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

Abstract

The present invention relates to a fan section (2) in which the fan bearings (20a, 20b) are positioned in the fan section (2) so as to satisfy the following formula: (I) where: d is an axial distance between a projection onto the axis of rotation (X) of a centre of gravity (G20a) of the front bearing (20a), along an axis of application of the forces of the front bearing (20b), and a projection of a centre of gravity (G20b) of the rear bearing (20b) onto the axis of rotation (X), along an axis of application of the forces of the rear bearing (20b), in metres (m); R is an average radius of the fan bearings (20a, 20b), in metres (m); Dg is the diameter of the fan rotor (9) in metres (m); and A = - 1.27 m-1 and B = 4.4.

Description

[0001] DESCRIPTION

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

[0003] TECHNICAL FIELD

[0004] The present application generally concerns the field of propulsion systems, and more particularly aeronautical propulsion systems comprising a ducted fan and having a high, or even very high, dilution ratio.

[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 rotated by the high-pressure turbine via a high-pressure shaft. The fan and, where appropriate, the low-pressure compressor are rotated 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 consideration the impact factors in all phases of design and development to obtain less energy-intensive, more environmentally friendly aeronautical components and products whose integration and use in civil aviation have moderate environmental consequences with the aim of improving the energy efficiency of aircraft.

[0008] Thus, in order to improve the propulsive efficiency of the propulsion system and to reduce its specific consumption as well as the noise emitted by the fan section, propulsion systems have been proposed having a high bypass ratio (BPR, corresponding to the ratio between the flow rate of the secondary air flow and the flow rate of the primary air flow). To achieve such bypass ratios, the fan section can be decoupled from the low-pressure turbine, thus making it possible to independently optimize their respective rotation speeds. Generally, the 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 rotor of the fan section is then driven by the low-pressure shaft via the reduction mechanism at a rotation speed lower than that of the low-pressure shaft.

[0009] However, reducing the fan rotor rotation speed has an impact on the dynamics of the fan section and in particular on the deformation modes of the fan shaft. This impact is all the more significant as we seek to increase the diameter of the fan rotor to improve its thrust.

[0010] PRESENTATION One aim of this application is to optimize the performance of the aeronautical propulsion system in terms of specific consumption, while controlling the dynamic behavior of the fan section.

[0011] For this purpose, according to a first aspect, a fan section of an aeronautical propulsion system is proposed, comprising:

[0012] - a fan rotor comprising a plurality of blades;

[0013] - a fan shaft configured to drive the fan rotor about an axis of rotation;

[0014] - fan bearings configured to center the fan shaft relative to the axis of rotation, the fan bearings comprising a front bearing and a rear bearing; the fan bearings being positioned in the fan section so as to comply with the following formula: where: d is an axial distance between a projection on the axis of rotation of a center of gravity of the front bearing, along an axis of application of the forces of the front bearing, and a projection of a center of gravity of the rear bearing on the axis of rotation, along an axis of application of the forces of the rear bearing, in meters (m);

[0015] R is an average radius of the fan bearings, in meters;

[0016] Dg is the fan rotor diameter in meters, measured in a plane normal to the axis of rotation at an intersection between a tip and a leading edge of the fan rotor blades; and

[0017] A = - 1.27 m-1 and B = 4.4.

[0018] Some preferred but non-limiting features of the blower section according to the first aspect are the following, taken individually or in combination:

[0019] - the fan bearings are also positioned in the fan section so as to respect the following formula: where C = 4.85;

[0020] - the diameter of the fan rotor is between 2.159 meters and 3.048 meters inclusive, for example equal to 2.286 meters;

[0021] - the front bearing and the rear bearing respectively comprise: tapered roller bearings; or a roller bearing and a ball bearing; or two angular contact ball bearings; or a double bearing comprising a row of balls and a row of tapered rollers;

[0022] - the fan bearings are without bearing mode damper;

[0023] - the fan section further has a fan compression ratio, corresponding to a pressure ratio between an outlet of the fan rotor and an inlet of the fan rotor less than or equal to 1.45, preferably less than or equal to 1.30 the fan section comprises at least twelve blades and at most twenty-four blades; and / or a hub-to-head ratio of the fan rotor is between 0.22 and 0.32.

[0024] According to a second aspect, an aeronautical propulsion system is proposed comprising:

[0025] - a blower section according to the first aspect;

[0026] - a drive turbine configured to drive the fan shaft in rotation about the axis of rotation; and

[0027] - a reduction mechanism coupling the drive shaft and the fan shaft in order to drive the fan shaft at a rotational speed lower than the rotational speed of the drive shaft.

