Abstract:
A fan portion of a dual flow turbojet engine including a plurality of fan blades, a disk supporting the blades and configured to be rotated relative to a stator portion of the fan, along a longitudinal axis of the fan, and a system for setting the angle of attack associated with each fan blade, the systems configured such that the angle of attack of each blade varies according to a same setting law according to the angular position of the blade relative to the stator portion, along the longitudinal axis, the same setting law being periodic with a period of P=360°/n, where n is an integer at least equal to 1.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of dual flow turbojet engines, preferably for aircrafts. 
     2. Description of the Related Art 
     Traditionally, such a turbojet engine comprises a plurality of fan blades, and a disk for supporting the blades and capable of being rotated relative to a stator portion of the fan, along a longitudinal axis of the fan. 
     The fan blades are generally mounted fixed on the support disk, with a predetermined angle of attack that is identical for each of them. This solution is relatively satisfactory when the fan is supplied with air substantially uniform. 
     Nevertheless, certain turbojet engine designs can result in a non-uniform supply of air to the fan, continuously during the operation of the turbojet engine. This is in particular the case when the aerodynamic fairing delimiting the air intake of the turbojet engine covers part of the set of blades of said fan, towards the longitudinal axis of the fan. For information, these configurations are said to have “partially buried turbojet engines” and differ from the traditional solutions, in which the aerodynamic fairing delimiting the air intake runs along the entire fixed outer casing of the fan, without covering the blades thereof. 
     BRIEF SUMMARY OF THE INVENTION 
     To deal with cases where the fan is supplied with air non-uniformly, occasionally or continuously, the invention provides a dual flow turbojet engine fan portion, a disk for supporting the blades capable of being rotated relative to a fan stator portion, along a longitudinal fan axis, and comprising a system for setting angles of attack associated with each fan blade, these systems being designed so that the angle of attack of each blade varies according to a same setting law as a function of the angular position of the blade relative to the stator portion, along said longitudinal fan axis, said same setting law being periodic with period P=360°/n, with n corresponding to a whole number greater than or equal to 1. 
     In other words, the invention provides that the vanes successively go to any angular position of the stator portion with the same angle of attack, which follows a setting law the evolves during a complete fan revolution along its longitudinal axis. 
     This cyclic setting makes it possible to provide an extremely satisfactory response to cases where the fan is supplied heterogeneously with air, continuously or occasionally. In the latter case, for example during variations in the angle of attack of the aircraft or during turns, the variation of the angle of attack of the fan blades is not continuous, but automatically or manually activated only when the flight conditions translate a heterogeneous supply of air to the fan. This scenario is therefore preferably encountered with the traditional solutions in which the aerodynamic fairing delimiting the air intake runs along the entire fixed outer casing of the fan, without covering the blades thereof. 
     However, when the air supply of the fan is continuously heterogeneous, the variation of the angle of attack of the fan blades is also continuously activated. This for example, as mentioned above, involves cases where the turbojet engine is partially buried in the aircraft structure, preferably in the main wing. 
     Thus, in this preferred case of a partially buried turbojet engine, the unique setting law is preferably provided to so that when any one fan blade enters the covered zone, also called buried zone, its angle of attack is decreased so that it claims less air, thereby reducing the risks of pumping and drop in output. However, when it leaves said covered zone, its normal angle of attack is reestablished without any risk, since it is located in the air stream. Naturally, much more complex setting laws can be contemplated, and can be determined as a function of the constraints and needs encountered. 
     Preferably, said system for setting angles of attack is steered passively by rotating the disk supporting the blades relative to the fan stator portion, along said longitudinal axis of the fan. The passive nature here translates the fact that no additional energy source other than that of the rotation of the fan is used. However, an active steering design could be contemplated without going beyond the scope of the invention. 
     According to a preferred first embodiment of the invention, the passively steered system for setting the angle of attack comprises a lug arranged eccentrically on the foot of the concerned blade, a first toothed wheel centered on said longitudinal fan axis and fastened to the fan stator portion, as well as a second toothed wheel rotated along the longitudinal fan axis by the support disk, and mounted freely rotating on the latter along a wheel axis of rotation separate from said longitudinal fan axis, said second toothed wheel meshing with said first toothed wheel and being mechanically connected to said lug. 
