Patent Publication Number: US-2023145551-A1

Title: Propulsion system for an aircraft

Description:
TECHNICAL FIELD 
     The present invention relates to the field of the propulsion systems for aircraft. In particular, it relates to a propulsion system capable of providing a lateral thrust component. 
     The present invention also relates to an aircraft comprising such a propulsion system. 
     Prior Art 
     The technical background comprises in particular the documents US 2011/030380 A1, EP 2184481 A2, US 5328098 A1 and FR 2310471 A1. 
     A propulsion system for aircraft comprises at least one rotor or a propeller comprising a plurality of blades mounted on a rotating shaft. 
     There are aircrafts, and in particular Vertical Take-Off and Landing (VTOL) Aircraft, with single rotor propulsion systems when they comprise only one rotor or counter-rotating when they comprise rotors grouped in pairs turning in opposite directions. 
     These propulsion systems are either with shrouded rotors (the rotor is then surrounded by an annular nacelle fairing), or with free rotors, the propulsion systems and in particular the rotors (free or shrouded) being able to be mounted on a pivot shaft allowing the orientation of the propulsion systems and thus of the rotors between a vertical position and a horizontal position, for example the vertical orientation for a vertical take-off or landing and the horizontal orientation for forward flight or airplane flight mode. 
     The shrouded rotors have several interesting advantages, such as:
     a significant reduction in the sound signature of the rotor in direct emission;   a protection of the blades of the rotor from surrounding obstacles;   an improvement in the performance of the rotor, in particular in hovering flight of the aircraft or at low forward speed.   

     However, the VTOL aircrafts with shrouded rotors, in particular in hovering flight, do not have the same manoeuvrability as the conventional helicopters. 
     For conventional helicopters, altitude and placement corrections can be made, in the X, Y, Z aeronautical standards reference trihedral, by:
     corrections around the roll axis X (axis of rotation of the helicopter around its longitudinal axis) by a left or right action on the cyclic pitch (acting on the orientation of the thrust of the rotor, here in a plane perpendicular to the axis X);   corrections around the pitch axis Y (axis of the helicopter around its transverse axis) by an action forward (nose down) or backward (nose up) on the cyclic pitch (acting on the orientation of the thrust of the rotor, here in a plane perpendicular to the axis Y);   corrections around the yaw axis Z (axis of rotation in a horizontal plane of the helicopter around its vertical axis) by action on the rudder (acting on the thrust of the anti-torque rotor of the helicopter);   corrections in longitudinal translation along the axis X of the reference trihedral in aeronautical standards (longitudinal axis) by a command of the cyclic/collective pitch (acting on the orientation of the thrust of the rotor, here forward or backward);   corrections in lateral translation according to the axis Y of the reference trihedral in aeronautical standards (transverse axis) by a command of the cyclic/collective pitch, (acting on the orientation of the thrust of the rotor, here towards the right or the left);   corrections in vertical translation along the reference axis Z by a command of the collective pitch upwards, downwards (acting on the thrust of the rotor, setting pitch: command pulled upwards).   

     Of course, coordinated corrections of the collective pitch and of the rudder, well known to pilots, are required to counter the induced effects of loss of lift component and torque effect. 
     For the VTOL aircrafts with shrouded rotors, the positioning corrections, or the corrections allowing to counteract gusty winds in hovering flight, are usually complex and make the flight uncomfortable for the passengers of the aircraft. 
     For example, in the case of a quadrotor aircraft (i.e. an aircraft with four rotors), in order to make a correction in the direction of the reference axis X, it is necessary to tilt the aircraft forward about the reference axis Y (along the pitch axis), then to make a correction of the position reached (for example to counter a gust of wind) and to return quickly to a neutral position, referred to as “flat”. Similarly, in order to make a correction in the direction of the reference axis Y, it is necessary to tilt, i.e. roll, the aircraft towards and around the reference axis X (along the roll axis), then make a correction of the position reached and quickly return to a neutral position. With such a quadrotor, when transporting passengers, they may experience front/back and/or right/left sways. 
     A gas turbine engine has been proposed comprising a two-dimensional nozzle comprising a flexible panel capable of changing position, under the action of a cylinder, so as to regulate the exhaust of the engine. 
     Gas turbine engines comprising variable nozzle geometries have also been proposed, so that an outlet surface area of the nozzle can be varied. 
     Thus, these proposed solutions propose nozzles in which the two-dimensional vector thrust technology is implemented. 
     However, none of these proposed solutions proposes the implementation of the three-dimensional vector thrust technology in the nacelle fairings of propulsion systems, in particular in the VTOL mode with rotors, guiding the secondary flow resulting from said rotors. 
