Abstract:
The present invention relates to a method of manufacture of a group of transverse internal stiffening ribs ( 46 ) for an aerodynamic fairing of an engine mounting device, including:
       production, for each rib ( 46 ), of a given rib preform ( 46   a ) by superplastic forming; and   in respect of at least one of the ribs ( 46 ) of the group having a smaller size than that of the preform ( 46   a ), division of the latter into four preform parts ( 56   a - 56   d ) each incorporating one of the four corners of the quadrilateral formed by the preform, followed by the joining of the four parts to one another so as to obtain the rib.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a lower aft aerodynamic fairing for an engine mounting device intended to be interposed between an aircraft wing surface and the engine concerned, this fairing also being called a “shield” or “APF” (“Aft Pylon Fairing”). 
         [0002]    Such a mounting device, also called an “EMS” (“Engine Mounting Structure”), allows, for example, a turbine engine to be suspended beneath the wing surface of the aircraft, or allows this turbine engine to be mounted above this same wing surface or, again, allows it to be mounted laterally in the aft part of the fuselage. An example of a mounting device is known from document WO 2009/037267. 
         [0003]    More specifically, the invention relates to the transverse internal stiffener ribs which are fitted to the lower aft aerodynamic fairing, and more specifically to their method of manufacture. 
         [0004]    The invention may be used in any type of aircraft fitted with turbojets or turboprop engines. 
       STATE OF THE PRIOR ART 
       [0005]    A mounting device is, indeed, designed to constitute the connecting interface between a turbine engine and a wing surface of the aircraft. It enables the forces generated by its associated turbine engine to be transmitted to the structure of this aircraft, and also allows conveyance of the fuel, and passage of the electrical, hydraulic and air systems, between the engine and the aircraft. 
         [0006]    To ensure the transmission of the forces the structure includes a rigid structure, also called the primary structure, often of the “box” type, i.e. formed by the assembly of upper and lower spars and of side panels connected one to the other through transverse stiffening ribs. 
         [0007]    Secondly, the device is fitted with mounting means interposed between the turbine engine and the rigid structure, and these means include globally two engine attachments, and a device for transmission of the thrust forces generated by the turbine engine. In the prior art this transmission device habitually includes two lateral connecting rods connected firstly to an aft part of the fan casing of the turbine engine, and secondly to an aft attachment attached to the central casing of the engine. 
         [0008]    Similarly, the mounting device also includes another series of attachments constituting a mounting system interposed between the rigid structure and the wing surface of the aircraft, and this system habitually consists of two or three attachments. 
         [0009]    In addition, the pylon is fitted with multiple secondary structures allowing the systems to be segregated and held in position, whilst supporting aerodynamic fairing elements, and where the latter generally take the form of assemblies of panels added on to the structures. In a manner known to the skilled man in the art, the secondary structures are differentiated from the rigid structure in that they are not intended to transfer forces originating from the engine which must be transmitted to the wing surface of the aircraft. 
         [0010]    The secondary structures include the lower aft aerodynamic fairing, also called APF, which has multiple functions, one of which is the formation of a thermal or anti-fire barrier, and the formation of aerodynamic continuity between the engine outlet and the mounting pylon. 
         [0011]    The lower aft aerodynamic fairing generally takes the form of a box including two side panels joined to one another by transverse internal stiffening ribs spaced relative to one another in a lengthways direction of the fairing, and a thermal protection floor. It is stipulated that this box cannot be closed at the end opposite the thermal protection floor, i.e. in the upper part when the engine is intended to be suspended under the wing surface of the aircraft, given that it is in this location that it is connected to the other structures of the pylon. 
         [0012]    The thermal protection floor is fitted with an outer surface intended to be followed by a primary flow of the engine which it demarcates, whereas the side panels are, for their part, designed to be followed externally by a secondary flow of the engine, due to their positioning in the annular channel of the engine&#39;s secondary flow, and/or at the engine&#39;s outlet. 
         [0013]    The internal ribs are habitually made from a titanium alloy, TA6V, due to the excellent mechanical characteristics which it confers, and to its relatively low mass, notably in comparison with the mass of steel. 
         [0014]    Machining is a first manufacturing technique for the rib. It has the advantage of giving precise tolerances, but does not enable thin ribs to be obtained. Indeed, below a certain rib thickness, for example of the order of 2 mm, the rib is no longer capable of supporting the machining stress. 
