Aeronautical propulsion system comprising an optimized fan section

[0001] This Application claims priority to French Application No. 2300739, filed Jan. 26, 2023, in the French Patent Office, the contents of which being herein incorporated by reference in its entirety.

[0002] The present application generally concerns the field of propulsion systems, and more particularly aeronautical propulsion systems having a high, or even very high, bypass ratio.

[0003] A propulsion system generally includes, from upstream to downstream in the gas flow direction, a fan section, a compressor section which can comprise a low-pressure compressor and a high-pressure compressor, a combustion chamber and a turbine section which can comprise 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.

[0004] The technological research efforts have already made it possible to very significantly improve the environmental performance of the aircrafts. The Applicant takes into account the impacting factors in all phases of design and development to obtain aeronautical components and products that are less energy-efficient, more respectful of the environment and whose integration and use in civil aviation have moderate environmental consequences with the aim of improving the energy efficiency of the aircrafts.

[0005] 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 BPR bypass ratio (corresponding to the ratio between the flow rate of the secondary air stream and the flow rate of the primary air stream). 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 rotational speed. 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 rotational speed lower than that of the low-pressure shaft.

[0006] The improvement of the propulsive efficiency of the system can also involve the dimensioning of the fan section. Indeed, due to its large diameter (in particular in order to reach high bypass ratios and low fan pressure ratios), the fan section represents an important portion of the propulsion system in terms of mass and therefore of specific consumption. At the same time, the fan section produces a very large portion of the thrust of the propulsion system.

[0007] One aim of the present application is to optimize the fan section of the propulsion system in order to make it more efficient without excessively penalizing the mass and therefore the specific consumption of the propulsion system.

[0008] For this purpose, according to a first aspect, a fan section of an aeronautical propulsion system is proposed, the fan section comprising a fan rotor comprising at least seventeen blades and at most twenty blades and having a solidity strictly less than 1.0, where the solidity is equal to a ratio between a chord at the blade tip and an inter-blade pitch at the blade tip, the fan section having:

• • [0009] a pressure ratio greater than or equal to 1.05 and less than or equal to 1.5; and
  • [0010] a peripheral speed at the blade tip greater than or equal to 260 m/s and 400 m/s;
  • [0011] the pressure ratio and the peripheral speed being measured at cruising speed.

[0012] Some preferred but non-limiting characteristics of the fan section according to the first aspect are as follows, taken individually or in combination:

• • [0013] the solidity of the fan rotor is strictly greater than 0.6;
  • [0014] a diameter of the fan rotor is greater than or equal to 127 cm and less than or equal to 304.8 cm, for example greater than or equal to 215.9 cm and less than or equal to 304.8 cm, for example of the order of 228.6 cm;
  • [0015] the peripheral speed at the tip of the blades is greater than or equal to 270 m/s and less than or equal to 380 m/s;
  • [0016] the fan section has a hub-tip ratio greater than or equal to 0.22 and less than or equal to 0.32, for example greater than or equal to 0.235 and less than or equal to 0.30, for example less than or equal to 0.27;
  • [0017] the fan section is housed in a fan casing, the blades of the fan rotor being fixed in rotation relative to a hub of the fan rotor so as to have a fixed pitch; and/or
  • [0018] the fan section further comprises a fan stator comprising at least 38 stator vanes and at most 48 stator vanes, for example exactly 40 stator vanes.

[0019] According to a second aspect, an aeronautical propulsion system is proposed comprising:

• • [0020] a drive shaft movable in rotation about an axis of rotation;
  • [0021] a fan shaft;
  • [0022] a fan section according to the first aspect, the fan rotor being driven in rotation by the fan shaft; and
  • [0023] 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.

