Abstract:
The nacelle of the invention includes an air-inlet front section, a middle section for encasing the fan of the turbofan ( 7 ) and a rear section including means for connection to a pylon to be connected to the rigid structure or an airplane. The rear section includes a structural framework ( 18 ) having at least one aerodynamic smoothing and acoustic panel ( 21 ). The acoustic panel ( 21 ) is secured on the structural framework ( 18 ) by floating or elastic fixation means so that said acoustic panel ( 21 ) can be deformed in a direction ( 31 ) essentially centrifugal and radial relative to the turbofan ( 7 ) in the presence of overpressure air ( 3 ) in the engine compartment.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a nacelle for a bypass turbofan. 
       BACKGROUND OF THE INVENTION 
       [0002]    An aircraft is propelled by several turbofans, each housed in a nacelle also accommodating a set of accessory actuation devices associated with its operation, such as a thrust-reverser device, and performing various functions when the turbofan is in operation or stopped. 
         [0003]    A nacelle usually has a tubular structure comprising an air intake upstream of the turbofan, a mid-section designed to surround a fan of the turbofan, a downstream section accommodating thrust-reverser means and designed to surround the combustion chamber of the turbofan, and is usually terminated by an exhaust nozzle the outlet of which is situated downstream of the turbofan. 
         [0004]    Modern nacelles are designed to accommodate a bypass turbofan capable of generating, via the air foils of the fan in rotation, a flow of hot air (also called the main flow) originating from the combustion chamber of the turbofan, and a flow of cold air (the bypass flow) which travels outside the turbojet through an annular passageway, also called the stream, formed between a fairing of the turbofan (or an internal structure of the downstream structure of the nacelle and surrounding the turbofan) and an internal wall of the nacelle. The two air flows are discharged from the turbofan through the rear of the nacelle. 
         [0005]    Each propulsive assembly of the aircraft is therefore formed by a nacelle and a turbofan, and is suspended from a fixed structure of the aircraft, for example beneath a wing or on the fuselage, by means of a mast attached to the turbofan in its front and rear portion by suspension elements. 
         [0006]    In such a configuration, it is the turbofan which supports the nacelle. Such an architecture is subjected to many combined external forces during the mission of the aircraft. Amongst other things this includes forces resulting from gravity, external and internal aerodynamic forces, gushes of wind, and thermal effects. 
         [0007]    These stresses applied to the propulsive assembly are transmitted to the turbofan and cause deformations of the casings which directly impact the efficiency of the various stages of the turbofan. More particularly, in the case of a propulsive assembly called wasp-waist, that is to say having a long and relatively thin downstream portion relative to the intermediate structures and air intake, these stresses result in a particularly harmful deformation called “banana effect”, the downstream portion bending considerably. 
         [0008]    Such a “banana effect” results in a deformation of the external structure of the nacelle formed by the various successive casings while the drive shaft, the blades of the fan and the internal blades of the turbofan remain rectilinear. The result of this is that the heads of the blades of the shaft come closer to the internal periphery of the casings. The general performance of the turbofan is thereby reduced relative to a configuration in which the casings sustain very little or no deformations, because it is then necessary to take account of this deformation in the design of the nacelle so as always to arrange a sufficient clearance between the heads of the blades and the periphery of the casings. This results in a portion of the supply air which is not compressed by the blades because it escapes through this considerable clearance. 
         [0009]    A solution to this problem has been proposed in the as yet unpublished French patent application registered under number 06.05912 in the name of the applicant.  FIGS. 1 and 2  of the appended drawing summarize the subject of this document FR 06.05912. 
         [0010]    The nacelle  1  illustrated in these figures is called structural, that is to say that it supports the engine  7  and connects it directly to a fixed structure  2  of an aircraft via a mast  15  incorporated into its build. The rear section of the nacelle comprises an internal structural framework  18  consisting of radial frames  20  associated with uprights  22 ; longitudinal reinforcements  26 ,  27  associated with upper longitudinal structures  23  and lower longitudinal structures  24  complete this structure. In addition, a set of thrust-absorbing link rods  29  help with transferring the forces from the engine  7  to the fixed structure  2  of the aircraft. An aerodynamic and acoustic smoothing panel  21  is mounted on the framework  18  and surrounds the engine  7 . 