[0028] Optionally, a dilution ratio of the propulsion system is greater than or equal to 10, for example between 10 and 35 inclusive, preferably between 10 and 18 inclusive.

[0029] According to a third aspect, there is provided an aircraft comprising at least one propulsion system according to the second aspect attached to the aircraft via a mast.

[0030] According to a fourth aspect there is provided a method of dimensioning or manufacturing a fan section comprising a fan rotor, wherein the fan section is dimensioned such that fan bearings of the fan section are positioned in the fan section so as to comply with the following formula: where: d is an axial distance between a projection on the axis of rotation of the center of gravity of the front bearing, along an axis of application of the forces of the front bearing, and a projection of the center of gravity of the rear bearing on the axis of rotation, along an axis of application of the forces of the rear bearing, in meters (m);

[0031] R is an average radius of the fan bearings, in meters;

[0032] Dg is a fan rotor diameter in meters, measured in a plane normal to the axis of rotation at an intersection between a tip and a leading edge of the fan rotor blades; and

[0033] A = - 1.27 m -1 and B = 4.4.

[0034] Optionally, the fan section is further dimensioned so that the fan bearings are positioned in the fan section to comply with the following formula: where C = 4.85.

[0035] DESCRIPTION OF FIGURES

[0036] Other characteristics, aims and advantages will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which: Figure 1a is a schematic, partial and sectional view of an example of a propulsion system according to a first embodiment;

[0037] Figure 1b is a detailed view of the fan bearings of Figure 1a;

[0038] Figure 1c is a detailed view of fan bearings of an exemplary propulsion system according to a second embodiment;

[0039] Figure 2 is a schematic sectional view of an example of a reduction mechanism according to a first variant;

[0040] Figure 3 is a schematic sectional view of an example of a reduction mechanism according to a second variant;

[0041] Figure 4 is an example of an aircraft that may include at least one propulsion system according to the first or second embodiment;

[0042] Figure 5 is a flowchart illustrating examples of steps in a sizing or manufacturing process.

[0043] Throughout the figures, similar elements have identical references.

[0044] DETAILED DESCRIPTION OF THE INVENTION

[0045] A propulsion system 1 has a main direction extending along a longitudinal axis X and comprises, from upstream to downstream in the direction of flow of the gases in the propulsion system 1 when it is in operation, a fan 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 is here an aeronautical propulsion system 1 configured to be fixed to an aircraft 100 via a pylon (or mast).

[0046] The compressor section 4, 5 comprises a succession of stages each comprising a moving blade wheel (rotor) 4a, 5a rotating in front of a fixed blade wheel (stator) 4b, 5b. The turbine section 7, 8 also comprises a succession of stages each comprising a fixed blade wheel (stator) 7b, 8b behind which a moving blade wheel (rotor) 7a, 8a rotates.

[0047] In the present application, the axial direction corresponds to the direction of the longitudinal axis X, in correspondence with the rotation of the shafts of the gas generator, and a radial direction is a direction perpendicular to this axis X and passing through it. Furthermore, the circumferential (or lateral, or even tangential) direction corresponds to a direction perpendicular to the longitudinal axis X and not passing through it. Unless otherwise specified, internal (respectively, interior) and external (respectively, exterior), respectively, are used in reference to a radial direction so that the internal part or face of an element is closer to the axis X than the external part or face of the same element.

[0048] In operation, an air flow F entering the propulsion system 1 is divided between a primary air flow F1 and a secondary air flow F2, which circulate from upstream to downstream in the propulsion system 1. The secondary air flow F2 (also called “bypass air flow”) flows around the primary body 3. The secondary air flow F2 makes it possible to cool the periphery of the primary body 3 and is used to generate the majority of the thrust provided by the propulsion system 1.

[0049] The primary air flow F1 flows in a primary vein 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 oxidant, and the turbine section 7, 8. The passage of the primary air flow F1 through the turbine section 7, 8 receiving energy from the combustion chamber 6 causes rotation of the rotor of the turbine section 7, 8, which in turn drives rotation of the rotor of the compressor section 4, 5 as well as a rotor part 9 of the fan section 2.

[0050] In a twin-spool propulsion system 1, the compressor section 4, 5 may comprise a low-pressure compressor 4 and a high-pressure compressor 5. The turbine section 7, 8 may comprise a high-pressure turbine 7 and a low-pressure turbine 8. The rotor of the high-pressure compressor 5 is rotated 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 fan section 2 are rotated 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 comprising the high-pressure compressor 5, the high-pressure turbine 7 and the high-pressure shaft 10, and a low-pressure body comprising the fan 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 spool is greater than the rotational speed of the low-pressure spool. In a triple-spool propulsion system 1, the turbine section 7, 8 further comprises 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 fan 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.

