Abstract:
A casing for a lifting aid for an aircraft, comprising at least one strake which extends essentially in a protruding manner in the direction of flight in relation to an outer surface of the casing. An aircraft comprising a lifting aid and said type of casing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to PCT/EP2010/056374 filed May 10, 2010 which claims the benefit of and priority to U.S. Provisional Application No. 61/177,831, filed May 13, 2009 and German Patent Application No. 10 2009 003 084.0, filed May 13, 2009, the entire disclosures of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a casing for a lifting aid for an aircraft, in particular for a flap track of an aircraft. The invention further relates to an aircraft with a lifting aid and a casing of this type. 
         [0003]    Although applicable to any aircraft or spacecraft casings, the present invention and the problem on which it is based will be explained in greater detail with reference to flap track casings of jet engine commercial aircraft. 
         [0004]    Aircraft of this type generally comprise so-called “landing flaps” which can be extended during the take-off and landing phases to provide increased aerodynamic lift at slow flight speeds. For this purpose, the landing flaps are usually arranged in the flap tracks on the underside of the aerofoils. In order to keep air resistance as low as possible at full cruising speed, the flap tracks are enclosed by corresponding aerodynamically optimised casings which can in addition be configured to produce aerodynamic lift. 
         [0005]    In particular in the case of aircraft with jet engines located on the underside of the aerofoils, undesirable vibrations of the flap track casing can, however, occur owing to the effect of the propulsive jet of an engine. Along with spatial closeness of the engine and flap track casing, particular operating states of the aircraft can promote the occurrence of vibrations, for example if the aircraft is still on the ground during maximum thrust or accelerates while still on the runway. The vibrations can lead to damage, for example hairline cracks in the flap track casing or supports thereof, to the point of a complete breakdown of the structure in the pivot region. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore the object of the present invention to eliminate damaging effects of the propulsive jets on casings of lifting aids such as flap tracks. 
         [0007]    The idea behind the present invention involves forming at least one strake which projects from an outer surface of the casing and extends substantially in the direction of flight. A strake is to be understood here as a planar (surface-optimised) structure such as a plate or plank. 
         [0008]    Because the strake projects from the outer surface of the casing and extends substantially in the direction of flight of the aircraft, the strake obstructs aerodynamic fluxes on the outer surface of the casing which comprise significant flux components perpendicular to the direction of flight. In this way, if the casing dips into the propulsive jet, for example during the take-off phase, the formation of unstable, oscillating vortex systems due to flux components of this type are suppressed and a stable aerodynamic flux is produced and flows around the casing. Because of the presence of oscillating vortex systems, no significant oscillating pressure variations and therefore no forces causing damaging vibrations occur on the outer surface of the casing. 
         [0009]    Advantageous embodiments and improvements of the invention are set out in the sub-claims. 
         [0010]    According to a preferred development, the strake projects substantially vertically from the outer surface. Flux components extending transverse to the strake are thus particularly effectively obstructed on both sides. 
         [0011]    According to a preferred development, the strake forms an angle of between 0° and 40° with a vertically downward direction. This has the effect that in the case of oncoming flow, a transverse force is applied which displays no zero passages, so that the casing and strake are particularly gently loaded in only one direction. 
         [0012]    According to a preferred development, the strake has a length of from 1 m to 3 m along the outer surface of the casing. A strake of this length adequately stabilises the aerodynamic flux in the case of low inherent weight. 
         [0013]    According to a preferred development, the strake comprises a leading edge on its front end and/or a trailing edge on its back end, which extend perpendicular to the outer surface of the casing. This produces a controlled oncoming flow of the leading edge or a controlled stall of the trailing edge. 
         [0014]    According to a preferred development, the strake has a lower height at its front end than at its back end. The height of the strake at its front end is preferably 1 cm or less, so that the formation of vortices on the leading edge is eliminated. 
         [0015]    According to a preferred development, the strake has a height of 15 cm or less at its back end. A strake of this height stabilises the aerodynamic flux adequately in the case of sufficient inherent stability and low inherent weight. Preferably, the strake has a maximum height at a position at a distance from its front and back ends. 
