Optimization of the behavior of the fan in an aeronautical propulsion system

Abstract

The invention relates to an assembly of an aeronautical propulsion system (1) comprising: • - a movable element (35) rotating about an axis (X) of the assembly (11a, 11b, 11c); and • - a mode damper (25) having an axial length and a maximum radial clearance such that a surface viscance of the mode damper (25), which corresponds to the ratio of the square of the axial length to the product of the cube of the maximum radial clearance multiplied by twice the value of Pi, is between 1.0 x 104 m-1 and 1.15 x 106 m1 and a surface clearance, which corresponds to the ratio of the maximum radial clearance to the product of twice the value of Pi multiplied by the implantation radius (R) of the mode damper (25) and the axial length (L) of the mode damper (25), is between 2.5 x 10-2 m-1 and 1.0 x 10-1 m-1.

Description

[0001] DESCRIPTION

[0002] TITLE: Optimization of fan behavior in an aeronautical propulsion system

[0003] TECHNICAL FIELD

[0004] The present application relates in general to the field of propulsion systems, and more particularly to mode dampers comprising a pressurized fluid ("squeeze film" in English) which can be mounted between the outer ring of a bearing or a reduction mechanism with epicyclic or planetary gear train and a rigid support fixedly attached to a stator part of the propulsion system.

[0005] STATE OF THE ART

[0006] A propulsion system generally comprises, from upstream to downstream in the direction of gas flow, a fan section, a compressor section which may include a low-pressure compressor and a high-pressure compressor, a combustion chamber, and a turbine section which may include, in particular, a high-pressure turbine and a low-pressure turbine. The high-pressure compressor is driven in rotation by the high-pressure turbine via a high-pressure shaft. The fan, and where applicable the low-pressure compressor, are driven in rotation by the low-pressure turbine via a low-pressure shaft.

[0007] Technological research efforts have already led to very significant improvements in the environmental performance of aircraft. The Applicant takes into account the impactful factors in all phases of design and development to obtain aeronautical components and products that are less energy-intensive, more environmentally friendly, and whose integration and use in civil aviation have moderate environmental consequences, with the aim of improving the energy efficiency of aircraft.

[0008] To minimize mechanical loads, vibrations, and displacements between a moving part, such as a rotor, and a stator part of the propulsion system, it has been proposed to interpose a mode damper (or "squeeze-film") and, where appropriate, a flexible cage (also known as a squirrel cage) between the moving and stator parts. For this purpose, a mode damper comprises an inner ring and an outer ring, which are fixed relative to the stator part, extending radially outward from the moving part, and a pressurized fluid confined between the inner and outer rings.

[0009] The performance and noise requirements of modern propulsion systems are constantly increasing. To improve propulsion system performance, one approach is to reduce the dynamic play in the engine. This could be achieved by increasing the radius of the mode dampers (the damping capacity of mode dampers being dependent on their radius). However, the available space between the rotors and their respective drive shafts is becoming increasingly limited to improve propulsion system efficiency and reduce specific fuel consumption. In the more specific case of short- and medium-range engines, the current trend is to increase the bypass ratio to reduce specific fuel consumption and improve propulsion system efficiency.To achieve this, the fan is driven by a reduction mechanism, allowing for independent optimization of their respective rotational speeds. As a result, the drive shaft rotates at higher speeds, increasing the loads transmitted from the rotor to the stators. However, mechanical integration constraints necessitate large-diameter bearings, particularly in the vicinity of the reduction mechanism. Consequently, the mode dampers mounted above these bearings also have an unusually large diameter. Furthermore, the properties of mode dampers at these diameters are poorly understood, requiring costly test campaigns to measure them and recalibrate predictive models.Indeed, when the inertial and damping forces generated by the fluid in the mode damper are too great and the bearing's rotational speed is high, the mode damper behaves like a rigid bearing, thus negating the beneficial effect of its damping. This is referred to as "locking" the mode damper.

[0010] The increased rotational speed of the low-pressure shaft, made possible by the reduction mechanism, also leads to a greater number of overall vibration modes within the operating range, unlike a conventional direct-drive design. However, the mode damper is generally optimized for only one specific overall vibration mode. It can therefore have detrimental effects on other modes, leading to increased dynamic loads, increased dynamic backlash consumption, and vibrations in those modes, instead of a reduction (due to a "lock-in" of the mode damper).

[0011] The increase in rotational speeds of the low-pressure shaft can also lead to the presence of a low-pressure shaft bending mode in the operating range (supercritical low-pressure shaft), which can lead to excessive deformations of the low-pressure shaft, or even contact between the low-pressure shaft and the high-pressure shaft when crossing the low-pressure shaft mode and / or to non-synchronous vibrations after crossing this mode, which can damage the propulsion system.

[0012] It is therefore necessary to optimize the mode damper to obtain sufficient damping effort to fall below the HCF (High Cycle Fatigue) loading targets (for vibration fatigue, for which structures are designed for infinite lifespan, typically corresponding to 10 7cycles) and consumption of dynamic games, and below the vibration and noise thresholds imposed by aircraft manufacturers, without risking blocking the damper mode.

[0013] EXPOSED

[0014] One objective of this application is to optimize the performance of aeronautical propulsion systems, particularly propulsion systems with high bypass ratios, specifically in terms of vibration damping, dynamic clearance consumption, and polycyclic loading, while taking into account integration constraints within the propulsion system. To this end, a first aspect of an aeronautical propulsion system is proposed, comprising:

[0015] - a movable element rotating around an axis of the assembly; and

[0016] - a mode damper comprising an inner ring, an outer ring, and a fluid confined between the inner and outer rings, the inner and outer rings being coaxial with the moving element; the inner ring extending radially between the moving element and the outer ring; and the mode damper having an axial length and a maximum radial clearance such that a surface viscosity of the mode damper, which corresponds to the ratio of the square of the axial length to the product of the cube of the maximum radial clearance multiplied by twice the value of Pi, is between 1.0 x 10 4 m -1 and 1.15 x 10 6 m 1 :

[0017] 1.0 x 10 4 1.15 x 10 6 Or :

[0018] L is the axial length of the damper and is expressed in meters; and

[0019] This is the maximum radial play of the damper mode and is expressed in meters.

[0020] Some preferred but not exhaustive characteristics of the set according to the first aspect are the following, taken individually or in combination:

[0021] - the surface viscosity of the mode damper is less than or equal to 7.1 x 10 5 m -1 ;

[0022] - the mode damper also exhibits a surface clearance, which corresponds to the ratio between the maximum radial clearance and the product of twice the value of Pi multiplied by the mounting radius of the mode damper and the axial length of the mode damper, is between 2.5 x 10 -2 m -1 and 1.0 x IO 1 m -1 :

[0023] 2.5 x 10" 2 < surface area 1.0 x 10 -1 where: R is the radius of implantation of the mode damper and is expressed in meters

[0024] - the surface clearance of the mode damper is greater than or equal to 3.7 x 10 -2 m-1 ;

[0025] - the moving element includes a moving part of a bearing of the propulsion system;

[0026] - the moving element includes a moving part of a propulsion system reduction mechanism; and / or

[0027] - the mode damper further includes at least one sealing segment, preferably two sealing segments, axially delimiting the fluid between the inner ring and the outer ring.

