Abstract:
The present invention relates to the field of blades, particularly fan blades, intended for turbojets, particularly of the aeronautical type. 
     Its objective is to provide a fan blade the features of which will enable the number of fan blades to be reduced while at the same time providing satisfactory performance. 
     According to the invention, the fan blade comprises a plurality of aerodynamic parts which are superposed in a radial direction Z and the number of aerodynamic profiles varies from one aerodynamic part to the other.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
       [0001]    The present invention relates to the field of blades, particularly fan blades, intended for turbojets, particularly of the aeronautical type. 
         [0002]    The turbojet conventionally comprises a compressor, a combustion chamber and a turbine. The purpose of the compressor is to increase the pressure of the air supplied to the combustion chamber. The purpose of the turbine is to drive the rotation of the compressor by tapping off some of the pressure energy of the hot gases leaving the combustion chamber and converting it into mechanical energy. 
         [0003]    A turbojet may be of the “bypass” type, that is to say one through which two air flows pass, a primary flow and a secondary flow. The primary flow is produced by elements that make up a single flow turbojet to which one or more additional turbines are added in order to drive a compression stage, the fan. This fan is equipped with large-sized blades, the fan blades, which produce the secondary flow. The fan slightly increases the pressure of the gases passing through it but, because its diameter is large, the energy produced for thrust is high. 
         [0004]    One well-known example of a turbojet with a fan, also known as a turbofan is the CFM56 fitted to numerous airplanes across the world for a number of decades now. The successive series of the CFM56 have seen a gradual decrease in the number of fan blades. 
         [0005]    Decreasing the number of fan blades on a turbojet is advantageous insofar as it allows a significant reduction in the mass of the turbojet and a reduction in procurement and maintenance costs. This reduction in the number of blades must not, however, be made at the expense of turbojet performance. Preferably, an increase in the chord length of the blades should be avoided in order to limit the size, and thus the mass, of the turbojet. 
         [0006]    Gradually reducing the number of blades involves increasing the relative pitch better known in English as the “pitch to chord ratio” and, for the same chord length, increasing the inter-blade distance, that is to say the distance separating two consecutive blades. 
         [0007]    The pitch to chord radio is defined as the ratio s/C where:
       s represents the inter-blade pitch (s=2πR/N) N being number of blades on a bladed disk, and   C represents the chord of the profile at the radius R considered over the height of a blade, the chord C representing the length of the segment connecting the leading edge to the trailing edge of a blade.       
 
       SUMMARY OF THE INVENTION 
       [0010]    The objective of the present invention is to provide a fan blade the features of which will enable the number of fan blades to be reduced while at the same time providing satisfactory performance. 
         [0011]    To this end, the present invention relates to a bypass turbojet fan blade which comprises a plurality of aerodynamic parts which are superposed in a radial direction Z and in which the number of aerodynamic profiles varies from one aerodynamic part to the other. 
         [0012]    One aerodynamic part has aerodynamic properties imparted by at least one aerodynamic profile, each aerodynamic profile comprising a suction face, a pressure face, a leading edge and a trailing edge. 
         [0013]    The radial direction z corresponds to the essentially longitudinal direction of a blade. It is usually termed the radial direction by the person skilled in the art because this direction corresponds to a radius leading from the axis of rotation X of the turbojet on which said blade is customarily positioned during operation. 
         [0014]    As a preference, the fan blade according to the invention comprises a lower aerodynamic part and an upper aerodynamic part which is superposed in a radial direction Z, each aerodynamic part having at least one aerodynamic profile, the number of aerodynamic profiles of the upper aerodynamic part being greater than the number of aerodynamic profiles of the lower aerodynamic part. 
         [0015]    Within the meaning of the present invention, the upper aerodynamic part denotes that part of the blade furthest from the axis of rotation X of the turbojet on which said blade is customarily positioned in operation and the lower aerodynamic part denotes that part of the blade closest to the axis of rotation X of the turbojet. 
         [0016]    As a preference, the lower aerodynamic part has a single aerodynamic profile and the upper aerodynamic part has at least two aerodynamic profiles. 
         [0017]    As a preference, the aerodynamic profiles of the one and the same aerodynamic part are identical. 
         [0018]    The fan blade according to the invention may further comprise a platform separating the lower aerodynamic part and the upper aerodynamic part. This platform may constitute an air splitter, particularly to separate the primary flow from the secondary flow in a bypass turbojet. 
         [0019]    The invention also relates to a turbojet comprising at least one fan blade fixed, either by its lower end to a hub, or by its upper ends to a rotary casing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Other advantages and features of the invention will become apparent from reading the detailed description which follows, with reference to the attached figures, provided by way of nonlimiting examples, in which: 
           [0021]      FIG. 1  depicts a front view of fan blades according to the invention positioned on a hub; 
           [0022]      FIG. 2  depicts a perspective view of fan blades according to the invention, positioned on a hub; 
           [0023]      FIG. 3  depicts a side view of a fan blade according to a first embodiment of the invention; and 
           [0024]      FIG. 4  depicts a side view of a fan blade according to a second embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]      FIGS. 1 and 2  depict three fan blades  1  according to the invention positioned on a hub  2  of axis of revolution X. The axis of revolution X of the hub  2  coincides with the axis of rotation X of the turbojet. 