         [0016]    According to a preferred development, the strake has a thickness between 2 mm and 5 mm. This enables a high stability of the strake in the case of low weight. The strake preferably has a substantially rectangular transverse profile. This enables a controlled stall on the upper edge. 
         [0017]    According to a preferred development, the invention provides an aircraft with a lifting aid which comprises a casing of this type. The aircraft preferably comprises a propulsive jet that is arranged in such a way that during the operating phase of the aircraft, the casing at least partially dips into a jet of the jet engine. In this way, jet engines and lifting aids can be optimised in their position without constructive restrictions and without the occurrence of damage to the casing. 
         [0018]    According to a preferred development, the strake is formed on one of the sides of the casing which faces the jet. The strake thus works particularly efficiently, directly at the location of the direct oncoming flow via the propulsive jet. 
         [0019]    The invention will be further explained using embodiments with reference to the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the figures: 
           [0021]      FIGS. 1A-B  are sectional views in the direction of flight of an aircraft aerofoil having a casing in accordance with two embodiments of the invention; 
           [0022]      FIG. 2  is a schematic perspective view of a strake of a casing in accordance with an embodiment; 
           [0023]      FIG. 3  shows simulated function curves of pressure distributions on strakes of casings in accordance with different embodiments; 
           [0024]      FIG. 4  is a perspective view of a back portion of a casing in accordance with an embodiment, with simulated flow; 
           [0025]      FIG. 5  is a bottom view of the portion of the casing from  FIG. 4 , with simulated flow; 
           [0026]      FIG. 6  is a perspective view of a back portion of a conventional casing, with simulated flow; and 
           [0027]      FIG. 7  is a bottom view of the portion of the casing from  FIG. 6 , with simulated flow. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0028]    In the figures, the same reference numerals denote identical or functionally identical components, unless indicated otherwise. 
         [0029]      FIG. 1A  is in a schematic rear view of a portion of an aerofoil  402  of a jet-driven commercial aircraft  118 , which aerofoil is fixed on a fuselage  120  of the aircraft  118 . The viewing direction of the viewer corresponds to the direction of flight of the aircraft  118 . On its trailing edge  404  which is facing the viewer, the aerofoil  402  comprises landing flaps  400 ,  401  which are supported so as to be retractable from a flap track as a lifting aid. The flap track itself is enclosed by a casing  100  and is not shown in the drawing of  FIG. 1A . The flap track casing  100  extends substantially in the direction of flight along a portion of the underside of the aerofoil  402 , in the present case, for example, from approximately the centre of the aerofoil  402  as far as its trailing edge  404  or marginally beyond this. 
         [0030]    Likewise, an engine mount  116  is fixed on the underside of the aerofoil  402  near the flap track casing  100 , and holds a jet engine  112  of the aircraft  118 . During operation the jet engine  112  produces a propulsive jet  114  counter to the direction of flight, the cross-section and direction of which depend on the operating state of the aircraft  118 . The dashed line in  FIG. 1A  marks highly schematically a cross-section of the propulsive jet  114  in the region of the trailing edge  404  of the aerofoil during operation of the jet engine  112  on the ground, for example when it is not moving, or during acceleration on the ground during take-off. In this operating state, the propulsive jet  114  is deflected outwards under the effect of the ground, the landing flap casing  100  dipping partially into the propulsive jet  114 , and to a greater extent when the casing  100  of the flap track is lowered further towards the bottom than shown, in order to extend the landing flaps  400 ,  401 . 
         [0031]    In the present embodiment the flap track casing  100  is arranged at substantially the same distance from the aircraft fuselage  120  as the engine mount  116 , purely by way of example. The engine mount  116  can, for example, comprise an engine mount casing which transitions into the casing  100  of the flap track. 
