Abstract:
The present invention relates to a thrust reverser for the nacelle of a turbojet engine comprising, on the one hand, means ( 11 ) for deflecting at least some of an air flow of the turbojet engine and, on the other hand, at least one hood ( 10 ) able to move translationally in a direction substantially parallel to a longitudinal axis of the nacelle and able to switch alternately from a closed position in which it ensures the aerodynamic continuity of the nacelle and covers the deflection means, and an open position in which it opens a passage in the nacelle and uncovers the deflection means, characterized in that the moving hood comprises at least one outer part ( 10   a ) having a downstream extension forming a nozzle and at least one internal part ( 10   b ) each of which parts is mounted such that it is translationally mobile and is connected to at least one actuating means able to allow it to be moved, each independently of the other, or together, in a substantially longitudinal direction of the nacelle. The present invention also relates to a turbojet engine nacelle comprising such a thrust reverser.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to a thrust reverser for a turbine engine nacelle comprising, on the one hand, means for deflecting at least a fraction of an air stream of the turbine engine and, on the other hand, at least one cowl which can move translationally in a substantially longitudinal direction of the nacelle and which is able to switch alternately from a closed position in which it provides aerodynamic continuity of the nacelle and covers the deflection means, to an open position in which it opens a passage in the nacelle and uncovers the deflection means. The present invention also relates to a turbine engine nacelle comprising such a thrust reverser. 
       BRIEF DESCRIPTION OF RELATED ART 
       [0002]    An aircraft is propelled by a number of turbine engines each housed in a nacelle which also accommodates a collection of auxiliary actuating devices associated with the operation thereof and performing various functions when the turbine engine is operating or not running. These auxiliary actuating devices comprise in particular a mechanical system for actuating thrust reversers. 
         [0003]    A nacelle generally has a tubular structure comprising an air intake upstream of the turbine engine, a central section intended to surround a fan of the turbine engine, a downstream section accommodating thrust reverser means and intended to surround the combustion chamber of the turbine engine, this section generally ending in an exhaust nozzle whose outlet is situated downstream of the turbine engine. 
         [0004]    Modern nacelles are intended to accommodate a bypass turbine engine which is able, by means of the blades of the rotating fan, to generate a hot air stream (also known as the primary stream) coming from the combustion chamber of the turbine engine, and a cold air stream (secondary stream) which flows around the outside of the turbine engine through an annular passage, also known as a duct, formed between a shroud of the turbine engine and an internal wall of the nacelle. The two air streams are ejected from the turbine engine through the rear end of the nacelle. 
         [0005]    The purpose of a thrust reverser is, when an aircraft is landing, to improve the ability of said aircraft to brake by redirecting forward at least some of the thrust generated by the turbine engine. During this phase, the reverser closes off the cold stream duct and directs this cold stream toward the front of the nacelle, thereby generating a reverse thrust which combines with the braking of the aircraft wheels. 
         [0006]    The means employed to achieve this reorientation of the cold stream vary according to the type of reverser. However, in all cases, the structure of a reverser comprises movable cowls which can move between, on the one hand, a deployed position in which they open, within the nacelle, a passage intended for the deflected stream and, on the other hand, a retracted position in which they close this passage. These cowls may perform a deflecting function or may simply activate other deflecting means. 
         [0007]    In the case of a cascade-type reverser, the air stream is reoriented by cascades of deflection vanes, the cowl merely having a simple sliding function with the aim of uncovering or re-covering these cascades, the translational movement of the movable cowl taking place along a longitudinal axis substantially parallel to the axis of the nacelle. Additional blocking doors, activated by the sliding movement of the cowling, are generally able to close off the duct downstream of the cascades so as to optimize the reorientation of the cold stream. 
         [0008]    It is possible to avoid having to fit blocking doors by adapting the shape of the duct such that it has an S shape, that is to say that the engine shroud has a bulge which matches the inner wall of the nacelle formed by the cowling at this point. The height of the bulge is calculated so that the reverser cowling by itself closes off the duct as it slides into the reverser open position. In this case, the cascade-type reverser is known as a natural blockage cascade reverser, the sliding cowling naturally blocking off the cold stream duct by virtue of its shape and the shape of said duct. 
         [0009]    A reverser of such type is described in documents FR 2 132 380 and U.S. Pat. No. 4,232,516, for example. 
