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

Abstract

An aeronautical propulsion system (1) comprises: - a fan section; - a combustion chamber (6); - a first turbine (8) configured to drive the fan rotor (9) and a first compressor (4) via a first shaft (11) about an axis of rotation (X); - a reduction mechanism (19) coupling the first shaft (11) and the fan shaft (20); - bearings (11a, 11b, 11c) configured to centre the drive shaft (11) with respect to the axis of rotation (X); and - a mode damper (25) comprising an inner ring (30) mounted on a stator part of one of the bearings (11a, 11b, 11c), an outer ring (32) and a fluid (29) confined between the inner ring (30) and the outer ring (32); and an axial length (L), an installation radius (R) and a maximum radial clearance (G) of the mode damper (25) being dimensioned according to a free length and a mean radius of the drive shaft (11) so as to comply with the formula: (I) as follows: I 2 1.0 x 10 < 2.5 x 102 x -^- where: L11 is the free length of the drive shaft (11) and is expressed in metres (m); and R11 is the mean radius of the drive shaft (11) between the first and the second support and is expressed in metres (m).

Description

[0001] Optimization of fan behavior in an aeronautical propulsion system

[0002] TECHNICAL FIELD

[0003] 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.

[0004] STATE OF THE ART

[0005] 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.

[0006] 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.

[0007] Thus, in order to improve the propulsion efficiency of the propulsion system and reduce its specific fuel consumption as well as the noise emitted by the fan section, propulsion systems with a high bypass ratio (BPR, the ratio between the secondary airflow rate and the primary airflow rate) have been proposed. To achieve such bypass ratios, the fan section can be decoupled from the low-pressure turbine, thereby allowing their respective rotational speeds to be optimized independently. Generally, decoupling is achieved using a reduction mechanism placed between the upstream end of the low-pressure shaft and a rotor of the fan section. The fan section rotor is then driven by the low-pressure shaft via the reduction mechanism at a rotational speed lower than that of the low-pressure shaft.

[0008] Improving the propulsive efficiency of the fan section also involves reducing its pressure ratio. However, since the propulsion system has a high dilution rate, this necessitates reducing the flow rate in the high-pressure section, which generally translates into a reduction in its radial dimensions. However, because the low-pressure shaft is housed within the high-pressure shaft, the clearance between the shafts is also reduced, increasing the risk of shaft contact.

[0009] The increase in rotational speeds of the low-pressure shaft leads to the appearance of a greater number of overall vibration modes in the operating range, or even to the appearance of a bending mode of the low-pressure shaft 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.

[0010] It is therefore necessary to minimize mechanical loads, vibrations, and displacements of the low-pressure shaft (in particular). To this end, it has been proposed to interpose a mode damper (or "squeeze-film") and, where appropriate, a flexible cage (also known as a squirrel cage) on all or part of the bearings supporting the low-pressure shaft. A mode damper, as is known, comprises an inner ring and an outer ring that are fixed relative to a stator portion of the propulsion system, extending radially outward from the bearing, and a pressurized fluid that is confined between the inner and outer rings.

[0011] However, because the low-pressure shaft bending mode (mode 1F) occurs at a relatively high frequency due to the presence of the reduction mechanism, the vibrations generated during the transition to this mode compress the oil in the annular space between the mode damper rings. This leads to significant displacement velocities in the oil film. Consequently, the inertial and damping forces generated by the fluid in the mode damper become too great compared to the inertial forces of the low-pressure shaft mode. As a result, the mode damper behaves like a rigid bearing, thus negating its beneficial damping effect. This is referred to as "locking" the mode damper.

[0012] EXPOSED

[0013] One aim of this application is to optimize the performance of the aeronautical propulsion system, in particular propulsion systems with high bypass ratios, in terms of vibration damping, dynamic clearance consumption and polycyclic loading, taking into account the constraints of integration into the propulsion system.

