Abstract:
In the case of an aircraft aft portion equipped with an engine mounting structure passing across the fuselage, assembly of the aft portion presents problems caused by the moving of the box structure of the mounting structure through lateral openings in the fuselage. In order to overcome these problems, a method of assembling an aircraft aft portion is proposed, in which method the box structure of the engine mounting structure is inserted into the fuselage through a top opening extending from one side of the fuselage to the other across a vertical midplane of the fuselage such that the top opening opens to the top and to the sides of the fuselage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to FR 14 53807 filed Apr. 28, 2014, the entire disclosure of which is incorporated by reference herein. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to the assembling of an aircraft aft portion equipped with engines attached to its fuselage. 
       BACKGROUND 
       [0003]    In order to create an aircraft aft portion such as this, the prior art has proposed interposing an attachment pylon between the fuselage and each engine, as described for example in International application WO 2011/086221. In this configuration, the pylon is fixed directly to the fuselage. In order satisfactorily to transfer engine load to the fuselage, substantial structural reinforcers prove necessary for this pylon, and for the part of the fuselage that supports it, and for the attachment structure interposed between these elements. That results in a significant mass penalty. 
         [0004]    Another solution is to provide an engine mounting structure passing across the fuselage, and across the interior space of the aircraft defined by this fuselage, as in International application WO 2010/031959 for example. 
         [0005]    This solution makes it possible to reduce the intensity of the load introduced into the fuselage at each of the two openings therein through which the mounting structure passes, particularly load directed in the main direction of the mounting structure, namely in a plane orthogonal to a longitudinal direction of the aircraft. 
         [0006]    However, assembling the engine mounting structure with the fuselage requires that the two half-structures of which the mounting structure is formed be moved one towards the other through openings in the fuselage. 
         [0007]    Such an operation presents risks of damaging the fuselage and the engine mounting structure, and proves to be difficult to carry out and costly in terms of time. 
       SUMMARY 
       [0008]    It is an object of the disclosure herein to provide a simple, economical and effective solution to this problem. 
         [0009]    To this end, the disclosure herein proposes a method for assembling an aircraft aft portion, comprising:
       on the one hand, assembling a fuselage outer skin and circumferential frames so as to form a fuselage of the aircraft aft portion having a top opening extending from one side of the fuselage to the other across a vertical midplane of the fuselage so that the top opening opens towards the top and towards the sides of the fuselage, and, on the other hand, assembling a mounting structure comprising a box structure and having opposite ends which are provided with attachment structure intended for attaching aircraft engines to the mounting structure, then   inserting the box structure of the mounting structure into the top opening of the fuselage, then   fixing the box structure of the mounting structure to the fuselage.       
 
         [0013]    The “top” and the “sides” are naturally defined with reference to an orientation adopted by the aircraft aft portion when it is fitted to an aircraft parked on a horizontal surface. This orientation is preferably also the orientation adopted by the fuselage of the aircraft aft portion during the method of assembly thereof. 
         [0014]    Inserting the box structure of the mounting structure into an opening in the fuselage that opens towards the top and towards the sides of the fuselage makes the operation of mounting the mounting structure on the fuselage easier and makes it possible to reduce the risks of damage to these elements, which risks are encountered when using the methods of assembly of the prior art that comprise inserting and moving the box structure through lateral openings in the fuselage. 
         [0015]    The method according to the disclosure herein also offers the option of assembling aerodynamic fairings on the lateral portions of the box structure before the mounting structure is mounted on the fuselage. 
         [0016]    Similarly, the method according to the disclosure herein also makes it possible for aircraft engines to be mounted on the mounting structure before this mounting structure is mounted on the fuselage. 
         [0017]    Preferably, the fuselage comprises two circumferential frames, these respectively being the forward frame and an aft frame, which extend forward of and aft of the top opening in the fuselage. 
         [0018]    Preferably, the method comprises a later step of connecting the forward and aft circumferential frames by at least one top cross beam extending above the middle portion of the box structure of the mounting structure. 
         [0019]    Preferably, one of the at least one top cross beams, hereinafter referred to as the central top cross beam, is centered in the vertical midplane of the aircraft aft portion, the method comprising fixing the central top cross beam to the box structure of the mounting structure. 
         [0020]    Preferably, the forward and aft circumferential frames are joined together by two lateral cross beams extending respectively one on each side of the fuselage, below the top opening in the fuselage. 
