Abstract:
A system of contra-rotating propellers for an aircraft jet engine, including: a free power turbine; a first and second propeller, contra-rotating, the first propeller being placed in a predetermined direction relative to the second propeller, each including a hub, an outer ferrule, and coupling arms linking the two; and a casing inserted between the free turbine and the propellers, the propellers arranged in the predetermined direction relative to the casing and the free turbine arranged in the opposite direction relative to the casing, the casing including a casing extension in the given direction, rotatably supporting the hub of the propeller. The coupling arms extend in the opposite direction, radially towards the outside.

Description:
TECHNICAL FIELD 
     The present invention generally relates to a system of contra-rotating propellers, for an aircraft turbomachine. 
     The invention also relates to a turbomachine for an aircraft comprising such a contra-rotating propeller system. 
     The invention preferably applies to aircraft turbomachines, for example of the turbojet engine or turboprop type. It more particularly applies to so-called “open rotor” turbomachines, within which a power free turbine drives the contra-rotating propellers, directly or indirectly via mechanical transmission device forming a reducer and in particular comprising an epicyclic gearing. In these contra-rotating propeller systems, the propellers therefore do not have a fairing at their outer radial ends. 
     BACKGROUND OF THE INVENTION 
     In the prior art, turbomachines are known with contra-rotating propeller systems, the propellers of which are driven by a mechanical transmission device, typically assuming the form of a differential reducer. This differential reducer has a particular epicyclical gearing, the sun gear of which is made to rotate by a rotor of a power free turbine, whereof the planet carrier drives the first propeller, and the crown of which drives the second propeller. In this respect, it is noted that as a function of the position of the contra-rotating propellers relative to the power free turbine driving them, the first propeller constitutes the downstream propeller and the second propeller, the upstream propeller, or vice versa. Whatever the case may be, unlike a simple epicyclical gearing, the crown is not stationary, but mobile. 
     Usually, each of the first and second propellers comprises a hub centered on the longitudinal axis, an outer ferrule being arranged concentrically thereto and participating in outwardly radially delimiting a main annular tunnel of the turbomachine, as well as coupling arms connecting the outer ferrule to the hub. 
     Moreover, a casing is provided, inserted between the power free turbine and the first and second propellers. This casing has a casing extension towards the closest propeller, this extension rotatably supporting the hub of said propeller. 
     This configuration means that a significant part of the aforementioned propeller, or even the entirety thereof, is axially offset from the casing extension ensuring its rotatable support. This results in a cantilever that is delicate to manage from a mechanical perspective, and generally requiring a substantial elongation of the casing extension, in particular with the aim of moving the rolling bearings apart so as to procure an acceptable rotational guiding. 
     This constraint amounts to an axial elongation of the propeller system, which is costly both in terms of overall mass and bulk. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention therefore aims to at least partially resolve the aforementioned drawbacks, relative to the embodiments of the prior art. 
     To that end, the invention first relates to a system of contra-rotating propellers for an aircraft jet engine, comprising: 
     a free power turbine; 
     a first propeller and a second propeller, contra-rotating, intended to be rotated around a longitudinal axis of the system of propellers, the former being placed in a predetermined direction relative to the latter, each one of the first and second propellers comprising a hub centered on the longitudinal axis, an outer ferrule being arranged concentrically thereto and participating in the outward radial definition of the main annular tunnel, as well as coupling arms linking the outer ferrule to said hub; 
     a mechanical transmission device driven by said power free turbine and driving said first and second propellers; and 
     a casing inserted between the power free turbine and the first and second propellers, the latter being arranged in the predetermined direction relative to the casing and the free turbine being arranged in the opposite direction relative to said same casing, the latter having a casing extension in the given direction, rotatably supporting the hub of the propeller. 
     According to the invention, the coupling arms of the second propeller extend in said opposite direction, radially towards the outside. 
     The invention therefore cleverly provides for inclining the coupling arms of the second propeller so that they come closer to the casing while going radially towards the outside, which globally makes it possible to bring the blades closer to said casing. 