         [0015]    Formation of such an internal rib by superplastic forming is also known. It has the advantage that it is not limited by any machining constraint, and is thus perfectly suited to obtaining thin ribs. Conversely, the ribs thus formed have imprecise tolerances. In addition, this manufacturing technique requires a special die for each rib of a different size, implying a high manufacturing cost. 
       OBJECT OF THE INVENTION 
       [0016]    The purpose of the invention is therefore to provide at least partially a solution to the disadvantages mentioned above, compared with the embodiments of the prior art. 
         [0017]    To accomplish this the purpose of the invention is a method of manufacture of a group of transverse stiffening internal ribs for a lower aft aerodynamic fairing of an engine mounting device intended to be interposed between a wing surface of an aircraft and the said engine, where the group includes several ribs of different sizes, and where the said method includes the following steps:
       production, for each rib in the rib group, of a given rib preform by superplastic forming, where the said preform has, seen from the front, an outline of a broadly quadrilateral shape, and a central opening traversing this rib preform; and   in respect of at least one of the ribs in the group having a smaller size than that of the said rib preform, division of the said rib preform into four preform parts each incorporating one of the four corners of the said quadrilateral, followed by joining of the four parts to one another so as to obtain the said at least one of the ribs of the group.       
 
         [0020]    The invention is remarkable in that each stiffening rib in the group is obtained from an identical rib preform produced by superplastic forming. Only one superplastic die is therefore required to obtain all the ribs in the group, reducing the manufacturing costs accordingly. 
         [0021]    Each of the ribs in the group, if applicable, in respect of one or more of them, consists of the rib preform, whereas one or more other ones of smaller dimensions are each produced by reducing the size of the rib preform, by splitting it into four parts which are then reassembled. This latter method of obtaining the ribs of smaller sizes than the preform from which all the ribs in the group are obtained is also remarkable in that it enables thin ribs to be obtained, and enables them to be given precise tolerances through the assembly of the four rib preform parts. For this reason it can be envisaged that all the ribs in the group, which includes all or only part of the ribs of the fairing in question, may each be produced by splitting the preform into four parts which are then reassembled. 
         [0022]    However, it can be envisaged that the group of ribs includes at least one rib constituted by the said rib preform, without going beyond the scope of the invention. In these cases, the rib or ribs concerned is/are directly constituted by the rib preform, and there is no need to alter it/them. 
         [0023]    The said group of ribs preferably includes at least two ribs of different sizes, each being produced by dividing the said rib preform, and by then joining the four preform parts to one another. As mentioned above, this technique for obtaining the ribs can be chosen for each of the ribs in the group. 
         [0024]    The joining of the four parts of the preform to one another is preferably undertaken by laser welding or by bolted fishplates. The adjustment of the relative positions of the four rib preform parts before their fishplating or welding enables precise dimensions to be obtained in both directions of the rib plane, namely height and width. To adjust them these parts are, for example, arranged in a simple and inexpensive positioning die, which is found widely in the industry. 
         [0025]    The rib preform is preferably made such that it has, seen from the front, an outline with a broadly trapezoid shape. However, other quadrilateral shapes could be envisaged, such as for example a square or a rectangle. 
         [0026]    The rib preform is preferably made from a titanium alloy TAV6. 
         [0027]    Another purpose of the invention is a lower aft aerodynamic fairing for an engine mounting device intended to be interposed between a wing surface of an aircraft and the engine, where the fairing forms a box including two side panels connected to one another by a group of transverse stiffening internal ribs spaced relative to another in a lengthways direction of the fairing, and also including a thermal protection floor having an outer surface intended to be followed by a primary engine flow. 
         [0028]    According to the invention, the group of transverse stiffening internal ribs is manufactured through the use of the method described above. 
         [0029]    Another purpose of the invention is an engine mounting device intended to be interposed between a wing surface of an aircraft and the engine, where this device includes a lower aft aerodynamic fairing as described above. 
         [0030]    In addition, another purpose of the invention is an engine assembly including an engine such as a turbojet and a device for mounting this engine in which this device is in keeping with the one which has just been mentioned. 
         [0031]    Finally, another purpose of the present invention is an aircraft including at least one such engine assembly. 