         [0011]    For a such a structural framework design to be able to be certified by the certification authorities and to be perfectly dimensioned with no other random reinforcement means, it is necessary to dissociate the acoustic panel from the transfer of the forces from the engine to the aircraft. In addition, the structure of the acoustic panel and the surrounding structure must not be too impacted if the pipework of the engine were to burst. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0012]    The present invention achieves these objectives and consists for this purpose in a nacelle for a bypass turbofan comprising a front air-intake section, a mid-section designed to surround a fan of the turbofan and a rear section having means for coupling to a mast designed to be connected to a fixed structure of an aircraft, said rear section comprising a structural framework onto which is mounted at least one aerodynamic and acoustic smoothing panel, wherein said acoustic panel is attached to the structural framework by floating or elastic attachment means, so as to allow the acoustic panel to deform in a substantially centrifugal radial direction relative to the turbofan in the presence of over pressurized air in the engine compartment. Therefore, the acoustic panel does not transmit forces originating from the turbofan to the fixed structure of the aircraft. 
         [0013]    Said attachment means may comprise a nut facing a hole passing through the structural framework, and designed to receive and retain the shank of a screw mounted in a reinforcing bush housed in a hole passing through the acoustic panel. 
         [0014]    In one embodiment, the reinforcing bush comes into direct contact with the structural framework, and the screw is mounted with a slight clearance in the reinforcing bush and has a nonconical head, so as to produce a floating attachment of the acoustic panel to the structural framework. 
         [0015]    In another embodiment, the reinforcing bush is associated with an elastic ring which comes into contact with the structural framework. 
         [0016]    In yet another embodiment, said nut comprises a base attached to the structural framework and having a housing in which the nut itself is mounted so as to move in translation on the axis of said hole passing through the structural framework, a return spring being provided in this housing in order to return the nut itself to a rest position at a distance from the structural framework. These arrangements, by playing on the stiffness of the spring or the tightness of the screw, make it possible to modulate the attachment of the panel to the framework in order, if necessary, to discharge internal over pressurized air. 
         [0017]    The acoustic panel may be in one piece or formed of several elements. This includes the adaptation of the panel to an internal fixed structure comprising bifurcations or its application to a structure of the O-duct type. 
         [0018]    According to one possibility, a heat-protection mat, designed to be interposed between the structural framework and the acoustic panel, is mounted by means of retention elements on the acoustic panel. Therefore, the panel incorporates its own heat protection. 
         [0019]    The acoustic panel may comprise at least two longitudinal slots for discharging the over pressurized air in the nacelle. In this case, the heat-protection mat may have, in the vicinity of the discharge slots a tile-like overlap capable of allowing a deformation of the panel without affecting the heat protection. 
         [0020]    According to another aspect of the invention, the panel may comprise at least one inspection hatch with rectilinear or scalloped cutouts. Peripheral seals are for example associated with the cutouts of the inspection hatch or hatches. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]    The application of the invention will be better understood with the aid of the detailed description that is given below with respect to the appended drawing in which: 
           [0022]      FIG. 1  (already commented on in the introduction) is a schematic view in perspective of a nacelle according to the prior art; 
           [0023]      FIG. 2  (already commented on in the introduction) is a schematic view in perspective, from another angle, of the same nacelle; 
           [0024]      FIG. 3  is a view in partial longitudinal section of an example of a nacelle according to the invention, which illustrates the behavior of the structure of an acoustic panel if the pipe work of the engine should burst; 
           [0025]      FIGS. 4 to 6  are similar views in perspective of a first, a second and a third exemplary embodiments of said acoustic panel; 
           [0026]      FIG. 7  is a view in section on the line VII-VII of  FIG. 6 ; 
           [0027]      FIGS. 8 ,  9 ,  10  and  11   a  are similar views, in section on the line VIII-VIII of  FIG. 4 , of a first, a second, a third and a fourth examples of a device for attaching the acoustic panel to a structural framework of the nacelle; 
           [0028]      FIGS. 11   b,    12  and  13  represent three operating configurations of a fifth example of an attachment device similar to that of  FIG. 11   a;    
           [0029]      FIG. 14  is a view in section on the line XIV-XIV of  FIG. 5 ; 
           [0030]      FIGS. 15 and 16  are views in the direction of the arrow XV of  FIG. 14  which illustrate two examples of the cutout of an inspection hatch in the acoustic panel. 