[0051] The low pressure shaft 11 is generally housed, over a section of its length, in 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, i.e. driven in the same direction around the longitudinal axis X. Alternatively, the low pressure shaft 11 and the high pressure shaft are counter-rotating, i.e. driven in opposite directions around the longitudinal axis X. Where appropriate, 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.

[0052] The fan section 2 comprises at least the fan rotor 9 adapted to be driven in rotation relative to a stator part 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 a variable pitch. In this case, the root of the blades 14 of each rotor 9 is pivotally mounted along a pitch axis and is connected to a pitch change mechanism 15 mounted in the propulsion system 1, the pitch being adjusted according to the flight phases by the pitch change mechanism 15. The pitch change mechanism 15 is illustrated in broken lines in Figure 1a to show that this feature is optional.

[0053] The fan section 2 may further comprise a fan stator 16, or rectifier, which comprises blades 17 mounted on a hub of the fan stator 16 and have the function of straightening the secondary air flow F2 which flows out of the fan rotor 9. The blades 17 of the fan stator 18 may be fixed relative to the hub or have a variable pitch. In a similar manner to the rotor blades 14, the root of the stator blades 17 is pivotally mounted along a pitch axis X and is connected to a pitch change 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 change mechanism.

[0054] The fan shaft 20 is supported by fan bearings 20a, 20b in order to control its deformation modes and take up the axial and radial forces of the fan rotor 8. The fan shaft 20 may thus comprise a front bearing 20a and a rear bearing 20b, the front bearing 20a being placed further upstream on the fan shaft 20, close to the hub 13, than the rear bearing 20b which is closer to the reduction mechanism 19. The front bearing 20a and the rear bearing 20b may comprise an inner ring mounted on the fan shaft 20 and an outer ring mounted on a stator part of the propulsion system 1, typically on an inlet casing 24, which is a casing through which forces pass in the propulsion system 1 and which extends between the fan rotor 9 and the low-pressure compressor 4.

[0055] In order to improve the propulsive efficiency of the propulsion system 1 and to reduce its specific consumption as well as the noise emitted by the fan section 2, the propulsion system 1 has a high bypass ratio. By high bypass ratio, we mean here a bypass 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 air flow F2 and the mass flow rate of the primary air flow F1 are measured when the propulsion system 1 is stationary, uninstalled, in takeoff mode in a standard atmosphere (as defined by the International Civil Aviation Organization (ICAO) manual, Doc 7488 / 3, 3 eedition) and at sea level. It will be noted that, in the present application, the parameters (pressure, flow rate, thrust, speed, etc.) are systematically determined under these conditions. By "not installed", it will be understood here that the measurements are carried out when the propulsion system 1 is in a test bench (and not installed on an aircraft 100), the measurements then being simpler to carry out. The distances (length, radius, diameter, etc.) are, on the other hand, measured at ambient temperature (approximately 20°C) when the propulsion system 1 is cold, that is to say when the propulsion system 1 has been stopped for a sufficient period for the parts of the propulsion system to be at ambient temperature, it being understood that these dimensions vary little compared to the conditions in which the propulsion system 1 is in take-off mode.

[0056] The fan rotor 9 is decoupled from the low-pressure shaft 11 by means of a reduction mechanism 19, placed between an upstream end of the low-pressure shaft 11 and the fan rotor 9, in order to independently optimize their respective rotational speed. In this case, the propulsion system 1 further comprises an additional shaft, called the fan shaft 20. The low-pressure shaft 11 connects the low-pressure turbine 8 to an inlet of the reduction mechanism 19 while the fan shaft 20 connects the outlet of the reduction mechanism 19 to the fan rotor 9. The fan rotor 9 is therefore driven by the low-pressure shaft 11 via the reduction mechanism 19 and the fan shaft 20 at a rotational speed lower than the rotational speed of the low-pressure turbine 8.

[0057] This decoupling makes it possible to reduce the rotational speed and the pressure ratio of the fan rotor 9 and to increase the power extracted by the low-pressure turbine 8. Indeed, the overall efficiency of the propulsion systems is conditioned to the first order by the propulsive efficiency, which is favorably influenced by a minimization of the variation in kinetic energy of the air when passing through the propulsion system 1. In a propulsion system 1 with a high bypass ratio, the majority of the flow generating the propulsive force is constituted by the secondary air flow F2 of the propulsion system 1, the kinetic energy of the secondary air flow F2 being mainly affected by the compression that the secondary air flow F2 undergoes when passing through the fan section 2. The propulsive 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 propulsive efficiency will be.In order to optimize the propulsive efficiency of the propulsion system 1, the pressure ratio of the fan, which corresponds to the ratio between the average pressure at the outlet of the fan stator 17 (or, in the absence of a stator, of the fan rotor 9) and the average pressure at the inlet of the fan rotor 9, is less than or equal to 1.70, for example less than or equal to 1.50, for example between 1.05 and 1.45. The average pressures are measured here over the height of the blade 14 (from the surface which radially delimits the flow path at the inlet of the fan rotor 9 to the tip 21 of the fan blade 14).