[0028] According to a second aspect, an aeronautical propulsion system is proposed comprising an assembly according to the first aspect and a rotor coaxial with the moving element.

[0029] Some preferred but not exhaustive characteristics of the system according to the first aspect are the following, taken individually or in combination:

[0030] - the rotor includes a shaft of the propulsion system and the moving element includes a bearing which supports the rotor;

[0031] - the rotor includes a drive shaft, the propulsion system further comprising a blower section including a blower shaft (20); a drive turbine driven in rotation by the rotor around the axis; and a reduction mechanism coupling the rotor and the blower shaft in order to drive the blower shaft at a rotational speed lower than the rotational speed of the rotor, the moving element comprising a moving part of the reduction mechanism;

[0032] - the drive shaft is supported by at least one front bearing and one or two rear bearings, the moving element corresponding to the front bearing and, where applicable, to one of the rear bearings; and / or

[0033] - a dilution ratio of the propulsion system is greater than or equal to 10, for example between 10 and 80 inclusive.

[0034] According to a third aspect, it is proposed an aircraft comprising at least one aeronautical propulsion system according to the second aspect fixed to the aircraft by means of a mast.

[0035] According to a fourth aspect, a manufacturing process for an aeronautical propulsion system assembly is proposed, comprising the following steps:

[0036] - determine an axial length and a maximum radial clearance of a mode damper such that a surface viscosity of the mode damper, which corresponds to the ratio between the square of the axial length and the product between the maximum radial clearance cubed multiplied by twice the value of Pi, is between 1.0 x 10 4 rrr 1 and 1.15 x 10 6 rrr 1 :

[0037] 10 4 1.15 x 10 6 Or :

[0038] L is the axial length of the damper and is expressed in meters; and

[0039] C is the maximum radial play of the mode damper and is expressed in meters;

[0040] - manufacture an inner ring and an outer ring of the mode damper according to the axial length and maximum radial play thus determined;

[0041] - assemble the inner and outer rings with a moving element; and

[0042] - inject a fluid between the inner ring and the outer ring.

[0043] Some preferred but not limiting characteristics of the manufacturing process according to the fourth aspect are as follows, taken individually or in combination:

[0044] - the axial length and maximum radial play are determined so that the surface viscosity is greater than or equal to 3.7 x 10 -2 rrr 1 ;

[0045] - the axial length, the maximum radial clearance and an installation radius can be determined so that a surface clearance of the mode damper is between 2.5 x 10 A -2 rrr 1 and 1.0 x 10 A -1 rrr 1 , where the surface clearance is defined as follows:

[0046] 2.5 x 10" 2 < surface area 1.0 x 10 -1 where: R is the mounting radius of the damper and is expressed in meters; and / or - the axial length, the maximum radial clearance, and the mounting radius are determined such that the surface clearance is greater than or equal to 3.7 x 10 -2 rrr 1 .

[0047] DESCRIPTION OF THE FIGURES

[0048] Other features, purposes, and advantages will become apparent from the following description, which is purely illustrative and not exhaustive, and should be read in conjunction with the attached drawings on which:

[0049] Figure 1 is a schematic, partial and cross-sectional view of an example of a propulsion system conforming to a first embodiment, in which the blower section is faired;

[0050] Figure 2 is a schematic, partial, cross-sectional view of an example of a propulsion system conforming to a second embodiment, in which the blower section is unfaired;

[0051] Figure 3 is an enlarged schematic cross-sectional view of part of an example assembly for a propulsion system according to a first variant;

[0052] Figure 4 is an enlarged schematic cross-sectional view of part of an example assembly for a propulsion system according to a second variant;

[0053] Figure 5 is an example of an aircraft that may include at least one propulsion system conforming to the first or second embodiment;

[0054] Figure 6 is a flowchart illustrating examples of steps in a manufacturing process for an assembly for a propulsion system; and

[0055] Figure 7 is a graph representing the radial displacement of a moving element (bearing) of a bearing (in millimeters) as a function of the rotational speed of the shaft on which it is mounted (in rpm), in the presence of an imbalance on the shaft.

[0056] Across all figures, similar elements bear identical references.

[0057] DETAILED DESCRIPTION

[0058] A propulsion system 1 has a principal direction extending along a longitudinal axis X and includes, from upstream to downstream in the direction of the gas flow in the propulsion system 1 when in operation, a blower section 2 and a primary body 3, often called a "gas generator", comprising a compressor section 4, 5, a combustion chamber 6 and a turbine section 7, 8. The propulsion system 1 here is an aeronautical propulsion system 1 configured to be fixed on an aircraft 100 by means of a pylon (or mast).

[0059] The compressor section 4, 5 comprises a series of stages, each including a rotating blade wheel (rotor) 4a, 5a in front of a stationary blade wheel (stator) 4b, 5b. The turbine section 7, 8 also comprises a series of stages, each including a stationary blade wheel (stator) 7b, 8b behind which a rotating blade wheel (rotor) 7a, 8a rotates.

[0060] In this application, the axial direction corresponds to the direction of the longitudinal axis X, corresponding to the rotation of the gas generator shafts, and a radial direction is a direction perpendicular to and passing through this axis X. Furthermore, the circumferential (or lateral, or tangential) direction corresponds to a direction perpendicular to and not passing through the longitudinal axis X. Unless otherwise specified, internal (respectively, inside) and external (respectively, outside) are used with reference to a radial direction such that the inner part or face of an element is closer to the X axis than the outer part or face of the same element.

[0061] In operation, an airflow F entering the propulsion system 1 is divided between a primary airflow F1 and a secondary airflow F2, which flow from upstream to downstream in the propulsion system 1.

[0062] The secondary airflow F2 (also called the "bypass airflow") flows around the primary body 3. The secondary airflow F2 cools the periphery of the primary body 3 and is used to generate most of the thrust provided by the propulsion system 1.

[0063] The primary airflow F1 flows in a primary channel inside the primary body 3, passing successively through the compressor section 4, 5, the combustion chamber 6 where it is mixed with fuel to serve as an oxidizer, and the turbine section 7, 8. The passage of the primary airflow F1 through the turbine section 7, 8 receiving energy from the combustion chamber 6 causes a rotation of the rotor of the turbine section 7, 8, which in turn drives the rotation of the rotor of the compressor section 4, 5 as well as a rotor part 9 of the blower section 2.