     There is therefore a need to provide a simple and effective solution to the above problems. 
     A purpose of the present invention is to provide a solution allowing to improve the manoeuvrability of the VTOL type aircrafts, while reducing the weight and aeronautical loss impacts associated with the flight commands. 
     In particular, the invention proposes to improve the manoeuvrability of the VTOL aircrafts by providing a propulsion system capable of providing a thrust component lateral to the axis of the rotors. 
     SUMMARY OFTHE INVENTION 
     To this end, the invention relates to a propulsion system for an aircraft, comprising at least one rotor and a nacelle fairing extending around said at least one rotor with respect to an axis of rotation of said rotor, this nacelle fairing comprising an upstream section forming an inlet cross-section of the nacelle fairing and a downstream section, a downstream end of which forms an outlet cross-section of the nacelle fairing; and characterised in that the downstream section comprises a radially internal wall and a radially external wall made of a deformable shape-memory material, and in that the radially external wall comprises a plurality of actuator mechanisms with at least one cylinder, each actuator mechanism being operable independently of the other actuator mechanisms and being configured to cooperate with means embedded in an internal surface of the radially external wall so as to deform the radially external wall in a radial direction with respect to the axis of rotation, under the effect of a predetermined displacement command. 
     The propulsion system according to the invention can be with single rotor or counter-rotating rotors, installed in a stationary or pivoting nacelle, with a through or offset pivoting axis. 
     According to the invention, the fairing is constituted at its air outlet by a semi-deformable shape-memory material and by a plurality of actuator mechanisms with cylinders configured to automatically vary the shape of the nacelle and, consequently, the orientation of the air flow at its outlet, in order to manoeuvre the aircraft on which said nacelle is installed. 
     The profile of the fairing advantageously has a semi-rigid downstream portion whose dimensions and shape of the trailing edge outlet cross-section can be varied, so that the trailing edge outlet cross-section can be oriented laterally under the effect of a controlled device, to form a current tube producing a thrust with a lateral component, referred to as vectoral or oriented thrust. 
     The propulsion system according to the invention thus allows to improve the manoeuvrability of the aircraft in which it is installed, in particular during manoeuvres at low forward speed, such as take-offs and landings, while minimizing the noise pollution induced by the rotor of the propulsion system and ensuring a safety of this rotor by the presence of the nacelle fairing. 
     In fact, the shape of the fairing is continuously adapted according to the deflection of the controlled thrust to obtain a precise placement of the aircraft. The profile of the fairing can thus be oriented laterally according to the mechanical constraints of the flight sought in VTOL mode, so as to produce a thrust component perpendicular to the rotor axis. 
     The actuator mechanism with cylinders allows advantageously a relatively rigid realization of the rear portion of the fairing, and an activation with short response time. 
     During the flight phases in airplane mode, the actuator mechanism with cylinders, which is useful during the phases with low forward speed, can be deactivated. 
     According to an embodiment, the means comprise a plurality of prisms distributed on the internal surface of the radially external wall in at least one annular row. Advantageously, the prisms may be angularly equidistant on the internal surface of the radially external wall. 
     In an embodiment, the prisms are actuated by the cylinders by means of at least one annular element. 
     Advantageously, the cylinders are angularly equidistant in the downstream section. 
     According to an embodiment, the plurality of actuator mechanisms with at least one cylinder comprises a first actuator mechanism with at least one cylinder, wherein the cylinder is configured to cooperate with the means so as to deform the radially external wall in a first radial direction relative to the axis of rotation, under the effect of a predetermined displacement command, and a second actuator mechanism with at least one cylinder, wherein the cylinder is configured to cooperate with the means so as to deform the radially external wall in a second direction which is opposite to the first direction, under the effect of a predetermined displacement command. 
     Advantageously, the propulsion system also comprises stiffening bridges connecting the radially internal and radially external walls of the downstream section and allowing to ensure a substantially constant distance between the radially internal and radially external walls of the downstream section. 
     The nacelle fairing may comprise an upstream section forming an inlet cross-section of the nacelle fairing and an intermediate section connecting the upstream and downstream sections. 
     Advantageously, the intermediate section is rigid and is connected by at least one mast to an engine of the propulsion system. This provides to the nacelle fairing of the propulsion system a rigid structure that can ensure a shielding function. 
