Abstract:
The jet engine of the invention comprises an exhaust nozzle of variable section, comprising at least one flap pivot mounted about a pin, the pin being joined to the fixed structure of the jet engine via two braces each comprising a bore for housing the pin, the flap being pivot commanded by an actuating cylinder, connected to two fasteners which each comprise a bore through which the pin passes and about which they rotate. The jet engine is characterized by the fact that the pin, the bores of the braces and bores of the fasteners are arranged so that the contacts between the pin with the bore walls of the braces and with the bore walls of the fasteners are made over convex surfaces. Therefore, when in operation, in the event of pin bending the contact surfaces follow this convex shape and are suitably distributed.

Description:
BACKGROUND OF THE INVENTION 
   The invention concerns a jet engine with a variable section nozzle. 
   A jet engine generally comprises, from upstream to downstream in the direction of the gas stream, a fan, one or more compressor stages, a combustion chamber, one or more turbine stages and an exhaust duct, in which a so-called reheat or afterburner device may be provided. 
   The exhaust duct comprises a casing and a nozzle, in which the combustion gases expand. It is usual to use a nozzle of variable section, optionally with an afterburner device, to regulate the flow rate of the exhaust gases in relation to jet engine speed. Variable section nozzles may be of different types, in particular axisymmetrical, two-dimensional or swivelling. The invention applies to all types of variable section nozzles. 
   Variable section nozzles comprise flaps. With axisymmetrical nozzles for example the flaps are arranged in a ring, to obtain a globally circular section, and they partly overlap. Every other flap, called controlled flap, is actuated by an actuating cylinder, whilst between two controlled flaps there is a servo or follower flap driven by the controlled flaps. 
   Each controlled flap is connected to the fixed structure of the jet engine via a pin, around which it is pivot mounted. The pin extends transverse to the axis of the jet engine, between two longitudinal connecting braces for connection to the fixed structure, upstream of the flap. It is housed, at each of its ends, in a bore arranged in the braces and locked in translation and rotation with respect to the latter. Two fasteners are also mounted on the pin at a bore in one of their ends, each fastener being arranged adjacent to a connecting brace. These fasteners are connected to the actuating cylinder of the controlled flap and are mounted so as to pivot about the pin with friction thereupon to drive the flap pivotally under the action of its actuating cylinder. 
   The pin is cylindrical and the bore walls of the fasteners and connecting braces in contact with it are also cylindrical. The bearing surfaces of the pin in contact with the fasteners and connecting braces are coated with a protective coating, generally in cobalt and chromium carbide. 
   On account of fastener displacements, induced aerodynamic loads, vibrations and varying operating temperatures, the pin undergoes bending and high stresses on its bearing surfaces causing its wear. The displacements to which the fasteners are subject are chiefly of two types: short displacements related to low variations in the nozzle section about stable positions resulting from low variations in pressure on the flaps, and extensive displacements related to changes in nozzle section when in operation. Wear of the pin bearing surfaces is the consequence of either one of these types of displacements, or a combination of both. 
   Therefore bending of the pin reduces the contact surfaces between the pin with the fasteners and braces, these surfaces therefore no longer being cylindrical. Contact occurs in particular along the sharp edges of the fasteners and generates high stresses. Combined with the increased forces on the pin due to the forces of the fasteners, these phenomena lead to major local overpressures which are ill distributed and cause stripping of the pin coating. As wear increases in time, the bearing contact surfaces of the pin are shifted in the direction of the pin centre. 
   This wear of the pin considerably reduces its lifetime. It is therefore generally necessary to change a pin, whose lifetime is theoretically 1000 hours, after 500 hours. In addition to maintenance and replacement costs, there is a non-negligible risk of nozzle seizing. One solution which could be considered entails strengthening the pin, for example by increasing its diameter; however, this would increase the weight of the jet engine. 
   The present invention sets out to overcome these drawbacks. 
   BRIEF SUMMARY OF THE INVENTION 
   For this purpose, the invention concerns a jet engine comprising a variable section exhaust nozzle comprising at least one flap pivot-mounted about a pin, the pin being connected to the fixed structure of the jet engine by two connecting braces, each comprising a bore for housing the pin, the flap comprising two fasteners actuated by an actuating cylinder, each comprising a bore through which the pin passes, on which they are rotatably mounted, characterized by the fact that the pin, the bores of the braces and the bores of the fasteners are arranged so that the contacts between the pin with the bore walls of the braces and with the bore walls of the fasteners are made over convex surfaces. 