[0058] The propulsion system 1 is configured to provide thrust between 18,000 Ibf (80,068 N) and 51,000 Ibf (22,2411 N), for example between 20,000 Ibf (88,964 N) and 35,000 Ibf (15,5688 N).

[0059] The fan section 2 may comprise a fan housing 12 and the fan rotor 9 is housed in the fan housing 12.

[0060] The fan section 2 comprises a fan rotor 9 extending upstream of a fan stator. The blades of the fan stator are then generally called outlet vanes (“Outlet Guide Vane” or “OGV” in English) and have a fixed pitch relative to the hub of the fan stator. 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 speed at the tip 21 of the blades of the fan rotor 9 can also be between 260 m / s and 400 m / s. The blades 14 of the fan rotor 9 can be fixed or have a variable pitch. The fan pressure ratio can then be between 1.20 and 1.45.

[0061] The reduction mechanism 19 may comprise, for example, a reduction mechanism 19 with an epicyclic gear train, for example of the “epicyclic” type or of the “planetary” type, single-stage or two-stage.According to a first variant, the reduction mechanism 19 may be of the planetary type (“star” in English) (Figure 2) and comprise a sun gear 19a (input of the reduction mechanism 19), centered on an axis X of rotation of the reduction mechanism 19 (generally confused with the longitudinal axis X) and configured to be driven in rotation by the low pressure shaft 11, a ring gear 19b (output of the reduction mechanism 19) coaxial with the sun gear 19a and configured to drive the fan shaft 20 in rotation around the axis X of rotation, and a series of satellites 19c distributed circumferentially around the axis X of rotation between the sun gear 19a and the ring gear 19b, each satellite 19c being meshed internally with the sun gear 19a and externally with the ring gear 19b.The series of satellites 19c is mounted on a planet carrier 19d which is fixed relative to a stator part 19e of the propulsion system 1, for example relative to a casing of the compressor section 4, 5. According to a second variant, the reduction mechanism 19 may be of the epicyclic type (“planetary” in English) (Figure 3), in which case the crown 19b is fixedly mounted on the stator part 19e of the propulsion system 1 and the fan shaft 20 is driven in rotation by the planet carrier 19d (which is therefore movable in rotation relative to a stator part 19e of the propulsion system 1, for example relative to a casing of the compressor section 4, 5).

[0062] Whatever the configuration of the reduction mechanism 19, the diameter of the crown 19b and the planet carrier 19d are greater than the diameter of the sun gear 19a, so that the rotation speed of the fan rotor 9 is lower than the rotation speed of the low pressure shaft 11.

[0063] The reduction ratio of the reduction mechanism 19 is greater than or equal to 2.5 and less than or equal to 11. For example, the reduction ratio may be greater than or equal to 2.7 and less than or equal to 6.0, typically around 3.0.

[0064] The dual-body propulsion system 1 may in particular comprise a 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.

[0065] The redline speed of the low pressure shaft 11, which corresponds to the absolute maximum speed likely to be encountered by the low pressure shaft 11 during the entire flight (according to the European certification regulation EASA CS-E 740 (or according to the American certification regulation 14-CFR Part 33.87)), is between 8500 rpm and 12000 rpm, for example between 9000 rpm and 11000 rpm. The redline speed corresponds to the maximum rotation speed when the propulsion system is healthy (and potentially at the end of its life). It is therefore likely to be reached by the low pressure shaft 11 in flight conditions. This redline speed is part of the data declared in the engine certification (type certificate data sheet).Indeed, this rotation speed is usually used as a reference speed for the dimensioning of propulsion systems 1 and in certain certification tests (such as blade loss or rotor integrity tests).

[0066] The implementation of a reduction mechanism 19 makes it possible to reduce the rotational speed of the fan rotor 9, in comparison with a direct drive motor (where the fan shaft 20 is driven directly by the low pressure shaft 11 and rotates at the same rotational speed as the low pressure shaft 11) and to increase the diameter Dg of the fan rotor 9. However, this has the effect of modifying the unbalance deformation modes of the fan rotor 9. As a result, the fan shaft 20 is likely to reach a critical speed and enter into resonance in the operating ranges of the fan rotor 9. At resonance, which occurs when the fan shaft 20 passes the critical speed, the latter undergoes overvoltage phenomena which amplify the deformations and forces caused by the (inevitable) unbalances of the fan rotor 9.