[0014] To this end, a first aspect of an aeronautical propulsion system is proposed, comprising:

[0015] - a blower section comprising a blower rotor connected to a blower shaft;

[0016] - a combustion chamber;

[0017] - a first turbine configured to drive the blower rotor and a first compressor via a first shaft around an axis of rotation;

[0018] - a reduction mechanism coupling the first shaft and the blower shaft to drive the blower shaft at a rotational speed lower than that of the first shaft; - bearings configured to center the drive shaft relative to the axis of rotation; and

[0019] - a mode damper comprising an inner ring mounted on a stator portion of one of the bearings, an outer ring, and a fluid confined between the inner and outer rings; and an axial length, a mounting radius, and a maximum radial clearance of the mode damper being dimensioned as a function of a free length and an average radius of the drive shaft so as to comply with the following formula: l 2

[0020] 1.0 2.5 x 10 2 x R rl Or :

[0021] L is the axial length of the damper and is expressed in meters;

[0022] C is the maximum radial play of the mode damper and is expressed in meters;

[0023] R is the radius of implantation of the mode damper and is expressed in meters;

[0024] Lu is the free length of the drive shaft, which corresponds to an axial distance between a first support and a second support immediately adjacent to the drive shaft, the first support being positioned upstream of the combustion chamber and the second support being positioned downstream of the combustion chamber, and is expressed in meters; and

[0025] R11 is the average radius of the drive shaft between the first and second supports and is expressed in meters.

[0026] Some preferred but not limiting characteristics of the aeronautical propulsion system according to the first aspect are the following, taken individually or in combination:

[0027] - The axial length, the mounting radius, and the maximum radial play of the mode damper are dimensioned according to the free length and the average radius of the drive shaft so as to comply with the following formula: l 2

[0028] 1.0 5.0 x 10 1 x -ii; R rl

[0029] - The axial length, the mounting radius, and the maximum radial play of the mode damper are dimensioned according to the free length and the average radius of the drive shaft so as to comply with the following formula: l 2

[0030] 5.0 x 10° 1.0 x 10 2 x -ii; R rl

[0031] - the first and second support correspond respectively to at least one of the following positions: a position of a center of gravity of one of the bearings; a position of a connection of a trunnion of the first compressor to the drive shaft; a position of a connection of a trunnion of the first turbine to the drive shaft;

[0032] - the mode damper comprises a determined number n of fluid supply grooves between the inner ring and the outer ring, where the determined number n is an integer greater than or equal to 0, so as to define n+1 groove-free portions between two seals of the mode damper, each groove-free portion having respectively a length and a height, each groove having respectively a length and a height, the maximum radial clearance being equal to the ratio between the sum, for each portion and for each groove, of the quotient between the length and the height, and the sum of the lengths of each portion and each groove;

[0033] - the average radius of the drive shaft is greater than or equal to 0.01 meters and less than or equal to 0.06 meters and / or the free length of the drive shaft is greater than or equal to 0.50 meters and less than or equal to 2.50 meters;

[0034] - the mounting radius of the mode damper is greater than or equal to 0.025 meters and less than or equal to 0.30 meters, and / or its axial length is greater than or equal to 0.01 meters and less than or equal to 0.05 meters and its maximum radial clearance is greater than or equal to 0.1 x10 -4 meters and less than or equal to 1.0 x 10 -3 meters;

[0035] - the bearings include at least one front bearing and one or two rear bearings, the damper being mounted on the front bearing;

[0036] - the aeronautical propulsion system further includes a mode damper mounted on one of the rear bearings;

[0037] - the aeronautical propulsion system further includes fan bearings configured to center the fan shaft relative to the axis of rotation and additional mode dampers, the additional mode dampers being mounted on all or part of the fan bearings and / or bearings when said bearings do not include a mode damper;

[0038] - a dilution ratio of the propulsion system is greater than or equal to 10, for example between 10 and 80 inclusive;

[0039] - the aeronautical propulsion system further comprises a second turbine configured to drive a second compressor via a second shaft around the axis of rotation, an overall compression ratio of the propulsion system, which corresponds to the pressure ratio between the pressure at the outlet of the second compressor and the pressure at the inlet of the fan rotor, is 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, and a maximum inlet temperature of the first turbine is greater than or equal to 1050°C and less than or equal to 1150°C.

[0040] According to a second aspect, it is proposed an aircraft comprising at least one aeronautical propulsion system according to the first aspect fixed to the aircraft by means of a mast.