[0064] In a twin-spool propulsion system 1, the compressor section 4, 5 may include a low-pressure compressor 4 and a high-pressure compressor 5. The turbine section 7, 8 may include a high-pressure turbine 7 and a low-pressure turbine 8. The rotor of the high-pressure compressor 5 is driven in rotation by the rotor of the high-pressure turbine 7 via a high-pressure shaft 10. The rotor of the low-pressure compressor 4 and the rotor portion 9 of the blower section 2 are driven in rotation by the rotor of the low-pressure turbine 8 via a low-pressure shaft 11. Thus, the primary body 3 comprises a high-pressure body including the high-pressure compressor 5, the high-pressure turbine 7, and the high-pressure shaft 10, and a low-pressure body including the blower section 2, the low-pressure compressor 4, the low-pressure turbine 8, and the low-pressure shaft 11.The rotational speed of the high-pressure casing is greater than the rotational speed of the low-pressure casing. In a three-casing propulsion system 1, the turbine section 7, 8 further includes an intermediate turbine, positioned between the high-pressure turbine 7 and the low-pressure turbine 8 and configured to drive the rotor of the low-pressure compressor 4 via an intermediate shaft. The blower rotor 9 and the rotor of the high-pressure compressor 5 remain driven by the low-pressure shaft 11 and the high-pressure shaft 10, respectively.

[0065] The low-pressure shaft 11 is generally housed, along a portion of its length, within the high-pressure shaft 10 and is coaxial with the high-pressure shaft 10. The low-pressure shaft 11 and the high-pressure shaft 10 may be co-rotating, that is, driven in the same direction around the longitudinal axis X. Alternatively, the low-pressure shaft 11 and the high-pressure shaft may be counter-rotating, that is, driven in opposite directions around the longitudinal axis X. If applicable, the intermediate shaft is housed between the high-pressure shaft 10 and the low-pressure shaft 11. The intermediate shaft and the low-pressure shaft 11 may be co-rotating or counter-rotating. The blower section 2 includes at least the blower rotor 9 suitable for being driven in rotation relative to a stator part of the propulsion system 1 by the turbine section 7, 8. Each blower rotor 9 includes a hub 13 and blades 14 extending radially from the hub 13.The blades 14 of each rotor 9 can be fixed relative to the hub 13 or have variable pitch. In this case, the base of the blades 14 of each rotor 9 is pivotally mounted about a pitch axis and is connected to a pitch-changing mechanism 15 mounted in the propulsion system 1, the pitch being adjusted according to the flight phases by the pitch-changing mechanism 15. The pitch-changing mechanism 15 is shown in dashed lines in Figure 1 to show that this feature is optional.

[0066] The fan section 2 may further include a fan stator 16, or rectifier, which comprises blades 17 mounted on a hub of the fan stator 16 and whose function is to rectify the secondary airflow F2 exiting the fan rotor 9. The blades 17 of the fan stator 18 may be fixed relative to the hub or have variable pitch. Similar to the rotor blades 14, the base of the stator blades 17 is pivotally mounted about a pitch axis X and is connected to a pitch-changing mechanism 15a, which is generally separate from that of the fan rotor 9, the pitch being adjusted according to the flight phases by the pitch-changing mechanism.

[0067] The fan rotor 9 also includes at least twelve blades 14 and at most twenty-four blades 14, for example at least fourteen blades 14 and at most twenty-two blades 14, for example sixteen blades 14. The number of blades 16 in the fan stator 17 depends on the acoustic criteria defined for the propulsion system 1 and is at least equal to the number of blades 14.

[0068] To improve the propulsive efficiency of propulsion system 1 and reduce its specific fuel consumption and the noise emitted by fan section 2, propulsion system 1 has a high bypass ratio. A high bypass ratio is defined here as a ratio greater than or equal to 10, for example, between 10 and 80 inclusive. To calculate the bypass ratio, the mass flow rate of the secondary airflow F2 and the mass flow rate of the primary airflow F1 are measured when propulsion system 1 is stationary, uninstalled, in takeoff mode under standard atmospheric conditions (as defined by the International Civil Aviation Organization (ICAO) Manual, Doc 7488 / 3, 3). eedition) and at sea level. It should be noted that, in this application, the parameters (pressure, flow rate, thrust, speed, etc.) are systematically determined under these conditions. "Not installed" here means that the measurements are taken when the propulsion system 1 is in a test bench (and not installed on an aircraft 100), as the measurements are then simpler to perform. The distances (length, radius, diameter) are, however, measured at ambient temperature (approximately 20°C) when the propulsion system 1 is cold, that is, when the propulsion system has been at rest for a sufficient period for the components of the propulsion system to reach ambient temperature.

[0069] The blower rotor 9 is decoupled from the low-pressure shaft 11 by means of a reduction mechanism 19, positioned between an upstream end of the low-pressure shaft 11 and the blower rotor 9, in order to independently optimize their respective rotational speeds. In this case, the propulsion system 1 further includes an additional shaft, referred to as the blower shaft 20. The low-pressure shaft 11 connects the low-pressure turbine 7 to an inlet of the reduction mechanism 19, while the blower shaft 20 connects the outlet of the reduction mechanism 19 to the blower rotor 9. The blower rotor 9 is therefore driven by the low-pressure shaft 11 via the reduction mechanism 19 and the blower shaft 20 at a rotational speed lower than the rotational speed of the low-pressure turbine 7.

[0070] This decoupling allows for a reduction in the rotational speed and pressure ratio of the fan rotor 9 and an increase in the power extracted by the low-pressure turbine 7. Indeed, the overall efficiency of the propulsion systems is primarily determined by the propulsion efficiency, which is favorably influenced by minimizing the variation in the kinetic energy of the air as it passes through the propulsion system 1. In a propulsion system 1 with a high bypass ratio, the majority of the flow generating the propulsive force consists of the secondary airflow F2 of the propulsion system 1, the kinetic energy of the secondary airflow F2 being mainly affected by the compression that the secondary airflow F2 undergoes as it passes through the fan section 2. The propulsion efficiency and the pressure ratio of the fan section 2 are therefore linked: the lower the pressure ratio of the fan section 2, the better the propulsion efficiency.In order to optimize the propulsive efficiency of the propulsion system 1, the blower pressure ratio, which corresponds to the ratio between the average pressure at the outlet of the blower stator 17 (or, in the absence of a stator, of the blower rotor 9) and the average pressure at the inlet of the blower rotor 9, is less than or equal to 1.70, preferably less than or equal to 1.50, for example greater than or equal to between 1.05 and less than 1.45. The average pressures are measured here over the height of the blade 14 (from the surface which radially delimits the flow duct inside the inlet of the blower rotor 9 to the apex 21 of the blower blade 14).

[0071] The propulsion system 1 is configured to provide a thrust of between 18,000 Ibf (80,068 N) and 51,000 Ibf (22,241 N), preferably between 20,000 Ibf (88,964 N) and 35,000 Ibf (15,568 N).

[0072] The blower section 2 may be shrouded or unshrouded. In the case of a shrouded blower section 2, the blower section 2 includes a blower housing 12 and the blower rotor 9 is housed in the blower housing 12.