[0024] Some preferred but non-limiting characteristics of the aeronautical propulsion system according to the second aspect are as follows, taken individually or in combination:

• • [0025] a thrust density per fan rotor blade of the fan section is greater than or equal to 0.5×10⁴ and less than or equal to 3.0×10⁴ N/m², where the thrust density per blade is defined by the following formula:

[0026] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$
and where: FN is the thrust generated by the propulsion system and is measured when the propulsion system is stationary system is stationary at cruising speed in a standard atmosphere and is expressed in Newton; n is the number of blades in the fan rotor and is at least equal to seventeen blades and at most equal to twenty blades; and D is the diameter of the fan rotor, measured in a plane normal to the axis of rotation at an intersection between the top and a leading edge of the fan rotor blades, and is expressed in meters; and/or

• • [0027] a bypass ratio of the propulsion system is greater than or equal to 10, for example comprised between 10 and 35 inclusive, for example between 10 and 18 inclusive;
  • [0028] a drive turbine comprises at least three and at most five stages;
  • [0029] a compressor comprises at least two and at most four stages;
  • [0030] the propulsion system further comprises a high-pressure turbine and a high-pressure compressor connected via a high-pressure shaft, the high-pressure shaft rotating more quickly than the drive shaft, the high-pressure turbine being a two-stage turbine; and/or
  • [0031] the high-pressure compressor comprises at least eight and at most eleven stages.

[0032] According to a third aspect, an aircraft is proposed comprising at least one propulsion system according to the second aspect fixed to the aircraft via a mast.

[0033] According to a fourth aspect, a method for dimensioning or manufacturing a fan section of an aeronautical propulsion system is proposed, the fan section comprising a fan rotor comprising at least seventeen blades and at most twenty blades and having a solidity strictly less than 1.0 and optionally strictly greater than 0.6, where the solidity is equal to a ratio between a chord at the blade tip and an inter-blade pitch at the blade tip, the fan section having:

• • [0034] a pressure ratio greater than or equal to 1.05 and less than or equal to 1.5; and
  • [0035] a peripheral speed at the blade tip greater than or equal to 260 m/s and 400 m/s;
  • [0036] the pressure ratio and the peripheral speed being measured at cruising speed.

[0037] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$
Some preferred but non-limiting characteristics of the dimensioning or manufacturing method according to the fourth aspect are as follows, taken individually or in combination:

[0038] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$

• • [0039] the fan section is further dimensioned so that a thrust density per fan rotor blade of the propulsion system is greater than or equal to 0.5×10⁴ and less than or equal to 3.0×10⁴ N/m² where the thrust density per fan rotor blade is defined by the following formula:

[0040] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$

[0041] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$
and where: FN is the thrust generated by the propulsion system and is measured when the propulsion system is stationary at cruising speed and is expressed in Newton; n is the number of blades in the fan rotor and is at least equal to seventeen blades and at most equal to twenty blades; and D is the fan diameter, measured in a plane normal to the axis of rotation at an intersection between the top and a leading edge of the fan rotor blades, and is expressed in meters.

[0042] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$

[0051] In all the figures, similar elements bear identical references.

[0052] A propulsion system 1 has a main direction extending along a longitudinal axis X and comprises, from upstream to downstream in the gas flow direction in the propulsion system 1 when in operation, a fan section 2 and a primary spool 3, often called “gas generator”, including 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 on an aircraft 100 via a pylon (or mast).

[0053] The compressor section 4, 5 comprises a succession of stages each comprising a blade wheel (rotor) 4a, 5a rotating in front of a vane wheel (stator) 4b, 5b. The turbine section 7, 8 also comprises a succession of stages each comprising a vane wheel (stator) 7b, 8b behind which a blade wheel (rotor) 7a, 8a rotates.

[0054] 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 therethrough. Moreover, the circumferential (or lateral, or tangential) direction corresponds to a direction perpendicular to the longitudinal axis X and not passing therethrough. Unless otherwise specified, inner (respectively, internal) and outer (respectively, external), respectively, are used with reference to a radial direction so that the inner portion or face of an element is closer to the axis X than the outer portion or face of the same element.

[0055] In operation, an air stream F entering the propulsion system 1 is divided between a primary air stream F1 and a secondary air stream F2, which circulate from upstream to downstream in the propulsion system 1.

[0056] The secondary air stream F2 (also called “bypass air stream”) flows around the primary spool 3. The secondary air stream F2 allows cooling the periphery of the primary spool 3 and serves to generate most of the thrust provided by the propulsion system 1.