         [0021]    Preferably, the method involves, before inserting the box structure of the mounting structure into the top opening in the fuselage, a step of assembling a top fairing with the middle portion of the box structure of the mounting structure so that the top fairing aerodynamically closes off a middle section of the top opening in the fuselage after the middle portion of the box structure has been fixed to the fuselage. 
         [0022]    Preferably, the top fairing is secured beforehand to a top aileron, the top aileron being positioned such that when the top fairing is assembled with the fuselage, the top aileron extends above the top fairing and in the forward continuation of an empennage of the aircraft aft portion. 
         [0023]    As an alternative, the method involves, after fixing the middle portion of the box section to the fuselage, a later step of mounting a top fairing on the fuselage, over the top of the middle portion of the box structure of the mounting structure, so that the top fairing aerodynamically closes off a middle section of the top opening in the fuselage. 
         [0024]    Preferably, the method involves a later step of mounting a top aileron on the top fairing in the forward continuation of an empennage of the aircraft aft portion. 
         [0025]    Preferably, the top fairing incorporates longitudinal stiffeners so that in normal operation, the top fairing participates in the transfer of load between a portion of the fuselage that is situated aft of the top opening in the fuselage and a portion of the fuselage that is situated forward of the top opening in the fuselage. 
         [0026]    In another alternative form, the middle portion of the box structure of the mounting structure comprises an aerodynamic top skin delimiting the box structure and shaped so as to extend in the continuation of the outer skin of the fuselage after the middle portion of the box structure has been inserted into the top opening in the fuselage. 
         [0027]    In general, the method preferably involves a moving of the mounting structure in a vertical direction culminating in the insertion of the box structure of the mounting structure in the top opening in the fuselage. 
         [0028]    Moreover, the method preferably involves a step of fixing aircraft engines to the mounting structure, this step taking place before the box structure of the mounting structure is inserted into the top opening in the fuselage. 
         [0029]    The disclosure herein also relates to an aircraft aft portion comprising a fuselage formed of a fuselage outer skin and of circumferential frames, and a mounting structure comprising a box structure and attachment structure intended for the attaching of aircraft engines to the mounting structure. 
         [0030]    According to the disclosure herein, the fuselage has a top opening extending from one side of the fuselage to the other across a vertical midplane of the fuselage so that the top opening opens towards the top and towards the sides of the fuselage. In addition, the box structure comprises a middle portion housed in the top opening in the fuselage and fixed to the fuselage. 
         [0031]    Preferably, the box structure comprises a top skin forming part of an outer fairing of the aircraft aft portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The disclosure herein will be better understood and further details, advantages and features thereof will become apparent from reading the following description given by way of nonlimiting example and with reference to the attached drawings in which: 
           [0033]      FIG. 1  illustrates two steps of a method of assembling an aircraft aft portion according to a first preferred embodiment of the disclosure herein; 
           [0034]      FIG. 2  illustrates the aircraft aft portion at the end of a later step of the method according to the first preferred embodiment of the disclosure herein; 
           [0035]      FIG. 3  illustrates the aircraft aft portion at the end of an alternative form of embodiment of the step of  FIG. 2 ; 
           [0036]      FIG. 4  illustrates a later step in the method according to the first preferred embodiment of the disclosure herein; 
           [0037]      FIG. 5  illustrates the aircraft aft portion at the end of the method step illustrated in  FIG. 4 ; 
           [0038]      FIG. 6  is a schematic perspective view of an engine mounting structure intended to be used for implementing a method of assembling an aircraft aft portion according to a second preferred embodiment of the disclosure herein; and 
           [0039]      FIG. 7  is a schematic partial perspective view of an aircraft aft portion obtained by the method according to the second preferred embodiment of the disclosure herein. 
       
    
    
       [0040]    Throughout all of these figures, identical references may denote elements that are identical or analogous. 
       DETAILED DESCRIPTION 
       [0041]      FIG. 1  illustrates a first step in a method of assembling an aircraft aft portion  10  according to a first preferred embodiment of the disclosure herein, from a fuselage  12  intended to delimit an interior space of the aircraft, a rear empennage  16  provided with a rudder  18  and an elevator  20 , two engines (not depicted) which are for example unducted turbojet engines with contrarotating propellers, of the type commonly referred to as “open rotor” engines and a mounting structure  24  for mounting the engines. The engines may of course be turboprop engines or alternatively ducted-fan turbojet engines, without departing from the scope of the disclosure herein. 