[0073] A shrouded fan section 2 comprises a fan rotor 9 extending upstream of a fan stator. The fan stator blades are then generally called outlet guide vanes (OGVs) and have a fixed pitch relative to the fan stator hub. Furthermore, the bypass ratio of the propulsion system 1 is, for example, greater than or equal to 10, for example, between 10 and 35 inclusive, for example, between 10 and 18 inclusive. The peripheral velocity at the tip 21 of the fan rotor blades 9 can also be between 260 m / s and 400 m / s. The fan rotor blades 14 can be fixed or have a variable pitch. The fan pressure ratio can then be between 1.20 and 1.45.

[0074] In an unshrouded fan section 2, the fan section 2 (which can also be referred to as the propeller) is not enclosed by a fan casing. The fan section

[0075] Since 2 is unshod, the blades 14 of the fan rotor 9 have variable pitch. Propulsion systems comprising at least one unshod fan rotor 9 are known as "open rotor" or "unducted fan." The propulsion system 1 may comprise two unshod, counter-rotating fan rotors 9. Such a propulsion system 1 is known by the English acronym CROR for "Contra-Rotating Open Rotor" or UDF for "Unducted Double Fan." The fan rotor(s) 9 may be positioned at the rear of the primary casing 3 to be of the pusher type or at the front of the primary casing 3 to be of the tractor type. Alternatively, the propulsion system 1 may comprise a single unshod fan rotor 9 and an unshod fan stator 16 (rectifier). Such a propulsion system 1 is known by the English acronym USF for "Unducted Single Fan".In the case of a USF-type propulsion system 1, the blades 17 of the stator 16 are fixed in rotation relative to the X-axis of rotation of the upstream fan rotor 9 and therefore do not experience centrifugal force. The blades 17 of the stator 16 also have variable pitch.

[0076] Removing the fairing around the fan section 2 significantly increases the bypass ratio without the propulsion system 1 being negatively impacted by the mass of the housings or nacelles intended to surround the fan section 2. The bypass ratio of the propulsion system 1, including an unfaired fan section 2, is thus greater than or equal to 40, for example, between 40 and 80 inclusive. Furthermore, the peripheral velocity at the tip 21 of the fan blades 14 of the fan rotor(s) 9 can be between 210 m / s and 260 m / s. The fan pressure ratio can then be, for example, between 1.05 and 1.20.

[0077] The reduction mechanism 19 may include an epicyclic or planetary reduction mechanism, single-stage or two-stage. For example, the reduction mechanism 19 may be of the planetary type ("star" in English) and include a sun pinion (input of the reduction mechanism 19), centered on an X axis of rotation of the reduction mechanism 19 (generally coinciding with the longitudinal X axis) and configured to be driven in rotation by the low-pressure shaft 11, a ring gear (output of the reduction mechanism 19) coaxial with the sun pinion and configured to drive in rotation the blower shaft 20 around the X axis of rotation, and a series of satellites distributed circumferentially around the X axis of rotation between the sun pinion and the ring gear, each satellite being internally meshed with the sun pinion and externally with the ring gear.The series of satellites is mounted on a satellite carrier which is fixed relative to a stator part of the propulsion system 1, for example relative to a housing of the compressor section 4, 5. Alternatively, the reduction mechanism 19 can be epicycloidal (“planetary” in English), in which case the ring is fixedly mounted on the stator part of the propulsion system 1 and the blower shaft 20 is driven in rotation by the satellite carrier.

[0078] Regardless of the configuration of the reduction mechanism 19, the diameter of the crown and the satellite carrier are greater than the diameter of the solar pinion, so that the rotational speed of the blower rotor 9 is less than the rotational speed of the low pressure shaft 11.

[0079] In the case of a propulsion system 1 comprising a shrouded fan rotor 9, the reduction ratio can be greater than 2.5 and less than or equal to 6.0, typically around 3.0. In the case of a propulsion system 1 comprising an unshrouded fan rotor, the reduction ratio can be greater than 7.0 and less than or equal to 11.0, typically around 9.0. The low-pressure shaft 11 is supported by three or four bearings 11a, 11b and / or 11c, in order to control the deformation modes of the low-pressure shaft 1. The low-pressure shaft 11 can thus include one or two forward bearings 11a, which extend upstream of the combustion chamber 6, and two rear bearings 11b, 11c, which extend downstream of the combustion chamber 6.The bearings 11a, 11b, 11c comprise a first ring mounted on the low-pressure shaft 11 and a second ring mounted on a stator portion of the propulsion system 1, typically on a housing of the propulsion system 1 through which forces are transmitted in the propulsion system 1. Thus, a first front bearing 11a can be mounted on the low-pressure shaft 11 and on an inlet housing 26 of the propulsion system 1, which extends between the blower rotor 9 and the low-pressure compressor 4. If necessary, a second front bearing can be mounted on the low-pressure shaft 11 and on an inter-compressor housing 23 (or intermediate housing) or on the inlet housing 26 of the propulsion system 1, i.e., between the low-pressure compressor 4 and the high-pressure compressor 5.A first rear bearing 11b can be mounted on the low-pressure shaft 11 and on the inter-turbine housing 24 (i.e., on the housing extending between the high-pressure turbine 7 and the low-pressure turbine 8), upstream of the low-pressure turbine 8. Alternatively, the first rear bearing 11b can be mounted on the exhaust housing 27, which extends immediately downstream of the low-pressure turbine 8. The first rear bearing 11b extends downstream of the most downstream bearing 12b of the high-pressure shaft 10. The second rear bearing 11c can be mounted on the exhaust housing 27 or, if integration permits, on the inter-turbine housing. If necessary, the first and second rear bearing 11b, 11c can be mounted on the same cylindrical ferrule, which is itself fixed to the exhaust casing 27.

[0080] In one embodiment, the low-pressure shaft 11 is supported by a front bearing 11a and two rear bearings 11b, 11c.

[0081] All or part of the bearings, particularly bearings 11a-11c of the low-pressure shaft 11, may also include a flexible cage mounted between the inner ring and a rigid support fixedly attached to the stator part of the propulsion system 1 in order to control the stiffness of the bearings and to fine-tune the position of the first mode of deformation. The cage comprises, for this purpose, a generally cylindrical wall mounted between the rigid support and the second ring of the bearing, and radially deformable columns to allow radial displacement of the generally cylindrical wall, and therefore of the bearing, relative to the rigid support. Examples of bearings with a flexible cage are described in documents WO 2021 / 001610 and WO 2022 / 195198 on behalf of the Applicant.

[0082] The twin-body propulsion system 1 may include in particular a single- or two-stage high-pressure turbine 7, a high-pressure compressor 5 comprising at least eight stages and at most eleven stages, a low-pressure turbine 8 comprising at least three stages and at most five stages and a low-pressure compressor 4 comprising at least two stages and at most four stages.

[0083] The overall compression ratio of the propulsion system 1, which corresponds to the pressure ratio between the pressure at the outlet of the high-pressure compressor 5 and the pressure at the inlet of the blower rotor 9 (measured at the base of the blower rotor 9), shall be greater than or equal to 40 and less than or equal to 70, preferably greater than or equal to 44 and less than or equal to 55.