         [0032]    On the outer surface  106  of the flap track casing  100  a strake  104  is fixed on the side facing the jet  114 , meaning in this case on the internal side of the board, the outwardly deflected jet of which encounters the casing  100 , which strake extends along the outer surface  106  of the casing  100  in the direction of flight and finishes in an approximately vertical trailing edge on the outer surface  106  at its end which is turned towards the viewer. The strake  104  extends in a plane which forms an angle γ of approximately 40° with the vertically downward direction  108  and is substantially vertical on the outer surface  106  of the casing. With respect to a longitudinal axis  122  of the casing  100 , the plane of the strake  104  extends substantially radially. 
         [0033]      FIG. 1B  shows an alternative embodiment in which, by way of example, the flap track casing  100  is arranged offset in relation to the engine mount  116  in the direction of the internal side of the board, that is to say towards the fuselage  120 . It is assumed here that the flap track casing  100  and the engine mount  116  are offset at a distance in this way so that during operation of the jet engine  112  on the ground and outward deflection of the jet  114  due to the divergence of the jet  114  from the external side of the board, the propulsive jet  114  encounters the casing  100 . A strake is fixed on the outer surface  106  of the flap track casing  100  on the side facing the jet  114 , meaning in this case on the external side of the board. 
         [0034]      FIG. 4  is a perspective view of a back portion of a flap track casing  100  in accordance with a further embodiment which is based on the casing of the furthest outside located flap track of the Airbus A380. In this type of aircraft, the outermost engine is arranged at an equal distance from the aircraft fuselage and the flap track casing  100  shown. A strake  104  has been attached to the internal side of the board, seen by the viewer of  FIG. 4 , which strake extends over a length of 3 m along the outer wall  106  of the casing, stands vertically thereon and forms an angle γ=20° with the vertically downward direction. The landing flap  400  is shown in a lowered position, in which the casing  100  dips into the propulsive jet of the engine during operation thereof. 
         [0035]    Furthermore, flux lines  410  are shown in  FIG. 4 , which have been numerically calculated by a hybrid Navier-Stokes method common in fluid mechanics, in order to simulate the aerodynamic flux around the casing  100  in the position shown, upon full thrust of the engine on the ground. A first group  414  of flux lines indicates a flow on the bottom wall of the casing  100 , which flow is orientated substantially along the strake  104  in the direction of flight as far as the back end  406  of the casing  100 . A second group  412  of flux lines indicates a flow on the board internal side wall of the casing  100 , which flow is also directed substantially along the strake  104  in the direction of flight as far as the back end  406  of the casing  100  where it combines with the flow of the first group  414 . There is a stable flow without oscillations that cause vibrations. In addition, the flow shown produces an initial stress of the casing  100  directed towards the internal side of the board (to the viewer), which, irrespective of the existence of oscillations, prevents the material of the casing  100  from being loaded alternately in compression and tension, and thus prevents material fatigue. 
         [0036]      FIG. 5  is a bottom view of the portion of the casing from  FIG. 4  with the simulated flow of the simulation from  FIG. 4 . The curve of the strake can clearly be recognised, the trailing edge  206  of which at its back end  204  is higher than the leading edge at the front end  200  of the same strake. 
         [0037]      FIGS. 6 and 7  show for comparison a back portion of a conventional landing flap casing, with simulated flow according to the same method as in  FIGS. 4 and 5 . The configuration shown corresponds to the conventional casing of the outermost flap tracks of the Airbus A380. 
         [0038]    The first group  414  of flux lines, which flows along the bottom wall of the casing, comprises a considerable number of flux lines with components orientated upwards in the direction of the side wall of the casing  100 . The second group  412  of flux lines which flows along the casing  100  on the side wall also comprises a considerable number of flux lines with components orientated downwards towards the floor wall. The flux fields formed by the first group  414  and second group  412  meet in the region of the radius of the casing  100 , which radius is inside the board, to form an unstable vortex system which, according to the given local distribution of compression, hunts around the radius which connects the floor and side wall of the casing  100 . The unstable vortex system leads to an oscillating pressure distribution on the upper surface of the casing, which distribution causes material-damaging vibrations. 
