Abstract:
In order to eliminate or at least to reduce a possible unbalance of the propulsive system, counterweights are brought in locations that are provided on the hub envelope, near the ends of the latter and away from the planes of the propellers. More particularly, the counterweights are provided as discrete elements located in two planes spaced from the propellers and orthogonal to an outer surface of the hub envelopes, or are provided as a non-uniform mass ring.

Description:
TECHNICAL FIELD 
     The present invention relates to a balancing method for a propulsive system with coaxial non streamlined contra-rotating propellers. 
     BACKGROUND 
     It is known that, in such propulsive system, generally so-called CROR (for “Contra-Rotating Open Rotor”), each propeller comprises a hub concentrically surrounded by a hub envelope, each hub envelope projecting on either part of the plan of the corresponding propeller. In such propulsive system type, the centre of gravity of each propeller is adapted to be deviated from the axis of rotation thereof, thereby resulting, upon its rotation, in a radial unbalance phenomenon. Now, such unbalance is able to generate mechanical vibrations being able to be transmitted to a piece of equipment—in particular the nacelle of an aircraft receiving passengers—to which the propulsive system is attached, which can be unpleasant for them. 
     In order to eliminate (or at least to reduce) such possible radial unbalance, it is already known to fit a set of counterweights on locations provided on the propulsive system, in the plan of each propeller, being orthogonal to the axis of rotation thereof. It will be understood that “plan” in this application means the same as “plane,” and these terms are used interchangeably herein. The centre of gravity of each propeller can then be repositioned relative to the axis of rotation thereof by appropriately arranging, on said locations, counterweights, the masses of which are adequately selected taking their locations into account, which can be for example performed upon test phases of the propulsive system, either on the ground, or in flight conditions. 
     However, such balancing for the propulsive system is frequently found insufficient. Indeed, when the propulsive system is in flight, the propeller blades are subjected to a set of aerodynamic forces, amongst which axial aerodynamic thrust and resistance forces. It results in additional forces and moments occurring, to which said propellers are subjected, thereby unbalancing all the more the propulsive system. 
     The object of the present invention is to remedy such disadvantage. 
     SUMMARY 
     With the end in view, according to the invention, the balancing method for a propulsive system with at least two coaxial non streamlined contra-rotating propellers, each of said propellers comprising a hub concentrically surrounded by a hub envelope and each of said hub envelopes projecting on either part of the plan of the corresponding propeller, said balancing method according to which, to eliminate, or at least to reduce a possible unbalance of said propulsive system, counterweights are arranged on locations being provided on the latter, is remarkable in that said locations are located on said hub envelopes, close to the ends of the latter being distant from the plans of said propellers. 
     Thus, according to the present invention, the counterweights being arranged, in the propulsive system, distant both radially and axially from the centre of gravity of each propeller, it results in the generation of forces and moments being able to precisely correct the radial and axial unbalancings. 
     Moreover, thanks to the present invention, the manual balancing of the propulsive system can be easily performed by an operator, since the locations for the counterweights are directly provided on the hub envelopes, which are external parts of the system and thus very accessible. 
     Furthermore, it will be noticed that the present invention, due to the positioning of the counterweights on the ends of each hub envelope, takes optimally advantage of the space being available at the level of each propeller. 
     In order to make the balancing both precise and simple, for at least one of the hub envelopes, the locations of the counterweights can be located in two plans being orthogonal to the propeller axis and arranged on either part of the plan of the corresponding propeller, on the ends of said hub envelope. 
     According to a first embodiment of the invention, at least one of the counterweights is at least partially embedded in a cavity arranged on one of said locations. 
     When at least some of the counterweights are at least partially embedded in cavities arranged on some of the locations, said cavities can be circumferentially distributed on the corresponding hub envelope. 
     So that the propulsive system balancing has no influence on the aerodynamic performances of said system, at least one of the cavities can be configured in such a way that the corresponding counterweight, once embedded within said cavity, does not project on the corresponding hub envelope. 
     Amongst the above defined cavities, at least one of them can comprise: 
     a threaded housing in which the corresponding counterweight can be inserted, said counterweight presenting then at least partially the shape of a screw, or 
     an orifice in which the lower part of the corresponding counterweight can be inserted, the upper end of said counterweight being provided with at least one fastening orifice through which the latter can be fastened to the corresponding hub envelope by means of at least a fastening member, such as a rivet. 