   With the invention, the pin is able to bend whilst maintaining suitably distributed contact surfaces with the braces and fasteners, since at each contact the surfaces are able to follow the convex shape. 
   Preferably, the convex surfaces are spherical. 
   Advantageously in this case the radii of curvature of the different convex surfaces are all identical. 
   According to a first embodiment, the pin comprises a cylindrical body, at each end of which there are two convex bearing surfaces intended to be in contact with the bore wall of a brace and the bore wall of a fastener respectively. 
   According to a second embodiment, a bushing comprising an inner convex wall is fixed in each bore for contact with the pin which is of cylindrical shape. 
   The invention also concerns a nozzle flap pin comprising a cylindrical body, at each end of which it comprises two convex bearing surfaces. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood with the following description of the preferred embodiment of the invention with reference to the appended drawing in which: 
       FIG. 1  shows a partial, schematic, downstream view of the jet engine nozzle of the invention; 
       FIG. 2  is a schematic, cross-sectional, profile view of the jet-engine nozzle of the invention, 
       FIG. 3  is a partial, schematic, side view of the jet engine nozzle of the invention, 
       FIG. 4  is a partial, schematic, cross-sectional upstream view of a first embodiment of an exhaust flap of the jet engine of the invention, and 
       FIG. 5  is a partial, cross-sectional, downstream, schematic view of a second embodiment of an exhaust flap of the jet engine of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIGS. 1 ,  2  and  3 , the jet engine of the invention, from upstream to downstream in the direction of the gas stream, which globally follows a direction forming the axis of the jet engine, comprises a fan, one or more compressor stages, a combustion chamber, one or more turbine stages, not shown, an exhaust duct  1  in which a so-called reheat or afterburner device is provided, through which the fuel is injected into the exhaust duct to generate additional thrust. 
   The exhaust duct  1  here comprises an afterburner casing  2  downstream of which extends a nozzle  3 , in which the combustion gases derived from the combustion chamber expand. The nozzle  3  is of variable section; here it is a convergent axisymmetrical nozzle. It comprises a plurality of flaps  4 ,  5  arranged in a ring on the downstream side of the afterburner casing  2 . Each flap  4 ,  5  is pivot mounted about a pin  12 . Every other flap, called controlled flap  4 , is actuated by an actuating cylinder, whilst between two controlled flaps  4  there is a servo flap  5  or follower flap  5  driven by the controlled flaps  4 . 
   With reference to  FIG. 2 , flaps  22  are arranged around the nozzle  3 . These are flaps  22  of the jet engine secondary airstream, here connected to flaps  4  of the nozzle by connecting rods  23 . These flaps  22  do not influence the invention however and are not shown in  FIG. 2 . 
   A particular controlled flap  4  of nozzle  3  is described below, all the other controlled flaps  4  being identical therewith. 
   Flap  4  is in the shape of a rectangular plate for example. It is pivot mounted on its upstream side about a pin  12  which extends perpendicular to the jet engine axis and is fixedly connected to the afterburner casing  2  via connecting braces  16 ,  16 ′ as will be seen below. Flap  4  is controlled by an actuating cylinder  11  mounted on the outer wall of the afterburner casing  2 . A single actuating cylinder  11  is shown, solely in  FIG. 1 , but each controlled flap  4  has a dedicated actuating cylinder  11 . The actuating cylinder  11  and flap  4  are joined together via a linkage  7  here a cross-piece  7  with a globally ovoid-shaped body. 
   The linkage  7  comprises an upstream clevis located centrally and projecting from its ovoid body which comprises two lugs  9 ,  9 ′ extending longitudinally towards upstream and connected to a pin  10  made integral with the actuating cylinder  11  and housed in the mobile head of the actuating cylinder  11 . It also comprises, projecting radially, a radial clevis which comprises two fasteners  6 ,  6 ′ of globally triangular shape fixed to the controlled flap  4 . One side of the triangle formed by each fastener  6 ,  6 ′ extends longitudinally along flap  4  on which it is fixed, from its upstream edge; the two other sides extend radially between the ovoid body of the linkage  7  and the flap  4 . The upstream clevis and the radial clevis together form an angle drive between the actuating cylinder  11  and the flap  4 . Each side end of the ovoid body of linkage  7  is joined to the side end of the linkage  7  of the following controlled flap  4  by a connecting clevis  8 . 