     One consequence of this approach of the blades towards the casing lies in the decrease of the length of the propeller system, in the axial direction. This results in mass and bulk gains. 
     Another consequence of this approach lies in the limitation of the part of the second propeller cantilevered relative to the casing extension. In other words, the specific incline of the coupling arms makes it possible to offset the center of the masses of the second propeller towards the casing, thereby reducing the cantilever relative to the embodiments previously encountered. As a result, the casing extension serving to guide the hub of the second propeller can be smaller in the axial direction, given that the spacing required between the rolling bearings is also decreased relative to the earlier solutions with a more substantial cantilever. This results in an additional gain in terms of mass and bulk. 
     The invention is applicable to all turbomachines, in particular so-called “open rotor” turbomachines. In the latter case, the invention applies whether the propeller system is arranged upstream or downstream of the gas generator. In each of these two cases, within the propeller system, it is possible to consider placing the power turbine upstream or downstream of the contra-rotating propellers. This is also applicable for the position of the epicyclic gearing relative to the propellers. 
     Preferably, said predetermined direction is the downstream direction. In this way, the first propeller is the downstream propeller, and said second propeller is the upstream propeller. This specific arrangement is in particular chosen when the propeller system is arranged downstream of the gas generator of the turbomachine, i.e. when the latter adopts a design ensuring propulsion, called “pusher” design. Naturally, an opposite design could be considered, in which said predetermined direction would be the upstream direction, without going beyond the scope of the invention. Said first propeller would then be the upstream propeller, and said second propeller would be the downstream propeller. This other solution is in particular chosen when the propeller system is arranged upstream of the gas generator of the turbomachine, i.e. when the latter adopts a design ensuring traction, called “puller” design. 
     Whatever the design considered amongst those mentioned above, it is preferably done so that at least one portion of each coupling arm belonging to the second propeller is located in said opposite direction relative to a rolling bearing inserted between said casing extension and said hub of the second propeller. 
     Similarly, it is preferable for the second propeller to comprise a plurality of blades each mounted so as to be steered in incidence around a pivot axis, and for said pivot axis to be situated in said opposite direction relative to a rolling bearing inserted between said casing extension and said hub of the second propeller. 
     The two configurations described above, made possible owing to the specific incline of the arms of the second propeller, indeed illustrate the ability offered by the present invention to re-center the masses of said second propeller towards the casing that separates it from the power free turbine. 
     Preferably, the hub of said second propeller rotatably supports said hub of said first propeller. 
     Preferably, said coupling arms of the first propeller extend in said predetermined direction going radially outwardly, although any other configuration could be chosen, without going beyond the scope of the invention. 
     Preferably, said coupling arms of the first propeller support a first intermediate ferrule participating in the radial inward definition of the primary annular tunnel, and said coupling arms of the second propeller support a second intermediate ferrule participating in the radial inward definition of the main annular tunnel, the first intermediate ferrule being situated in the continuity of said second ferrule in said given direction. 
     Preferably, the coupling arm of the first propeller and its associated outer and intermediate ferrules form a part made in a single piece, and the coupling arms of the second propeller and its associated outer and intermediate ferrules also form a part made in a single piece. Alternatively, each of these two single-piece assemblies could be made by several parts attached to each other. 
     As mentioned above, said predetermined direction is preferably the downstream direction. 
     Preferably, said mechanical transmission device comprises an epicyclic gearing provided with a sun gear centered on said longitudinal axis and driven by the rotor of the power free turbine, at least one satellite meshing with said sun gear, a planet carrier driving said first propeller, as well as a crown meshing with each satellite and driving said second propeller. 
     Preferably, said planet carrier is secured to the hub of said second propeller, and said crown is secured to the hub of said second propeller. 
     Preferably, each outer ferrule supports a retaining ring of the blades of the concerned propeller. 
     Preferably, the first and second propellers each have a variable calibration system for their blades. In a known manner, these systems are steered so that the rotational speed of the two propellers is kept substantially constant during operation, irrespective of the rating. 