     According to a second embodiment of the invention, some of the counterweights can form a circumferentially non uniform mass ring inserted within a housing being coaxial to the corresponding hub envelope and arranged on one of the locations, said housing being bound, on the one side, by the latter, and, on the other side, by a shoulder arranged under said hub envelope. 
     In such a case, so as to correctly position the circumferentially non uniform mass ring and to hold it furthermore in such position, the latter and the corresponding hub envelope can each be provided with at least one lock orifice, said lock orifices being able to be put opposite each other with a view to introduce a lock member. 
     In order to easily position the circumferentially non uniform mass ring, the latter can comprise an angular graduation being visible thru said hub envelope. 
     The invention also relates to a propulsive system with at least two coaxial non streamlined contra-rotating propellers, each of said propellers comprising a hub concentrically surrounded by an hub envelope and each of said hub envelopes projecting on either part of the plan of the corresponding propeller, said propulsive system comprising a counterweights arranged on locations provided on the latter so as to eliminate, or at least to reduce, a possible unbalance of said propulsive system, said propulsive system being remarkable in that said locations are located on said hub envelopes, close to the ends of the latter being far from the plans of said propellers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The FIGS. of the accompanying drawing will make well understood how the invention can be implemented. On such FIG., identical reference annotations designate similar technical elements. 
         FIG. 1  is a schematic partial sectional view of a propulsive system with coaxial non streamlined contra-rotating propellers, provided with a set of counterweights according to the present invention. 
         FIG. 2  is a schematic perspective view of one of the propellers of the propulsive system of  FIG. 1 , the counterweights being screws inserted in threaded housings arranged in the hub envelope of said propeller. 
         FIG. 3  is a sectional view by a plan passing thru the threaded housings, the propeller blades being not represented. 
         FIGS. 4A and 4B  are views of different counterweights under the form of a screw, being able to be inserted in threaded housings previously arranged in the hub envelope of the propeller of  FIGS. 2 and 3 . 
         FIG. 5  is a schematic perspective view of one of the propellers of the propulsive system of  FIG. 1 , the counterweights being masses inserted and fastened in orifices arranged in the hub envelope of said propeller. 
         FIG. 6  is a partial plan view of the hub envelope of the propeller of  FIG. 5 . 
         FIG. 7  is a sectional view by a plan passing thru one counterweight of  FIGS. 5 and 6 . 
         FIG. 8  is a schematic perspective view of one of the propellers of the propulsive system of  FIG. 1 , the counterweights forming circumferentially non uniform mass rings. 
         FIG. 9  is a schematic perspective view of one of the rings of  FIG. 8 . 
         FIG. 10  is a sectional view of the hub envelope of the propeller of  FIG. 8 , at the level of the circumferentially non uniform mass rings. 
         FIG. 11  is a partial plan view of the hub envelop of  FIG. 8 , at the level of the circumferentially non uniform mass rings. 
     
    
    
     DETAILED DESCRIPTION 
     The propulsive system  1 , represented on  FIG. 1 , comprises two non streamlined propellers, respectively a front propeller  2  and a rear propeller  3 , arranged to rotate around a common axis A-A′, in opposed rotation directions. The front propeller  2  comprises a plurality of blades  20  (only two of which are represented on  FIGS. 2 ,  5  and  8 ), arranged in a plane B-B′ (orthogonal to the axis A-A′) forming the plane of the propeller  2 , as well as a hub  21  being concentrically surrounded by a hub envelope  22 , the latter projecting on either part from said plane B-B′. Similarly, the rear propeller  3  comprises a plurality of blades  30  arranged in a plane C-C′ (orthogonal to the axis A-A′) forming the plane of the propeller  3 , as well as a hub  31  concentrically surrounded by a hub envelope  32 , the latter projecting on either part of said plane C-C′. 
     In order to eliminate—or at least to reduce—a possible unbalance due to a shift of the centre of gravity of each propeller relative to the axis of rotation A-A′, on the hub envelopes  22  and  32 , according to the present invention, a set of counterweights respectively  40 ,  50  and  60 ,  70  is arranged, the respective masses and positions of which are determined so as to correctly reposition said centres of gravity. 