   Flap  4  is articulated about pin  12  via the upstream tip of fasteners  6 ,  6 ′ through which pin  12  passes via bores  13 ,  13 ′ respectively provided for this purpose. A sleeve  14 , made integral with fasteners  6 ,  6 ′ surrounds pin  12  between the two bores  13 ,  13 ′ of fasteners  6 ,  6 ′ without contacting pin  12  outside bores  13 ,  13 ′. Also, two lugs  15 ,  15 ′ join the sleeve  14  to the pin  10  integrally joined to the actuating cylinder  11 . 
   The fasteners  6 ,  6 ′ are pivot mounted at their bores  13 ,  13 ′ about pin  12 . The actuating cylinder  11  can therefore actuate the controlled flap  4  and drive it pivotally about this pin  12  via fasteners  6 ,  6 ′ forming levers. The integral assembly formed by the linkage  7 , fasteners  6 ,  6 ′ and sleeve  14  form a lever. 
   In the upstream continuation of the controlled flap  4 , the afterburner casing  2 , on its outer wall, comprises two flanges, here longitudinal flanges  16 ,  16 ′ called braces  16 ,  16 ′. These braces  16 ,  16 ′ extend either side of the actuating cylinder  11  which is fixed to them, and their downstream end projects outside the afterburner casing  2  towards the flap  4 . The end of each brace  16 ,  16 ′ comprises a bore  17 ,  17 ′ respectively housing the ends of pin  12  about which fasteners  6 ,  6 ′ are pivot mounted. The braces  16 ,  16 ′ are used to fix the pin  12  with respect to the fixed structure of the jet engine. The pin  12  at one of its ends comprises a D-shaped head  18  which is locked in rotation in a housing  19  of the corresponding brace  16 , whilst the other end is locked in translation by a bolt  20  screwed against the other brace  16 ′. 
   By “bearing surfaces” is meant the surfaces of pin  12  intended to come into contact with the bore walls  13 ,  13 ′ of fasteners  6 ,  6 ′ and the bore walls  17 ,  17 ′ of braces  16 ,  16 ′. The invention lies in the arrangement of pin  12  and these various bores  13 ,  13 ′,  17 ,  17 ′ so that the contact between them is made over convex surfaces i.e. curved surfaces in the direction of pin  12 , in other words surfaces with revolution symmetry derived from curved generating lines. 
   According to a first embodiment shown  FIG. 4 , the pin  12  comprises a cylindrical body with, at each of its ends, two convex bearing surfaces ( 12   a ,  12   b ), ( 12   a ′,  12   b ′) following each other on pin  12 . It is to be noted that  FIG. 4  is not drawn to scale to facilitate comprehension. The bearing surfaces  12   a ,  12   a ′ located the nearest to the centre of pin  12  are intended to come into contact with the walls of bores  13 ,  13 ′ of fasteners  6 ,  6 ′; the bearing surfaces  12   b ,  12   b ′ the furthest away from the centre of pin  12 , namely those located in the vicinity of D-shaped head  18  and bolt  20  respectively are intended to come into contact with the walls of bores  17 ,  17 ′ of braces  16 ,  16 ′. Bores  13 ,  13 ′ of fasteners  6 ,  6 ′ and bores  17 ,  17 ′ of braces  16 ,  16 ′ are cylindrical. 
   Therefore the contact between bearing surfaces  12   a ,  12   a ′,  12   b ,  12   b ′ of pin  12  with the bore walls  13 ,  13 ′ of fasteners  6 ,  6 ′ and the bore walls  17 ,  17 ′ of braces  16 ,  16 ′ is made over convex surfaces. The convex surfaces are preferably spherical, i.e. each derived from a generatrix in the arc of a circle. Here pin  12  has a diameter of 15 mm, outside its convex bearing surfaces, and the radii of curvature of bearing surfaces ( 12   a , 12   b ), ( 12   a ′, 12   b ′) are all identical and equal to approximately 300 mm. 
   Here there is tangential continuity between the two convex bearing surfaces ( 12   a , 12   b ), ( 12   a ′, 12   b ′) following each other at each end of pin  12 ; the shape of the pin between the convex bearing surfaces is cylindrical with rounded connecting portions having a reverse radius of curvature with respect to that of the convex portions ( 12   a , 12   b ), ( 12   a ′, 12   b ′). 