[0041] According to a third aspect, a manufacturing process for an aeronautical propulsion system is proposed, based on the first aspect and comprising the following steps:

[0042] - determine an axial length, a radius and a maximum radial clearance of a mode damper as a function of a free length and an average radius of the drive shaft so as to respect the following formula: l 2

[0043] 1.0 2.5 x 10 2 x R rl Or :

[0044] L is the axial length of the damper and is expressed in meters;

[0045] C is the maximum radial play of the mode damper and is expressed in meters;

[0046] R is the radius of implantation of the mode damper and is expressed in meters;

[0047] Lu is a free length of the drive shaft, which corresponds to an axial distance between a first support and a second support immediately adjacent to the drive shaft, the first support being positioned upstream of the combustion chamber and the second support being positioned downstream of the combustion chamber, and is expressed in meters; and

[0048] R11 is the average radius of the drive shaft between the first and second supports and is expressed in meters

[0049] - manufacture an inner ring and an outer ring of the mode damper according to the axial length and maximum radial play thus determined;

[0050] - assemble the inner and outer rings with a bearing according to the mounting radius of the damper; and

[0051] - inject a fluid between the inner ring and the outer ring.

[0052] Optionally, the axial length, mounting radius, and maximum radial play of the mode damper are determined based on the free length and mean radius of the drive shaft so as to comply with the following formula: I 2

[0053] 1.0 5.0 x 10 1 x -ii. R rl

[0054] Optionally, the axial length, mounting radius, and maximum radial play of the mode damper are determined based on the free length and mean radius of the drive shaft so as to comply with the following formula: l 2

[0055] 5.0 x 10° 1.0 x 10 2 x R, i

[0056] DESCRIPTION OF THE FIGURES

[0057] 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:

[0058] 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 shrouded and the low-pressure shaft is supported by two supports in accordance with a first configuration;

[0059] 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 and the low-pressure shaft is supported by two supports in accordance with a first configuration;

[0060] Figure 3 is an enlarged schematic cross-sectional view of part of an example assembly for a propulsion system according to a first variant;

[0061] Figure 4 is an enlarged schematic cross-sectional view of part of an example assembly for a propulsion system according to a second variant;

[0062] Figure 5 is an example of an aircraft that may include at least one propulsion system conforming to the first or second embodiment; and

[0063] Figure 6 is a flowchart illustrating examples of steps in a manufacturing process for an assembly for a propulsion system.

[0064] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION

[0065] 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).

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] The fan section 2 includes at least the fan rotor 9, which is driven in rotation relative to a stator portion of the propulsion system 1 by the turbine section 7, 8. Each fan rotor 9 comprises a hub 13 and blades 14 extending radially from the hub 13. The blades 14 of each rotor 9 may be fixed relative to the hub 13 or have variable pitch. In this case, the root 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 indicate that this feature is optional.

[0074] 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.

[0075] The fan rotor 9 also comprises at least twelve and at most twenty-four blades 14, for example, at least fourteen 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. In order to improve the propulsive efficiency of the propulsion system 1 and to reduce its specific fuel consumption as well as the noise emitted by the fan section 2, the propulsion system 1 has a high bypass ratio. A high bypass ratio is understood here to be a ratio greater than or equal to 10, for example, between 10 and 80 inclusive.To calculate the dilution ratio, the mass flow rate of the secondary airflow F2 and the mass flow rate of the primary airflow F1 are measured when the propulsion system 1 is stationary, uninstalled, in takeoff regime in a standard atmosphere (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.

[0076] 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 thus 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.

[0077] 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). 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).

[0078] 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.

[0079] 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.

[0080] In an unducted fan section 2, the fan section 2 (which may also be referred to as the propeller) is not enclosed by a fan casing. Because the fan section 2 is unducted, the blades 14 of the fan rotor 9 have variable pitch. Propulsion systems comprising at least one unducted fan rotor 9 are known as "open rotors" or "unducted fans." The propulsion system 1 may comprise two unducted, counter-rotating fan rotors 9. Such a propulsion system 1 is known by the acronym CROR for "Contra-Rotating Open Rotor" or UDF for "Unducted Double Fan." The blower rotor(s) 9 can be placed at the rear of the primary body 3 so as to be of the pusher type or at the front of the primary body 3 so as to be of the tractor type.Alternatively, the propulsion system 1 may comprise a single unducted fan rotor 9 and an unducted 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 rectifier 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 rectifier 16 also have variable pitch.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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 comprise one or two front 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 within 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 forward 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 on the high-pressure shaft 10. The second rear bearing 1 1c can be mounted on the exhaust housing 27 or, if integration allows, 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 housing 27. In one embodiment, the low-pressure shaft 11 is supported by a front bearing 11a and two rear bearings 11b, 11c.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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 its integration below the flow path. The mode damper 25 can, in particular, be mounted between a fixed portion of a bearing of the low-pressure shaft 11 and a stator portion of the propulsion system 1 (e.g., inlet housing, intermediate housing, turbine housing, or exhaust 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 fan shaft 20.