     With this specific design, it is preferably provided that the systems for setting the angles of attack of all of the fan blades share the same first toothed wheel. This results in significant gains in terms of mass and bulk. 
     Preferably, said second toothed wheel is mechanically connected to said lug via a connecting rod. 
     Still preferably, it is done so that the first and second toothed wheels form a conical gear, i.e. they adopt tapered shapes whereof the axes intersect, preferably orthogonally. 
     According to a second preferred embodiment of the invention, said system for setting the angle of attack comprises a lug fixedly mounted eccentrically on the foot of the concerned blade, as well as a guide path fixed relative to the fan stator portion and arranged around said longitudinal fan axis, said guide path having a geometry making it possible to apply to said lug, during its rotational movement relative to the stator portion along the longitudinal axis, an additional movement parallel to said same axis, ensuring a variation of the angle of attack of the blade according to said setting law. 
     Here again, it is therefore provided that the blades successively go to any angular position of the stator portion with the same angle of attack, which follows a setting law that evolves during a complete revolution of the fan along its longitudinal axis. 
     This cyclic setting makes it possible to provide an extremely satisfactory response to cases in which the air supply for the fan is continuously heterogeneous, given that the variation of the angle of attack of the fan blades is also obtained continuously, owing to the cooperation between the lug and its guide path having a suitable geometry. This for example involves, as mentioned above, cases where the turbojet engine is partially buried in the structure of the aircraft, preferably in the main wing. 
     Thus, in this preferred case of a partially buried turbojet engine, the unique setting law is preferably provided so that when any fan blade enters the covered zone, also called buried zone, its angle of attack is decreased so that it claims less air, thereby reducing the risks of pumping and output drop. However, when it leaves this covered zone, its normal angle of attack is reestablished without any risk, since it is located in the stream of air. Naturally, much more complex setting laws can be contemplated, and can be determined as a function of the constraints and needs encountered. 
     As emerges from the preceding, said system for setting the angle of attack is steered passively by rotating the disk supporting the blades relative to the fan stator portion, along said longitudinal fan axis. The passive nature here translates the fact that no additional energy source other than that of the rotation of the fan is used. In fact, the rotation of the fan results in applying a rotational movement relative to the stator portion, along the longitudinal axis, to the lug of a given blade. During this rotational movement, the lug following the guide path, the specific geometry of which results in applying an additional movement to this lug parallel to the longitudinal axis, ensuring the desired variation of the angle of attack of the blade. In other words, the guide path serves to transform the rotational movement of the lug into an axial movement of the latter, relative to the stator portion. 
     With this specific design, it is preferably provided that the systems for setting angles of attack for all of the blades of the fan share the same guide path. It could nevertheless be otherwise, without going beyond the scope of the invention. 
     Preferably, said system for setting angles of attack also has a roller secured to said lug, slidingly housed in said guide path. Alternatively, the lug could itself be housed directly in the guide path, without going beyond the scope of the invention. 
     Preferably, said guide path assumes the form of a groove radially outwardly open. Its shape is then substantially annular centered on the longitudinal fan axis, while having an evolution of its position in the axial/longitudinal direction so as to apply said desired setting law. 
     Irrespective of the embodiment contemplated, preferably, said whole number n defining the period of the setting law is equal to 1, which translates a period equal to one revolution. 
     The invention also relates to a turbojet engine comprising a fan portion as described above. 
     Preferably, the turbojet engine is provided to be partially buried, i.e. it comprises an aerodynamic fairing delimiting the air intake of the turbojet engine and covering, when seen in the direction of the longitudinal fan axis, a portion of the set of blades of said fan. In other words, in this scenario, the surface in transverse-cross section of the air intake is smaller than a disk-shaped surface with a diameter corresponding to that of the rotary portion of the fan, the ratio between these two surfaces being able to be in the vicinity of 0.33. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other advantages and features of the invention will appear in the non-limiting detailed description below. 