     The present invention also relates to an aircraft characterised in that it comprises at least one propulsion system having at least one of the above-mentioned characteristics, the propulsion system being mounted so as to pivot on the aircraft by means of a pivot shaft that is offset from or passes through the rotor. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The invention will be better understood and other details, characteristics and advantages of the present invention will become clearer from the following description made by way of non-limiting example and with reference to the attached drawings, in which: 
         FIG.  1 A a    is a schematic perspective view of a first embodiment of a propulsion system shown with a nacelle mounted on an offset pivot axis, the propulsion system being in a horizontal position; 
         FIG.  1 B  is a view similar to  FIG.  1 A , showing the propulsion system in a vertical position; 
         FIG.  1 C  is a schematic perspective view of a second embodiment of a propulsion system shown with a nacelle mounted on a through pivot axis, the propulsion system being in horizontal position; 
         FIG.  2    is a schematic view in longitudinal cross-section of the propulsion system according to the invention with its downstream section of nacelle fairing in symmetrical thrust mode, i.e. axial thrust; 
         FIGS.  3  and  4    are views similar to  FIG.  2    of the propulsion system according to the invention with its downstream section of nacelle fairing in asymmetric thrust mode; 
         FIG.  5    is a schematic rear view of an embodiment of an actuator mechanism according to the invention, in axial thrust mode; 
         FIGS.  6  and  7    are schematic rear views of an embodiment of the actuator mechanism according to the invention, in asymmetrical thrust mode; and 
         FIG.  8    is a schematic cross-sectional view of the propulsion system according to another embodiment of the invention with its downstream section of nacelle fairing in axial thrust mode. 
     
    
    
     The elements having the same functions in the different embodiments have the same references in the figures. 
     DESCRIPTION OF THE EMBODIMENTS 
     In this disclosure, the terms “axial,” “internal,” and “external” are used in reference to the axis of rotation of the propulsion system according to the invention. 
     A propulsion system generally consists:
     of a nacelle;   of an engine and its command and control system;   and, in the case of a propeller or rotor propulsion, of its propeller or rotor or rotors.   

     The nacelle is the element which allows to integrate the engine with the aircraft, it is made up:
     of nacelle fairings (allowing to cover the engine, to shroud the rotors, to capture the air flow during the operation of the aircraft, to create a thrust effect, to reverse the thrust on the propulsion systems, etc.);   of equipment to be mounted on the engine (such as the engine build-up unit -EBU, including the electrical, hydraulic and pneumatic networks); and   of systems for attachment to the aircraft.   

       FIGS.  1 A and  1 B  illustrate, in a simplified manner, a first embodiment of an aircraft propulsion system  1  according to the invention. 
     Here, the propulsion system  1  comprises at least one rotor  2  and one nacelle fairing  3  extending around said at least one rotor  2  with respect to a rotation axis X of the rotor  2 . The fairing  3  advantageously acts, by its shape and its materials, as a sound barrier. The propulsion system  1  can be fixedly mounted on the aircraft. The propulsion system  1  can also be mounted on a pivot shaft  4 , offset from the axis of rotation X of the rotor  2 . The pivot shaft  4  is attached by any means to the propulsion system  1 , on the one hand, and to the aircraft, on the other hand, and allows the orientation of the propulsion system  1  on the aircraft, authorizing the tilting of the propulsion system  1  around the pivot shaft  4 , according to the arrow F1, by means of known actuators, between a horizontal position as illustrated in  FIG.  1 A , and a vertical position as illustrated in  FIG.  1 B . This tilting allows the aircraft to be switched from a conventional aircraft mode to a VTOL or helicopter mode. 
     The rotor  2  of the propulsion system  1  is connected to the aircraft by a mast  5  supporting an engine  6 , for example an electric motor, which drives the rotor  2  in rotation by means of a power shaft, in a manner known per se. According to the illustrated example in a non-limiting way, each rotor  2  comprises two blades  7 . 
       FIG.  1 C  illustrates a second embodiment of an aircraft propulsion system  1 ′ according to the invention in which the propulsion system  1 ′ can be mounted on a pivot shaft  4 ′, passing through the rotor  2  perpendicular to the axis of rotation X of the rotor  2 . The rotor  2  of the propulsion system  1 ′ is connected to the aircraft by a mast  5  supporting an engine  6 , for example an electric motor, which drives the rotor  2  in rotation by means of a power shaft, in a manner known per se. According to the embodiment, the mast  5  of the rotor  2  is merged with the pivot shaft  4 ′. 
     With reference to  FIGS.  2  to  4   , the nacelle fairing  3  of the propulsion system  1 ,  1 ′ according to the invention comprises:
     an upstream section  10 ;   a downstream section  20 ; and   an intermediate section  30  connecting said upstream  10  and downstream  20  section.   

     The upstream section  10  forms an inlet cross-section (or in other words a leading edge) BA or air inlet of the nacelle fairing  3 . 
     The upstream section  10  is made of a material that can withstand temperatures that make it suitable for anti-icing when supplied with hot air. 