         [0010]    Apart from its thrust reversal function, the sliding cowl belongs to the rear section and has a downstream side forming an exhaust nozzle designed to channel the exhaust of the air streams. This nozzle may complement a primary nozzle which channels the hot stream and is then known as a secondary nozzle. 
         [0011]    The thrust reverser performance is obtained in a satisfactory manner with the known devices. However, there remains a problem of adapting the powerplant to the various flight phases which it encounters, in particular the aircraft takeoff and landing phases for which the optimum cross sections of the secondary exhaust nozzle that have been defined for cruise flight conditions are no longer suited. 
         [0012]    This problem has been solved for a cascade-type reverser in document FR 2 622 929, but it still remains for a natural blockage cascade reverser having an S-shaped secondary duct. 
         [0013]    Document FR 2 622 929 solves this problem by proposing a cascade-type thrust reverser having a variable exhaust cross section and to this end provides a movable cowl comprising two portions which are able to be interconnected by locking means. More precisely, a movable reverser cowl according to FR 2 622 929 comprises a downstream portion which is able to be moved alone or with an upstream portion to which it can be optionally locked so as to allow, in a first instance, a movement of the entire movable cowl during a deployment of the reverser and, in a second instance, a movement of the downstream portion alone, thus modifying the nozzle outlet cross section. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    The invention proposes a configuration adapted for a nacelle comprising a natural blockage cascade reverser installed more particularly, but without limiting it thereto, around a turbofan engine having a high bypass ratio. 
         [0015]    The invention further proposes an alternative to the solution implemented in document FR 2 622 929. 
         [0016]    The invention comprises a thrust reverser for a turbine engine nacelle comprising, on the one hand, means for deflecting at least a fraction of an air stream of the turbine engine and, on the other hand, at least one cowl which can move translationally in a direction substantially parallel to a longitudinal axis of the nacelle and which is able to switch alternately from a closed position in which it provides aerodynamic continuity of the nacelle and covers the deflection means, to an open position in which it opens a passage in the nacelle and uncovers the deflection means, characterized in that the movable cowl comprises at least one external portion having a nozzle-forming downstream extension and at least one internal portion, each of these portions being mounted such that they can move translationally and being connected to at least one actuating means enabling them to move, independently of one another or together, in a substantially longitudinal direction of the nacelle. 
         [0017]    Thus, by dividing the movable cowl into an internal portion and an external portion which can move independently of one another at least in part, it is possible to adapt the relative positions of the external portion and the internal portion so as to vary the cross section of the nozzle formed by the movable cowl by varying the length of the internal aerodynamic line of said movable cowl, both when the movable cowl is in a closed position and covers the deflection means and when the movable cowl is in an open position. In this way, it is easy to adapt the cross section of the exhaust nozzle formed by the movable cowl to the flight conditions so as to maintain an optimum configuration. 
         [0018]    Preferably, the external portion can be equally caused to make an advancing movement in the upstream direction of the nacelle or a retreating movement in the downstream direction of the nacelle with respect to the internal portion. 
         [0019]    Advantageously, the external portion and the internal portion are separated at the location of a recess of an internal aerodynamic line of the movable cowl. This makes it possible to minimize the impact of the aerodynamic discontinuity that is represented by the interruption between the external portion and the internal portion. 
         [0020]    Advantageously still, the internal aerodynamic line recess is intended, when the movable cowl is in the closed position, to be situated facing a bulge of a casing of the turbine engine that, together with the internal aerodynamic line of the movable cowl, defines an inner duct. 
         [0021]    According to a first embodiment, the movable cowl is equipped, on the one hand, with a means for actuating one of the external or internal portions and, on the other hand, with locking means which are able to switch alternately from a locking position in which the external portion is connected to the internal portion, to an unlocking position in which the external portion or the internal portion connected to the actuating means is able to move independently of the other portion. 
         [0022]    Advantageously, the actuating means is connected to the external portion. 
         [0023]    According to a second embodiment, the movable cowl is equipped with an actuating means for the external portion and with an actuating means specific to the internal portion which are able to be activated independently of one another so as to allow, on the one hand, a simultaneous movement of the external portion and the internal portion and, on the other hand, a relative movement between the external portion and the internal portion. 
         [0024]    Preferably, the actuating means comprise rams of the pneumatic, electric and/or hydraulic ram type. 
         [0025]    Preferably, the actuating means comprise a telescopic ram having a first rod which is able to allow the movement of the internal portion and a second rod which is able to allow the movement of the external portion, the two rods being able to be controlled synchronously or independently of one another. 