[0090] 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). The inner ring 30 therefore extends radially between the moving element and the outer ring 32.

[0091] Fluid 29 can include any type of viscous fluid 29, for example oil or fuel.

[0092] The supply parameters of the mode damper 25 can be conventional. For example, the supply pressure of the fluid 29 (which corresponds to a relative pressure variation with respect to the pressure of the pressurized enclosure in which the mode damper is located) can be between 0.5 bar (50 kPa) and 30 bar (3000 kPa), preferably between 4 bar (400 kPa) and 25 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.

[0093] The damping chamber 34 is radially delimited internally by the inner ring 30 and radially 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.

[0094] When the mode 25 damper is mounted on the bearing 11a-11c, the inner ring 30 of the mode 25 damper corresponds to the second ring of the bearing (or is one piece with the second ring of the bearing).

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] Optionally, the mode damper 25 includes a fluid supply groove 36 29 configured to distribute the fluid 29 circumferentially in the damping chamber 34. The supply groove 36 can, for example, be formed in the external surface 31 of the inner ring 30 and extend circumferentially and continuously over the entire circumference of the inner ring 30.

[0100] Mode dampers are designed to dampen the suspension modes of modules (particularly the fan section 2 or the low-pressure turbine 8), which have significant inertia and kinetic energy. Thus, these mode dampers are intended to provide substantial damping forces. However, because the mass of the low-pressure shaft 11 is low, primarily due to the reduction in its radius resulting from the smaller size of the high-pressure body and made possible by the increased rotational speed, the bending mode 1F of the low-pressure shaft 11 has very little inertia. Therefore, despite significant deformations that could lead to inter-shaft contact (i.e., contact between the low-pressure shaft 11 and the high-pressure shaft 10), it generates very little kinetic energy.Conventional mode dampers generating significant high-frequency forces therefore lead to an embedding of this bending mode, thus short-circuiting the flexible cage, which limits stability and amplifies the response of the mode damper and is the opposite of the desired objective.

[0101] To avoid these mode damper locking phenomena, it is therefore proposed to dimension the mode damper of at least one of the bearings 11a-11c of the low-pressure shaft 11 taking into account the free length Lu of the low-pressure shaft 11 and its mean radius Rn, in order to optimize the damping on the bending mode 1F. For this purpose, the axial length L, the mounting radius R and the maximum radial clearance c of at least one mode damper 25 of the low-pressure shaft 11 are dimensioned so as to comply with the following formula:

[0102] 1.0 2.5 x 10 2 (1) preferably: l 2

[0103] 2.5 1.0 x 10 2 x R rl Or :

[0104] L is the axial length of the mode 25 damper and is expressed in meters (m); c is the maximum radial clearance of the mode 25 damper and is expressed in meters (m);

[0105] R is the implantation radius of the mode 25 damper, that is to say the maximum radius between the X axis and the external surface 31 of the inner ring 30, the implantation radius R being measured in a plane normal to the X axis and expressed in meters (m);

[0106] Li 1 is the free length of the low-pressure shaft 11, which corresponds to an axial distance between a first support and a second support immediately adjacent to the drive shaft (11), the first support being positioned upstream of the combustion chamber (6) and the second support being positioned downstream of the combustion chamber (6), and is expressed in meters (m); and

[0107] Rn is the average radius of the low-pressure shaft 11 between the first and second supports and is expressed in meters (m). R, L, c, Lu and Ru being distances, these parameters are determined when the propulsion system 1 (and therefore the assembly including the mode damper 25) is cold, as specified above.

[0108] The axial length L of the mode damper 25 corresponds to the axial distance between the axial terminals 34a, 34b of the damping chamber 34, where applicable between the sealing segments of the damping chamber 34.

[0109] The maximum radial clearance c of the mode damper 25 corresponds to the maximum radial distance between the inner surface 33 of the outer ring 32 and the outer surface 31 of the inner ring 30. The maximum radial clearance therefore corresponds to the maximum thickness of the portion of the oil film that plays a role in mode damping. When the damper 25 includes a feed groove 36 (FIG. 4), the maximum radial clearance takes the feed groove 36 into account. For this purpose, an axial distance (Li, L2) and a maximum height (radial distance hi, h2) are defined for each portion without a groove. Similarly, for each feed groove 36, an axial distance of the feed groove (L3), which corresponds to the distance between the axial walls of the feed groove 36, and an axial height (hs) are defined.The maximum radial clearance then corresponds to the ratio between the sum, for each portion and for each feed groove 36, of the quotient between their length and their height, and the sum of the lengths of each portion and each feed groove 36:.