     More precisely, the counterweights  40  and  50  are respectively inserted on locations being provided on the hub envelope  22 , in two planes D-D′ and E-E′, both orthogonal to the axis A-A′ (and thus parallel to the plane B-B′ of the propeller  2 ) and located on either part of said plane B-B′, close to the front  22 A and rear  22 B ends of said hub envelope  22 . Similarly, the counterweights  60  and  70  are respectively inserted on locations being provided on the hub envelope  32 , in two planes F-F′ and G-G′, both orthogonal to the axis A-A′ (and thus parallel to the plane C-C′ of the propeller  3 ) and located on either part of the plane C-C′, close to the front  31 A and rear  32 B ends of said hub envelope  32 . 
     The arrangement of the counterweights  40 ,  50 ,  60  and  70  in four distinct planes (two planes for each propeller), as defined above, thus gives an operator the possibility to balance the propulsive system, not only easily from the most accessible parts of said system (the hub envelopes), but also precisely. It will be noticed that, due to their arrangement, the locations for the counterweights can be directly arranged upon the manufacture of the hub envelope. 
     Subsequently, only will be described the locations and counterweights located in the plane D-D′ of the front propeller  2 , but it goes without saying that the following can apply in a similar way to the locations and counterweights arranged in the planes E-E′, F-F′ and G-G′. 
     According to a first embodiment of the invention, represented on  FIGS. 2 ,  3 ,  4 A and  4 B, the counterweights  40  are present as screws  40 . 1 ,  40 . 2 ,  40 . 3 ,  40 . 4 ,  40 . 5  and  40 . 6  arranged equidistant on a circumference of the hub envelope  22  (i.e. angularly separated by 60 .degree.), located in the vicinity of its front end  22 A, in the plane B-B′. To obtain a precise balancing, the arrangement of at least six equidistant counterweights (as shown on FIGS.  2  and  3 )—preferably eight ones separated by 45 .degree.—can be appropriate. 
     The locations for the counterweights  40 . 1  to  40 . 6  are represented on  FIG. 4 . At the level of such locations, cavities  41 . 1 ,  41 . 2 ,  41 . 3 ,  41 . 4 ,  41 . 5  and  41 . 6  are regularly arranged on the circumference of the hub envelope  22 , at a distance from the axis A-A′ being substantially equal to the radius R 22  of said hub envelope  22 . 
     One of such cavities, designated by the reference  41  and which corresponds to anyone of cavities  41 . 1  to  41 . 6 , is represented more in details on  FIG. 4A . It comprises, in the lower part thereof, a threaded housing (preferably of a circular shape) in which the screw  40  can be nested (such screw indifferently designates one of the screw  40 . 1  to  40 . 6 ), and, in its upper part, a housing in which the head of said screw  40  can be embedded so that the latter does not project on the hub envelope  22 . 
     It will be noticed that the mass of each counterweight  40  is in particular a function of its diameter φ 40 , of its length L 40  and of its constitutive material. Thus, an equivalent counterweight effect can be produced by the counterweight  40  if, for an equal diameter φ 40  (for example 10 mm), the latter is made in a high density material (such as tungsten of a density equal to 19,500 kg/m 3 ) and present a short length L 40  (for example 35 mm) as shown on  FIG. 4A , or if it is made with a low density material (for example steel) and presents a high length L 40 , as shown on  FIG. 4B . Consequently, a high density material will be preferred because it enables to use screws (and thus cavities) of a smaller length, thereby allowing for space spare within the hub envelope. 
     The cavities  41  themselves can be made in low density materials (which allows for a reduction of the extra mass induced by the balancing device), but preferably high temperatures resistant. Furthermore, the counterweights  40  and the cavities  41  generating local mechanical constraints on the hub envelope  22 , there can be added to said hub envelope a reinforcing ring (non represented)—or at least a portion of such ring—at least at the level of said cavities  41 . Moreover, a lock mechanism can be joined to at least one of the counterweights  40 , once the latter is inserted in the corresponding cavity  41  so as to avoid any detachment of said counterweight  40  upon the operation of the propulsive system  1 . 
     Thus, the balancing of the propulsive system  1  can be effected by appropriately inserting the counterweight  40 , presenting at least partially the screw shape and the masses, diameters and lengths of which are correctly selected, within the cavities  41 . If there are cavities in which no counterweight must be inserted to balance the propulsive system, there can be used screws made in light materials or hollow screws so as to occupy such cavities. 