       This description will be done in reference to the appended drawings, in which: 
         FIG. 1  shows a longitudinal half-cross-sectional view of a front portion of a turbojet engine according to a first preferred embodiment of the present invention; 
         FIG. 2  shows a front view of the turbojet engine shown in  FIG. 1 ; 
         FIGS. 3   a  to  3   d  show a system for setting the angle of attack of one of the fan blades of the turbojet engine shown in  FIGS. 1 and 2 , in different configurations adopted during a revolution of said concerned blade; 
         FIG. 4  shows a graph diagramming the setting law applied to said blade; 
         FIG. 5  shows a longitudinal half-cross-sectional view of a front portion of a turbojet engine according to a second preferred embodiment of the present invention; 
         FIG. 6  shows a partial perspective view of part of the system for setting angles of attack shown in  FIG. 5 ; and 
         FIG. 7  shows a system for setting the angle of attack of one of the fan blades of the turbojet engine shown in  FIG. 5 , in different configurations adopted during a revolution of said concerned blade. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a front portion of a turbojet engine  1  according to a first preferred embodiment of the present invention. This front portion is essentially made up of a fan portion  1   a , centered on a longitudinal axis  2  also corresponding to the turbojet engine. It globally comprises a stator portion  4 , as well as a portion  6  rotatably mobile around said stator portion, along the axis  2 . The stator portion  4  is fixed and comprises, inter alia, a fixed outer fan casing  8 , secured to an intermediate casing making it possible to ensure the delimitation between the primary stream  12  and the secondary stream  14  of the turbojet engine. Moreover, this fixed outer fan casing  8  supports, toward the front, an aerodynamic fairing  16  delimiting the air inlet  18  of the turbojet engine. 
     The rotary portion  6  also comprises a plurality of fan blades  20 , only one of which is visible in  FIG. 1 , as well as a blade support disk  22  centered on the axis  2 . For information, the mobile portion  6  is rotated using a rotary driving shaft  24  centered on the axis  2 , and meshing directly or via a gearbox with said support  22 , also called hub. The latter has a plurality of orifices  28  spaced circumferentially apart from each other, each dedicated to the housing of one of the fan blades  20 . 
     Thus, as shown for one of the blades  20  in  FIG. 1 , the orifice  28  receives the foot  30  of the blade so as to allow a rotation thereof along a blade pivot axis  32 , preferably substantially radial. This is in particular allowed by the placement of rolling bearings  34  between the orifice  28  and the blade foot  30 . 
     This rotary assembly of the blade  20  relative to the hub  22  along the axis  32  is provided to be able to steer the angle of attack of said blade, owing to a system for setting angles of attack that will now be described, and which is preferably substantially identical for each of the blades. 
     It first incorporates a lug  36  arranged on the blade foot  30  eccentrically relative to the pivot axis  32 , and protruding radially inwardly. 
     Moreover, a first toothed wheel  38  is provided centered on the longitudinal axis  2  of the fan, and fastened to the stator portion  4 . Preferably, this wheel  38 , which assumes a substantially tapered shape, is shared by all of the setting systems equipping the blades of the fan. 
     The system for setting angles of attack also incorporates a second toothed wheel  40  rotated along the axis  2  by the support disk  22 , and mounted freely rotating thereon along the wheel axis of rotation  42 , secant and preferably orthogonal to the axis  2 . A lug  44  is for example provided on the hub  42  to allow the freely rotating assembly of the second wheel  42  with a substantially tapered shape, which meshes directly or indirectly with the first toothed wheel  38 . In this respect, a reduction ratio of 1 is preferably provided between the two wheels, so that one hub  22  revolution around the axis  2  creates a revolution of the second wheel  40  around its axis  42 . 
     Furthermore, the second wheel  40  also fixedly supports a lug  46 , eccentric relative to the axis of rotation  42  and radially protruding, and connected to the lug  36  by a connecting rod  48 . 
     This system is steered passively, since the rotation of the mobile portion  6  of the fan results in making the second mobile wheel  40  mesh with the first fixed wheel  38 , and therefore making it rotate around its axis of rotation  42 , causing the connecting rod  48  actuating the eccentric lug  36  of the blade foot  30  in motion. 
     This system for setting angles of attack is specifically designed so that the blade  20  to which it is connected has an angle of attack that varies according to a cyclic setting law, preferably with a period corresponding to one fan revolution. This setting law, which provides the angle of attack of the blade as a function of its angular position relative to the stator portion  4  along the axis  2 , is the same for all of the blades  20  of the fan. It is determined as a function of the needs encountered. In the example shown in  FIG. 2 , the turbojet engine  1  is intended to be semi-buried in the structure of the aircraft, preferably in the main wing. Thus, seen from the front as shown in  FIG. 2 , the aerodynamic fairing  16  delimiting the air intake  18  covers the lower half of the set of blades  20 . 
