Abstract:
A turbomachine casing including a substantially cylindrical wall and an annular one-piece acoustic insulation panel mounted inside the wall, the panel including an annular surface that is radially external opposite a radially internal annular surface of the wall, wherein the wall includes on its internal angular surface first projecting members which bear axially against second projecting members belonging to the external annular surface of the panel and which are fixed to these second members in a dismountable manner.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a turbomachine casing, more particularly a turbomachine such as a turbojet or a turboprop engine in a plane. 
     Description of the Related Art 
     A fan casing typically comprises a substantially cylindrical wall which extends about the turbomachine fan blades, and the internal surface of which is covered with acoustic insulation panels. Such panels generally comprise an annular honeycomb structure the internal and external surfaces of which are each covered with a skin which can be multi-perforated to improve the acoustic treatment. They are intended to absorb the sound waves generated by the fan of the turbomachine. 
     The applicant&#39;s previous applications EP-A1-2318679, EP-A1-2,088,290 and FR-A12965859 describe acoustic insulation panels of this type. 
     An acoustic insulation panel in a turbomachine is generally sectorized, i.e. formed of several panel sectors circumferentially arranged end-to-end and secured to the casing wall by gluing or fixing screws which extend radially with respect to the longitudinal axis of the turbomachine. Fixing the sectors by screwing has disadvantages in that it requires using many fixing screws which, on the one hand, increase the weight of the turbomachine and are liable to damage the fan blades in case of loss or breakage. On the other hand, more or less wide interface areas between the sectors induce alternating smooth and treated areas in the vicinity of the fan (and thus the creation of acoustic impedance discontinuities), which cause an increase in the noise levels in the turbomachine for some operating conditions. Besides, mounting the numerous fixing screws on the panels requires the provision therein of densified zones that affect the acoustical effectiveness. 
     In order to limit the number of fixing screws, the above-mentioned patent application FR-A1 2,935,017 provides to extend the fan casing and to fix an annular one-piece acoustic insulation panel on the casing wall and the air inlet duct. In practice, the maintenance of such an acoustic panel is difficult to achieve since the turbomachine has to be immobilized to substitute a new panel for the damaged panel. 
     Furthermore, a casing wall may be deformed and have an internal surface which is not perfectly cylindrical. This is more particularly the case in a casing wall made of a composite material which, because of the method used for the manufacturing thereof, may have relatively important manufacturing tolerances. In the currently available technique, the acoustic panels which are fixed on a casing wall of this type do not make it possible to compensate the above-mentioned tolerances. 
     This is more particularly the case for acoustic insulation panels which are glued to the casing wall by means of a hot-setting adhesive. Implementing such gluing is long (several hours are requested in a large-sized autoclave) and, if the panel is damaged, the engine must be dismounted prior to replacing the panel. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention makes it possible to remedy at least some of the above-mentioned drawbacks in a simple, efficient and economical way. 
     For this purpose, it provides for a turbomachine casing having a longitudinal axis and comprising a substantially cylindrical wall and an annular one-piece acoustic insulation panel mounted inside the wall, about said longitudinal axis, with the panel comprising an annular surface which is radially external opposite a radially internal annular surface of the wall, characterized in that the wall comprises, on its internal annular surface, first projecting members which are supported parallel to said longitudinal axis by said second projecting members of the external annular surface of the panel and which are fixed to such second members in a dismountable or removable manner. 
     The present invention thus provides a new system for fixing a one-piece acoustic insulation panel on a casing wall. As the panel is formed of a single annular piece (called OPB, the acronym for One Piece Barriel), no discontinuity in acoustic impedance occurs on the whole surface. Besides, the panel fixing means do not significantly increase the weight of the casing and do not go through the whole radial dimension of the panel. No discontinuity is thus created in the acoustic insulation. Furthermore, as will be described in greater details hereafter, the invention facilitates the mounting and enables the dismounting of the panel, in particular under the wing of an aircraft equipped with the turbomachine (i.e. without having to dismount the engine), and further optimizes the jet tolerances in that it enables to adjust the possible cylindricality defects in the casing wall (from +/−2 mm up to +/−0.4 mm in a particular embodiment of the invention). 
     The one-piece panel may comprise an annular honeycomb structure the internal and external surfaces of which are covered with a skin, with the internal skin being preferentially of the multi-perforated type. 
     The first members may be fixed to the second members by means of the screw-nut type. Such means of the screw-nut type advantageously extend parallel to the longitudinal axis of the casing. The panel mounting and dismounting tool then comprises a screwing/unscrewing tool which is intended to be axially inserted between the casing wall and the panel. 
     According to another characteristic of the invention, the first and second members comprise mounting lugs. Such lugs may be formed integral with the wall and the panel, respectively, or be fixed to the wall and the panel, respectively, for example by gluing, welding, screwing or riveting. 