[0067] In order to control the dynamic behavior of the fan rotor 9 and to move the deformation modes of the fan shaft outside the operating range of the fan rotor 9, the embedding of the fan shaft 20 is modified in order to place the fan bearings 20a, 20b radially and axially so as to comply with the following formula: > A * D9+ B (1 ) where: d is an axial distance between a projection on the axis of rotation X of a center of gravity G20a of the front bearing 20a, along an axis of application of the forces of the front bearing 20b, and a projection of a center of gravity G20b of the rear bearing 20b on the axis of rotation X, along an axis of application of the forces of the rear bearing 20b, in meters m the center of gravity G20a of the front bearing 20a and the center of gravity G20b of the rear bearing 20b,i.e. the distance between the orthogonal projection of the center of gravity G20a of the front bearing 20a on the axis of rotation X and the orthogonal projection of the center of gravity G20b of the rear bearing 20b on the axis of rotation X, in meters (m);,

[0068] R is an average radius of the fan bearings 20a, 20b, in meters (m);

[0069] Dg is a fan rotor diameter 9 in meters (m); and

[0070] A = - 1.27 m -1 and B = 4.4 (dimensionless). d, R and Dg being distances, they are determined when the propulsion system 1 (and therefore the fan section 2) is cold, as specified above.

[0071] The mean radius R of the fan bearings 20a, 20b corresponds to the arithmetic mean of the mean radius R20a of the front bearing 20a and the mean radius R20b of the rear bearing 20b. The mean radius R20a, R20b of a given bearing (front 20a or rear 20b) is equal to the distance, measured in a plane normal to the longitudinal axis X, between the axis X and the center of gravity G20a, G20b of the bearing 20a, 20b. It will be noted here that a bearing comprises at least one outer ring and at least one inner ring which are coaxial as well as bearings (balls, rollers, etc.) mounted between the inner ring and the outer ring and configured to allow relative movement of the inner ring with respect to the outer ring. The inner ring is mounted on the fan shaft and rotated by the fan shaft; the outer ring is mounted on a stator part (typically, the inlet casing 24) of the propulsion system 1.The center of gravity G20a, G20b of a bearing 20a, 20b then corresponds to the center of gravity of the assembly formed by the inner ring, the outer ring and the bearings. When the inner ring and / or the outer ring is formed integrally and in a single piece with a support (flange, ferrule, enclosure, etc.) of the propulsion system 1 so that the ring cannot be distinguished from the support, the center of gravity G20a, G20b is determined without taking into account the ring in question.

[0072] It will be noted that, when the direction of the load of the fan bearings 20a, 20b is oblique, that is to say when the axis (called the axis of application of the forces) between the point of application of the load between the rolling bearing and the outer ring of the bearing 20a, 20b and the center of gravity G20a, G20b of the bearing 20a, 20b, forms a non-zero angle with a plane normal to the axis of rotation X, the distance d is greater than the distance between the orthogonal projection of the center of gravity G20a of the front bearing on the axis of rotation X and the center of gravity G20b of the rear bearing 20b on the axis of rotation X.

[0073] Examples of angular contact bearings include, for example, an angular contact ball bearing or a tapered roller bearing. Note that, for a given bearing 20a, 20b, the direction of the load depends, among other things, on the mean radius F oa, F ob of the bearing 20a, 20b and the size of the bearing of the bearing 20a, 20b. Thus, two fan bearings 20a, 20b may have the same distance d but different distances d' depending on their position in the fan section 2 and their configuration.

[0074] The axial distance d can be between 180 mm and 350 mm.

[0075] The average radius of the R bearings can be between 120 mm and 180 mm.

[0076] Thus, different bearing arrangements can be considered. Among bearing arrangements, one example is the type having two angular contact ball bearings (or tapered ball bearings) with opposing loads (see Figures 1a and 1b). In these bearings, a front ball bearing has angular contacts oriented to transmit forces in an upstream direction associated with a rear ball bearing with angular contacts oriented to transmit forces in a downstream direction. Among bearing arrangements, another is the type having tapered roller bearings with opposing loads (see Figure 1c).In these bearings, the front bearing 20a with tapered roller bearing has its lines of force oriented to transmit forces in an upstream direction, while being associated with the rear bearing 20b with tapered roller bearing oriented to transmit forces in a downstream direction.

[0077] The determinations of the oblique lines of the forces or axes of application of the forces are known to the person skilled in the art of rolling bearing assemblies, both for assemblies with angular contact balls and for assemblies with tapered rollers, typically in works often called manuals of mechanical construction or industrial designer's guide (for example the reference book published by ISBN 2-09-194004-6).