     The intermediate section  30  is rigid. It is for example made of aluminium alloy, 6% aluminium and 4% vanadium (TA6V) filled titanium, or organic matrix carbon fibre composite. The intermediate section  30  is advantageously connected to the engine  6  of the propulsion system  1 ,  1 ′ by at least one mast  5 , and preferably by two masts  5  so as to mechanically fit the nacelle fairing  3  to the engine  6  of the propulsion system  1 ,  1 ′. The intermediate section  30  thus confers, by its material and its configuration, a shielding function to the propulsion system  1 ,  1 ′. 
     A downstream end  21  of the downstream section  20  forms an outlet cross-section (or in another word a trailing edge) BF or an air outlet of the nacelle fairing  3 . 
     The downstream section  20  comprises a radially internal wall  20   a  and a radially external wall  20   b . The radially internal  20   a  and radially external  20   b  walls of the downstream section  20  not only provide a structural function of the downstream section  20  but also an aerodynamic function. 
     The radially internal  20   a  and radially external  20   b  walls of the downstream section  20  are made of a semi-rigid deformable shape-memory material. In other words, the material constituting the radially internal  20   a  and radially external  20   b  walls of the downstream section  20  is both rigid to provide the downstream section  20  with a structural shape and flexible to provide the downstream section  20  with a deformability. Thus, the radially internal  20   a  and radially external  20   b  walls of the downstream section  20  are made of a material able to react under the effect of actuators as described below. When the radially internal  20   a  and radially external  20   b  walls of the downstream section  20  are energized by one or more actuators, the walls deform and when the energizing stress of the actuator or of the actuators stop, the walls return to their original shape. For example, the radially internal  20   a  and external  20   b  walls are made of a composite or a nickel-titanium alloy (also referred to as “Kiokalloy”) such as NiTiNol or NiTiCu. 
     The shape-memory material constituting the radially internal  20   a  and external  20   b  walls is fail-safe, i.e. it is such that its rest position, in other words when no actuator acts on the shape-memory material to deform it, corresponds to a natural storage geometry of said material or to a longer duration of use. Thus, in case of failure of the actuator, the shape-memory material will return to its natural shape at rest and the nacelle fairing  3  will return to a safe geometry ensuring an axial thrust to ensure the proper operation of the propulsion system  1 ,  1 ′ of the aircraft. 
     The radially internal  20   a  and external  20   b  walls may also have a varying thickness axially and also azimuthally in the vicinity of stiffening bridges  22  so as to locally modify the elasticity of the structure. It is further possible to locally optimize the mechanical characteristics of the shape-memory material constituting the radially internal  20   a  and external  20   b  walls according to the desired local properties along the downstream section  20 . Thus, it can be envisaged that the downstream section  20  consists of a plurality of sections of different materials. 
     The downstream section  20  is made of a deformable, semi-rigid material which guarantees a rigid structural shape so as to avoid its collapse both at rest and under the action of an air flow during operation of the propulsion system  1 ,  1 ′ and thus allowing the nacelle fairing  3  to maintain a homogeneous aerodynamic profile of its outlet cross-section BF. Advantageously, the downstream end  21  of the downstream section  20  can be made of an orthotropic material with suitable elastic moduli. 
     In addition, in order to ensure a substantially constant distance between the radially internal  20   a  and radially external  20   b  walls of the downstream section  20 , stiffening bridges  22  are provided at regular angular intervals between these walls  20   a ,  20   b . 
     One purpose of the present invention is to be able to take advantage of a nacelle fairing  3  of the propulsion system  1 ,  1 ′ whose outlet cross-section BF, as well as the shape of the latter, can be varied in order to direct the thrust of the propulsion system. 
     Thus, the downstream section  20  comprises means allowing for varying the shape of the outlet cross-section BF. 
     For this purpose, the downstream section  20  comprises a plurality of actuator mechanisms with cylinders  23 ,  23 ′. For example, the propulsion system  1 ,  1 ′ may comprise two actuator mechanisms with cylinders  23 ,  23 ′, one being for example angularly positioned at 6 o’clock, with reference to a time dial, (i.e., in the down position) when the propulsion system  1 ,  1 ′ is mounted on an aircraft, the other being diametrically opposite, i.e., positioned at 12 o’clock, with reference to a time dial, (i.e., in the up position). Each actuator mechanism  23 ,  23 ′ is configured to cooperate with means  24  integral with an internal surface  20   b ′ of the radially external wall  20   b , so as to deform the radially external wall  20   b  in a direction radial to the axis X of rotation, under the effect of a predetermined displacement command. 