         [0042]    In the description that follows, the longitudinal direction of the aircraft is referred to as X, the vertical direction as Z, and the transverse direction, which is orthogonal to the previous two directions, as Y. These directions are defined with reference to an orientation adopted by the aircraft aft portion when it is fitted to an aircraft parked on a horizontal surface, which also preferably corresponds to an orientation of the fuselage  12  of the aircraft aft portion  10  during the method of assembling the latter. 
         [0043]    The mounting structure  24  comprises a forward spar  26  and an aft spar  28  joined together by a plurality of ribs extending substantially orthogonal to the spars so as to form a box structure  32 . 
         [0044]    The box structure  32  has opposite lateral ends  34  which are provided with attachment structure (not depicted) intended for attaching the engines. 
         [0045]    The box structure  32  is strengthened and closed by a top skin  37   a  and a bottom skin  37   b  which are fixed respectively to the top edge and to the bottom edge of the spars  26 ,  28  and ribs of the box structure  32 . 
         [0046]    In general, the spars  26 ,  28  of the box structure  32  are preferably each produced as a single piece, or as an alternative may each be formed of several beams joined end to end, for example using bolts or fishplates. Similarly, the top  37   a  and bottom  37   b  skins may each be produced as a single piece or as parts joined together. 
         [0047]    The box structure  32  is broken down into a middle portion  38  intended to be housed inside the fuselage  12 , and into two lateral parts  40  intended to extend outside of the fuselage  12  and ending respectively in the lateral ends  34 . 
         [0048]    The fuselage  12  in general comprises an outer skin  50 , which may be reinforced by internal longitudinal stiffeners (the latter not being visible in  FIG. 1 ), and circumferential frames supporting the outer skin  50 .  FIG. 1  shows five of these circumferential frames, particularly a forward circumferential frame  66  and an aft circumferential frame  68 , as well as an intermediate circumferential frame  70 . The front  66  and aft  68  circumferential frames define forward and aft ends of a top opening  72  in the fuselage  12  which opening is intended to house the box structure  32  of the mounting structure  24 , whereas the intermediate circumferential frame  70  extends between the forward  66  and aft  68  circumferential frames and is truncated, namely devoid of its top section, so that the intermediate circumferential frame  70  extends wholly below the top opening  72 . 
         [0049]    The forward  66  and aft  68  circumferential frames are joined together by two fuselage lateral cross beams  74  extending respectively one on each side of the fuselage  12 , below the top opening  72  of the fuselage. The lateral cross beams extend symmetrically with respect to a vertical midplane P of the aircraft aft portion  10 . These lateral cross beams each extend at least as far as the circumferential frame  77   a  that is next after the forward circumferential frame  66  in the forward direction, and as far as the circumferential frame  77   b  that is next after the aft circumferential frame  68  in the aft direction. The intermediate circumferential frame  70  is truncated by the two lateral fuselage cross beams  74  and thus has two opposite circumferential ends connected respectively to the two lateral fuselage cross beams  74 . 
         [0050]    Each of the forward  66  and aft  68  circumferential frames further comprises a strengthening cross beam  79  extending in the transverse direction Y, substantially at the same vertical height as the lateral cross beams  74 . 
         [0051]    The fuselage  12  comprises two forward mounts  80 , two aft mounts  82 , two intermediate mounts  84 , and four load-reacting link rods  86 , which are intended for fixing the mounting structure  24  to the fuselage  12 . All of these mounts and link rods are arranged symmetrically on each side of the vertical midplane P. 
         [0052]    The forward  80 , aft  82 , and intermediate  84  mounts adopt for example the form of clevis mounts articulated about respective axes substantially parallel to the transverse direction Y so as to allow load to be transmitted in the longitudinal direction X and vertical direction Z. 
         [0053]    The forward mounts  80  are configured to connect the forward spar  26  of the box structure  32  to the forward circumferential frame  66 , whereas the aft mounts  82  are configured to connect the aft spar  28  of the box structure  32  to the aft circumferential frame  68 . In the example illustrated, the forward  80  and aft  82  mounts are configured to extend respectively in front of and behind the box structure  32 . 
         [0054]    Moreover, the load-reacting link rods  86  split into two forward link rods and two aft link rods (one link rod of each type being visible in  FIG. 2 , as will become more clearly apparent in what follows). Each forward link rod has one end intended to be connected to a clevis secured to the forward spar  26  and an opposite end intended to be connected to a clevis secured to the forward circumferential frame  66 . Similarly, each aft link rod has one end intended to be connected to a clevis secured to the aft spar  28  and an opposite end intended to be connected to a clevis secured to the aft circumferential frame  68 . These load-reacting link rods  86  extend substantially in the transverse direction Y so as mainly to transmit load oriented in this transverse direction Y. 
