Abstract:
The present invention relates to a turbine engine comprising two respectively upstream and downstream external impellers ( 7, 9 ) that are nonstreamlined, coaxial and contrarotating. The engine is noteworthy in that the downstream impeller ( 7 ) is retractable so as to reduce its diameter. The blades ( 7   a ) of the downstream impeller are mounted so as to pivot about a pivot ( 71 ), the axis of which forms a nonzero angle, notably perpendicular, with the axis ( 2 ) of rotation of the impeller, the blades in the retracted position being tilted about the pivot ( 71 ).

Description:
[0001]    The present invention relates to the field of aircraft turbine engines with nonstreamlined dual impellers. 
       BACKGROUND OF THE INVENTION 
       [0002]    An engine of this type, designated in the present field by the terms “open rotor” or “unducted fan” comprises a gas turbine engine feeding a free turbine with contrarotating coaxial rotors each associated with an impeller. The two impellers extend substantially radially to the outside of the nacelle of the turbine engine and are themselves coaxial and contrarotating. The drive of the two impellers is either direct, the two impellers are mounted on the periphery of the two turbine rotors, or by means of a mechanical gearbox, the two impellers each being connected to an output of the gearbox. 
         [0003]    Turbine engines with nonstreamlined impellers are currently being studied because they have the advantage of being powerful by being capable of supplying a considerable thrust and of consuming less fuel than other equivalent turbojets with streamlined fan. 
         [0004]    However, the high noise levels generated by the mechanisms of aerodynamic interaction between the two impellers are a drawback for this type of propulsion. 
         [0005]    One of the sources of this noise arises from the interaction of vortexes generated at the tips of the blades of the upstream impeller, with the blades of the downstream impeller. The vortex generated by the upstream impeller interacts with the downstream impeller extremely energetically which generates high noise levels. 
         [0006]    Ways of reducing the impeller noise are aimed at controlling the flows around the profiles; these means are not mature in the current state of the art. 
         [0007]    One solution for suppressing this noise consists in reducing the external diameter of the downstream impeller so that the vortices generated by the upstream impeller pass outside the envelope of the downstream impeller and do not interact with the latter. Such a solution is unsatisfactory because it results in a reduction in the thrust produced by the downstream impeller and therefore in a reduction in the performance of the engine. It would be possible to increase the load of the downstream impeller in order to compensate for the reduction in its diameter but this would also increase the aeromechanical difficulty of the design of the pair of impellers and it would become extremely complex and difficult to achieve. 
       SUMMARY OF THE INVENTION 
       [0008]    The object of the invention is therefore to reduce the noise levels generated by the impellers in order, in particular, to comply with the relatively strict acoustic certification standards that apply to the take-off and landing phases of the aircraft fitted with this engine. 
         [0009]    Another object is to provide good aerodynamic performance at cruising speed. 
         [0010]    These objects are achieved according to the invention with a turbine engine comprising two respectively upstream and downstream external impellers that are nonstreamlined, coaxial and contrarotating, wherein the downstream impeller is retractable so as to reduce its diameter and the mechanism for actuating the downstream impeller in the retracted position comprises a brake for braking the rotation of the rotor and a means for actuating the blades into a retracted position. 
         [0011]    By means of the invention, the diameter of the downstream impeller is reduced sufficiently for it not to see the vortexes in the phases in which it is desired to attenuate the engine noise, that is to say during take-off and landing of the aircraft. The downstream impeller is retracted preferably sufficiently so that, during these phases in the vicinity of airports, only one impeller is in use. In this manner, the totality of the interactions is suppressed. 
         [0012]    The blades of the downstream impeller are mounted so as to pivot about a pivot, the axis of which forms a nonzero angle with the axis of rotation of the impeller, the blades in the retracted position being tilted about the pivot. Advantageously, the axis of the pivot is perpendicular to the axis of rotation of the impeller. In the latter case, the blades can be placed along the nacelle while reducing drag. 
         [0013]    The impeller is braked until it stops rotating in order to make the retraction easier. 
         [0014]    According to one feature, the actuation mechanism comprises a means for compensating for the braking of the downstream impeller with an acceleration of the upstream impeller. Such an arrangement makes it possible to maintain the power of the rotor in an awkward phase of the flight. 
         [0015]    The actuation means comprises for example springs acting in opposition to the centrifugal force or else cylinders. 