     Specifically, for each actuator mechanism  23 ,  23 ′, a cylinder arm  26 ,  26 ′ is configured to extend or retract under the effect of the displacement command so as to act on and radially displace the means  24 , causing a thrust on the radially internal surface  20   b ′ of the radially external wall  20   b  and thereby vary the size and the shape of the outlet cross-section BF. Each cylinder  23 ,  23 ′ can be in a retracted state or in a fully extended state. The actuator mechanism with cylinders  23 ,  23 ′ can be electric, hydraulic, pneumatic or a screw-nut system. 
     The means  24  are arranged on different consecutive angular sectors around the axis X. One or more prisms may be associated with a single predetermined angular sector. Each actuator mechanism  23 ,  23 ′ cooperates with means  24  which face the cylinder  23 ,  23 ′, and which are thus associated with a predetermined angular sector, so as to deform the radially external wall  20   b  in a direction which is radial to the axis X of rotation and which is angularly centred on the predetermined angular sector over which the prisms associated with the cylinder extend. 
     The means  24  are, for example, embedded (e.g., by vulcanization) in the radially internal surface  20   b ′ of the radially external wall  20   b  and move radially under the action of the actuator mechanisms with cylinders  23 ,  23 ′. 
     According to an embodiment, the means  24  comprise a plurality of prisms, for example of triangular cross-section, distributed in at least one annular row, this plurality of prisms being actuated by the cylinders  23 ,  23 ′ by means of at least one annular element  25 . 
     Preferably, the prisms  24  are angularly equidistant on the radially internal surface  20   b ′ of the radially external wall  20   b . 
     Advantageously, the contacting faces of the prisms  24  and the annular element  25  are covered with an anti-friction coating. 
     Each actuator mechanism  23 ,  23 ′, and more specifically each cylinder, is operable independently of the other actuator mechanisms, and is configured to deform the radially external wall in a radial direction with respect to the axis of rotation X, under the effect of a predetermined displacement command. 
     For this purpose, each actuator mechanism  23 ,  23 ′ is connected to an automatic device allowing to actuate or not the cylinder  23 ,  23 ′ by applying a displacement command adapted according to the desired configuration for the nacelle fairing  3  to obtain the desired lateral thrust component. In particular, the automatic device operates the actuator mechanisms, and thus the cylinders  23 ,  23 ′, independently of the other actuator mechanisms (and thus the other cylinders). 
     Preferably, the cylinders  23 ,  23 ′ are angularly equidistant in the downstream section  20 . According to the embodiment illustrated in  FIGS.  2  to  7   , the propulsion system  1 ,  1 ′ comprises two actuator mechanisms with cylinders  23 ,  23 ′: a first actuator mechanism with one cylinder  23  extending in a first portion (the upper portion in the figures) of the downstream section  20 , and a second actuator mechanism with one cylinder  23 ′ extending in a second portion (the lower portion in the figures) of the downstream section  20 . In the figures, the actuator mechanisms with cylinders are shown in the upper (top) portion and lower (bottom) portion of the downstream section, but can of course be integrated in different angular sectors (e.g. right and left in the rear views), these angular sectors depending on the orientation of the propulsion system when installed on the aircraft. Of course, the invention is by no means limited to this embodiment, and the propulsion system  1 ,  1 ′ may comprise more actuator mechanisms with cylinders. 
       FIG.  2    shows a propulsion system  1 ,  1 ′ according to the invention with the nacelle fairing  3  shown in the neutral position, i.e. in pure axial thrust.  FIG.  5    shows the cylinders  23 ,  23 ′ in this neutral position. The arms  26 ,  26 ′ of the cylinders  23 ,  23 ′ of the actuator mechanisms with cylinders  23 ,  23 ′ are fully retracted. The radially internal wall  20   a  and the radially external wall  20   b  are in a state of zero extension (in other words, the walls  20   a  and  20   b  are at rest). The local deformation of the radially external wall  20   b  is then zero. 