       
    
    
       [0031]    Certain elements represented in  FIGS. 3 to 16  are similar to elements in  FIGS. 1 and 2  and are designated by the same reference numbers. The nacelle according to the invention differs essentially from that illustrated in  FIGS. 1 and 2  in that the acoustic panel  21  is attached to the structural framework  18  by floating or elastic attachment means. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    If pipe work of the engine  7  were to be burst for example (see  FIG. 3 ), the panel  21  sustains a uniform pressure  3  over the whole inside of its structure and, by virtue of its floating or elastic attachment means to the framework  18 , deforms in a substantially centrifugal radial direction  31  in the direction of an expansion of its internal volume in order to discharge the over pressurized air  30  in the engine compartment through the rear of the panel  21 , without damaging the structure of the nacelle  1  and the engine  7 . Therefore, the panel  21  does not transmit forces originating from the turbofan  7  to the fixed structure  2  of the aircraft. The upstream attachment of the panel  21  to the structure of the engine  7  remains sufficiently compact so as not to scoop the air  17  coming from the fan into the engine compartment and to amplify the harmful effect of the pressurization. 
         [0033]      FIGS. 4 to 7  show exemplary embodiments of the acoustic panel  21 , in which the panel  21  consists of a central shroud and upper and lower vertical panels. The acoustic panel  21  could also consist of only one shroud, or of a shroud associated with an upper or lower vertical panel. 
         [0034]    The lines of attachment  32  of the panel  21  of  FIG. 4  are situated opposite structural elements of the framework  18 . The number and location of the attachments are defined by those skilled in the art according to the dimensional and geometric requirements of the overall structure. 
         [0035]    In order to provide rapid and targeted maintenance on the engine  7 , a removable inspection hatch  33  forms an upstream portion of the panel  21 . The hatch  33  is situated in a zone of the engine  7  to be inspected. 
         [0036]    In  FIG. 5 , the acoustic panel  21  comprises an inspection hatch  34  which is situated downstream and which may serve as an overpressure hatch and for this purpose may comprise elastic attachments that are more flexible than for the rest of the acoustic panel  21  in order to promote the venting of air in this zone. 
         [0037]      FIG. 6  represents an acoustic panel  21  comprising downstream two longitudinal slots  35  designed to discharge the over pressurized air in the nacelle. The slots  35  comprise seam abutments at the end. These slots  35  are placed and oriented at the convenience of those skilled in the art, with no requirement for overlapping with the structural elements of the framework  18 . 
         [0038]      FIG. 7  shows an exemplary embodiment of a slot  35 . A seal  36  is associated with it. A heat-protection mat  37  designed to be interposed between the structural framework  18  and the acoustic panel  21 , is mounted on the acoustic panel  21 . The mat  37  has, in the vicinity of the discharge slot  35 , a tile-like overlap  38  designed to allow a local deformation of the panel  21  without affecting the heat protection. Therefore, it is sufficient to provide elastic attachments downstream of the slots  35  in order to allow the opening, by deformation in the direction  31 , of the region delimited by the two slots  35  of the panel  21 . 
         [0039]      FIG. 8  represents a device for the flexible attachment of the acoustic panel  21  to the framework  18 . In this example, the acoustic panel  21  requires no heat protection because it is made of titanium. 
         [0040]    This elastic attachment device comprises a nut  43  attached opposite a hole passing through the structural framework  18  and designed to receive and retain the shank of a screw  41  mounted in a local reinforcing bush  40  housed in a hole passing through the acoustic panel  21 . The reinforcing bush  40  is associated with an elastomer washer  42  interposed between the panel  21  and the structural framework  18 . 