     The wall and the panel may each comprise at least one annular row of lugs regularly distributed about the longitudinal axis of the casing. They may for example each comprise two annular rows of lugs, with the lugs of the first row being preferably angularly offset with respect to the second row of lugs for an easy mounting thereof. Each row includes for example twelve lugs. 
     At least a part of the lugs may be substantially L-shaped. Such lugs may include a substantially flat or cylindrical part intended to be fixed or made integral with one of the elements among the wall and the panel, and a substantially radial part having a hole intended to be gone through by a fixing means. 
     The lugs of the wall may be at least partially accommodated in recesses in the internal annular surface of the wall, and/or the lugs of the panel may be at least partially accommodated in recesses in the external annular surface of the panel. 
     The lugs advantageously each comprise a radial surface for bearing on another lug and a flat or substantially cylindrical centring surface intended to cooperate with said other lug. 
     The flat or cylindrical surfaces of the lugs of the wall are preferably made of a machinable material so as to correct a possible defect, if any, of cylindricality of the internal annular surface of the wall. 
     In the mounting position, the panel can be separated from the wall by an annular space, which has for example a thickness or a radial dimension of the order of about 10 mm. 
     The present invention also relates to a turbomachine, such as a turbojet or a turboprop engine, characterized in that it comprises a casing as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other advantages and characteristics of the invention will appear upon reading the following description given by way of not restrictive example and while referring to the appended drawings wherein: 
         FIG. 1  is a schematic half-view in axial section of a turbomachine fan casing according to the invention; 
         FIG. 2  is a front view, from upstream, of the casing of  FIG. 1  (axial arrow II in  FIG. 1 ); 
         FIG. 3  is a partial schematic half-view of an acoustic insulation panel in axial section (thus along the longitudinal axis of the casing); 
         FIG. 4  is an enlarged view of the detail I 4  in  FIG. 1 , and shows means for fixing an acoustic insulation panel; 
         FIG. 5  is a schematic sectional view along the V-V line of  FIG. 4  (a section perpendicular to the longitudinal axis of the casing); 
         FIG. 6  is a schematic view corresponding to  FIG. 5  and showing an alternative embodiment of the invention; and 
         FIG. 7  is a schematic view corresponding to  FIG. 4  and showing another alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is first made to  FIG. 1  which shows a fan casing  10  of a turbomachine such as a turbojet or a turboprop engine, with such casing belonging to a nacelle which surrounds the engine of the turbomachine and inside which a fan rotates which generates a secondary air flow which circulates between the nacelle and the engine and forms a part of the thrust generated by the turbomachine. 
     The casing  10  comprises a substantially cylindrical wall  12  which comprises fixing annular flanges  14 ,  16  at its longitudinal ends. The downstream flange  14  is secured by means of the screw-nut type to a flange (not shown) of an intermediate casing and the upstream flange  16  is secured by means of the screw-nut type to a flange (not shown) of an air inlet duct in the nacelle. 
     The casing comprises acoustic insulation annular panels  18 ,  20 ,  22  which cover the cylindrical internal surface of the wall  12  and which are fixed to same wall. 
     In the example shown, the wall  12  carries three annular panels  18 ,  20 ,  22 , two respectively upstream and median one-piece panels  18 ,  20  according to the invention, and a downstream panel  22  which is sectorized according to the prior art. 
     The downstream panel  22  comprises panel sectors which are positioned circumferentially end-to-end and which are fixed to the wall  12  by screws  24  which radially go through the sectors and are engaged in holes of the wall  12 . Such fixing system has many disadvantages described above. 
     The invention makes it possible to remedy such drawbacks thanks to annular one-piece (i.e. not sectorized) panels  18 ,  20  which are fixed to the wall  12  using a new technology which enables the dismounting of the panels, in particular under the wing of an aircraft during a maintenance operation. 
     In the example shown in  FIGS. 1 to 5 , the panels  18 ,  20  are mounted inside the wall  12  and secured to same wall by means of the screw-nut type, with each panel comprising lugs  26  for axially (axis  70  in  FIG. 4 ) and radially bearing on lugs  28  of the wall  12 , with such lugs comprising holes for the means  32  of the screw-nut type to go therethrough. 
       FIG. 3  shows an exemplary embodiment of a panel  18 ,  20  according to the invention, with such panel comprising an annular honeycomb structure  34 , the internal and external surfaces of which are each covered with a laminated skin  36 ,  38 , with such internal skin  36  comprising multiple perforations  40 . The panel may further comprise a layer of an abradable material, especially in the area of the panel surrounding the fan blades, as is the case in the panel which comprises an internal layer  42  made of an abradable material under its internal skin  36  (see  FIG. 1 ). 
     As shown in  FIG. 2 , each panel  18 ,  20  is formed in one piece without discontinuity, with the lugs  26  being fixed to the external skin  38  of the panel and being located in an annular space  40  which extends between the panel  18   20  and the wall  12 . Such annular space may have a thickness or a radial dimension of the order of 10 mm. 