       FIG.  3    shows a propulsion system  1 ,  1 ′ according to the invention with the nacelle fairing  3  shown in an upwardly deflected flow position.  FIG.  6    shows the radially external wall  20   b  and the prisms  24  when the actuator mechanisms with cylinders  23 ,  23 ′ are in an asymmetric downward thrust position. A command to create a downward displacement of the nacelle requires deforming the output cross-section of the nacelle upward, so as to create a force directed from the top down. The actuator mechanism with cylinders  23  is actuated. Specifically, an extension setpoint is calculated based on the input of the flight command and then sent to the actuator mechanism with cylinders  23 . The actuation of the actuator mechanism with cylinders  23  causes the cylinder arm  26  to extend, which causes the axial displacement of the upper portion of the annular element  25  (in the direction of the arrow F2 in  FIG.  3   ), which in turn causes the radially external displacement of the row of opposing prisms  24  (in the direction of the arrow F3 in  FIGS.  3  and  6   ). In particular, the annular element  25  moves on the faces of the prisms  24  facing the cylinder  23  so as to be, in the neutral position of the downstream section  20  of the nacelle fairing  3 , in a position flushing with the internal surface  20   b ′ of the radially external wall  20   b  and, in asymmetrical position of the downstream section  20  of the nacelle fairing  3 , in a position close to the summits of the prisms  24  facing the cylinder  23 . Because the prisms  24  are embedded in the radially internal surface  20   b ′ of the radially external wall  20   b  and the annular element  25  is rigid, the dimensions of the upper portion of the radially internal wall  20   a  increase under the effect of the radial thrust of the prisms  24 . This increase of the dimensions leads to a change in the shape of the upper portion of the outlet cross-section BF. Indeed, the radially internal  20   a  and the radially external  20   b  walls of the downstream section  20  being made of a deformable material, the modification of the geometry of the radially external wall  20   b  leads to the modification of the geometry of the radially internal wall  20   a , thanks in particular to the bridges  22 , having the effect of making the downstream section of the nacelle fairing  3  change to an asymmetrical configuration such as illustrated in  FIG.  6   . The cooperation between the cylinder  23 , the annular element  25  and the prisms  24  deforms the radially external wall  20   b  in a radial direction (arrow F3 in  FIG.  6   ) with respect to the axis X of rotation and angularly centred with respect to the angular sector over which the prisms  24  associated with the cylinder  23  extend, i.e., vertically and upward in  FIG.  6   . The cylinder  23  is thus activated, while the cylinder  23 ′ is not activated. In other words, the cylinder  23 ′ is in the rest position. In particular, the lower portion of the annular element  25  does not move on the faces of the prisms  24  facing the cylinder  23 ′. The actuation of the cylinder  23  results in a local deformation, i.e. a local and radial expansion E of the outlet cross-section BF. The outlet cross-section of the fairing is therefore asymmetrical, and its shape results from the mechanical characteristics of the structure subjected to the thrust of the cylinder  23 . When the cylinder  23  is activated, there is a radial and local expansion E of the outlet cross-section BF of the fairing  3 , i.e. there is a local deflection of the structure on the side of the activated cylinder. The radial dimension (in the direction of the arrow F3) of the set of the actuator mechanisms with cylinders  23 ,  23 ′ (the cylinder  23  being activated and the cylinder  23 ′ being at rest) is equal to the sum of the radial dimension DE of the radially external wall  20   b  when the actuator mechanisms with cylinders are at rest and of the expansion E. The radially external wall  20   b  forms an annulus, a portion of which (in this case the upper portion) is deformed. There is a radial and local increase in the radially external wall  20   b  (at the level of the prisms  24  that face the upper actuated cylinder  23 ).  FIG.  4    shows a propulsion system  1 ,  1 ′ according to the invention with the nacelle fairing  3  shown in a position where the flow is deflected downwards.  FIG.  7    shows the radially external wall  20   b  and the prisms  24  when the actuator mechanisms with cylinders  23 ,  23 ′ are in an asymmetrical upward thrust position. A command to create an upward displacement of the nacelle requires deforming the output cross-section of the nacelle downward, so as to create a force directed from the bottom up. The actuator mechanism with cylinders  23 ′ is activated. Specifically, an extension setpoint is calculated based on the input of the flight command and then sent to the actuator mechanism with cylinders  23 ′. The actuation of the actuator mechanism with cylinders  23 ′ causes the extension of the arm  26 ′ of the cylinder, which causes the axial displacement of the lower portion of the annular element  25  (in the direction of the arrow F4 in  FIG.  4   ), which in turn causes the radially external displacement of the row of prisms  24  (in the direction of the arrow F5 in  FIGS.  4  and  7   ). In particular, the annular element  25  moves on the faces of the prisms  24  facing the cylinder  23 ′ so as to be, in the neutral position of the downstream section  20  of the nacelle fairing  3 , in a position flushing with the internal surface  20   b ′ of the radially external wall  20   b  and, in the asymmetrical position of the downstream section  20  of the nacelle fairing  3 , in a position close to the summits of the prisms  24  facing the cylinder  23 ′. Because the prisms  24  