         [0026]    Alternatively or additionally, the actuating means comprise a screw/nut actuating system which can be actuated pneumatically, electrically and/or hydraulically. 
         [0027]    Advantageously, the external and internal portions are equipped with guide means which are able to cooperate with complementary guide means connected to a fixed portion of the nacelle. 
         [0028]    Preferably, the guide means are rails which are able to cooperate with corresponding grooves. 
         [0029]    According to a first variant embodiment, the rails of the external portion and of the internal portion are separate. 
         [0030]    According to a second variant embodiment, the rail of the external portion is integrated into the rail of the internal portion. 
         [0031]    The present invention also relates to a turbine engine nacelle, characterized in that it comprises at least one thrust reverser according to the invention. 
         [0032]    Advantageously, it is a nacelle for a bypass turbine engine, preferably with a high bypass ratio. 
         [0033]    Preferably, the thrust reverser is a natural blockage thrust reverser. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0034]    The implementation of the invention will be better understood from the detailed description which is explained below with reference to the appended drawing, in which: 
           [0035]      FIG. 1  is a schematic representation in longitudinal section of a nacelle of a bypass turbine engine with a high bypass ratio according to the prior art, equipped with a natural blockage cascade-type thrust reverser. 
           [0036]      FIG. 2  is a detailed representation of a thrust reverser according to the invention. 
           [0037]      FIG. 3  is a representation of a first variant arrangement of the actuating means shown in  FIG. 2 . 
           [0038]      FIG. 4  is a representation of a second variant arrangement of the actuating means shown in  FIG. 2 . 
           [0039]      FIG. 5  is a representation of a third variant arrangement of the actuating means shown in  FIG. 2 . 
           [0040]      FIG. 6  is a representation of a fourth variant arrangement of the actuating means shown in  FIG. 2 . 
           [0041]      FIG. 7  is a schematic representation of a first embodiment of the actuating means for the movable cowl of the reverser shown in  FIG. 2 . 
           [0042]      FIG. 8  is a schematic representation of the reverser shown in  FIG. 3  in a closed position forming an exhaust nozzle of minimum cross section. 
           [0043]      FIG. 9  is a schematic representation of the reverser shown in  FIG. 3  in a closed position forming an exhaust nozzle of maximum cross section. 
           [0044]      FIG. 10  is a schematic representation of the reverser shown in  FIG. 3  in an open position forming an exhaust nozzle of maximum cross section. 
           [0045]      FIG. 11  is a schematic representation of the reverser shown in  FIG. 3  in an open position forming an exhaust nozzle of minimum cross section. 
           [0046]      FIG. 12  is a representation of a second embodiment of the actuating means for the reverser shown in  FIG. 2 . 
           [0047]      FIG. 13  is a representation of a third embodiment of the actuating means for the reverser shown in  FIG. 2 . 
           [0048]      FIG. 14  is a schematic representation of the embodiment shown in  FIG. 13  in a closed position and forming an exhaust nozzle having a minimum cross section. 
           [0049]      FIG. 15  is a schematic representation of the embodiment shown in  FIG. 13  in an open position and forming an exhaust nozzle having a maximum cross section. 
       
    
    
       [0050]      FIG. 1  represents a nacelle  1  for a bypass turbine engine with a high bypass ratio according to the prior art. 
       DETAILED DESCRIPTION OF THE INVENTION  
       [0051]    The nacelle  1  is intended to form a tubular housing for a bypass turbine engine (not shown) with a high bypass ratio and serves to channel the air streams that it generates via the blades of a fan (not shown), namely a hot air stream passing through a combustion chamber (not shown) of the turbine engine, and a cold air stream flowing around the outside of the turbine engine. 
         [0052]    The nacelle  1  has a structure comprising a forward section which forms an air intake  4 , a central section  5  surrounding the fan of the turbine engine, and a rear section surrounding the turbine engine and comprising a thrust reversal system. 
         [0053]    The air intake  4  has an internal surface  4   a  intended to channel the incoming air and an external shroud surface  4   b.    
         [0054]    The central section  5  comprises, on the one hand, an internal casing  5   a  surrounding the fan of the turbine engine and, on the other hand, an external structure  5   b  shrouding the casing and extending the external surface  4   b  of the air intake section  5 . The casing  5   a  is attached to the air intake section  4  that it supports and extends the internal surface  4   a  thereof. 