         [0055]    The method of assembling the fuselage aft portion  10  first of all comprises assembling elements that make up the fuselage  12 , notably the circumferential frames  66 ,  68 ,  70 ,  77   a ,  77   b , the outer skin  50 , the lateral cross beams  74  and the mounts  80 ,  82 ,  84  and load-reacting link rods  86 , so as to obtain the fuselage as depicted in part in  FIG. 1 , with its top opening  72  extending from one side of the fuselage  12  to the other across the vertical midplane P of the fuselage. The top opening  72  thus opens towards the top and towards the sides of the fuselage. 
         [0056]    Moreover, the method comprises assembling the elements that make up the box structure  32  of the engine mounting structure  24 , notably the spars  26  and  28 , the ribs, the skins  37   a  and  37   b  and, as appropriate, the engine attachment structure and the engines. For the sake of clarity, the engines and the structure for attaching these engines have not been depicted in  FIG. 1 . 
         [0057]    Assembly of the engine mounting structure  24  may be performed prior to assembly of the fuselage  12 , or after assembly of the fuselage, or alternatively, in parallel therewith. 
         [0058]    Next, the method comprises inserting the box structure  32  of the mounting structure  24  into the top opening  72  in the fuselage, as symbolized by the arrow  90  in  FIG. 1 , followed by attachment of the box structure  32  of the mounting structure  24  to the fuselage, for example using the mounts  80 ,  82 ,  84  and the load-reacting link rods  86  all described hereinabove. 
         [0059]    In parallel with these operations of inserting and attaching the box structure  32  of the mounting structure, or alternatively before or after these operations, the empennage  16  is also mounted on the fuselage  12  as illustrated by the arrow  92  in  FIG. 1 . 
         [0060]    In the first embodiment of the disclosure herein, the method next comprises attaching one or more upper cross beams  76  to the forward  66  and aft  68  circumferential frames so that each upper cross beam connects these two circumferential frames together, as shown by  FIG. 2 . 
         [0061]    In the example illustrated in  FIG. 2 , a single upper cross beam  76  is depicted and this is centred with respect to the vertical midplane P. This upper cross beam is referred to as the “central upper cross beam” in what follows. In addition, the central upper cross beam  76  extends in the axial continuation of two upper longitudinal beams  76   a  and  76   b  of the fuselage  12  which are respectively arranged forward of and aft of the central upper cross beam  76  and also extend in the vertical midplane P. 
         [0062]    In the example illustrated in  FIG. 2 , the central upper cross beam  76  extends some distance from the box structure  32  of the mounting structure  24 . 
         [0063]    In general, each upper cross beam  76  is able to transmit some of the load, notably load oriented in the longitudinal direction X, from the part of the fuselage  12  that is situated aft of the opening  72  to the part of the fuselage  12  that is situated forward of the opening  72 . 
         [0064]      FIG. 3  illustrates an alternative form of embodiment of the method, in which the method further comprises fixing the central upper cross beam  76  to the box structure  32  of the mounting structure  24 . 
         [0065]    To do that, the central upper cross beam  76  adopts for example the form of a panel or of a beam extending in the vertical midplane P, and having a forward end connected to a clevis  132  secured to the forward circumferential frame  66  so as to form a pivot-type connection or ball joint able to react longitudinal and vertical load, a rear end connected to a clevis  134  secured to the aft circumferential frame  68  so as likewise to form a pivot type connection or ball joint able to react longitudinal and vertical load, and a lower edge  136  secured to a rib  30  of the box structure  32 . For that purpose, the lower edge  136  may be fixed to the rib of the box structure by fishplating, or the central upper cross beam  76  may be produced as one piece with the rib. 
         [0066]    Some of the longitudinal and vertical load imparted by the mounting structure  24  to the fuselage  12  can thus pass directly via the central upper cross beam  76 . 
         [0067]    In the example illustrated, the central upper cross beam  76  is secured to the box structure  32  of the mounting structure  24  before the latter is assembled with the fuselage  12 . Thus, the method of assembling the mounting structure  24  with the fuselage  12  comprises attaching the central upper cross beam  76  to the circumferential frames  66  and  68 . 