         [0041]    The reinforcing bush  40  makes it possible to prevent the screw  41  from crushing the internal structure of the panel  21 . The bush  40  could also be incorporated into the very structure of the panel  21  by means of a “honeycomb” cell filler product forming the panel  21 . 
         [0042]      FIG. 9  represents a second example of the device for attaching the acoustic panel  21  to the framework  18 . As in  FIG. 8 , a nut  43  is attached facing a hole passing through the structural framework  18 , and designed to receive and retain the shank of a screw  54  mounted in a reinforcing bush  45  housed in a hole passing through the acoustic panel  21 . In this instance, however, the screw  54  is mounted with a slight clearance in the reinforcing bush  45  and has a nonconical head, so as to achieve a floating, nonelastic attachment of the acoustic panel  21  to the structural framework  18 . 
         [0043]    This type of attachment may be employed for an acoustic panel requiring no heat protection, or else requiring heat protection provided by an aluminum panel  37 . 
         [0044]    The bush  45  in this instance is not associated with an elastic element but comes into direct contact with the framework  18 . An annular recess is made in the heat-protection mat  37  around this bush  45 ; a tooth lockwasher  44  placed in this annular recess holds the mat  37  on the panel  21 . 
         [0045]    The elastic attachment device illustrated in  FIG. 10  differs from the previous one in that the reinforcing bush  46  has, at one end, one or more lugs for the retention of an elastic ring  47  interposed between the panel  21  and the structural framework  18 . The elastic ring  47  also has an annular shoulder designed to hold the mat  37  against the panel  21 . 
         [0046]      FIGS. 11   a,    11   b,    12  and  13  represent another elastic attachment device comprising a nut assembly, the base  50  of which is attached to the structural framework  18  facing a hole passing through the latter. The base  50  has a housing in which the nut itself  48   a  or  48   b  is mounted so as to be able to move in translation along the axis of the hole passing through the structural framework  18 . A return spring  49  is provided in this housing to return the nut  48   a  or  48   b  to a rest position (see  FIGS. 11   a  and  11   b ) at a distance from the structural framework  18 . 
         [0047]    As above, the nut  48   a  or  48   b  is designed to receive and retain the shank of a screw  41  mounted in a reinforcing bush  47  housed in a hole passing through the acoustic panel  21 . In this instance, the reinforcing bush  47  is incorporated into the acoustic panel  21 . Depending on the stiffness of the spring  49 , a hold that is more rigid (for example upstream of the panel  21 ) or less rigid (for example downstream of the panel  21 ) of the nut assembly may be obtained. 
         [0048]    In  FIG. 11   a,  a tightening abutment of the screw  41  is incorporated into the nut  48   a.  It is sufficient to tighten up to contact in order to ensure a good installation. To ensure the necessary differential tension, the spring  49  may have several stiffness levels, or the nut  53  may have different lengths. 
         [0049]    In  FIG. 11   b,  the nut  48   b  does not have a tightening abutment of the screw  41 . Mounting the screw  41  by acting on the tightening torque tends to give more or less tension to the spring  49  in the rest position and therefore allows the differential movement of the panel  21  under pressure. 
         [0050]    In the case of overpressure, the acoustic panel  21  sustaining the air pressure in the direction of the arrows  3  (see  FIGS. 12 and 13 ) moves away from the framework  18  and the nut  48   b,  pulled by the screw  41 , slides in its housing made in the base  50 , against the return action of the spring  49 . 
         [0051]      FIG. 14  represents a junction zone between the panel  21  and its inspection hatch  34 . Two peripheral seals  51  are mounted between the structural framework  18  and the heat-protection mat  37  on either side of this junction zone, in order to insulate it from the air flow  17  originating from the fan of the engine  7 . 
         [0052]    As indicated in  FIGS. 15 and 16 , the cutouts  52  or  53  of the inspection hatch  34  in the panel  21  may be rectilinear ( 52 ) with a functional clearance for mounting, or scalloped ( 53 ) to optimize the width of the framework  18 . 
         [0053]    Although the invention has been described with particular exemplary embodiments, it is evident that it is in no way limited thereto and that it comprises all the technical equivalents of the means described and their combinations if the latter enter the scope of the invention.