         [0055]    The rear section comprises an external structure comprising a thrust reversal system and an internal engine-shrouding structure  8  which defines, together with the external surface, a duct  9  through which a cold stream is intended to flow in the case of a nacelle  1  for a bypass turbine engine like the one represented here. 
         [0056]    Each thrust reversal system comprises a cowl  10  which can move translationally along a substantially longitudinal axis of the nacelle and which is able to switch alternately from a closed position in which it shields deflection cascades  11  and provides structural continuity of the central section  5 , thus allowing the cold stream to be discharged through the duct  9  as a direct jet  3   a,  to an open position in which it uncovers the deflection cascades  11 , thus opening a passage in the nacelle  1 , and blocks off the duct  9  downstream of the deflection cascades  11 , thus allowing the cold stream to be reoriented into a reverse jet  3   b.    
         [0057]    More specifically, the cascade-type reversal system depicted here is a natural blockage cascade reversal system. This means that the movable cowl  10  naturally blocks off the duct  9  in the open position without requiring the presence of any additional blocking doors. 
         [0058]    To this end, the internal structure  8  of the rear section has, downstream of the deflection cascades  11 , a bulge  12  which is large enough that it substantially reaches the level of the casing  5   a  of the nacelle  1 . Thus, the inside diameter of the nacelle  1  at the outlet of the casing  5   a  of the central section  5  is substantially equal to the diameter of the internal structure  8  in the region of the bulge  12 . 
         [0059]    The movable cowl  10  has, on the one hand, an external surface  13  which is able to provide the external structural continuity of the nacelle  1  with the external structure  5   b  of the shroud of the casing  5   a  and, on the other hand, an internal surface  14  which is able to provide the internal structural continuity of the nacelle  1  with the casing  5   a,  the internal surface  14  substantially following the curvature of the internal structure  8  such that the duct  9  maintains a substantially constant cross section and therefore has a recess corresponding to the bulge  12  that is situated substantially facing said bulge when the movable cowl  10  is in the closed position. Furthermore, the internal surface  14  and the external surface  13  meet downstream of the movable cowl  10  to form an exhaust nozzle capable of ejecting the cold stream at a desired angle. 
         [0060]    Thus, in the open position, the movable cowl  10  completely blocks off the duct  9 , the bulge  12  bringing the internal structure  8  virtually into contact with an upstream portion of said movable cowl  10 , give or take the functional operating clearance. 
         [0061]    According to the invention, as represented in  FIG. 2 , the movable cowl  10  comprises an external portion  10   a  and an internal portion  10   b  which are independent of one another and which can each be moved parallel to a substantially longitudinal axis A of the nacelle. 
         [0062]    The external portion  10   a  and the internal portion  10   b  are separated in the region of the recess of the internal surface  14  such that the internal surface  14  is formed, upstream of the recess, by a wall of the internal portion  10   b  of the movable cowl  10  and, downstream of the recess, by an internal wall of the external portion  10   a,  the external surface  13  being formed by an external wall of the external portion  10   a.  In this way, the internal surface  14  has a minimum aerodynamic discontinuity due to the interruption between the external portion  10   a  and the internal portion  10   b.    
         [0063]    In order to provide aerodynamic continuity of the internal surface  14  when the external portion  10   a  is moved away from the internal portion  10   b,  the internal wall of the external portion  10   a  has an extension  15  toward the inside of the movable cowl  10 , the length of this extension depending on the degree of the maximum relative movement desired between the external portion  10   a  and the internal portion  10   b.    
         [0064]    In the same way, arrangements are provided to ensure the external aerodynamic continuity of the nacelle when the external portion  10   a  is moved. To this end, the central section  5  has, in the region of its interface with the movable cowl  10  of the thrust reverser, a slot  16  intended to receive a longitudinal wall  17  which extends the external wall of the external portion  10   a  of the movable cowl  10  over a distance which is slightly greater than the maximum distance of relative movement of the external portion  10   a  with respect to the internal portion  10   b.  The length of the longitudinal extension wall  17  and also the depth of the slot  16  are dependent on the degree of maximum separating and approach movements between the external portion  10   a  and the central structure  5 . 
         [0065]      FIGS. 3 to 6  show various configurations of the guiding of the external  10   a  and internal  10   b  portions. To this end, each of the external  10   a  and internal  10   b  portions is equipped with at least one lateral guide rail  18 ,  19  able to slide inside a corresponding groove  20 ,  21  formed in a structure  22 , preferably a common structure, connected directly or indirectly to a fixed structure  23  of the reverser or of the nacelle  1 , such as the central structure  5 , by means of a bearing structure  24 . 