[0110] This can be generalized to any number n of feed grooves 36, with n > 0:

[0111] The free length Lu of the low-pressure shaft 11 corresponds to the length of the deformable part of the low-pressure shaft 11, that is to say the length of the part which undergoes the bending mode

[0112] I F. This is therefore the length of the portion of the low-pressure shaft 11 that extends between its supports on either side of the combustion chamber 6. The position of these supports can therefore correspond to at least one of the following positions: a position of the center of gravity Gna, Giib of one of the bearings 11a, 11b, 11c of the low-pressure shaft 11; a position of a connection of a trunnion 38 of the low-pressure compressor 4 (hereinafter referred to as the compressor trunnion) to the low-pressure shaft 11; a position of a connection 39 of a trunnion of the low-pressure turbine 8 (hereinafter referred to as the turbine trunnion) to the low-pressure shaft 11. For example, in the case of a low-pressure shaft 11 comprising a front bearing 11a and two rear bearings 11b, 11c, the two supports can correspond respectively to:

[0113] - at the position of the connection 38 of the trunnion of the compressor 4, when this connection 38 extends downstream of the front bearing 1 1a, and of the connection 39 of the trunnion of the turbine 8, when this connection extends upstream of the rear bearings 11 b, 11c (see FIG. 1);

[0114] - at the position of the front bearing 11a, when this bearing 11a extends downstream of the connection 38 of the trunnion of the compressor 4, and at the first rear bearing 11 b, when this bearing 11 b extends upstream of the connection of the trunnion of the turbine 8;

[0115] - at the position of the front bearing 11a, when this bearing 11a extends downstream of the connection 38 of the compressor journal 4, and at the position of the connection 39 of the turbine journal 8, when this bearing

[0116] II b extends upstream of the turbine trunnion connection 8 (see FIG. 2); or - to the position of the compressor trunnion connection 38 4, when this connection 38 extends downstream of the front bearing 11a, and to the position of the first rear bearing 11 b, when this bearing 11 b extends upstream of the turbine trunnion connection 8.

[0117] The average radius of the low pressure shaft 11 then corresponds to the arithmetic mean of the radii of the low pressure shaft 11 between these two supports (knowing that the radius of the low pressure shaft 11 is globally constant in this portion of the shaft 11).

[0118] When the mode damper 25 is dimensioned to comply with formula (1), it generates little force when the rotational speed of the low-pressure shaft 11 reaches the frequency of the first bending mode 1F, thus limiting the risk of dynamic locking. Since the mode damper 25 is active (as opposed to "locked"), it is capable of generating sufficient damping force to effectively dampen the bending mode 1F of the low-pressure shaft 11 by reducing its amplitude.

[0119] Furthermore, the sizing, taking into account the dimensions L, c of the low-pressure shaft 11 as well as the mounting radius R, allows for improved damping that considers the available space in the propulsion system 1, and in particular the fact that the high-pressure compressor 5 is small. Typically, the propulsion system 1 can include a shrouded fan and exhibit a bypass ratio greater than or equal to 11 for a fan pressure ratio less than 1.5, an overall pressure ratio greater than or equal to 40, and a maximum low-pressure turbine inlet temperature of around 1080°C. This implies that the high-pressure body 5 is small at constant thrust (a bypass ratio coupled with a low fan pressure ratio allows for high fan cross-section efficiency and propulsion efficiency, while a high overall compression ratio results in high thermal efficiency).However, a propulsion system with high propulsive and thermal efficiency requires lower power (generated by the primary casing). Since power is the product of flow rate and temperature, a high low-pressure turbine outlet temperature implies that it is possible to reduce the high-pressure compressor inlet flow rate (and therefore its size). Similarly, in the case of a propulsion system with an unshod fan, the bypass ratio can be greater than or equal to 40 for a fan pressure ratio less than 1.4, an overall pressure ratio greater than or equal to 40, and a maximum low-pressure turbine inlet temperature of around 1110°C. This implies, for the same reasons, that the high-pressure casing is smaller for the same thrust.