     In this situation, the unique setting law Lc diagrammed in  FIG. 4  is preferably provided so that when any blade  20  is in an angular position of 0° relative to the stator portion  4 , it has a so-called normal angle of attack, which is preserved until it occupies an angular position close to 90°.  FIGS. 3   a  and  3   b  respectively show the state of the setting system for the two angular positions of 0° and 90° of the blade. In these two figures, it is in fact possible to see that while the configuration of the setting system has been modified due to the meshing of the two wheels  38 ,  40  over a quarter revolution, the angle of attack of the blade remains invariable over this cycle portion. 
     From the angular position of 90° and up to its angular position of 180°, corresponding to 6:00, the angle of attack of the blade  20  is decreased, preferably to a null or nearly null value. During this portion of the cycle, the blade in question effectively passes through the covered/buried portion, and it is therefore done so that it claims less air so as to reduce the risks of pumping and output drop. Conversely, from the angular position of 180° and up to its angular position of 270° coinciding with the exit of covered/buried portion of the fan, the angle of attack of the blade  20  is increased until it again reaches its so-called normal value, which it then keeps until its angular position of 0°, also corresponding to 360°. In this respective,  FIGS. 3   c  and  3   d  respectively show the state of the setting system for the two angular positions of 180° and 270° of the blade. 
     To obtain the desired setting law Lc, it is done here so that, as shown in  FIGS. 3   a  to  3   d , the lug  46  is slidingly mounted on the connecting rod  48 , along the latter. 
     Naturally, the unique setting law Lc applied to all of the fan blades  20  could differ, but with a same air intake configuration  18 , without going beyond the scope of the invention. 
       FIG. 5  shows a front portion of a turbojet engine  1  according to a second preferred embodiment of the present invention. This second embodiment has similarities with the first embodiment described above. Moreover, in the figures, the elements bearing identical reference numbers correspond to identical of similar elements. 
     Here again, the front portion is essentially made up of a fan portion  1   a , centered on a longitudinal axis  2  also corresponding to the axis of the turbojet engine. It globally comprises a stator portion  4 , as well as a portion  6  rotatably mobile around said stator portion, along the axis  2 . The stator portion  4  is fixed and comprises, inter alia, a fixed outer fan casing  8 , secured to an intermediate casing making it possible it to ensure the delimitation between the primary stream  12  and the secondary stream  14  of the turbojet engine. Moreover, this fixed outer fan casing  8  supports, towards the front, an aerodynamic fairing  16  delimiting the air intake  18  of the turbojet engine. 
     The rotary portion  6  comprises a plurality of fan blades  20 , only one of which is visible in  FIG. 5 , as well as a disk supporting the blades  22  centered on the axis  2 . For information, the mobile portion  6  is rotated via a rotary drive shaft  24  centered on the axis  2 , and meshing directly or via a gearbox with said support  22 , also called a hub. The latter has a plurality of orifices  28  circumferentially spaced apart from each other, each dedicated to the housing of one of the fan blades  20 . 
     Thus, as shown for one of the blades  20  in  FIG. 5 , the orifice  28  receives the foot  30  of the blade so as to allow a rotation thereof along a blade pivot axis  32 , preferably substantially radial. This is in particular allowed by the placement of rolling bearings  34  between the orifice  28  and the blade foot  30 . 
     This rotary assembly of the blade  20  relative to the hub  22  along the axis  32  is provided to be able to steer the angle of attack of said blade, owing to a system for setting angles of attack that will now be described, and which is preferably substantially identical for each of the blades. 
     It first incorporates a lug  36  fixedly mounted on the blade foot  30 , eccentrically relative to the pivot axis  32 , while protruding radially inwardly. Its outer radial end is therefore secured to the foot  30 , while its inner radial end preferably supports a roller  37 . 