         [0066]    Advantageously, attempts will be made to minimize the overall size of the guide means. 
         [0067]    A preferred arrangement of the guide means is to obtain a substantially balanced positioning between, on the one hand, the axis of the guide rail  18  of the external portion  10   a  and the top of the external wall of said external portion  10   a  and, on the other hand, between the axis of the guide rail  19  of the internal portion  10   b  and the most remote point of the internal portion  10   b.    
         [0068]    This makes it possible to minimize the dimensions of an aerodynamic appendage  25  required to shroud the downstream external end of the nacelle  1  around the guide rail  18  of the external portion  10   a  and to minimize the dimensions of an aerodynamic appendage  26  required to shroud the duct  9  around the guide rail  19  of the internal portion  10   b.    
         [0069]    Advantageously still, the shape and the arrangement of the guide rails  18 ,  19  must be chosen such that the spacing between said guide rails  18 ,  19  is as small as possible in order to reduce the dimensions of the aerodynamic appendages  25 ,  26  to a minimum. 
         [0070]    According to  FIGS. 3 to 5 , the grooves  20 ,  21  and guide rails  18 ,  19  of each external  10   a  and internal  10   b  portion of the movable cowl  10  can be superposed ( FIG. 3 ), slightly offset ( FIG. 4 ), or else aligned ( FIG. 5 ), the spacing between the guide rails  18 ,  19  being minimal in the latter configuration and greatest in the first configuration. 
         [0071]    Another possible configuration ( FIG. 6 ) can consist in arranging the guide rail  18  inside the guide rail  19 , which then serves as a groove therefor. In such a configuration, the guide rail  18  of the external portion  10   a  makes a smaller movement than its groove formed by the guide rail  19  of the internal structure. 
         [0072]    According to a first embodiment represented in  FIGS. 6 to 10 , each of the external  10   a  and internal  10   b  portions is connected to a ram  28 ,  29  of pneumatic, hydraulic or electric type, preferably electric type, able to allow a longitudinal movement of the corresponding external  10   a  or internal  10   b  portion. 
         [0073]      FIG. 7  shows the relative positions of the external portion  10   a  and the internal portion  10   b  of the movable cowl  10  when the latter is in a closed position in which it covers the deflection cascades  11  and has a conventional exhaust nozzle cross section. 
         [0074]    The cross section of the nozzle can be easily modified by moving the external portion  10   a  and the internal portion  10   b  independently by means of their respective rams  28 ,  29 . 
         [0075]      FIG. 8  represents a thrust reverser in a closed position forming an exhaust nozzle of reduced cross section, the ram  28  of the external portion  10   a  being retracted to a maximum. 
         [0076]      FIG. 9  represents a thrust reverser in a closed position forming an exhaust nozzle with an enlarged cross section, the ram  28  of the external portion  10   a  being deployed to move the external portion  10   a  relative to the internal portion  10   b  without interrupting the internal aerodynamic line by virtue of the extension  15  of the internal wall of the external portion  10   a  providing aerodynamic continuity with the internal portion  10   b.    
         [0077]      FIG. 10  represents a thrust reverser in an open thrust reversal position forming an exhaust nozzle of enlarged cross section, the external  10   a  and internal  10   b  portions being moved simultaneously from the position represented in  FIG. 8 . 
         [0078]      FIG. 11  represents a thrust reverser in an open thrust reversal position forming an exhaust nozzle of conventional cross section. 
         [0079]    The movable cowl  10  opens from the position represented in  FIG. 8 . In this position, only the ram  29  of the internal portion  10   b  is powered and moves the internal portion  10   b  in order to bring it into a position relative to the external portion  10   a  identical to that represented in  FIG. 6  or identical to that represented in  FIG. 7 , a so-called compensating position. Once the compensating position has been reached, the rams  28 ,  29  of the external  10   a  and internal  10   b  portions are actuated simultaneously until the desired retreated reversal position is obtained. Such an opening method makes it possible to reduce the length of rectilinear movement of the ram  28  and to therefore reduce the length of the drive rail, thereby consequently making it possible to reduce the length of the aerodynamic shrouding appendage  25  protruding from the nacelle  1 . 
         [0080]    The movable cowl  10  closes in the same way in reverse. The important thing is to ensure that the opening cross section obtained between the external portion  10   a  and the central section  5  of the nacelle  1  or a fixed structure of the reverser is outside or equal to the opening cross section existing between the internal portion  10   b  and the central section  5  of the nacelle  1  or a fixed structure of the reverser. 