[0120] The lower the numerical value of the ratio R, the lower the forces exerted by the mode 25 damper, and therefore the more suitable it will be for damping the bending mode 1F of the low-pressure shaft, particularly a very flexible low-pressure shaft with a very small diameter and dynamically isolated from the rest of the turbomachine. Conversely, the closer the numerical value is to the upper limit of formula (1), the more the mode 25 damper will be able to dampen both a compressor / turbine suspension mode and the shaft mode, while ensuring that the mode 25 damper is not locked. Thus, when the mode 25 damper is sized to comply with formula (1), and preferably formula (2) below:

[0121] 1.0 x 10~' x -^ < R x -Y < 5.0 x 10 1 x -^ (2)

[0122] 7?n \Cz and preferably: l 2

[0123] 2.5 x 10 1 x R rlThe mode damper is particularly effective at damping the bending mode 1F of the low-pressure shaft 11. However, it will be less effective at damping the suspension modes of modules, and in particular of the blower section 2. The propulsion system 1 can then include additional, conventional mode dampers 37 configured to dampen the suspension modes. "Conventional" here means that the value of the ratio R(x) is greater than 5.0 x 10 1 x for these additional mode dampers 37 and are therefore more effective at damping the suspension modes of modules but generate significant forces. They can then be mounted on all or part of the other bearings, for example on the bearings supporting the blower shaft, or on the bearings supporting the low pressure shaft 11 which do not include a mode damper 25 conforming to formula (1).

[0124] When the mode 25 damper is sized to comply with formula (1), and preferably formula (3) below:

[0125] 1.0 x 10° 2.5 x 10 2 and preferably: l 2

[0126] 5.0 x 10° 1.0 x 10 2 x R rl The mode damper remains effective in damping the bending mode 1F of the low-pressure shaft 11. However, it generates more force than a mode 25 damper conforming to formula (2). On the other hand, such a mode 25 damper is also capable of damping module suspension modes with high kinetic energy, such as the suspension modes of the fan section 2. A mode 25 damper dimensioned according to formula (3) thus allows for the combined damping functions of the bending mode 1F and the module suspension modes.

[0127] Preferably, the mode 25 damper is mounted on all or part of the bearings 11a, 11b, 11c of the low-pressure shaft 11 that are affected by the deformation of the low-pressure shaft 11, i.e., on the front bearing 11a and / or the first rear bearing 11b (the one closest to the combustion chamber 6) when these bearings correspond to the supports. In other words, when the front bearing 11a extends downstream of the connection 38 of the trunnion of the low-pressure compressor 4, the mode 25 damper conforming to formula (1) can be mounted on the front bearing 11a. This example of a configuration is illustrated, for example, in FIG. 2. Alternatively, when the rear bearing 11 b extends upstream of the connection 39 of the trunnion of the low pressure turbine 8, the mode damper 25 conforming to formula (1 ) can be mounted on the first rear bearing 1 1 b.When the front bearing 1 1a extends downstream of the connection 38 of the trunnion of the low pressure compressor 4 and the rear bearing 11 b extends upstream of the connection 39 of the trunnion of the low pressure turbine 8, a mode 25 damper conforming to formula (1 ) can be mounted on the front bearing 1 1a and / or on the rear bearing 11 b.

[0128] Conversely, when the trunnion connections 38, 39 correspond to the first and second supports of the low-pressure shaft 11, the front bearing 11a and the rear bearing 11b are only slightly impacted by the deformation mode 1F of the low-pressure shaft 11, since they are located near and on either side of these supports: the mode 25 damper is then preferably mounted on the second rear bearing 11c, which extends furthest downstream of the low-pressure shaft 11. This example of a configuration is illustrated, for example, in FIG. 1.

[0129] The other bearings of the low pressure shaft 1 1, which do not include a mode damper 25 conforming to formula (1), can then either be without a mode damper, or include a conventional mode damper 37.

[0130] It should be noted that the positions of the first and second supports shown in Figures 1 and 2 are not limiting and do not depend on the type of motor (faired or unfaired). Thus, the position of the supports for the low-pressure shaft 11 shown in Figure 2 can be used in a faired motor as shown in Figure 1, and vice versa. Furthermore, these two motors (faired and unfaired) can also have a different configuration, for example, a second support formed by the first low-pressure bearing 11b (the connection 39 can then be located axially between the two low-pressure bearings 11b and 11c).

[0131] The axial length, maximum radial clearance and mounting radius of a mode 25 damper conforming to formula (1) can, for example, have the following values, depending on the free length and average radius of the low-pressure shaft:

[0132] 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 US 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).