[0078] The diameter Dg of the fan rotor 9 is measured in a plane normal to the axis X of rotation at an intersection between a tip 21 and a leading edge 22 of the blades 14 of the fan rotor 9, and is expressed in meters. Note that since Figure 1a is a partial view, the diameter Dg is only partially visible.

[0079] The Applicant has in fact noticed that the fan bearings 20a, 20b have an impact on the embedding of the fan shaft 20. In particular, the further the fan bearings 20a, 20b are from the axis of rotation X (high R) and / or from each other (high d), the more the stiffness of the fan shaft 20 increases and the more the deformation modes of the fan shaft 20 are weak and are therefore likely to be found in the operating range of the fan rotor 9. On the contrary, when the radial and axial position of the fan bearings 20a, 20b complies with formula (1), the stiffness of the fan shaft increases sufficiently so that its critical speed is at least 15% higher than its limiting speed, i.e. the absolute maximum speed likely to be encountered by the fan shaft 20 throughout the flight.This is the maximum permissible speed defined in the engine certification (“type certification data sheet”).

[0080] The diameter Dg of the fan rotor 9 can then be between 2.032 meters (m) (80 inches) and 4.699 meters (m) (185 inches) inclusive, which makes it possible to obtain thrusts in the ranges described above. The diameter Dg is for example between 2.159 meters (m) (85 inches) and 3.048 meters (m) (120 inches) inclusive, for example equal to 2.286 meters (m) (90 inches), which makes it possible to integrate the propulsion system 1 in a conventional manner, in particular under the wing of an aircraft.

[0081] The fan rotor 9 further comprises at least twelve blades 14 and at most twenty-four blades 14, for example at least 16 blades 14 and at most twenty-two 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.

[0082] The rotational speed of the fan shaft 20 can also be between 1,800 and 5,000 revolutions per minute.

[0083] In one embodiment, the position of the fan bearings 20a, 20b is chosen so as to also comply with the following formula: where: C = 4.85 (dimensionless).

[0084] Compliance with formula (2) for the positioning of the fan bearings 20a, 20b ensures acceptable mode positioning and good absorption of radial forces and the moment of the fan rotor 9, while maintaining an acceptable axial size of the fan bearings 20a, 20b. In addition, an engine whose d / R value does not comply with formula (2) will have a significant distance d, which impacts the length of the engine (in particular when the engine includes a reduction mechanism) and therefore the mass and drag of the engine.

[0085] The fan bearings 20a, 20b comprise exactly two bearings, namely the front bearing 20a and the rear bearing 20b.

[0086] It will be noted that formulas (1) and (2) apply to any type of bearing. For example, the front bearing 20a and the rear bearing 20b may each comprise a tapered roller bearing or an angular contact ball bearing. Alternatively, the front bearing 20a may comprise a roller bearing and the rear bearing 20b a ball bearing.

[0087] It will be noted here that the invention also applies in the case where the two fan bearings 20a, 20b form a double (or duplex) bearing comprising two rows of bearings which are twinned and can be housed in the same cage. The two rows of bearings are therefore considered as two separate bearings, the bearings of which are separated by the distance d and the average radius R of which corresponds to the average of the average radii R20a, R20b measured at the center of gravity of each bearing. Indeed, in the case of such a double bearing, one of the bearings ensures the absorption of radial forces while the other of the bearings ensures the absorption of axial forces. An example of a double bearing may for example comprise a row of balls (absorption of radial forces) followed by a row of tapered rollers (absorption of axial forces).

[0088] Compliance with formula (1) and where appropriate with formula (2) therefore makes it possible to control the dynamic behavior of the fan shaft 20, its deformation modes being displaced outside the operating range of the fan rotor 9. It therefore becomes possible to eliminate any bearing mode dampers (such as dampers comprising a pressurized fluid ("squeeze film" in English) or flexible cages usually mounted between the outer ring of the bearing and a rigid support fixedly secured to a stator part of the propulsion system 1), in particular when the propulsion system 1 comprises a reduction mechanism 19.Indeed, it is then no longer necessary to dampen the mode transition, which makes it possible to reduce the space required for integrating the fan bearings 20a, 20b (and where appropriate the pitch change mechanism 15) under the fan rotor 9, and consequently to reduce the hub-head ratio of the fan rotor 9. A bearing mode damper can however be retained to reduce vibration levels, for example to provide a response function to an unbalance.