         [0081]    According to a second embodiment represented in  FIG. 12 , the actuating means comprise a telescopic ram  30  having a first rod  30   a  connected to the external portion  10   a  and a second rod  30   b  connected to the internal portion  10   b.  As above, this telescopic ram  30  can be hydraulic, pneumatic or electric, preferably electric. 
         [0082]    The assembly is supplemented by means  31  (means not shown) for locking the external  10   a  and internal  10   b  portions. 
         [0083]    In the case of a hydraulic ram, the operations of reducing and increasing the cross section of the exhaust nozzle are carried out by means of a hydraulic pressure acting on the cross sections of the rods  30   a,    30   b.  First of all, the first rod  30   a,  connected to the external portion  10   a,  is the one which is actuated. At the end of the retreating movement of the first rod  30   a,  the latter butts against the second rod  30   b  which in turn drives along the internal portion  10   b  of the movable cowl  10  after unlocking the means  31  for locking said internal portion  10   b.  The internal portion  10   b  may be attached to the second rod  30   b  by way of oblong eyes  32  arranged on either side of the second rod  30   b,  so as to reduce the overhang of the attachment point and avoid any hyperstaticity stress in the alignment of the internal portion  10   b  and the drive points of the external  10   a  and internal  10   b  portions. 
         [0084]    The operation of the external  10   a  and internal  10   b  portions by the telescopic ram  30  allows a successive opening of the two portions, or a simultaneous combined opening, or the opening of the external portion  10   a  over at least part of its stroke. 
         [0085]      FIGS. 13 to 15  show a drive system for the external  10   a  and internal  10   b  portions that comprises a mechanical ball screw or roller screw system  35  connected to the external portion  10   a  and a fixed nut  36  connected to a fixed structure of the reverser or to the central structure  5  of the nacelle  1 . The external portion  10   a  is driven either by a fixed screw on the external portion  10   a  or by a fixed nut on the external portion  10   a.  The drive power can be hydraulic, pneumatic or electric. More precisely, a sleeve  37  connected to a fixed structure of the reverser or to the central section  5  of the nacelle  1  supports the drive nut  36  used to drive the fixed screw  35  connected to the external portion  10   a.    
         [0086]    At least one lock  37  keeps the internal portion  10   b  at a fastening point  38  when the movable cowl  10  is in the closed position. When the lock  37  is thus closed, an element for locking the internal portion  10   b  with the external portion  10   a  is kept in an open position. Thus, the external portion  10   a  can slide independently of the internal portion  10   b  until an additional means for locking the external portion  10   a  engages with the internal portion  10   b.    
         [0087]    In this instance, the locking element is a rocker arm  39  articulated at a point  40  of the internal portion  10   b  and able to cooperate with a hook  41  which terminates the extension  15  of the internal wall of the external portion  10   a.    
         [0088]    As above, the cross section of the exhaust nozzle can be easily reduced by retracting the drive screw  35 . 
         [0089]    The cross section of the exhaust nozzle is increased in the same way by deploying the drive screw  35  until the hook  41  butts against the internal portion  10   b.    
         [0090]    The thrust reverser opens starting from the preceding position. The lock  37  is disengaged in order to release the internal portion  10   b.  In so doing, it returns, by way of a spring  42 , the rocker arm  39  to the locking position behind the hook  41 . The drive screw  35  is then deployed, driving both the external portion  10   a  and the internal portion  10   b  by way of the hook  41 . 
         [0091]    The movable cowl ( 10 ) is returned from its open position to its closed position in the same manner in reverse. The drive screw  35  is retracted and drives the external portion  10   a.  Since the hook  41  is blocked by the rocker arm  39 , the movement of the external portion  10   a  also causes the movement of the internal portion  10   b  until the fastening point  38  engages with the lock  37 . With the locking of the internal portion  10   b,  the rocker arm  39  returns to its position in which it releases the hook  41 , and the external portion  10   a  on its own continues its movement into the selected position for obtaining the desired exhaust nozzle cross section in direct jet mode. 
         [0092]    Although the invention has been described with specific exemplary embodiments, it is quite obvious that it is in no way limited thereto and that it covers all technical equivalents of the means described and combinations thereof where they come within the scope of the invention. In particular, it is possible to combine the various drive means described or to use other drive and locking means known to a person skilled in the art.