, each other. Each rib arm  206 ,  208  comprises a pivot mount  210 ,  212  respectively. The rib arms  206 ,  208  are substantially identical and arranged to mirror each other. Cross braces  214 ,  216  and  218  join the rib arms  206 ,  208  and retain them in their relative orientation. 
     The rib arms  206 ,  208  are spot welded to the load distribution plates  202 ,  204  via welds  220  such that the beam  200  projects perpendicularly from the spar  112 . The pivot mounts  210 ,  212  are aligned to define a pivot axis X. A flap (not shown in  FIG. 3   a ) can be mounted to the wing  104  to pivot about the pivot axis X. 
     Referring to  FIG. 3   b , two ribs  200 ,  222  are shown mounted onto the same spar to support a flap  106  via flap mounts  224 ,  226 . As shown, the footprint F 2  in the direction of the LOF is significant. 
     It will be understood that the cantilever ribs form the main support and hinge points for the flap  106 . 
       FIGS. 4   a  and  4   b  show a cantilever rib  300  and a cantilever rib arrangement components similar to cantilever rib  200  numbered  100  greater. 
     The rib arms  306 ,  308  are orientated with their long axes  307 ,  309  respectively at an angle C to the spar  112 . Angle C is such that each of the arms  306 ,  308  and hence the rib  300  is oriented in the direction of the line of flight, L. Specifically, angle C=angle B (i.e. the sweep angle of the trailing edge  110  of the wing  104 ). 
     Like rib arms  206 ,  208  of the rib  200 , the rib arms  306 ,  308  comprise pivot mounts  310 ,  312 . A cross-brace  318  between the rib arms  306 ,  308  is also provided. 
     It should be noted that the geometry of the rib  300  differs from that of the rib  200 . The pivot mounts  210 ,  212  are oriented to rotate about an angle to the main axes  307 ,  309  of the rib arms  306 ,  308  such that the pivot axis X is substantially parallel to the spar  112 . 
     Furthermore, the load distribution plates  302 ,  304  are shaped to account for the non-perpendicular orientation of the rib arms  306 ,  308  with respect to the spar  112 . 
     Referring to  FIG. 3   b , it can be clearly seen that the footprint F 3  of the arrangement is significantly less than the footprint F 2 . As such, a fairing of lower lateral width can be used and hence drag is reduced. 
     Cantilever ribs are often assembled with actuators for moving the flap between deployed and stowed positions. It will be appreciated that the actuator will therefore be oriented in the same direction as the rib (i.e. non-perpendicular to the spar) and as such will be non-perpendicular to the pivot axis. This may require that a sliding joint is required at the point at which the actuator joins the flap to take account of any relative motion. The sliding joint will allow relative motion between the actuator and flap in a direction parallel to the pivot axis during deployment/stowage. 
     Variations of the above embodiments fall within the scope of the present invention. The concept is equally applicable to other aircraft control surfaces such as slats, rudders and ailerons.