         [0039]      FIG. 2  is a schematic perspective view of a strake of a casing according to an embodiment, for example the embodiment from  FIG. 1 . The strake  104  has a rectangular cross-sectional profile of thickness D and a rectangular upper edge  208 . A lower edge  210  is fit to the contour curve of the outer surface of the flap track casing. On the leading end  200  the strake begins with a leading edge  202  substantially perpendicular to the outer surface of the casing, which edge is of height H 1 , extends along the upper edge  208  over its length in the form of a spline which reaches a maximum height H 3  in a position at a distance from both ends, and finishes at the back end  204  of the strake with a trailing edge which is also substantially vertical on the outer surface of the casing  100  and has the height H 2 . 
         [0040]      FIG. 3  shows simulated function curves of pressure distributions on differently formed strakes which all correspond to the basic form described by means of  FIG. 2 , assuming that the thickness D is always 3 mm, the height H 1  is always 1 cm, and the height H 2  is always 15 cm. In each case, the simulation is based on a configuration such as those shown in  FIGS. 4 to 7 , irrespective of the form of the strake. 
         [0041]    The curves  311  to  313  each correspond to a strake of length L=300 cm, the longitudinal coordinates along the horizontal axis  304  denoting the distance from the centre of the aerodynamic system of the aircraft. The strake forms an angle of γ=0° with the downward vertical in the case of curve  311 , an angle of γ=30° in the case of curve  312 , and an angle of γ=20° in the case of curve  312 . Along the vertical axis  302 , each curve shows the pressure distributions on the strake at the respective longitudinal coordinates, the atmospheric pressure  300  having been marked separately on the bottom. 
         [0042]    In the case of curves  314  to  316 , a strake of length L=203 cm has been assumed, which is arranged further towards the back end of the casing, as can be seen from the longitudinal coordinates. The strake forms an angle of γ=0° with the downward vertical in the case of curve  314 , an angle of γ=20° in the case of curve  315 , and an angle of γ=40° in the case of curve  316 . A particularly advantageous pressure distribution is shown by curve  313 , in which the pressure varies gently over the length of the strake and the line of the atmospheric pressure  300  is not crossed, so that the strake and the casing are not exposed to varying pressure and tensile stresses. 
         [0043]    A strake of this type can, for example, be produced from a composite material and connected to the outer wall of the casing by means of fastening elements integrated into this wall. 
         [0044]    Although the present invention has presently been described on the basis of preferred embodiments, it is not restricted thereto, but can be modified in many different ways. 
         [0045]    For example, the invention can also be applied to types of lifting aids other than flap track casings, as well as to actual loads, inter alia, which are mounted underneath the aerofoils or in other places which are accessible to the propulsive jet of a jet engine. 
       LIST OF REFERENCE NUMERALS 
       [0046]      100  flap track casing 
         [0047]      101  longitudinal axis of the casing 
         [0048]      102  flap track 
         [0049]      104  strake 
         [0050]      106  outer surface 
         [0051]      108  vertically downward direction 
         [0052]      110  transverse profile 
         [0053]      112  jet engine 
         [0054]      114  jet 
         [0055]      116  engine mount casing 
         [0056]      118  aircraft 
         [0057]      120  aircraft fuselage 
         [0058]      200  front end of the strake 
         [0059]      202  leading edge 
         [0060]      204  back end of the strake 
         [0061]      206  trailing edge 
         [0062]      208  upper edge 
         [0063]      210  lower edge 
         [0064]      300  atmospheric pressure 
         [0065]      302  simulated pressure distribution 
         [0066]      304  x-coordinates of the aerodynamic system 
         [0067]      311 - 316  pressure distribution curve 
         [0068]      400 ,  401  landing flap 
         [0069]      402  aerofoil 
         [0070]      404  trailing edge of the aerofoil 
         [0071]      406  back end of the casing 
         [0072]      410  flux lines 
         [0073]      412  flux on the side wall 
         [0074]      414  flux on the floor wall 
         [0075]    D thickness of the strake 
         [0076]    H 1  height of the leading edge 
         [0077]    H 2  height of the trailing edge 
         [0078]    H 3  maximum height 
         [0079]    γ angle to downward vertical