         [0032]    Other advantages and characteristics of the invention will appear in the non-restrictive detailed disclosure below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    This description will be made with reference to the attached illustrations, among which 
           [0034]      FIG. 1  represents a schematic side view of an aircraft engine assembly, including a mounting device according to a preferred embodiment of the present invention; 
           [0035]      FIG. 2  represents a more detailed sideways schematic view of the lower aft aerodynamic fairing fitted to the mounting device shown in  FIG. 1 , where this fairing is also the subject of the present invention; 
           [0036]      FIG. 3  represents a perspective view of a part of the lower aft aerodynamic fairing in  FIG. 2 ; and 
           [0037]      FIGS. 4   a  to  4   d  represent various steps of the manufacture of one of the internal ribs fitted to the fairing shown in  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0038]    With reference to  FIG. 1 , an aircraft engine assembly  1  can be seen intended to be attached to a wing  2  of this aircraft, where this assembly  1  includes a mounting device  4  according to a preferred embodiment of the present invention, together with an engine  6 , such as a turbojet, mounted under this device  4 . 
         [0039]    Globally, the mounting device  4  includes a rigid structure  8 , also called primary structure, supporting means for mounting the engine  6 , where these means for mounting have multiple engine attachments  10 ,  12 , together with a device for transmission of the thrust forces  14  generated by the engine  6 . 
         [0040]    As an indication, it should be noted that the assembly  1  is intended to be surrounded by a nacelle (not represented), and that mounting device  4  includes another series of attachments (not represented) added on to this rigid structure  8  and enabling this assembly  1  to be suspended under the wing surface  2  of the aircraft. 
         [0041]    In the remainder of the following description, by convention, X refers to the lengthways direction of the device  4  which is also comparable to the lengthways direction of turbojet  6  and to that of the lower aft aerodynamic fairing which will be presented below, and where this direction X is parallel to a lengthways axis  5  of this turbojet  6 . Further, Y refers to the direction aligned transversely relative to the device  4  and also comparable to the transverse direction of turbojet  6  and to that of the lower aft aerodynamic fairing, and Z to the vertical direction or the height, and where these three directions X, Y and Z are mutually orthogonal. 
         [0042]    In addition, the terms “front” and “aft” must be considered relative to the forward direction of the aircraft imparted due to the thrust exerted by the turbojet  6 , and this direction is represented schematically by the arrow  7 . 
         [0043]    In  FIG. 1  the two engine attachments  10 ,  12 , the thrust forces transmission device  14 , the rigid structure  8  of the mounting device  4 , and multiple secondary structures added on to the rigid structure are therefore depicted. These secondary structures segregating the systems and holding them in position, whilst supporting the elements of the aerodynamic fairing, will be described below. 
         [0044]    As can be seen, turbojet  6  has at the front a large fan casing  18  demarcating an annular bypass duct  20 , and includes towards the aft a smaller central casing  22 , enclosing the core of this turbojet. Casings  18  and  22  are of course securely connected to one another. 
         [0045]    As can be seen in  FIG. 1 , the device  4  has two engine attachments  10 ,  12 , called respectively the front engine attachment and the aft engine attachment. 
         [0046]    In this preferred embodiment of the present invention the rigid structure  8  takes the form of a box extending from the aft to the front, roughly in direction X. 
         [0047]    The box  8  then takes the form of a pylon of a similar design to that habitually observed for turbojet mounting pylons, notably in the sense that it is fitted with transverse ribs (not represented), each taking the form of a rectangle aligned in a YZ plane. 
         [0048]    The mounting means of this preferred embodiment firstly include the front engine attachment interposed between a front end of the rigid structure  8 , also called a pyramid, and an upper part of the fan case  18 . The front engine attachment  10  is designed in a traditional manner known to the skilled man in the art. 
         [0049]    Secondly, the aft engine attachment  12 , which is also produced in a traditional manner known to the skilled man in the art, is for its part interposed between the rigid structure  8  and the central casing  22 . 
         [0050]    Again with reference to  FIG. 1 , the secondary structures of the pylon  4  include a front aerodynamic structure  24 , an aft aerodynamic structure  26 , a fairing  28  connecting the front and aft aerodynamic structures, and a lower aft aerodynamic fairing  30 . 
         [0051]    Globally, these secondary structures are traditional structures identical or similar to those found in the prior art, and known to the skilled man in the art, except for the lower aft aerodynamic fairing  30 , which will be described in detail below. 