         [0068]      FIG. 4  illustrates a later step in the method according to the first embodiment of the disclosure herein, comprising mounting a top fairing  102  on the fuselage  12 , above the middle portion  38  of the mounting structure  24  so as to close off a middle section of the top opening  72  in the fuselage and thus ensure the aerodynamic continuity of the outer skin  50  of the fuselage. 
         [0069]    The top fairing  102  is, for example, fixed to the forward  66  and aft  68  circumferential frames and to the top cross beam  76 , for example using fixings with countersunk heads. 
         [0070]    In the example illustrated, the top fairing  102  incorporates four longitudinal stiffeners  103  distributed symmetrically on each side of a midplane of the fairing. The mounting of the top fairing  102  therefore also comprises fixing the longitudinal stiffeners  103  to the forward  66  and rear  68  circumferential frames, for example using fixings with countersunk heads. The circumferential frames comprise notches open radially to the outside to accept the longitudinal stiffeners  103 . 
         [0071]    Aside from its aerodynamic function, the top fairing  102  can thus perform a structural role, namely participate in the transfer of load, notably longitudinal load, between a part of the fuselage situated aft of the top opening  72  of the fuselage, notably the aft circumferential frame  68 , and a part of the fuselage situated forward of the top opening  72  in the fuselage, notably the forward circumferential frame  66 . 
         [0072]    In addition, the top fairing  102  may participate in protecting each of the engines in the event of an incident that causes a blade or vane, or a fragment thereof, torn out of the opposite engine, to be thrown out. Specifically, the top fairing  102  lies in the potential paths connecting the respective rotors of the engines with one another. 
         [0073]    In addition, in the example illustrated, the top fairing  102  is secured beforehand to an upper aileron  104 , also referred to as a “dorsal fin”, positioned in such a way as to extend above the fuselage top fairing  102  when the latter is mounted on the fuselage  12 , as shown by  FIG. 5 . The top aileron  104  thus extends in the forward continuation of the empennage  16 , and thus forms a bottom part of a leading edge of the empennage  16 . 
         [0074]    As an alternative, the top aileron  104  may be assembled with the top fairing  102  after the latter has been assembled with the fuselage  12 . 
         [0075]    The top aileron  104  is designed to participate in protecting each of the engines in the event of an incident causing a blade or vane, or a fragment thereof, torn from the opposite engine, to be thrown out. 
         [0076]    The method furthermore comprises a step (not illustrated in the figures) comprising mounting aerodynamic fairings on the front and rear of the lateral parts  40  of the mounting structure  24  so that these aerodynamic fairings are connected to the fuselage  12  and form respective leading edges and respective trailing edges of the two lateral parts  40  of the mounting structure. These aerodynamic fairings may be assembled with the mounting structure  24  beforehand before the latter is joined to the fuselage  12 . As an alternative, these aerodynamic fairings may be installed on the mounting structure  24  after the latter has been assembled with the fuselage  12 . 
         [0077]    Furthermore, if an upper cross beam  76  is preassembled with the box structure  32  of the mounting structure  24 , as described hereinabove with reference to  FIG. 3 , the top fairing  102  may be fixed beforehand to the upper cross beam  76  before the box structure  32  of the mounting structure is inserted into the top opening  72  in the fuselage. The upper cross beam  76  thus provides mechanical support for the top fairing  102  before the top fairing  102  is assembled with the fuselage. 
         [0078]      FIG. 6  illustrates an engine mounting structure  24  intended to be used for implementing a method of assembling an aircraft aft portion according to a second preferred embodiment of the disclosure herein. 
         [0079]    This mounting structure  24  differs from the mounting structure of  FIGS. 1 to 5  in that the middle portion  38  of the box structure  32  is covered with an aerodynamic top skin  140  forming part of the top skin  37   a  of the box structure  32  and configured to extend in the continuation of the outer skin  50  of the fuselage  12  when the box structure  32  of the mounting structure  24  is fixed to the fuselage  12 , as illustrated by  FIG. 7  which shows the aircraft aft portion  10  equipped with this mounting structure  24 . 
         [0080]    In the example described, the box structure  32  of the mounting structure  24  is fixed to the opposite top ends of three truncated consecutive circumferential frames respectively referred to as the forward circumferential frame  142 , intermediate circumferential frame  144  and aft circumferential frame  146 . 
         [0081]    The middle portion  38  of the box structure  32  thus joins together the upper ends of each of the circumferential frames  142 ,  144  and  146 . 
         [0082]    While at least one exemplary embodiment of the invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.