         [0016]    According to a preferred embodiment, the brake is arranged between the turbine rotor driving the downstream impeller and the shaft of the downstream impeller so as to reduce the rotation speed of the impeller relative to the rotation speed of the turbine rotor. More particularly, the downstream impeller is driven by means of an epicyclic gear train. The gear train may drive both impellers. Thus the epicyclic gear train comprises planet gears mounted on a planet carrier between a ring gear and a central gear, the downstream impeller being driven by the planet gears and the brake being arranged to brake the rotation of the planet carrier. The upstream impeller is connected to said ring gear so as to be rotated by the ring gear. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention is now described in greater detail without the description being limiting, with the aid of the appended drawings in which: 
           [0018]      FIG. 1  represents in axial section the diagram of a turbine engine with nonstreamlined dual impellers; 
           [0019]      FIG. 2  shows the engine of  FIG. 1  with the transmission via an epicyclic gear train. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    With reference to  FIG. 1 , it shows a turbine engine  1  of the “open rotor” type, according to a preferred embodiment of the present invention. 
         [0021]    In the figures, the direction A corresponds to the longitudinal direction or axial direction, parallel to the longitudinal direction  2  of the turbine engine. The direction B corresponds for its part to the radial direction of the turbine engine. Moreover, the arrow  4  schematizes the direction of travel of the aircraft under the action of the thrust of the turbine engine  1 , this direction of travel being opposite to the main direction of flow of the gases within the engine. The terms “front”, “upstream”, “rear”, “downstream” used in the reset of the description are to be considered with respect to said direction of travel  4 . 
         [0022]    In the front portion, the turbine engine has an air inlet  6  continuing toward the rear through a nacelle  8 , the latter comprising in general an external casing  10  centered on the axis  2 . 
         [0023]    The turbine engine comprises a gas generator  14  consisting of a gas turbine engine. The latter in this instance is a twin-spool engine with 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 , thus forming a first low-pressure spool, while the compressor  18  and the turbine  22  are mechanically connected by a drum  28 , forming a higher-pressure spool. 
         [0024]    Downstream of the turbine  24 , there is a system of contrarotating impellers  30 , forming a receiver of the gas generator. 
         [0025]    This system  30  comprises a free power turbine  32 , which forms a low-pressure turbine, and which has the particular feature of being contrarotating. Specifically, with reference more precisely to  FIG. 2 , it comprises a first rotor  32   a  forming the internal rotor of the contrarotating turbine, and a second rotor  32   b  forming the external rotor of this turbine. 
         [0026]    The impeller system  30  comprises a stator or housing  34 , centered on the longitudinal axis  2  of the system, and enclosing notably said free power turbine  32 . This stator  34  is, in a known manner, designed to be secured to the other housings of the turbine engine. In this respect, it is indicated that the impeller system  30  is preferably designed so that the impellers have no external radial streamlining surrounding them, as can be seen in the figures. 
         [0027]    Moreover, downstream of the contrarotating turbine  32 , the impeller system  30  incorporates a first impeller  7  or downstream impeller, supporting blades  7   a.  Similarly, the system  30  comprises a second impeller  9  or upstream impeller, supporting blades  9   a.  Thus, the impellers  7 ,  9  are offset relative to one another in the direction  4 , and both are situated downstream of the free turbine  32 . 
         [0028]    The two impellers  7 ,  9  are designed to rotate in opposite directions about the axis  2  on which they are centered, the rotations taking place relative to the stator  34  remaining immobile. 
         [0029]    According to the invention, provision is made to allow the retraction of the downstream impeller  7  for the purpose of reducing the noise emitted by the interaction of the upstream impeller on the downstream impeller. Shown in  FIG. 1  are the two possible configurations of the downstream impeller. The blades of the downstream impeller are each mounted on a pivot  71  with its axis perpendicular to the axis of rotation  2 , and are articulated about the latter so as to be able to take a deployed position in the transverse plane and a retracted position in the downstream direction along the nacelle. In the latter position, with a blade shown in dashed lines, the diameter of the impeller is thus reduced. Therefore, the current lines travel along the blade tip and give rise to blade-tip vortexes on the upstream impeller. When the downstream impeller is in the retracted position, the impact of these vortexes on the downstream impeller and the source of noise are prevented. 