         [0052]    More specifically, the front aerodynamic structure  24  is positioned as a lower front extension of the wing surface  2  and above the primary structure  8 . It is assembled securely on the rigid structure  8 , and has an aerodynamic profile between an upper part of the fan access door assemblies connected to it, and the leading edge of the wing surface. This front aerodynamic structure  24  then has not only an aerodynamic fairing function, but also allows the positioning, segregation and routing of the various systems (air, electrics, hydraulics, fuel). In addition, since the front part of this structure  24  is not in contact with the rigid structure  8 , it is habitually interposed between a thermal exchanger in the space defined between these two elements. 
         [0053]    As a direct aft extension of this structure  24 , again under the wing surface and assembled above the rigid structure  8 , is the “connecting fairing”  28 , also called the “karman”. After this, again in the aft direction, the karman  28  is extended by the aft aerodynamic structure  26 , which contains most of the hydraulic equipment. This structure  26  is preferably located fully to aft of the rigid structure  8 , and is therefore attached under the wing surface of the aircraft. 
         [0054]    Finally, under the rigid structure  8  and the aft aerodynamic structure  26  is the lower aft aerodynamic fairing  30 , also called the “shield” or “Aft Pylon Fairing”. Its essential functions are the formation of a thermal barrier, also called the anti-fire barrier, used to protect the pylon and the wing surface from the heat released by the primary flow, and the formation of an aerodynamic continuity between the engine outlet and the mounting pylon. 
         [0055]    The abovementioned fairing  30  includes a thermal protection floor  32 , or lower spar fitted with an outer surface intended to be followed by a primary flow of the engine which it partially and radially demarcates towards the outside, and where this primary flow escaping from the exhaust nozzle  33  of the engine is represented schematically by the arrow  36 . In addition, the fairing  30  also includes two side panels  44  which, for their part, are designed to be followed externally by a secondary flow of the engine represented schematically by the arrow  38 , due to their positioning in the annular bypass duct  40  of the engine&#39;s secondary flow, and/or at the outlet of the latter. 
         [0056]    It should be noted that in the described preferred embodiment in which the engine  6  is intended to be suspended under the wing surface of the aircraft, the thermal protection floor  32  of the pylon and of the wing surface with regard to the primary flow  36  constitutes a lower portion of the fairing  30 . Naturally, this floor would constitute an upper portion of the fairing in the alternative case in which the engine was intended to be installed above the wing surface. 
         [0057]    Lastly, as can be seen in  FIG. 1 , it is intended that the front end of the floor  32  should follow the upper aft end of the exhaust nozzle  33 , or again that it should be positioned very closely to this same aft nozzle end  33 . 
         [0058]    With reference, at present, to  FIGS. 2 and 3 , the lower aft aerodynamic fairing  30  can be seen in greater detail, taking as it does the general shape of a box which is closed at the top by an upper spar  35 , i.e. in the direction of the other structures of the pylon  4  on which it is intended to be assembled, namely the aft aerodynamic structure  26  and the rigid structure  8 . The fairing  30  preferably has a plane of symmetry P corresponding to an XZ plane, where this plane P also constitutes a vertical plane of symmetry for the entire mounting device  4 , and for the engine  6 . 
         [0059]    The lower aft aerodynamic fairing  30  in the shape of a box also includes the two side panels  44  (only one is visible in each of the  FIGS. 2 and 3  for reasons of clarity), where each of these panels  44  is, substantially, aligned in a plane XZ, either side of plane P. They are mounted together by a group of transverse internal stiffening ribs  46 , with spaces between one another in the direction X, where each of these ribs  46  is aligned in a plane YZ. These transversely aligned ribs  46  are also mounted on the upper  35  and lower  32  spars, thereby enabling all the external elements  32 ,  35 ,  44  forming the box to be held together. 
         [0060]    In  FIG. 3  it can be seen that the fairing  30  here includes a group of five transverse ribs  46 , which are manufactured by a method which will be described in due course, and which is also a purpose of the invention. 
         [0061]    Each rib  46  adopts a roughly planar shape, for example of a thickness of several millimetres, preferably less than or equal to 2 mm. It has, globally, the shape of a trapezium perforated in its centre by an opening, with the large base located towards the top and the small base towards the bottom. The large base is thus intended to be connected to the upper spar of the fairing, the lower base is intended to be connected to the lower spar of the fairing, and both sides are respectively intended to be connected to the two side panels of this same fairing. 
         [0062]    As an indication, the opening area occupies 40 to 60% of the total area of the rib traversed by this opening. 