[0133] 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 Dg 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 11 is supported by three bearings: a front bearing 11a mounted on the inlet housing and two rear bearings 11b, 11c mounted on the exhaust housing. These two bearings 11b, 11c may, for example, be roller or ball bearings. 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.

[0134] 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 Dg between 70 and 120 inches (between 1.778 and 3.048 meters (m)) and is driven via a reduction mechanism by a drive turbine whose maximum speed is between 7,000 and 14,000 revolutions per minute (rpm). The low-pressure shaft 11 is supported by three bearings: a front bearing 11a mounted on the intermediate housing and two rear bearings 11b, 11c mounted on the exhaust housing. These two bearings 11b, 11c may, for example, be roller or ball bearings. The dilution 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 further configured to provide a thrust of between 50,000 and 350,000 N.

[0135] In these two examples of propulsion systems 1, the first support of the low-pressure shaft 11 corresponds to its front bearing 11a and the second support to its first rear bearing 11b. The connections to the journals are therefore positioned on either side of the bearings 11a and 11b. The mode damper 25 can then be mounted on the front bearing 11a and / or the first downstream bearing 11b. The second downstream bearing 11c can, for its part, include a conventional mode damper 37.

[0136] 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).

[0137] This description applies, however, to any type of bearing that may include a mode 25 damper, and in particular to roller bearings, ball bearings, or tapered roller bearings. 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 fan rotor blades 14, and is expressed in meters (m). Note that Figures 1 and 2 are partial views, and therefore the diameter D9 is only partially visible.

[0138] Manufacturing process

[0139] A propulsion system conforming to this description can be manufactured according to the following steps. A mode 25 damper comprising an inner ring 30 and an outer ring 32 is dimensioned so that the mode 25 damper has the axial length, the maximum radial clearance and the mounting radius as defined in formula (1).

[0140] The mode 25 damper is then manufactured and assembled with the bearing 11a-11c, 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 the inner ring 30 extends radially between the bearing and the outer ring 32.

[0141] 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.

Claims

DEMANDS 1. Aeronautical propulsion system (1) comprising: - a blower section comprising a blower rotor (9) connected to a blower shaft (20); - a combustion chamber (6); - a first turbine (8) configured to drive the blower rotor (9) and a first compressor (4) via a first shaft (11) around an axis of rotation (X); - a reduction mechanism (19) coupling the first shaft (11) and the blower shaft (20) in order to drive the blower shaft (20) at a rotational speed lower than the rotational speed of the first shaft (11); - bearings (11a, 11b, 11c) configured to center the drive shaft (11) with respect to the axis of rotation (X); and - a mode damper (25) comprising an inner ring (30) mounted on a stator portion of one of the bearings (11a, 11b, 11c), an outer ring (32) and a fluid (29) confined between the inner ring (30) and the outer ring (32); and an axial length, a mounting radius and a maximum radial clearance of the mode damper (25) being dimensioned as a function of a free length and an average radius of the drive shaft (11) so as to satisfy the following formula: l 2 1.0 2.5 x 10 2 x -22- R rl Or : L is the axial length of the mode damper (25) and is expressed in meters (m); C is the maximum radial play of the mode damper (25) and is expressed in meters (m); R is the radius of implantation of the mode damper (25) and is expressed in meters (m); Lu is the free length of the drive shaft (11), which corresponds to an axial distance between a first support and a second support immediately adjacent to the drive shaft (11), the first support being positioned upstream of the combustion chamber (6) and the second support being positioned downstream of the combustion chamber (6), and is expressed in meters (m); and R11 is the average radius of the drive shaft (11) between the first and second support and is expressed in meters (m).

2. Aeronautical propulsion system (1) according to claim 1, wherein the axial length, the mounting radius and the maximum radial clearance of the mode damper (25) are dimensioned as a function of the free length and the mean radius of the drive shaft so as to satisfy the following formula: l 2 1.0 5.0 x 10 1 x -22-, R, i 3. Aeronautical propulsion system (1) according to claim 1, wherein the axial length, the mounting radius and the maximum radial play of the mode damper (25) are dimensioned as a function of the free length and the average radius of the drive shaft so as to comply with the following formula: I 2 / I \ 3 I 2 5.0 x 10° x < R x (-) < 1.0 x 10 2 x R rl \C / R rl 4. Aeronautical propulsion system (1) according to any one of claims 1 to 3, wherein the first and second support correspond respectively to at least one of the following positions: a position of a center of gravity of one of the bearings; a position of a connection (38) of a trunnion of the first compressor (4) to the drive shaft (11); a position of a connection (39) of a trunnion of the first turbine (8) to the drive shaft (11).

