the propulsive system 1 , represented on fig1 , 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 fig2 , 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 fig2 , 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 fig2 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 fig4 . 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 fig4 a . 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 fig4 a , or if it is made with a low density material ( for example steel ) and presents a high length l 40 , as shown on fig4 b . 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 fig5 , 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 fig2 and 3 and intended to be inserted in cavities 81 ( fig6 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 fig2 , 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 fig8 , 9 , 10 and 11 . each of such rings — for example the ring 90 ( fig9 )— 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 fig1 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 .