         [0026]    The fan blade  1  according to the invention, which extends radially from the axis X, comprises a lower aerodynamic part  11  and an upper aerodynamic part  12 . The lower aerodynamic part  11  consists of one aerodynamic profile. 
         [0027]    In the example illustrated in  FIGS. 1 and 2 , the upper aerodynamic part  12  of the fan blade  1  comprises two aerodynamic profiles  14 . Alternative forms of embodiment of the invention with a fan blade  1  comprising more than two aerodynamic profiles  14  are also conceivable. A fan blade  1  comprising three aerodynamic profiles  14  is also advantageous. A blade such as this is depicted in dotted line in  FIG. 2 . These aerodynamic profiles  14  are preferably identical and directed radially. 
         [0028]    When the upper aerodynamic part  12  of the fan blade  1  comprises at least two aerodynamic profiles, the number of blades increases and this appreciably reduces the pitch to chord ratio in the upper aerodynamic part  12  of the blade. The pitch to chord ratio at the upper end  16  of the fan blade  1  therefore has more limited values for which the compression ratio is satisfactory. 
         [0029]    By keeping a higher pitch to chord ratio on the lower aerodynamic part  11  of the fan blade, it is possible to safeguard against possible aerodynamic lock problems that arise when it becomes difficult to provide the primary flow with a sufficiently high flow rate. 
         [0030]    A platform  10  separates the lower aerodynamic part  11  from the upper aerodynamic part  12  of the fan blade  1 . This platform  10  connects the upper end of the aerodynamic profile  13  to the lower ends of the other two aerodynamic profiles  14 . In order to cause the least possible disturbance to the flow of the primary and secondary flows, this platform  10  needs to lie at a point on the fan blade  1  where the primary and secondary flows form. As a preference, it itself constitutes an air splitter dividing the primary flow from the secondary flow. 
         [0031]    As a preference, the platform  10  has an aerodynamic shape so as to guide the flow of air to which it is liable to be subjected. 
         [0032]    The platform  10  may also be of the contiguous type, that is to say that it has a shape capable of espousing the complementary and identical shape of an adjacent platform  10  when the fan blades  1  from which they hang are duly positioned on a hub  2 . 
         [0033]    According to a first embodiment depicted in  FIG. 6 , the fan blade  1  is fixed by its lower end  15  to the hub  2 , its upper ends  16  being free. Attachment may be performed using techniques known to those skilled in the art such as, for example, collaboration between a tenon situated at the lower end  15  and sliding in a mortise slot in the hub  2 . 
         [0034]    Advantageously, the axial size of the aerodynamic profiles  13  and  14  may be substantially identical, for example where they meet one another. Thus, for example, at the platform  10 , the axial size of an aerodynamic profile  13  is identical to the axial size of an aerodynamic profile  14 . As a result, at the platform  10 , the leading edges of the aerodynamic profiles  13  and  14  are axially aligned. Likewise, at the platform  10 , the trailing edges of the aerodynamic profiles  13  and  14  are also axially aligned. 
         [0035]    In this first embodiment, the fan blade  1  is conventionally subjected to tensile stresses in a radial direction Z with respect to the axis X of the hub  2 . 
         [0036]    According to a second embodiment depicted in  FIG. 4 , the fan blade  1  is fixed by each of its upper ends  16  to a rotary casing  3  of axis X, its lower end  15  possibly being free. The rotary casing  3  is in the form of a shell ring surrounding the fan and secured to the latter. The assembly formed by the rotary casing  3  and the fan blades  1  can be rotated about the axis X. Rotational drive of the assembly is performed through a system of gears  4  mechanically connecting the rotary casing  3  to the turbine of the turbojet. 
         [0037]    This second embodiment also has the objective of eliminating the clearance between the blades and the casing  3  surrounding them. 
         [0038]    In this second embodiment, the fan blade  1  is subjected to compressive stresses. This configuration is advantageous because a mechanical component is better able to withstand compressive stresses than tensile stresses. 
         [0039]    Furthermore, the special shape of the fan blade  1  according to the invention contributes to its good mechanical integrity. As the fan rotates, centrifugal forces push the aerodynamic profile  13  and the platform  10  radially outward toward the rotary casing  3 . The stresses exerted by these components  10  and  13  are advantageously spread over the two aerodynamic profiles  14  of the upper aerodynamic part  12  of the fan blade  1 . The risk of the fan blade  1  buckling, that is to say of lateral deformation due to a normal compressive load, is therefore low. 
         [0040]    Attaching a fan blade  1  via the upper aerodynamic part  16  also presents advantages in terms of turbojet efficiency because the clearance between the upper end  16  of a fan blade  1  and the rotary casing  3  becomes nonexistent. Thus, the loss of efficiency due to this clearance in more conventional designs of turbojet disappears. 
         [0041]    Furthermore, this type of attachment advantageously makes it possible to reduce the mass of the turbojet by reducing the hub Ri/Re ratio, that is to say the ratio between the internal radius Ri and the external radius Re, Ri being distance to the point on the leading edge of the blade  1  closest to the axis (X) of the turbojet, and Re being the distance to the point on the leading edge of the blade  1  that is furthest from said axis (X). Because the hub  2  is no longer used, in this second embodiment, to attach the blades, the internal radius can be small or even zero. In one extreme case, the turbojet may have no hub  2  for the fan blades  1 . For the same external radius Re, the mass of the hub  2  can thus be small or even zero. The mass of the turbojet is thus reduced.