         [0030]    In cruising configuration, during which it is not necessary to attenuate the noise of the impellers and which corresponds to more than 90% of the mission, the two impellers are deployed; in particular, the blades of the downstream impeller  7  extend in a radial direction relative to the axis of the engine. It is on take-off or on landing that the retracted position of the downstream impeller is activated and that generally represents only 10% of the mission. 
         [0031]    Various mechanisms allow the retraction of the downstream impeller. One means of actuating the blades into the retracted position includes for example spring means, not shown, which exert a force for tilting the blades about their pivot  71  in the downstream direction. These spring means are advantageously associated with a braking means  72  for braking the downstream impeller in order to form the mechanism for actuating the downstream impeller into the retracted position. In this manner, when the brake is actuated, the spring means exceed the centrifugal forces applied to the blades, which causes them to tilt in the downstream direction. 
         [0032]    The spring means may be reinforced by cylinders or any other equivalent means, exerting a rotational torque on the blade. According to another embodiment, the latter may even replace them. 
         [0033]    According to the embodiment shown here, the braking means  72  acts on the shaft  29  for driving the impeller  7 . 
         [0034]    The mode for driving the impellers according to this embodiment is not direct but is done via a gear mechanism such as a speed-reduction box and more particularly by means of an epicyclic gear mechanism. 
         [0035]    With reference to  FIG. 2 , for the rotation of these two impellers  7 ,  9 , a transmission device  13  is provided, forming a reduction gear and notably comprising an epicyclic gear train  15 . Such a drive by epicyclic gear train is described in the patent application in the name of Snecma FR200080058822, filed on 19 Dec. 2008. 
         [0036]    The gear train  15  is furnished with a sun gear  17  centered on the longitudinal axis  2 , and supported by a sun-gear shaft  19  with the same axis, securely connected in the upstream direction to the first rotor  32   a  via a flange  38 . Therefore, the rotor  32   a  rotates the sun gear  17  directly, the latter taking the form of a gearwheel with external teeth. 
         [0037]    The gear train  15  also comprises a planet gear  21 , and preferably several as can be seen in  FIG. 2 , each of them meshing with the sun gear  17 . Each planet gear  21  is supported by a planet-gear shaft  23  with an axis that is off center relative to the axis  2 , and takes the form of a gearwheel with external teeth. 
         [0038]    Moreover, the gear train  15  is fitted with a planet carrier  25  centered on the longitudinal axis  2  and supporting in a rotary manner each of the planet gears  21 , by means of the shafts  23  respectively. The planet carrier  25  is supported by a planet-carrier shaft  29  with the same axis, secured to the first impeller  7 , as can be seen in  FIG. 2 , so as to be able to rotate it directly. 
         [0039]    Finally, the gear train  15  has a ring gear  31  centered on the axis  2  and supported by a ring-gear shaft  33  with the same axis, this ring gear  31  meshing with each planet gear  21 . The shaft  33  extends in the downstream direction while being secured to the second impeller  9 , so as to be able to rotate it directly. For example, this shaft  33  is situated around the planet-carrier shaft  29  with which it is concentric, as shown in the figures. 
         [0040]    The ring gear  31 , taking the form of a gearwheel with internal teeth, has the additional particular feature of also being supported by another ring-gear shaft  35 , with the same axis, and extending for its part in the upstream direction. This ring-gear shaft  35 , situated around the sun-gear shaft  19  with which it is concentric, is securely connected to the second rotor  32   b,  via a flange  40 . Thus, the rotor  32   b  also participates directly in the driving of the ring gear  31 , and therefore in the driving of the upstream impeller  9 . This makes it possible to obtain a unitary ratio between the torques transmitted respectively to the downstream impeller  7  and to the upstream impeller  9 , in order to obtain a better output from the turbine engine. 
         [0041]    When the braking means  72  is activated, the planet-gear carrier  25  no longer rotates about the sun gear  17  causing an increase in the rotation speed of the upstream impeller  9 . This therefore compensates for the reduction in thrust resulting from the downstream impeller. In order to complete the device, the blades at least of the upstream impeller are variable-setting blades in order to optimize the performance of the latter. 
         [0042]    Finally, it is noted that, in this preferred embodiment, in which each impeller is fitted with a system for the variable setting of its blades, the epicyclic gear train  15  is situated in line with and inside a housing  42  separating the contrarotating free power turbine  32  and the impellers  7 ,  9 . 
         [0043]    This housing  42 , also called the exhaust housing or else “static frame”, supports an engine mount  44  designed to couple the turbine engine to the structure of the aircraft.