         [0063]    As can be seen in  FIG. 3 , the five ribs in the form of trapezoid frames are of different sizes, although several of them could be of identical size. In the preferred embodiment represented the ribs become smaller towards the aft, i.e. the heights and/or weights of the ribs located towards the front of the fairing  30  are greater, and those located towards the aft of this fairing are smaller. 
         [0064]    With reference to  FIGS. 4   a  to  4   d , the manufacture of one of the four ribs  46  of the group which are located aft of the most forward rid of this same group of five ribs is represented. 
         [0065]    With reference firstly to  FIG. 4   a , the manufacture of the rib is begun by the production of a rib preform  46   a , by superplastic forming, also called SPF (“Superplastic Forming”). This preform  46   a  has a geometry similar to that of the final ribs  46 . In particular, seen from the front as represented in  FIG. 4   a , preform  46   a  has an outline  50  in a broadly quadrilateral shape, and more specifically in the form of a trapezium. In addition, it has a central opening  52  fully traversing the preform  46   a , which also has a broadly trapezoid shape, such that the preform  46   a  has the general shape of a frame following the outline of a trapezium. 
         [0066]    The rib preform  46   a  is produced from a single piece of titanium alloy TAV6, which has appropriate characteristics for the implementation of the step of superplastic forming enabling it to be obtained, in a manner known to the skilled man in the art. 
         [0067]    One of the features of the present invention lies in the fact that the five ribs of the group are all obtained from this same rib preform  46   a , requiring only one single superplastic forming die. 
         [0068]    The size of this preform  46   a  is greater than the size of the desired rib, although it can be used in its unmodified state for the production of the largest rib in the group, i.e. the most forward one. 
         [0069]    Conversely, for the desired rib manufacturing continues by reducing the dimensions of the preform  46   a , both the height and the width. To accomplish this, as has been represented schematically in  FIG. 4   b , the said rib preform  46   a  is divided, so as to split it into four rib parts  56   a  to  56   d . This division of the four parts  56   a  to  56   d , each of which includes one of the corners  58   a  to  58   d  of the trapezium, is therefore accomplished so as to remove material from the preform, in order to achieve the desired dimensions. In  FIG. 4   b  the four parts  60  shown in grey represent the removed material which is, for example, removed by machining or drawing. 
         [0070]    Each of them is preferably centred on their associated trapezium sides which they traverse as far as opening  52 , although another configuration could be possible, without going beyond the scope of the invention. 
         [0071]    Thus, the amount of the parts  60  to be eliminated is larger the greater the reduction in size of the preform to be accomplished. In addition, it should be noted that the sizes of the two parts  60  located on the sides of the trapezium is identical, just as the size is identical for the two parts  60  located on the bases of the trapezium. 
         [0072]      FIG. 4   c  shows the four parts  56   a  to  56   d  obtained after the removal of material from the rib preform, where these mutually independent parts are then positioned in a traditional positioning die (not represented), in order to make them occupy their definitive relative positions enabling the desired rib to be obtained. This also guarantees accurate dimensions in both directions of the rib plane, i.e. in the direction of the height in which direction the two bases of the trapezium are separated, and in the direction of the width in which the two sides of this same trapezium are separated. 
         [0073]    After this, when the four preform parts  56   a  to  56   d  have been repositioned relative to one another, they are joined by bolted fishplates, or by laser welding, or again by any other technique deemed appropriate by the skilled man in the art, so as to obtain the smaller internal transverse stiffening rib. In  FIG. 4   d  showing the welding beads  64  connecting two-by-two the preform parts  56   a  to  56   d , it can be seen that the recomposed rib has an outline  50  which is still of a roughly trapezoid shape, without any level differences at the junctions between these parts. In order to obtain this characteristic, which is advantageous in that it enables the rib to follow better the external elements of the fairing which it is holding, the amount of the parts  60  removed from the bases of the trapezium is correlated with the amount of the parts  60  removed from the sides of the trapezium. 
         [0074]    Naturally, in order to obtain the three other ribs in the group of the same preform size  46   a , the amount of the parts  60  to be removed is larger the greater the reduction of size of the preform to be accomplished. 
         [0075]    Naturally, various modifications can be made by the skilled man in the art to the invention which has just been described, solely as non-restrictive examples. With this regard, it can be stated notably that if engine assembly  1  has been presented in an appropriate configuration for it to be suspended under the wing surface of the aircraft, this assembly  1  could also have been presented in a different configuration enabling it to be mounted above this same wing surface, or in an aft part of the fuselage.