[0084] The propulsion system 1 further includes an assembly comprising at least one mode damper 25 (known in English as a "squeeze film") capable of sufficiently damping the vibrations generated by a moving element 35 of the propulsion system 1 while ensuring that it can be integrated below the flow path. The mode damper 25 can, in particular, be mounted between a fixed part of a bearing of the propulsion system 1 and a stator part of the propulsion system 1 (e.g., inlet housing, intermediate housing, turbine housing, or exhaust housing), or between a fixed part of the reduction mechanism 19 (ring support or planet carrier, depending on the configuration of the reduction mechanism 19) and a stator part of the propulsion system 1 (e.g., inlet housing or intermediate housing).The propulsion system 1 may include a mode damper 25 on each bearing supporting the low-pressure shaft 11 and / or on each bearing supporting the blower shaft 20. Preferably, the propulsion system 1 includes a mode damper 25 on the upstream bearing 11a of the low-pressure shaft 11 (or at least one of the upstream bearings when the low-pressure shaft 11 has several) insofar as this upstream bearing has a larger radius due to the presence of the reduction mechanism. Optionally, the propulsion system 1 may further include a mode damper on the downstream bearing 11b, 11c of the low-pressure shaft 11 (or at least one of the downstream bearings 11b, 11c when the low-pressure shaft 11 has several).

[0085] The mode damper 25 comprises an inner ring 30 and an outer ring 32 which define a damping chamber 34, and a fluid 29 confined within the damping chamber 34. The fluid 29 is pressurized within the damping chamber 34 and is configured to dampen the mode change (viscous damping) of the stationary part on which the mode damper 25 is mounted. The inner ring 30 and the outer ring 32 are annular and coaxial with the moving element 35 (the bearing, a moving part (satellites or ring gear) of the reduction mechanism 19). The inner ring 30 therefore extends radially between the moving element and the outer ring 32.

[0086] Preferably, the mode 25 shock absorber is simple, that is to say, it includes only one damping chamber 34, as opposed to a double shock absorber which includes at least two damping chambers.

[0087] Fluid 29 can include any type of viscous fluid 29, for example oil or fuel.

[0088] The supply parameters for the mode damper 25 can be conventional. For example, the supply pressure of the fluid 29 can be between 0.5 bar (50 kPa) and 30 bar (3000 kPa), preferably between 4 bar (400 kPa) and 18 bar (1800 kPa); the temperature of the fluid 29 can be between 10°C and 300°C, preferably between 50°C and 200°C; the supply flow rate of the fluid 29, which depends on the supply pressure and the sealing system of the mode damper 25, can be between ten liters per hour (L / h) and several hundred liters per hour. The damping chamber 34 is radially bounded internally by the inner ring 30 and radially bounded externally by the outer ring 32.The inner surface 33 of the outer ring 32 and the outer surface 31 of the inner ring 30 which delimit the damping chamber 34 may be smooth, in which case the inner ring 30 and the outer ring 32 are spaced radially apart from each other so as to define the damping chamber 34. Alternatively, a recess may be formed in at least one of the inner surface 33 of the outer ring 32 and the outer surface 31 of the inner ring 30, the damping chamber 34 then being delimited by the walls of the recess.

[0089] When the mode damper 25 is mounted on a bearing, the inner ring 30 of the mode damper 25 is aligned with the second ring of the bearing (or is integral with the second ring of the bearing). Similarly, when the mode damper 25 is mounted on a reduction mechanism 19, the inner ring 30 is integral with the crown gear or planet carrier, depending on the configuration of the reduction mechanism 19.

[0090] The inner ring 30 is fixed relative to the outer ring 32. For example, the assembly further includes an anti-rotation device configured to lock the inner ring 30 relative to the outer ring 32. The anti-rotation device may, for example, include a radial spacer mounted in notches formed in the inner ring 30 and in the stator part of the propulsion system 1 (or, alternatively, in the outer ring 32 of the mode damper 25) so as to circumferentially lock the inner ring 30 relative to the outer ring 32.

[0091] The damping chamber 34 is axially delimited by axial terminals 34a, 34b configured to prevent the fluid 29 from leaking during the operation of the propulsion system 1. The axial terminals 34a, 34b may correspond to the upstream and downstream axial limits of the damping chamber 34, typically the upstream and downstream faces of the recess formed in the outer ring 32 and / or the inner ring 30. Alternatively, at least one of the axial terminals 34a, 34b may be formed by a sealing segment mounted between the inner ring 30 and the outer ring 32 so as to axially delimit the damping chamber 34. Preferably, the mode damper 25 comprises two sealing segments, each forming one of the axial terminals 34a, 34b.

[0092] Each sealing segment 34a, 34b can be made of a metallic material, a composite material, a ceramic or an elastomeric material, preferably a metallic or elastomeric material.

[0093] In one embodiment, the mode damper 25 is circumferentially continuous around the X axis, i.e. the damping chamber 34 is devoid of separating means forming independent damping chamber sectors 34.

[0094] Optionally, the mode damper 25 includes a fluid feed groove 36 for fluid 29 configured to circumferentially distribute the fluid 29 within the damping chamber 34. The feed groove 36 can, for example, be formed in the outer surface 31 of the inner ring 30 and extend circumferentially and continuously around the entire circumference of the inner ring 30. In order to improve the performance of the propulsion system 1 and reduce the noise generated by the propulsion system 1, the axial length and maximum radial clearance of the mode damper 25 are configured such that the surface viscosity of the mode damper 25, which is the ratio of the axial length squared to the product of the maximum radial clearance cubed and twice the value of Pi, is between 1.0 x 10 4 rrr 1 and 1, 15 x 10 6 rrr 1 :

[0095] L 2

[0096] 1.0 x 10 4< surface viscosity = < 1.15 x 10 6 (1) where: L is the axial length of the mode 25 damper and is expressed in meters (m); and

[0097] C is the maximum radial play of the mode 25 damper and is expressed in meters (m).

[0098] The axial length L of the mode 25 damper corresponds to the distance along which the oil film operates (viscous damping). When the mode 25 damper lacks a feed groove (FIG. 3), the axial length L therefore corresponds to the axial distance between the axial terminals 34a, 34b of the damping chamber 34, and, where applicable, between the sealing segments of the damping chamber 34. When the mode 25 damper includes a feed groove 36 (FIG. 4), the axial length does not include the length of the feed groove 34; it therefore corresponds to the sum of the axial distance L1 between the upstream axial terminal 34a and the upstream limit of the groove, and the axial distance L2 between the downstream limit of the groove and the downstream axial terminal 34b.

[0099] The maximum radial play of the mode damper 25 corresponds to the maximum radial distance, outside the feed groove 36, between the inner surface 33 of the outer ring 32 and the outer surface 31 of the inner ring 30. The maximum radial play therefore corresponds to the maximum thickness of the part of the oil film which plays a role in the damping of the modes.