[0089] The fan rotor 9 may then have a hub-to-head ratio of between 0.22 and 0.32. In the case of a fixed-pitch fan rotor 9, the hub-to-head ratio may be between 0.24 and 0.32. In the case of a variable-pitch fan rotor 9, the hub-to-head ratio is preferably between 0.24 and 0.32 to allow the integration of the pitch-changing mechanism 15. The hub-to-head ratio corresponds to the ratio between the internal radius Ri and the external radius R e of the fan rotor 9. The internal radius Ri corresponds to the distance between the axis of rotation X and the point of intersection between the leading edge 22 and the surface which radially delimits on the inside the flow vein at the inlet of the fan rotor 9 (and corresponds to the point of connection of the leading edge 22 with the aerodynamic surface of a platform of the fan rotor 9). The external radius R ecorresponds to the distance between the axis of rotation X and the point of intersection between the leading edge 22 and the tip 21 of the fan blades (and corresponds to half the diameter Dg of the fan rotor 9). The lower the hub-to-head ratio, the more efficient the fan rotor 9 is. When the fan bearings 20a, 20b are positioned so as to comply with formula (1) and, where appropriate, formula (2) and the hub-to-head ratio of the fan rotor 9 is between 0.22 and 0.32, it is possible to obtain an efficient fan section 2 while ensuring the possibility of integrating the fan bearings 20a, 20b under the fan rotor 9 in the fan section 2, upstream of the reduction mechanism 19.

[0090] Comparative example:

[0091] Engine 1 is a double-body propulsion system comprising a shrouded fan section 2 corresponding to the current technical standard (at the date of filing of this application) which we seek to improve.

[0092] The engine 2 is a double-body propulsion system 1 comprising a shrouded fan section 2 in accordance with the teaching of the present application, the fan section of which comprises bearings whose position complies with the formulas (1) and (2) defined above. The fan bearings of the engines 1 and 2 both comprise angular contact ball bearings, and their front bearing is without a bearing mode damper. Their low-pressure shaft is further supported by exactly one bearing upstream of the combustion chamber and two bearings downstream of the combustion chamber. In addition, the front bearing of the low-pressure shaft comprises damping by a film of pressurized fluid.

[0093] The d / R ratio of engine 1 does not comply with formula (1). It follows that the first deformation mode of the fan rotor of engine 1 occurs at a rotation speed that is very close to the redline of its fan rotor (107% of the redline). In comparison, engine 2, whose d / R ratio complies with formula (1), the first deformation mode of the fan rotor occurs at a rotation speed of the fan rotor that is much higher than its redline. The unbalanced loads of the fan rotor of engine 1 are therefore higher, which has an impact on the sizing of the structures of the fan section of engine 1 and its suspension. The clearance consumptions of the fan section of engine 1 are also higher, which reduces its performance. The fan section of engine 2 is therefore more efficient due to its larger diameter and lower clearance consumptions.

[0094] To move from engine 1 (reference) to engine 2 (compliant with the disclosure), the fan diameter Dg and the bypass ratio BPR were increased. The increase in the fan diameter Dg compared to engine 1 has the effect of increasing the mass of the fan rotor and the loading of the fan bearings, reducing the frequency of deformation modes (if the fan shaft diameter and / or the distance between its bearings are not increased) and increasing the fan clearance consumptions under unbalance or load factor of engine 2. However, this increase in the fan diameter (and BPR) in engine 2 made it possible to improve the propulsive efficiency and to maintain a comparable fan thrust given the decrease in the pressure ratio of fan section 2.Furthermore, modifying the d / R ratio to comply with formula (1) made it possible to control the unbalanced loads and the fan rotor clearance consumption, despite the increase in the fan diameter.

[0095] Furthermore, the overall compression ratio was further increased between engine 1 and engine 2, as well as the inlet temperature of the high-pressure turbine 7, which made it possible to increase the thermal efficiency of the propulsion system 1.

[0096] It can be noted that the integration of angular contact bearings or tapered roller bearings is favorable in a motor. To obtain a motor respecting formulas (1) and (2), the diameter of the motor bearings can be obtained via their dimensioning by respecting the practices relating to their form factors. Indeed, the loads acting on the bearings are axial forces (fan thrust) and radial forces (unbalance). The dimensioning of the bearings must respect contact pressure criteria between the rolling element and the ring and proportionality criteria between the different dimensions (in particular average diameter, diameters of the rolling elements). The dimensioning of the bearings takes into account the different life situations of the motor (normal loads, limit loads, ultimate loads).

[0097] The distance d can be obtained by increasing, for example, the distance between the fan and the reduction mechanism. This impact can be advantageously reduced by dimensioning the bearings allowing their integration under the fan disc and by improving the shape of the connection (fan shaft) with the reduction mechanism. The design constraints of the reduction mechanism impose a stiffness requirement on the fan shaft, which can be adapted by adjusting the positioning of the rear fan bearing in diameter and axial position.