     The invention also relates to an aircraft turbomachine comprising a contra-rotating propeller system as described above, this turbomachine for example being a turboprop, but alternatively being able to be a turbojet engine with a contra-rotating fan. Naturally, in the latter case, the aforementioned mechanical transmission device is intended to move the contra-rotating fan of the turbojet engine. Preferably, as mentioned above, the invention more particularly applies to so-called “open-rotor” turbomachines, within which the power free turbine drives the two contra-rotating propellers, indirectly via a mechanical transmission device forming a reducer and in particular comprising an epicyclic gearing. 
     Other advantages and features of the invention will appear in the non-limiting detailed description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This description will be provided in reference to the appended drawings, in which: 
         FIG. 1  shows a longitudinal half-cross-sectional diagrammatic view of an aircraft turbomachine, according to one preferred embodiment of the present invention; 
         FIG. 2  shows a cross-sectional view along line II-II of  FIG. 1 ; 
         FIGS. 3   a  and  3   b  show partial perspective views, following different angles, of the contra-rotating propeller system equipping the turbomachine shown in  FIG. 1 ; 
         FIG. 4  shows an enlarged cross-sectional view of part of the contra-rotating propeller system shown in the preceding figures; and 
         FIG. 5  shows a longitudinal half-cross-sectional diagrammatic view of an aircraft turbomachine, according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a turbomachine  1  of the “open rotor” type, according to one preferred embodiment of the present invention. 
     In the figures, direction A corresponds to the longitudinal or axial direction, parallel to the longitudinal axis  2  of the turbomachine. Direction B corresponds to the radial direction of the turbomachine. Furthermore, the arrow  4  diagrams the direction of travel of the aircraft under the actions of the thrust of the turbomachine  1 , this direction of travel being opposite the main flow direction of the gases inside the turbomachine. The terms “front,” “upstream,” “back,” “downstream” used in the rest of the description must be considered relative to said direction of travel  4 . 
     In the front part, the turbomachine has an air inlet  6  continuing towards the rear through a nacelle  8 , the latter globally including an outer skin  10  and an inner skin  12 , both centered on the axis  2  and radially offset relative to each other. 
     The inner skin  12  forms an outer radial casing for a gas generator  14 , traditionally comprising, from front to back, a low-pressure compressor  16 , a high-pressure compressor  18 , a combustion chamber  20 , a high-pressure turbine  22 , and an intermediate-pressure turbine  24 . The compressor  16  and the turbine  24  are mechanically connected by a shaft  26 , thereby forming a low-pressure body, while the compressor  18  and the turbine  22  are mechanically connected by a shaft  28 , forming a higher-pressure body. Consequently, the gas generator  14  preferably has a traditional design, called “dual body”. 
     Downstream of the intermediate pressure turbine  24 , there is a contra-rotating propeller system  30 , forming a receiver for the turbomachine. 
     This system  30  comprises a power free turbine  32 , forming a low-pressure turbine. It includes a rotor  32   a  constituting the inner part of the turbine, as well as a stator  32   b  constituting the outer part of said turbine, which is fixedly connected to a fixed casing assembly  34  of said propeller system, centered on the longitudinal axis  2  of the system. In a known manner, this casing assembly  34  is intended to be secured to the other casings of the turbomachine. In this respect, it is indicated that the propeller system  30  is preferably designed so that the propellers do not have an outer radial fairing surrounding them, as shown in the figures. 
     Furthermore, downstream of the power free turbine  32 , the propeller system  30  incorporates a first propeller  7  or downstream propeller, supporting blades  7   a . Similarly, the system comprises a second propeller  9  or upstream propeller, supporting blades  9   a . In this way, the propellers  7 ,  9  are offset from each other in direction  4 , and both situated downstream of the free turbine  32 . 
     The two propellers  7 ,  9  are intended to rotate in opposite directions around the axis  2  on which they are centered, the rotations being done relative to the stator  34 , which remains immobile. 
     To drive these two propellers  7 ,  9  in rotation, a mechanical transmission device  13  is provided, forming a reducer and in particular comprising an epicyclic gearing  15 . 