[0100] When the moving element 35 rotates at a given rotational speed co, the inner ring 30 is subjected to a precessional motion (i.e., an orbit without relative rotation of the inner ring 30 around its axis) of the same frequency, resulting from the presence of an (inevitable) imbalance on the moving element 35, with a relative eccentricity e with respect to the outer ring 32. The damping generated by the mode damper 25 then depends on the tangential force generated by the compression of the fluid film 29 between the inner and outer rings. The tangential force Ft can be estimated by the following formula:

[0101] Ft = — Cv xexa> where: Cv is the viscosity of the mode 25 damper.

[0102] Viscosity is a drag coefficient that reflects the ability of a mode 25 damper to dissipate energy. This viscosity can be estimated using the following formula: where: p depends on the properties of the fluid 29 of the mode 25 damper;

[0103] R is the mounting radius of the mode 25 damper, i.e., the maximum radius between the X-axis and the external surface 31 of the inner ring 30, the mounting radius being measured in a plane normal to the X-axis and expressed in meters (m); and

[0104] A is a constant.

[0105] The surface viscosity defined above is therefore a parameter that allows us to approximate the viscosity of the mode 25 damper, taking into account the surface area of ​​the mode 25 damper: surface viscosity = -2n - RL = 2nC 3 ( '2 ')

[0106] Since R, L and C are distances, these parameters are determined when the propulsion system 1 (and therefore the assembly including the mode 25 damper) is cold, as specified above.

[0107] When the mode damper 25 is dimensioned to comply with formula (1), the damping provided by the mode damper 25 is improved while taking into account the mechanical integration constraints of the assembly including the damper. In particular, increasing the surface viscosity allows for an increase in the damping of the moving element 35. However, this increase does not need to be achieved at the expense of the available space in the propulsion system 1.

[0108] The present exposition applies mainly to propulsion systems in which at least one of the rotors (low pressure shaft 11, or high pressure shaft 10, rotating part of the reduction mechanism 19, etc.) has several natural modes of deformation included in the operating range of the propulsion system 1. In general, this will be more particularly the low pressure shaft 11 (in the case of direct drive propulsion systems or systems including a reduction mechanism 19).

[0109] The Applicant observed that optimizing the damping of the first deformation mode could increase the stiffness of the mode 25 damper at high frequencies in the assembly comprising the mode 25 damper and the moving element 35, and short-circuit the flexible cage, as the forces applied by the moving element 35 would then be transmitted directly to the stator part of the propulsion system 1 by the mode 25 damper. Conversely, dimensioning the mode 25 damper to comply with formula (1) optimizes the damping provided by the mode 25 damper for the first three deformation modes and avoids the risk of locking. In particular, a mode 25 damper with a surface viscosity greater than or equal to 1.0 x 10 4 and less than or equal to 1.15 x 10 6 has the following effect:

[0110] - to reduce the amplitude of the first mode of deformation of the moving element 35 without changing its frequency; and

[0111] - to increase the frequency of the second and third modes of deformation and to reduce their amplitude.

[0112] Preferably, the surface viscosity of the mode 25 damper is less than or equal to 7.1 x 10 5 Indeed, beyond this value, the amplitude of the second and third modes of deformation of the moving element 35 increases again (as does the frequency of the second and third modes of deformation).

[0113] In order to further take into account the integration constraints in the damping optimization, the mode 25 damper has a surface clearance, which corresponds to the ratio between the maximum radial clearance C (between the external surface 31 of the inner ring 30 and the internal surface 33 of the outer ring 32) and the product of twice the value of Pi multiplied by the mounting radius R of the mode 25 damper and the axial length L of the mode 25 damper, between 2.5 x 10 -2 m -1 and 1.0 x 10 -1 m -1 : 2.5 x 10 2 < surfing game acique = < 1.0 x 10 1 (3)

[0114] The surface clearance parameter takes into account, in particular, the mounting radius of the mode 25 damper, and therefore the available space in the propulsion system 1. Indeed, the smaller the mounting radius R, the greater the surface clearance, but the less effective the damping. A mode 25 damper exhibiting a surface viscosity between 10 A 4 rrr 1 and 1.15 x 10 A 6 rrr 1 and a surface clearance between 2.5*10 -2 rrr 1 and 1.0*10 -1 rrr 1is therefore capable of sufficiently reducing the vibration response of the moving element 35 around which it is mounted, while respecting strict space requirements, thanks to the adaptation of the axial length and radial play of the mode damper 25. Thus, the dimensioning of the mode damper(s) 25 is adapted according to their position in the propulsion system 1 taking into account, in particular, their radius of implantation R.

[0115] In a conventional engine, given the values ​​usually selected for the axial length L and the maximum radial clearance C of the mode damper, the surface viscosity is greater than 1.15 x 10 6 , which leads to a blocking of the mode damper. Conversely, by selecting values ​​for the axial length L and the maximum radial clearance C such that the surface viscosity of the mode damper 25 remains less than 1.15 x 10 6The Mode 25 damper is no longer locked and returns to its normal damper behavior. To optimize the Mode 25 damper's performance, increase the surface viscosity (without exceeding the limit of 1.15 x 10). 6 ) then allows us to reduce the response on the deformation modes.

[0116] Preferably, the surface clearance of the mode 25 damper is greater than or equal to 3.7 x 10 -2 rrr 1 in order to optimize the integration of the mode 25 damper. Advantageously, when the viscosity is high, a surface clearance greater than or equal to 3.7 x 10 -2 rrr 1 has the effect of moderating the damping of the mode 25 damper and therefore limiting its stiffness in high frequencies.

[0117] This description is particularly applicable to propulsion systems comprising a reduction mechanism 19 and a high-speed low-pressure shaft 11. The term "high-speed low-pressure shaft 11" here refers to a low-pressure shaft 11 whose redline speed, which corresponds to the absolute maximum speed likely to be encountered by the low-pressure shaft 11 during the entire flight (according to European certification regulation EASA CS-E 740 (or according to American certification regulation 14-CFR Part 33.87)), is greater than or equal to 8000 revolutions per minute. For example, the redline speed may be between 8500 and 12000 revolutions per minute, preferably between 9000 and 11000 revolutions per minute. The redline speed corresponds to the maximum rotational speed when the propulsion system is functioning properly (and potentially at the end of its service life). It is therefore likely to be reached by the low-pressure shaft 11 under flight conditions.This speed limit is part of the data declared in the engine certification (the "type certificate data sheet"). Indeed, this rotational speed is usually used as a reference speed for the sizing of propulsion systems and in certain certification tests (such as blade loss or rotor integrity tests).