[0098] If necessary, the fan section can be adapted. For example, if the distance between the fan bearings must be increased to comply with formulas (1) and (2), it may be advantageous to modify the fan rotor hub-to-head ratio in order to place the upstream bearing under the fan disc and not incur the increase in engine length that would otherwise be necessary.

Claims

CLAIMS 1. Fan section (2) of an aeronautical propulsion system (1) comprising: - a fan rotor (9) comprising a plurality of blades (14); - a fan shaft (20) configured to drive the fan rotor (9) around an axis of rotation (X); - fan bearings (20) configured to center the fan shaft (20) relative to the axis of rotation (X), the fan bearings (20) comprising a front bearing (20a) and a rear bearing (20b); the fan bearings (20a, 20b) being positioned in the fan section (2) so as to comply with the following formula: where: d is an axial distance between a projection on the axis of rotation (X) of a center of gravity (G20a) of the front bearing (20a), along an axis of application of the forces of the front bearing (20b), and a projection of a center of gravity (G2ot>) of the rear bearing (20b) on the axis of rotation (X), along an axis of application of the forces of the rear bearing (20b), in meters (m); R is an average radius of the fan bearings (20a, 20b), in meters (m); Dg is the diameter of the fan rotor (9) in meters (m), measured in a plane normal to the axis of rotation (X) at an intersection between a tip (21) and a leading edge (22) of the blades (14) of the fan rotor (9); and A = - 1.27 m -1 and B = 4.

4.

2. Fan section (2) according to claim 1, wherein the fan bearings (20a, 20b) are also positioned in the fan section (2) so as to comply with the following formula: where C = 4.

85.

3. Fan section (2) according to one of claims 1 and 2, in which the diameter (Dg) of the fan rotor (9) is between 2.159 meters (m) (85 inches) and 3.048 meters (m) (120 inches) inclusive, for example equal to 2.286 meters (m) (90 inches).

4. Fan section (2) according to one of claims 1 to 3, wherein the front bearing (20a) and the rear bearing (20b) respectively comprise: - tapered roller bearings; - a roller bearing and a ball bearing; - two angular contact ball bearings; or - a double bearing comprising a row of balls and a row of tapered rollers.

5. Fan section (2) according to one of claims 1 to 4, wherein the fan bearings (20) are without bearing mode damper.

6. Fan section (2) according to one of claims 1 to 5, further having a fan compression ratio, corresponding to a pressure ratio between an outlet of the fan rotor (9) and an inlet of the fan rotor (9) less than or equal to 1.45, preferably less than or equal to 1.

30.

7. Fan section (2) according to one of claims 1 to 6, comprising at least twelve blades (14) and at most twenty-four blades (14).

8. Fan section (2) according to one of claims 1 to 7, wherein a hub-to-head ratio of the fan rotor (9) is between 0.22 and 0.

32.

9. Aeronautical propulsion system (1) comprising: - a fan section (2) according to one of claims 1 to 8; - a drive turbine (8) configured to drive the fan shaft (20) in rotation around the axis of rotation (X); and - a reduction mechanism coupling the drive shaft (11) and the fan shaft (20) in order to drive the fan shaft (20) at a rotational speed lower than the rotational speed of the drive shaft (11).

10. Propulsion system (1) according to claim 9, in which a dilution ratio of the propulsion system (1) is greater than or equal to 10, for example between 10 and 35 inclusive, preferably between 10 and 18 inclusive.

11. Aircraft (100) comprising at least one propulsion system (1) according to one of claims 9 and 10 fixed to the aircraft by means of a mast.

12. A method of dimensioning a fan section (2) comprising a fan rotor (9), wherein the fan section (2) is dimensioned such that fan bearings (20a, 20b) of the fan section (2) are positioned in the fan section so as to comply with the following formula: where: d is an axial distance between a projection on the axis of rotation (X) of a center of gravity (G20a) of the front bearing (20a), along an axis of application of the forces of the front bearing (20b), and a projection of a center of gravity (G2ot>) of the rear bearing (20b) on the axis of rotation (X), along an axis of application of the forces of the rear bearing (20b), in meters (m); Dg is a diameter of the fan rotor (9) in meters, measured in a plane normal to the axis of rotation (X) at an intersection between a tip (21) and a leading edge (22) of the blades (14) of the fan rotor (9); and A = - 1 .27 rrr 1 and B = 4.

4.

13. A sizing method according to claim 12, wherein the fan section (2) is further sized such that the fan bearings (20a, 20b) are positioned in the fan section so as to comply with the following formula: where C = 4.85.

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