5. Aeronautical propulsion system (1) according to any one of claims 1 to 4, wherein the mode damper (25) comprises a determined number n of fluid supply grooves (36) between the inner ring (30) and the outer ring (32), where the determined number n is an integer greater than or equal to 0, so as to define n+1 groove-free portions between two seals of the mode damper (25), each groove-free portion having respectively a length and a height, each groove having respectively a length and a height, the maximum radial clearance being equal to the ratio between the sum, for each portion and for each groove, of the quotient between the length and the height, and the sum of the lengths of each portion and each groove.

6. Aeronautical propulsion system (1) according to any one of claims 1 to 5, wherein the average radius of the drive shaft (11) is greater than or equal to 0.01 meters and less than or equal to 0.06 meters and / or the free length of the drive shaft (11) is greater than or equal to 0.50 meters and less than or equal to 2.50 meters.

7. Aeronautical propulsion system (1) according to any one of claims 1 to 6, wherein the mounting radius of the mode damper (25) is greater than or equal to 0.025 meters and less than or equal to 0.30 meters, and / or its axial length is greater than or equal to 0.01 meters and less than or equal to 0.05 meters and its maximum radial clearance is greater than or equal to 0.1 x 10 -4 meters and less than or equal to 1.0 x 10' 3 meters.

8. Aeronautical propulsion system (1) according to any one of claims 1 to 7, wherein the bearings (11a, 11b, 11c) comprise at least one forward bearing (11a) and one or two rear bearings (11b, 11c), the mode damper (25) being mounted on the forward bearing (11a).

9. Aeronautical propulsion system (1) according to claim 8, further comprising a mode damper mounted on one of the rear bearings (11b, 11c).

10. Aeronautical propulsion system (1) according to any one of claims 1 to 9, further comprising fan bearings configured to center the fan shaft with respect to the axis of rotation (X) and additional mode dampers (37), the additional mode dampers (37) being mounted on all or part of the blower bearings and / or the bearings (11, 11b, 11c) when said bearings (11a, 11b, 11c) do not include mode damper (25).

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

12. Aeronautical propulsion system (1) according to any one of claims 1 to 11, further comprising a second turbine (7) configured to drive a second compressor (5) via a second shaft (10) around the axis of rotation (X), an overall compression ratio of the propulsion system (1), which corresponds to the pressure ratio between the pressure at the outlet of the second compressor (5) and the pressure at the inlet of the fan rotor (9), is 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, and a maximum inlet temperature of the first turbine (8) is greater than or equal to 1050°C and less than or equal to 1150°C.

13. Aircraft (100) comprising at least one aeronautical propulsion system (1) according to any one of claims 1 to 12 fixed to the aircraft by means of a mast.

14. A method for manufacturing an aeronautical propulsion system (1) according to any one of claims 1 to 12 comprising the following steps: - determine an axial length, a radius and a maximum radial clearance of a mode damper (25) as a function of a free length and an average radius of the drive shaft so as to satisfy the following formula: l 2 1.0 2.5 x 10 2 x R rl Or : L is the axial length of the mode damper (25) and is expressed in meters (m); C is the maximum radial play of the mode damper (25) and is expressed in meters (m); R is the radius of implantation of the mode damper (25) and is expressed in meters (m); Lu is a free length of the drive shaft (11), which corresponds to an axial distance between a first support and a second support immediately adjacent to the drive shaft, the first support being positioned upstream of the combustion chamber (6) and the second support being positioned downstream of the combustion chamber (6), and is expressed in meters (m); and R11 is an average radius of the drive shaft between the first and second supports 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 and maximum radial clearance thus determined; - assemble the inner ring (30) and the outer ring (23) with a bearing (11a, 11b, 11c) according to the implantation radius of the mode damper (25); and - inject a fluid (29) between the inner ring (30) and the outer ring (32).

15. A manufacturing method according to claim 14, wherein the axial length, the mounting radius, and the maximum radial clearance of the mode damper (25) are determined as a function of the free length and the mean radius of the drive shaft so as to comply with the following formula: 2 1.0 5.0 x 10 1 x -ii. R rl 16. A manufacturing method according to claim 15, wherein the axial length, the mounting radius and the maximum radial play of the mode damper (25) are determined as a function of the free length and the average radius of the drive shaft so as to comply with the following formula: 5.0 x 10° 1.0 x 10 2

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