[0118] A first example of a propulsion system 1 to which the present description applies is a USF-type engine 1 comprising a single unshod fan rotor 9 and an unshod fan stator 16, which are fixed in rotation with respect to the X-axis of the fan rotor 9. The fan rotor 9 has a diameter D9 greater than 132 inches (3.3528 meters (m)) and is driven via a reduction mechanism by a drive turbine whose limiting speed is between 7,000 and 14,000 revolutions per minute (rpm). The low-pressure shaft is supported by three bearings, one front bearing mounted on the inlet casing and two rear bearings mounted on the exhaust casing. The dilution ratio of this propulsion system 1 is greater than or equal to 40, for example between 40 and 80 inclusive, and its overall compression ratio is greater than 40. This propulsion system 1 is also configured to provide a thrust of between 50,000 and 350,000 N.

[0119] A second example of a propulsion system 1 to which the present description applies is an engine comprising a shrouded fan section. The fan rotor 9 has a diameter D9 of between 70 and 120 inches (between 1.778 and 3.048 meters (m)) and is driven via a reduction mechanism by a drive turbine with a limiting speed of between 7,000 and 14,000 revolutions per minute (rpm). The low-pressure shaft is supported by three bearings: a front bearing mounted on the intermediate housing and two rear bearings mounted on the exhaust housing. The bypass ratio of this propulsion system 1 is greater than 10, less than or equal to 35, preferably less than or equal to 18, and its overall compression ratio is greater than 40. This propulsion system 1 is also configured to provide a thrust of between 50,000 and 350,000 N.

[0120] In these two examples of propulsion systems 1, the mode damper 25 can be mounted on the reduction mechanism 19 and / or at least one bearing of the low pressure shaft 11. For example, the mode damper 25 can be mounted on the front bearing 11a and on one of the downstream bearings 11b, 11c which are mounted on the exhaust housing, these two bearings 11b, 11c being for example roller or ball bearings.

[0121] The invention applies, however, to any type of bearing capable of including a mode 25 damper, and in particular to roller bearings, ball bearings or tapered roller bearings.

[0122] It should be noted here that, regardless of the configuration of the propulsion system 1, the diameter D9 of the fan rotor 9 is measured in a plane normal to the axis of rotation of the fan rotor, at the intersection between a vertex 21 and a leading edge 22 of the blades 14 of the fan rotor 9, and is expressed in meters (m). Note that since Figures 1 and 2 are partial views, the diameter D9 is only partially visible.

[0123] It should be noted that these two examples of propulsion systems present severe mechanical integration constraints (related in particular to the dimensions of the high-pressure body, which is increasingly compact, and to the integration of the reduction mechanism, leading to bearings with a large radius due to the axial proximity of the reduction mechanism) leading to constrained bearing environments (bearings supporting shafts radially distant from the X axis).

[0124] Examples of sizing for mode 25 shock absorbers, particularly suitable for assemblies 1 to 7 including a bearing supporting the low-pressure shaft 11 of a propulsion system 1, are as follows:

[0125] These examples of mode damper 25 all allow the vibration response of the moving element 35 (bearing or reduction mechanism 19) around which they are mounted to be reduced and can in particular be implemented in propulsion systems 1 conforming to the first and second examples of propulsion systems described above.

[0126] In these examples, the Mode 25 dampers Nos. 1 to 7 exhibit high surface viscosity and low surface clearance, which optimizes the damping of bearing deformation modes, particularly in propulsion systems such as the examples described above. Their low surface clearance also helps to limit cavitation effects. The Mode 25 damper No. 2 is particularly optimized in terms of size (small mounting radius and axial length) and damping (high surface viscosity and low surface clearance) and can, for example, be mounted on a low-pressure shaft bearing 11 due to its small mounting radius R and short axial length L. The Mode 25 damper No. 3 is also optimized in terms of radial size due to its small mounting radius and damping (high surface viscosity).

[0127] Mode 25 shock absorbers No. 4 to 7 will be particularly advantageous for damping located above the reduction mechanism 19.

[0128] Figure 7 illustrates the influence of the surface viscosity of a mode 25 damper on the displacement of the front bearing 11a of the second example of propulsion system 1 as a function of the rotational speed of the low-pressure shaft 11 of this propulsion system 1. In this figure:

[0129] - The Cv- curve corresponds to a mode 25 damper with a surface viscosity much less than 10 4 rrr 1 ;

[0130] - The Cv- curve corresponds to a mode 25 damper with a surface viscosity greater than or equal to 10 4 rrr 1 ;

[0131] - The Cv curve corresponds to a mode 25 damper with a surface viscosity between 10 4 rrr 1 and 7.1 x 10 5 rrr 1 ;

[0132] - the Cv+ curve corresponds to a mode 25 damper with a surface viscosity greater than 7.1 x 10 5 rrr 1 and less than 1.15 x 10 6 rrr 1 ;

[0133] - The Cv++ curve corresponds to a mode 25 damper with a surface viscosity greater than 1.15 x 10 6 rrr 1 ; and - the Cv+++ curve corresponds to a mode 25 damper exhibiting a surface viscosity even greater than Cv++.

[0134] As can be seen from this figure, the amplitude of the three deformation modes of the bearing comprising a mode damper whose surface viscosity is between 1.0 x 10 4 and 1.15 x 10 6 rrr 1is reduced. Such a mode damper is therefore capable of damping vibrations and reducing HCF loads on such a bearing throughout the entire operating range of the propulsion system. On the other hand, the amplitude of the second mode of deformation of the bearing comprising a mode damper whose surface viscosity is less than 1.0 x 10 4 rrr 1 is very high (C- and C- curves), as is the amplitude of the third mode of deformation of the bearing comprising a mode damper whose surface viscosity is greater than 1.15 x 10 6 rrr 1 .

[0135] Manufacturing process

[0136] An assembly for a propulsion system conforming to this exposition can be manufactured in accordance with the following steps, after defining the damping requirement of the mode 25 damper (surface viscosity) and specifying the geometry of the mode 25 damper (in particular its axial length L and its maximum radial clearance C as a function of the defined surface viscosity).

[0137] A mode 25 damper comprising an inner ring 30 and an outer ring 32 is dimensioned such that the mode 25 damper has the maximum axial length and radial clearance such that the surface viscosity of the mode 25 damper is between 1.0 x 10 4 and 7.1 x 10 5 m- 1 .

[0138] The mode damper 25 is then manufactured and assembled with a movable element 35, for example a bearing, so that the inner ring 30 and the outer ring 32 are coaxial with the bearing and the first ring (which are movable around the X axis) of the bearing and that the inner ring 30 extends radially between the bearing and the outer ring 32.

[0139] A fluid 29 is then injected under pressure into the damping chamber 34 of the mode damper 25, between the inner ring 30 and the outer ring 32.

[0140] Comparative example:

[0141] The engine 1 is a twin-body propulsion system comprising a shrouded blower section 2 driven in rotation by means of a gear reduction mechanism corresponding to the current technical standard (at the date of filing of this application) which we seek to improve.

[0142] Engine 2 is a twin-body propulsion system 1 comprising a shrouded fan section 2 conforming to the second example described above.

[0143] The low-pressure shaft of motors 1 and 2 is each supported by a front ball bearing comprising a mode 25 damper conforming to formulas (1) and (3) and two rear roller bearings, the downstream roller bearing also comprising a mode 25 damper conforming to formulas (1) and (3). The reduction mechanism and the second downstream bearing do not include a mode damper.