     In reference to  FIGS. 1 and 2 , the gearing  15  is provided with a sun gear  17  centered on the longitudinal axis  2 , and supported by a sun gear shaft  19  with the same axis, integrally connected upstream of the rotor  32   a , via a flange  38 . In this way, the rotor  32   a  directly drives the sun gear  17  in rotation, the latter assuming the form of an outwardly toothed wheel. 
     The gearing  15  also has a satellite  21 , and preferably as shown in  FIG. 2 , each of them mesh with the sun gear  17 . Each satellite  21  is supported by a sun gear shaft  23  with an axis off-centered relative to the shaft  2 , and assumes the form of an outwardly toothed wheel. 
     Moreover, the gearing  15  is equipped with a planet carrier  25  centered on the longitudinal axis  2 , and rotatably supporting each of the planets  21 , via shafts  23 , respectively, the planet carrier  25  is supported by a planet carrier shaft  29  with the same axis, secured to the first propeller  7 , as visible in  FIG. 1 , so as to be able to drive it directly in rotation. 
     Lastly, the gearing  15  has a crown  31  centered on the axis  2  and supported by a crown shaft  33  with the same axis, this crown  31  meshing with each planet  21 . The shaft  33  extends downstream and is secured to the second propeller  9 , so as to be able to drive it directly in rotation. For example, this shaft  33  is situated around the planet carrier shaft  29  with which it is concentric. The crown  31  also assumes the form of an inwardly toothed wheel. 
     In the described preferred embodiment, in which each propeller is equipped with a system for variable calibration of its blades, the epicyclic gearing  15  is situated at and inside a casing  42  inserted between the power free turbine  32  and the propellers  7 ,  9 . This casing  42 , also called escapement casing or “static frame,” supports an engine mount  44  intended to ensuring mounting of the turbomachine on the aircraft structure, as in particular is visible in  FIGS. 1 ,  3   a  and  3   b . In general, it is indicated that the mechanical transmission device is housed in the hub of the casing  42 . 
     The casing  42 , downstream of which the propellers are located and upstream of which the power turbine  32  is located, comprises a casing extension  46  extending downstream relative to a central portion of said casing. This extension  46  assumes the form of a hollow cylinder centered on the longitudinal axis  2 , rotatably supporting a hub  48   b  of the second propeller, said hub  48   b  being combined with the crown shaft  33 , as shown in  FIG. 1 . This rotatable support is done via two rolling bearings  50  spaced apart from each other in direction A, and inserted between the extension  46  and the hub  48   b . To increase the rigidity and mechanical maintenance of the extension  46 , the latter is connected to the central portion of the casing  42  via reinforcing ribs  52 , distributed all around said extension and each extending radially. 
     The second propeller  9  also has an outer ferrule  56   b  arranged concentrically to the hub  48   b , and participating in outwardly radially defining a main annular tunnel  58 . 
     Furthermore, it also comprises a plurality of coupling arms  60   b  connecting the outer ferrule  56   b  to the hub  48   b . One of the particularities of the present invention consists of providing that each arm  60   b  extends from an inner radial arm, secured to a portion of the hub  48   b  protruding downstream of the casing extension  46 , towards an outer radial end secured to the ferrule  56   b , going in the upstream direction. In cross-sectional as shown in  FIG. 1 , the angle between the arms  60   b  and the direction B can be between 20 and 50°. 
     Moreover, the coupling arms  60   b  of the second propeller support a second intermediate ferrule  62   b  arranged between the hub  48   b  and the outer ferrule  56   b , this ferrule  62   b  participating in inwardly radially defining the main annular tunnel  58 . Naturally, the hub  48   b , the arms  60   b  arranged in a star as shown in  FIGS. 3   a  and  3   b , and the ferrules  62   b ,  56   b  form an assembly rotatably secured along the axis  2 . 
     In the illustrated preferred embodiment, it is made so that, to limit the cantilever at the casing extension  46 , at least one portion of each coupling arm  60   b  is situated upstream of the rolling bearing  50  furthest downstream. Due to the specific incline of the arms  60   b , this involves the radially external portion of these arms. Moreover,  FIG. 1  shows that each blade  9   a  is mounted so that it can be steered in incidence around its pivot axis  64   b , by its variable calibration system (not shown). Still to limit the cantilever, it is done so that this pivot axis  64   b , and more generally the transverse plane incorporating the set of axes  64   b  of the blades  9   a , is situated upstream of the furthest downstream rolling bearing  50 . 