are embedded in the radially internal surface  20   b ′ of the radially external wall  20   b  and the annular element  25  is rigid, the dimensions of the upper portion of the radially internal wall  20   a  increase under the effect of the radial thrust of the prisms  24 . This increase in dimensions leads to a change in the shape of the lower portion of the outlet cross-section BF. Indeed, the radially internal  20   a  and the radially external  20   b  walls of the downstream section  20  being made of a deformable material, the modification of the geometry of the radially external wall  20   b  leads to the modification of the geometry of the radially internal wall  20   a , thanks in particular to the bridges  22 , having the effect of making the downstream section of the nacelle fairing  3  change to an asymmetrical configuration such as illustrated in  FIG.  7   . The cooperation between the cylinder  23 ′, the annular element  25  and the prisms  24  deforms the radially external wall  20   b  in a radial direction (arrow F5 in  FIG.  7   ) with respect to the axis X of rotation and angularly centred with respect to the angular sector over which the prisms  24  associated with the cylinder  23 ′ extend, i.e., vertically and downward in  FIG.  7   . The cylinder  23 ′ is thus activated, while the cylinder  23  is not. In other words, the cylinder  23  is in the rest position. In particular, the upper portion of the annular element  25  does not move on the faces of the prisms  24  facing the cylinder  23 . The actuation of the cylinder  23 ′ results in a local deformation, i.e. a local and radial expansion E of the outlet cross-section BF. The outlet cross-section of the fairing is therefore asymmetrical, and its shape results from the mechanical characteristics of the structure subjected to the thrust of the cylinder  23 ′. When the cylinder  23 ′ is activated, there is a radial and local expansion E of the outlet cross-section BF of the fairing  3 , i.e. there is a local deflection of the structure on the side of the activated cylinder. The radial dimension (in the direction of the arrow F5) of the set of the actuator mechanisms with cylinders  23 ,  23 ′ (the cylinder  23  being at rest and the cylinder  23 ′ being activated) is equal to the sum of the radial dimension DE of the radially external wall  20   b  when the actuator mechanisms with cylinders are at rest and of the expansion E. The radially external wall  20   b  forms an annulus, a portion of which (in this case the lower portion) is deformed. There is a radial and local increase in the radially external wall  20   b  (at the level of the prisms  24  that face the lower cylinder  23 ′ that is operated). 
     The gradual increase in the extension of the cylinders induced by the automatic device gradually changes the shape and the dimensions of the outlet cross-section BF. In particular, there is a deformation, radially to the axis X and locally, of the radially internal  20   a  and radially external  20   b  walls made of deformable shape-memory material of the downstream section  20  associated with the prisms facing the actuated cylinder. This consequently changes the dimensions and the shape of the outlet cross-section BF, which thus changes from a minimum radial dimension DE and a circular shape in the axial thrust mode configuration as shown in  FIG.  2    to a radial dimension DE + E greater than the minimum radial dimension DE, with a deformation in at least one radial direction of the circular shape of the set of the actuators in the asymmetric thrust mode configuration of the nacelle fairing  3  as shown in  FIGS.  3  and  4   . 
     Similarly, the progressive reduction of the extension of the cylinders induced by the automatic device progressively shifts the nacelle fairing  3  from an asymmetrical thrust mode configuration as illustrated in  FIGS.  3  and  4    to an axial thrust mode configuration as illustrated in  FIG.  2   . 
     The passage from the configuration of  FIG.  5    to the configuration of  FIG.  6    or  FIG.  7    of the downstream section  20  of the nacelle fairing  3 , and vice versa, is done continuously as a function of the extension or retraction of the arm  26 ,  26 ′ of cylinder  23 ,  23 ′ of the actuator mechanism with cylinders induced by the automatic device, the stiffening bridges  22  ensuring a substantially constant distance between the radially internal  20   a  and radially external  20   b  walls of the downstream section  20  during the changes of configuration of the outlet cross-section BF (dimensions and shape) of the nacelle fairing  3 . 
     According to an embodiment, the propulsion system  1 ,  1 ′ may comprise several rows of prisms  24 ,  24 ′ and several actuator mechanisms with cylinders  23 ,  23 ′ for each annular row of prisms  24 ,  24 ′, independent of each other. 
     Advantageously, but in a non-limiting way, each annular row may comprise at least four prisms  24 ,  24 ′ distributed azimuthally in an equidistant manner along the radially internal surface  20   b ′ of the radially external wall  20   b  of the downstream section  20 . 
     With reference to  FIG.  8   , the means  24 ,  24 ′ may comprise a plurality of prisms, for example of triangular cross-section, distributed in two annular rows. A first annular row of prisms  24  is connected to the or each arm  26 ,  26 ′ of cylinder  23 ,  23 ′ of the actuator mechanisms with cylinders  23 ,  23 ′ by means of a first annular element  25 . A second annular row of prisms  24 ′ is connected to the first annular row of prisms  24 , and more precisely to the first annular element  25 , by means of a second annular element  28  and a plurality of rods  27  (here arranged at 12 o’clock and at 6 o’clock). The prisms  24 ,  24 ′, the annular elements  25 ,  28  and the connecting rods  27  are made of a rigid material, for example metal. 