[0144] Engine 1 has a subcritical shaft dynamic situation, meaning that the first bending mode of the low-pressure shaft is outside its operating range with a sufficient margin. This characteristic limits the engine's performance in terms of overall compression ratio / high-pressure turbine temperature and in terms of mechanical loading (blade root stress) of the low-pressure turbine.

[0145] Engine 2 has a supercritical shaft dynamic state, meaning that the first bending mode of the low-pressure shaft 11 is within its operating range. This dynamic state is made possible by the use of mode 25 dampers on the upstream bearing 1a and the second downstream bearing of the low-pressure shaft 11, as defined by equations (1) and (3). Achieving this modal positioning of the low-pressure shaft 11 allows for an increase in the compression ratio of the high-pressure compressor (and therefore the overall compression ratio), the temperature of the high-pressure turbine, and the mechanical load (blade tip stress) of the low-pressure turbine. This improves the performance of the propulsion system 1 by enhancing its thermal efficiency and reducing its mass.

Claims

DEMANDS 1. Assembly (11a, 11b, 11c) of an aeronautical propulsion system (1) comprising: - a movable element (35) rotating about an axis (X) of the assembly (11a, 11b, 11c); and - a mode damper (25) comprising an inner ring (30), an outer ring (32) and a fluid (29) confined between the inner ring (30) and the outer ring (32), the inner ring (30) and the outer ring (32) being coaxial with the moving element (35); the inner ring (30) extending radially between the moving element (35) and the outer ring (32); and the mode damper (25) having: - an axial length and a maximum radial clearance such that a surface viscosity of the mode damper (25), which corresponds to the ratio between the square of the axial length and the product between the cube of the maximum radial clearance multiplied by twice the value of Pi, is between 1.0 x 10 4 rrr 1 and 1.15 x 10 6 m- 1: 1.0 x 10 4 1.15 x 10 6 Or : L is the axial length of the mode (25) damper and is expressed in meters (m); and C is the maximum radial clearance of the mode damper (25) and is expressed in meters (m); and - a surface clearance, which corresponds to the ratio between the maximum radial clearance and the product between twice the value of Pi multiplied by the mounting radius (R) of the mode damper (25) and the axial length (L) of the mode damper (25), is between 2.5 x 10 -2 m -1 and 1.0 x 10 -1 m -1 : 2.5 x 10" 2 < surface area 1.0 x 10 -1 where: R is the radius of implantation of the mode damper (25) and is expressed in meters (m).

2. Assembly (11a, 11b, 11c) according to claim 1, wherein the surface viscosity of the mode damper (25) is less than or equal to 7.1 x 10 5 m -1 .

3. Assembly (11a, 11b, 11c) according to any one of claims 1 and 2, wherein the surface clearance of the mode damper (25) is greater than or equal to 3.7 x 10 -2 m -1 .

4. Assembly (11a, 11b, 11c) according to any one of claims 1 to 3, wherein the moving element (35) comprises a moving part of a bearing (11a, 11b, 11c) of the propulsion system (1).

5. Assembly (11a, 11b, 11c) according to any one of claims 1 to 4, wherein the moving element (35) comprises a moving part of a reduction mechanism (19) of the propulsion system (1).

6. Assembly (11a, 11b, 11c) according to any one of claims 1 to 5, wherein the mode damper (25) further comprises at least one sealing segment (34a, 34b), preferably two sealing segments (34a, 34b), axially delimiting the fluid (29) between the inner ring (30) and the outer ring (32).

7. Assembly (11a, 11b, 11c) according to any one of claims 1 to 6, wherein the mode damper (25) comprises a single damping chamber (34).

8. Aeronautical propulsion system comprising an assembly (11a, 11b, 11c) according to any one of claims 1 to 7 and a rotor coaxial with the moving element (35).

9. System according to claim 8, wherein the rotor comprises a shaft (11) of the propulsion system and the moving element (35) comprises a bearing (11a, 11b, 11c) which supports the rotor.

10. Aeronautical propulsion system (1) according to claim 9, wherein the rotor comprises a drive shaft (11), the propulsion system further comprising: - a blower section (2) comprising a blower shaft (20); - a drive turbine (8) driven in rotation by the rotor (11) around the axis (X); and - a reduction mechanism (19) coupling the rotor (11) and the blower shaft (9) in order to drive the blower shaft (20) at a rotational speed lower than the rotational speed of the rotor (11), the moving element (35) comprising a moving part of the reduction mechanism (19).

11. Aeronautical propulsion system (1) according to claim 10, wherein the drive shaft (11) is supported by at least one front bearing (11a) and one or two rear bearings (11b), the moving element corresponding to the front bearing (11a) and where applicable to one of the rear bearings (11b).

12. Aeronautical propulsion system (1) according to any one of claims 10 and 11, wherein a dilution ratio of the propulsion system (1) is greater than or equal to 10, for example between 10 and 80 inclusive.

13. Aeronautical propulsion system (1) according to any one of claims 10 to 12, wherein a critical speed of the drive shaft is greater than or equal to 8000 revolutions per minute.

14. Aircraft (100) comprising at least one aeronautical propulsion system (1) according to any one of claims 10 to 13 fixed to the aircraft by means of a mast.

15. Method for manufacturing an assembly (11a, 11b, 11c) of an aeronautical propulsion system (1) comprising the following steps: - determine an axial length, a maximum radial clearance and an implantation radius of a mode damper (25) such that a surface viscosity of the mode damper (25), which corresponds to the ratio between the square of the axial length and the product between the cube of the maximum radial clearance multiplied by twice the value of Pi, is between 1.0 x 10 4 rrr 1 and 1.15 x 10 6 rrr 1 : 10 4 1.15 x 10 6 and that a surface clearance of the mode damper (25) is between 2.5 x 10 A -2 rrr 1 and 1.0 x 10 A -1 m -1 , where the surface clearance is defined as follows: 2.5 x 10" 2 < surface area 1.0 x 10 -1 Or : L is the axial length of the mode damper (25) and is expressed in meters (m); C is the maximum radial clearance of the mode (25) damper and is expressed in meters (m); and R is an implantation radius of the mode (25) damper and is expressed in meters (m); - manufacture an inner ring (30) and an outer ring (32) of the mode damper (25) according to the axial length, the maximum radial play and the implantation radius thus determined; - assemble the inner ring (30) and the outer ring (23) with a movable element (35); and - inject a fluid (29) between the inner ring (30) and the outer ring (32).

16. A manufacturing method according to claim 15, wherein the axial length and maximum radial clearance are determined such that the surface viscosity is less than or equal to 7.1 x 10 5 rrr 117. A manufacturing method according to any one of claims 15 and 16, wherein the axial length, maximum radial clearance, and mounting radius are determined such that the surface clearance is greater than or equal to 3.7 x 10 -2 m -1 .

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