     The crown shaft  33  assumes the form of a hollow cylinder centered on the axis  2 , rotatably supporting a hub  48   a  of the first propeller, this hub  48   a  being combined with the planet carrier shaft  29 , as shown in  FIG. 1 . This rotatable support is done via two rolling bearings  66  spaced apart from each other in direction A, and inserted between the two hubs  48   b ,  48   a.    
     The first propeller  7  also has an outer ferrule  56   a  arranged concentrically to the hub  48   a , and participating in outwardly radially defining the main annular tunnel  58 . It is situated in the downstream aerodynamic extension  56   b  of the second propeller. 
     Moreover, it also comprises a plurality of coupling arms  60   a  connecting the outer ferrule  56   a  to the hub  48   a . Here, each arm  60   a  extends from an inner radial end, secured to a portion of the hub  48   a  protruding towards the downstream direction of the hollow hub  48   b , towards an external radial end secured to the ferrule  56   a , going in the downstream direction. In cross-section like that shown in  FIG. 1 , the angle between the arms  60   a  and the direction B can be between 20 and 50°. Nevertheless, any other configuration can be considered, the inclines chosen for the arms  60   a ,  60   b  depending in particular on the desired spacing between the blades  9   a  and  7   a , in direction A, in particular to respond to the acoustic stresses. As an illustrative example, the arms  60   a  could be inclined in the upstream direction in the same way as the coupling arms  60   b.    
     Aside from their incline in the axial direction, the coupling arms can also be calibrated relative to the flow in the tangential direction, so as to limit the drag caused by them. 
     Furthermore, the coupling arms  60   a  of the first propeller support a first intermediate ferrule  62   a  arranged between the hub  48   a  and the outer ferrule  56   a , this ferrule  62   a  participating in radially inwardly defining the main annular tunnel  58 . It is situated in the downstream aerodynamic extension of the intermediate ferrule  62   b  of the second propeller. Naturally, the hub  48   a , the arms  60   a  arranged in a star as shown in  FIGS. 3   a  and  3   b , and the ferrules  62   a ,  56   a  form an assembly rotatably secured along the axis  2 . For information, it is noted that in  FIGS. 3   a  and  3   b , for clarity reasons, the outer ferrules  56   a ,  56   b  have voluntarily been omitted, while the intermediate ferrules  62   a ,  62   b  have been shown interrupted. 
     In  FIG. 4 , one can see that the outer ferrule  56   b  of the second propeller, which extends between the casing  42  and the ferrule  56   a  in one or several parts attached to each other, supports a retaining ring  68   b  of the blades  9   a . This ring  68   b , having a plurality of holes each intended to receive the foot of a blade  9   a , is globally situated towards the upstream direction relative to the outer radial ends of the coupling arms  60   b . Similarly, the outer ferrule  56   a  of the first propeller, which extends in the downstream direction from the ferrule  56   b  in one or several parts attached to each other, supports a retaining ring  68   a  of the blades  7   a . This ring  68   a , having a plurality of holes each intended to receive the foot of a blade  7   a , is globally situated at the outer radial ends of the coupling arms  60   a.    
     According to one particular embodiment, a first single-piece assembly is formed by the coupling arm  60   a  and by the outer ferrule  56   a  and the intermediate ferrule  62   a , just as another single-piece assembly is formed by the coupling arms  60   b  and by the outer ferrules  56   b  and the intermediate ferrule  62   b.    
       FIG. 5  shows another embodiment, in which the propeller system is arranged upstream of the gas generator of the turbomachine. In the “puller design” shown in  FIG. 5 , the predetermined direction would be the upstream direction. The first propeller is the upstream propeller, and the second propeller is the downstream propeller. 
     Of course, various modifications can be made by one skilled in the art to the invention described above, purely as non-limiting examples.