     The longitudinal axis of the casing which is also that of the cylindrical wall  12  and of the panel  18 ,  20  is referred to by number  100 . 
     It should be noted that the wall  12  and the panel are coaxial. 
     The panel  18 ,  20  extends about the longitudinal axis  100 . 
     Each panel  18 ,  20  is equipped with two annular rows of lugs, an upstream row of lugs  26 ,  28  and a downstream row of lugs  26 ′,  28 ′. The lugs in each row are regularly distributed about the longitudinal axis of the casing and are diametrically opposed in pairs. The lugs  26 ,  28  of the upstream row are further angularly offset with respect to the lugs  26 ′,  28 ′ of the downstream row, with respect to the longitudinal axis of the casing ( FIG. 2 ). Each row comprises for example twelve lugs  26 ,  26 ′,  28 ,  28 ′. 
     The lugs  26  carried by the panel  18 ,  20  are substantially L-shaped and each comprise a longitudinal part  42  applied to the external skin  38  of the panel and fixed to such skin by screws  43  cooperating with hub nuts of the self-locking type ( FIG. 5 ). Such longitudinal part  42  has the shape of a cylindrical portion and follows the external shape of the panel. 
     One of the longitudinal ends of the part  42  of the lug  26  is connected to a substantially radial part  44  which extends outwardly and which comprises a hole for the screw  32  (axis  70 ) to go therethrough. 
     The part  42  of the lug  26  comprises a radially external bearing surface  46  having a cylindrical shape and the radial part  44  comprises a radial bearing surface  48 . 
     As can be seen in  FIG. 5 , the part  44  of the lug  26  has a circumferential dimension smaller than that of its part  42 . 
     The lugs  28  carried by the wall  12  each comprise a radially external substantially flat portion  50  applied to the radially internal surface of the wall  12  and fixred thereto by screws  52  cooperating with hub nuts of the self-locking type, and a part  54  which extends radially inwardly and which comprises a through-hole aligned with the hole in the lug  26  for the screw  32  fixing such lugs to go therethrough. 
     Such part  54  comprises a radial surface  56  (axis  70  parallel to the axis  100  of the casing) for axially bearing on the radial surface  48  of the lug  26  and a flat  58  or substantially cylindrical surface bearing on the cylindrical surface  46  of the lug  26 . 
     As can be seen in  FIG. 5 , the part  54  of the lug  28  has a circumferential dimension smaller than that of its part  50 . Besides, the part  50  of the lug  28  is partially engaged into a recess  60  having a shape matching that of the wall  12 . 
     The thickness of the panel  18 ,  20  honeycomb structure  34  could be increased so as to improve the acoustic insulation performances thereof. In the present case, this can be achieved by increasing the external diameter of the panel. The lugs  26  of the panel would then be accommodated in longitudinal grooves  62  of the radially external surface of the panel  18 ′,  20 ′, as schematically shown in  FIG. 6 . This figure also shows the above-mentioned angular offset between a lug  26  of an upstream annular row and a lug  26 ′ of a downstream annular row of the panel  18 ′,  20 ′, with such lugs  26 ,  26 ′ being accommodated in the same groove  62  of the panel. 
     The panels  18 ,  20  described above may be mounted inside the wall  12  of the casing as follows. 
     Each panel  18 ,  20  is positioned upstream of the wall  12 , coaxially thereto, and is positioned angularly about the longitudinal axis of the casing so that such lugs  26 ,  26 ′ are axially aligned with those  28 ,  28 ′ of the casing. The panel is then displaced in axial translation in the downstream direction until it is accommodated inside the wall and its lugs  26 ,  26 ′ are axially bearing on those  28 ,  28 ′ of the casing. 
     A tool such as a ratchet wrench equipped with an extension is then used to screw the screws  32  into the lugs to secure the panel to the casing. Such tool is inserted axially from upstream into the annular space  40  which extends between the panel and the wall. If this space  40  does not have a sufficient radial dimension and if the lugs of the panel are accommodated in longitudinal grooves  62  of the panel  18 ′,  20 ′, the screws can be accessed through the tool or the extension thereof in the groove of the panel. 
       FIG. 7  shows an alternative embodiment of the invention which is different from the previously described embodiment in that the lugs  126 ,  128  are here formed with sheet metals folded into an L-shape. The lug  126  carried by the panel  118 ,  120  comprises a longitudinal part fixed to the external skin of the panel and a radial part axially bearing (axis  170  parallel to that of the casing  100 ) on a radial part of the other lug  128 , a longitudinal part of which is fixed to the internal surface of the wall of the casing  112 . 
     The radial parts of the lugs  126 ,  128  each comprise a hole. The hole in the lug  128  receives a hub nut  133 . A screw  132  is axially engaged (axis  170 ) into the hole of the lug  126  and is then screwed into the nut  133  carried by the lug  128  for fixing the lugs together. This operation is performed for each pair of lugs  126 ,  128  so as to secure the panel  118   120  to the wall  112 .