     According a second embodiment of the invention, represented on  FIGS. 5 ,  6  and  7 , the counterweights  40  in a screw shape  40 . 1  to  40 . 6  are replaced by mass-shape counterweights  80 ,  80 . 1 ,  80 , 2 ,  80 . 3 ,  80 . 4 .  80 . 5  and  80 . 6 , arranged just like the counterweights  40  of  FIGS. 2 and 3  and intended to be inserted in cavities  81  ( FIGS. 6 and 7 ), which are present under the shape of orifices arranged in the hub envelope  22 . 
     More precisely, each of such counterweights, designated by the reference  80  (indifferently corresponding to the counterweights  80 . 1  to  80 . 6 ) comprises, on the one side, a lower part  80 A constituting the largest part of the mass of the counterweight, and, on the other side, an upper end  80 B of a dimension higher than the lower part  80 A and being able to be embedded in the orifice  81  of the hub envelope  22 . Such upper end  80 B is provided with two fastening orifices  80 A and  80 B. A ring  83  of a circumferentially uniform mass and a higher width than the dimensions of the orifice  81 , is further arranged so as to be abutted against the internal wall of the hub envelope  22 , at the level of said orifices  81 . Such ring  83  is furthermore provided with a plurality of orifices, amongst which orifices  83 A and  83 B intended to be faced to the fastening orifices  82 A and  82 B of the upper end  80 B of each counterweight  80 , as well as orifices  80 C intended to be crossed by the lower part  80 A of each counterweight  80  (but not by their upper end  80 B). 
     Thus, thru the ring  83 , each counterweight  80  can be fastened to the hub envelope  22  by welding fastening members  84 A and  84 B respectively inside the orifices  82 A and  83 A and inside the orifices  82 B and  83 B, when said counterweight  40  is correctly inserted in the orifices  81  and  83 C. To do so, the fastening members  84 A and  84 B may be rivets. 
     It will be noticed that the use of counterweights  80  under the form of masses being joined to the hub envelope  22  thru fastening members enables, with respect to the screw-shaped counterweights  40  such as represented on  FIGS. 2 ,  3 ,  4 A and  4 B, to strengthen the fastening of said counterweights in their respective cavities and thus to avoid any detachment risk for said counterweights. However, in this case, when a counterweight has to be replaced by another one for a re-balancing, it is important to withdraw the fastening members and, in the case of rivets, to weld new ones, which needs specific tools. 
     According to a third embodiment of the invention, the counterweights  40  and  80  are replaced by two circumferentially non uniform mass rings  90  and  100  (i.e. the mass of which is not uniform on their circumference), as shown on  FIGS. 8 ,  9 ,  10  and  11 . Each of such rings—for example the ring  90  (FIG.  9 )—presents a radius R  90  being substantially lower than the one R 22  of the hub envelope  22  and is provided with a plurality of lock orifices  91 . Moreover, each ring  90  (respectively  100 ) is arranged in a housing  91  ( 101 ) being coaxial to the hub envelope  22  and located under the internal wall of said hub envelope  22 , such housing  91  ( 101 ) being circumferentially bound, on the one side, by said hub envelope  22  and, on the other side, by a shoulder  92  ( 102 ) arranged under said hub envelope  22 . 
     Thus, each ring  90  (respectively  100 ) can be moved into its housing  91 - 101  by a simple sliding along the shoulder  92  ( 102 ), which then plays a guiding role, so that the distribution of the extra mass (circumferentially non uniform) being induced by the ring  90  ( 100 ) allows the propulsive system  1  to be re-balanced on an adequate way. In order to slide the ring  90 ( 100 ), an operator can use any adequate tool being at his disposal. Subsequently, when the ring  90 ( 100 ) is positioned on a desired way within its housing, the position thereof can be locked by putting opposite a lock orifice  91 ( 101 ) provided on said ring ( 100 ) and a lock orifice  93  ( 103 ) provided on the hub envelope  22 , and then by coupling them to each other by means of an appropriate lock member (for instance, screw, rivet, etc.). 
     It should be noticed herein that the angular positioning of each ring (and thus of balancing of the propulsive system  1 ) is all the more precise than the number of lock orifices  91  provided on the ring  90  is high. 
     Furthermore, as it is represented on  FIG. 11  as far as the ring  90  is concerned (but that also applies to the ring  100 ), the hub envelope  22  is provided with an additional orifice  95  thru which an angular graduation  96 , arranged on said ring  90 , can be read, so that the operator who manipulates said ring  90  can easily determine the angular position of the latter and deduce therefrom the displacement to be conferred on him. With this end in view, the angular position  96  can be directly written on the ring  90 .