   According to the second embodiment illustrated very schematically  FIG. 5 , the pin  12  is of cylindrical shape and bushings ( 21   a ,  21   a ′), ( 21   b ,  21   b ′) with inner convex surface are mounted in the bores  13 ,  13 ′ of fasteners  6 ,  6 ′ and bores  17 ,  17 ′ of braces  16 ,  16 ′ respectively. Here, bores  13 ,  13 ′ of fasteners  6 ,  6 ′ are cylindrical in the alignment of the inner wall of sleeve  14  joining fasteners  6 ,  6 ′ together, and therefore forming with this wall a continuous channel of constant diameter, bushings  21   a ,  21   a ′ being fixed to the walls of bores  13 ,  13 ′ and ensuring contact with pin  12  via their inner convex surface. Similarly, bores  17 ,  17 ′ of braces  16 ,  16 ′ are cylindrical and bushings  21   b ,  21   b ′ are fixed to their wall to ensure contact with pin  12  via their inner convex surface. 
   The inner convex surfaces of bushings  21   a ,  21   b ,  21   a ′,  21   b ′ are preferably spherical i.e. each derived from a generating line in an arc of a circle and their radii of curvature here are all identical and equal to approximately 300 mm, the diameter of pin  12  being 15 mm. It is to be noted that  FIG. 5  is not drawn to scale to facilitate comprehension. 
   The functioning of the jet engine of the invention will now be described in more detail. It is comparable for the two above-described embodiments. 
   A change in section of nozzle  3  is obtained by pivoting the controlled flaps  4  about their pin  12 , the controlled flaps  4 , through their pivoting, causing the pivoting of the servo flaps  5 . Each servo flap  5  also pivots about a pin, not shown, but their structure is different to that of the controlled flaps. The invention could however also be applied to the pins of the servo flaps and to their receiving bores. The pivoting of a controlled flap  4  is obtained by its actuating cylinder  11  which drives linkage  7  via its integrally joined lugs  9 ,  9 ′, to cause pivoting of fasteners  6 ,  6 ′ and hence of the controlled flap  4  about pin  12 . 
   The aerodynamic loads transmitted by fasteners  6 ,  6 ′ to pin  12  may cause its bending. With the invention, this bending does take place but it is accompanied by contact on convex surfaces between the bearing surfaces of pin  12  with the bore walls of fasteners  6 ,  6 ′ and braces  16 ,  16 ′, this contact here being ensured either directly by the convex bearing surfaces of pin  12  or via bushings with inner convex wall fixed in the bores. In other words, the contact follows the generating line of the convex contact surface. It is to be noted that the bore walls of fasteners  6 ,  6 ′ and braces  16 ,  16 ′ comprise the walls of bushings  21   a ,  21   b ,  21   a ′,  21   b′.    
   Therefore pin  12 , when it bends, follows these convex contact surfaces. The distribution of the contact surfaces therefore remains globally constant irrespective of the bending state of pin  12 . The Applicant has found that it is not necessary to make provision for different radii of curvature between the convex contact surfaces located the nearest to the centre of pin  12  and the convex contact surfaces located the furthest distant from the pin centre, in particular due to the size of these radii with respect to the diameter of pin  12 . However, this evidently could be the case, for example to further fine-tune the distribution of stresses. 
   Through the invention it has therefore been possible to eliminate the additional stresses related to changes in the contact surfaces and in particular to the onset of contact surfaces on the sharp edges of the bore walls, which set up edge effect stresses, as was the case with prior art flaps. The stresses undergone by pin  12  are therefore continuous in time and only result from aerodynamic stresses and no longer from changes in the distribution of the contact surfaces. 
   With the invention the lifetime of the pins  12  is considerably increased and the risk of nozzle seizing is reduced. Maintenance and replacement costs are lowered as well as manufacturing costs since it is no longer necessary to provide a coating on the bearing surfaces of pin  12 . 
   The pin  12  here is in a nickel-based alloy, for example of the registered Waspalloy or Inco 718 trade mark, but evidently another material could be chosen. Also, in the invention the convex contact surfaces are either convex bearing surfaces directly formed on pin  12  or the inner surfaces of bushings fixed in the bores of fasteners  6 ,  6 ′ and braces  16 ,  16 ′, but evidently other configurations are possible; it could be proposed for example that the bore walls of fasteners  6 ,  6 ′ and braces  16 ,  16 ′ be machined so as to be convex and to cooperate with the cylindrical bearing surfaces of the pin, or convex surfaces could be proposed both on pin  12  and on the bore walls or the inner walls of the bushings fixed therein, contact then being made between two convex surfaces, or further various combinations of these configurations. A varnish could also be added to the convex contact surfaces or the surfaces on which they are in contact in order to reinforce their resistance.