     Moreover, a guide path  41  is provided stationary relative to the stator portion  4 , this path running around the longitudinal fan axis  2 . More precisely, the guide path  41 , housing the cam follower  37 , is a path following a closed line, and preferably adopts the form of a groove centered on the axis  2  and open radially outwardly. This is in particular visible in  FIG. 6 , which shows a portion of said path  41 , which is axially delimited in the downstream and upstream direction respectively by two substantially annular flanks  39 , whereof the constant spacing “e” is substantially identical to the diameter of the roller  37 . Thus, in the second illustrated preferred embodiment, the path  41  is arranged radially inwardly relative to the foot of the blade  30 , so that the roller  37  can be housed between the two flanks  39  serving for guiding thereof. 
     Preferably, this path  41  is shared by all of the setting systems equipping the blades of the fan, i.e. it houses a plurality of rollers  37  each associated with the lug  36  of a given blade. 
     One of the particularities of the preferred embodiment lies in the fact that the guide path  41  has a geometry making it possible to apply to the lug  36 , during its rotational movement relative to the stator portion  4 , an additional movement parallel to the axis  2 , ensuring a variation in the angle of attack of the blade. In this way, this system is steered passively, since the rotation of the mobile portion  6  of the fan results in making the roller  37  run along the path  41 , and thereby causing the additional movement of the lug  36  along the direction of the axis  2 . To that end, the path  41  extends in the annular direction around the axis  2 , but also in the direction of said axis so as to obtain the evolution of the desired setting. 
     The system for setting the angle of attack is specifically designed so that the blade  20  to which it is connected has an angle of attack that varies according to a cyclic setting law, preferably with a period corresponding to one revolution of the fan. This setting law, which yields the angle of attack of the blade as a function of its angular position relative to the stator portion  4  along the axis  2 , is the same for all of the blades  20  of the fan. It is determined according to the needs encountered. Still in the example shown in  FIG. 2 , the turbojet engine  1  is intended to be semi-buried in the structure of the aircraft, preferably in the primary wing. Thus, in front view as visible in  FIG. 2 , the aerodynamic fairing  16  delimiting the air intake  18  covers the lower half of the set of blades  20 . 
     In this situation, the unique setting law Lc diagrammed in  FIG. 4  is preferably provided so that when any blade  20  is located in an angular position of 0° relative to the stator portion  4 , it has a so-called normal angle of attack, which is preserved until it occupies an angular position close to 90°. The two left illustrations in  FIG. 7  respectively show the state of the setting system for the two angular positions of 0° and 90° of the blade. In this figure, to facilitate understanding, the guide path  41  has been shown in a developed manner in the plane, whereas it is recalled that the latter has a globally annular shape. 
     It can be seen that between these two first positions, the path  41  does not have any evolution in the axial direction, so that the angle of attack of the blade is not modified during this first quarter revolution. 
     From the angular position 90° and up to its angular position of 180°, corresponding to 6:00, the incidence of the blade  20  is decreased, preferably up to a null or nearly null value. During this portion of the cycle, the blade in question effectively passes through the covered/buried portion, and it is therefore done so that it claims less air so as to reduce the risks of pumping and output drop. Conversely, from the angular position of 180° and up to its angular position 270° coinciding with the exit of the covered/buried portion of the fan, the angle of attack of the blade  20  is increased until it again reaches its so-called normal value, which it then keeps until its angular position of 0°, also corresponding to 360°. In this respect, the two right illustrations in  FIG. 3  respectively show the state of the setting system for the two angular positions of 180° and 270° of the blade. 
     It is thus possible to see that between the 90° and 180° positions, the path  41  becomes gradually offset in the downstream direction in the direction of the axis  2 , ensuring the additional movement of the lug in this same direction. Consequently, during this quarter revolution, the setting of the blade in fact evolves while gradually closing. However, it can be seen that between the 180° and 270° positions, the path becomes progressively offset in the upstream direction in the direction of the axis  2 , ensuring the additional movement of the lug  36  in this same direction. Consequently, during this quarter revolution, the setting of the blade evolves, re-opening again until its normal angle of attack position. 
     Lastly, it is possible to see that between the 270° and 360° positions, the path  41  does not have any evolution in the axial direction, so that the angle of attack of the blade is not modified during this last quarter revolution. 
     Naturally, the unique setting law Lc applied to all of the fan blades  20  could differ, but with a same air intake configuration  18 , without going beyond the scope of the invention. 
     Of course, various modifications can be made by one skilled in the art to the invention just described, solely as non-limiting examples.