     Preferably, the downstream section  20  comprises at least two angularly equidistant means  24 ,  24 ′ (e.g., angularly positioned at 6 o’clock and 12 o’clock) for, in particular, pushing the annular elements  25 ,  28  and the connecting rods  27 . 
     As can be seen in  FIG.  8   , the prisms  24  of the first annular row (which is closest to the intermediate section  30 ) and the prisms  24 ′ of the second annular row (which is closest to the outlet cross-section BF) have different dimensions. In particular, the prisms  24 ′ have a summit angle θ′ less than the summit angle θ of the prisms  24 . In fact, in order to move the downstream section  20  of the nacelle fairing  3  from a neutral position to an asymmetrical position, the internal  20   a  and external  20   b  walls undergo displacements of variable amplitude increasing as they approach the outlet cross-section, the prisms  24 ,  24 ′ thus being configured to allow this variable amplitude of radial displacement of the internal  20   a  and external  20   b  walls for a same axial displacement of the annular elements  25 ,  28 . 
     In order to divert the flow upwards, for example, the actuator mechanism with cylinders  23  is operated. The actuation of the actuator mechanism with cylinders  23  causes the extension of the cylinder arm  26 , which causes the axial displacement of the annular element  25  (in the direction of the arrow F6), which in turn causes the radially external displacement of the row of prisms  24  (in the direction of arrow F7) and the axial displacement of the rods  27  and of the second annular element  28  (in the direction of the arrow F8), which in turn causes the radially external displacement of the second row of prisms  24 ′ (in the direction of the arrow F9). Because the prisms  24 ,  24 ′ are embedded in the radially internal surface  20   b ′ of the radially external wall  20   b  and the annular elements  25 ,  28  are rigid, the dimensions of the upper portion of the radially internal wall  20   a  increase under the effect of the radial thrust of the prisms  24 ,  24 ′. This increase in dimensions leads to a change in the shape of the upper portion of the outlet cross-section BF. The cooperation between the cylinder  23 , the annular elements  25 ,  28 , the rods  27  and the prisms  24 ,  24 ′ deforms the radially external wall  20   b  in a direction radial relative to the axis X of rotation and angularly centred with respect to the angular sector over which the prisms  24 ,  24 ′ associated with the cylinder  23  extend. The cylinder  23  is thus activated, while the cylinder  23 ′ is not activated. In particular, the lower portions of the annular elements  25 ,  28  do not move on the faces of the prisms  24 ,  24 ′ facing the cylinder  23 ′. The actuation of the cylinder  23  results in a local deformation, i.e. a local and radial expansion of the outlet cross-section BF. There is a local radial increase in the radially external wall  20   b  (at the level of the prisms  24 ,  24 ′ which face the actuated cylinder  23 ). In order to divert the flow downwards, for example, the actuator mechanism with cylinders  23 ′ is actuated. The actuation of the actuator mechanism with cylinders  23 ′ causes the extension of the cylinder arm  26 ′, which causes the axial displacement of the annular element  25  (in the direction of the arrow F10), which in turn causes the radially external displacement of the row of prisms  24  (in the direction of the arrow F11) and the axial displacement of the rods  27  and of the second annular element  28  (in the direction of the arrow F12), which in turn causes the radially external displacement of the second row of prisms  24 ′ (in the direction of the arrow F13). Because the prisms  24 ,  24 ′ are embedded in the radially internal surface  20   b ′ of the radially external wall  20   b  and the annular elements  25 ,  28  are rigid, the dimensions of the lower portion of the radially internal wall  20   a  increase under the effect of the radial thrust of the prisms  24 ,  24 ′. This increase in dimensions leads to a change in the shape of the lower portion of the outlet cross-section BF. The cooperation between the cylinder  23 ′, the annular elements  25 ,  28 , the rods  27  and the prisms  24 ,  24 ′ deforms the radially external wall  20   b  in a direction radial to the axis X of rotation and angularly centred with respect to the angular sector over which the prisms  24 ,  24 ′ associated with the cylinder  23 ′ extend. The cylinder  23 ′ is thus activated, while the cylinder  23  is not. In particular, the upper portions of the annular elements  25 ,  28  do not move on the faces of the prisms  24 ,  24 ′ facing the cylinder  23 . The actuation of the cylinder  23 ′ results in a local deformation, i.e. a local and radial expansion of the outlet cross-section BF. There is a local radial increase in the radially external wall  20   b  (at the level of the prisms  24 ,  24 ′ which face the actuated cylinder  23 ′). 
     The invention has primarily been described for a propulsion system comprising one annular element and two cylinders, but the propulsion system may of course comprise a plurality of annular elements and more cylinders.