[0057] The primary air stream F1 flows in a primary flowpath inside the primary spool 3, successively passing through the compressor section 4, 5, the combustion chamber 6 where it is mixed with fuel to serve as oxidizer, and the turbine section 7, 8. The passage of the primary air stream 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 in rotation the rotor of the compressor section 4, 5 as well as a rotor portion 9 of the fan section 2.

[0058] In a two-spool propulsion system 1, the compressor section 4, 5 can comprise a low-pressure compressor 4 and a high-pressure compressor 5. The turbine section 7, 8 can comprise 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 fan section 2 are driven in rotation by the rotor of the low-pressure turbine 8 via a low-pressure shaft 11. Thus, the primary spool 3 comprises a high-pressure spool comprising the high-pressure compressor 5, the high-pressure turbine 7 and the high-pressure shaft 10, and a low-pressure spool 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.

[0059] 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 can be co-rotating, that is to say driven in the same direction about the longitudinal axis X. As a variant, the low-pressure shaft 11 and the high-pressure shaft are counter-rotating, that is to say driven in opposite directions about the longitudinal axis X. Where 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 can be co-rotating or counter-rotating.

[0060] The fan section 2 comprises at least the fan rotor 9 capable of being 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 can be fixed relative to the hub 13.

[0061] The fan section 2 can further comprise a fan stator 16, or straightener, which comprises vanes 17 mounted on a hub of the fan stator 16 and have the function of straightening the secondary air stream F2 which flows at the outlet of the fan rotor 9. The vanes 17 of the fan stator 18 can be fixed relative to the hub or have variable pitch angle.

[0062] 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”, it is meant here a bypass ratio greater than or equal to 10, for example comprised between 10 and 80 inclusive. To calculate the bypass ratio, the mass flow rate of the secondary air stream F2 and the mass flow rate of the primary air stream F1 are measured when the propulsion system 1 is stationary, uninstalled, in take-off rating in a standard atmosphere (as defined by the International Civil Aviation Organization (ICAO) Manual, Doc 7488/3, 3^rd edition) and at sea level (conditions known as SLS, for Sea Level Standard). By “uninstalled”, it is meant here that the measurements are performed when the propulsion system 1 is in a test bench (and uninstalled on an aircraft 100), the measurements then being simpler to perform.

[0063] It will be noted that, in the present application, some parameters are determined in cruising conditions that is to say at 10,668 m altitude (35,000 feet), 0.8 Mach and in ISA (International Standard Atmosphere) conditions defined by the standard ISO2533/edition 1975/addendum 1985. In addition, the distances (length, radius, diameter, etc.) are measured at ambient temperature (approximately 20° C.) when the propulsion system 1 is cold, that is to say when the propulsion system 1 is 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 rating.

[0064] The fan rotor 9 is decoupled from the low-pressure shaft 11 using 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 “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.

[0065] 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 crossing the propulsion system 1. In a high bypass ratio propulsion system 1, most of the flow rate generating the propulsive force is constituted by the secondary air stream F2 of the propulsion system 1, the kinetic energy of the secondary air stream F2 being mainly affected by the compression that the secondary air stream F2 undergoes when crossing 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.

[0066] The propulsion system 1 is configured to provide a thrust comprised between 18,000 lbf (80,068 N) and 51,000 lbf (222,411 N), for example between 20,000 lbf (88,964 N) and 35,000 lbf (155,688 N), when the propulsion system 1 is stationary, uninstalled, in take-off rating in a standard atmosphere (as defined by the International Civil Aviation Organization (ICAO) Manual, Doc 7488/3, 3^rd edition) and at sea level.

[0067] The fan section 2 can comprise a fan casing 12, the fan rotor 9 being housed in the fan casing 12.

[0068] The fan rotor 9 extends upstream of a fan stator. The vanes 17 of the fan stator 16 are then generally called Outlet Guide Vane (or OGV) and have a fixed pitch angle relative to the hub of the fan stator. Moreover, the bypass ratio of the propulsion system 1 is for example greater than or equal to 10, for example comprised between 10 and 35 inclusive, for example between 10 and 18 inclusive.

[0069] Each fan blade 14 has a leading edge 14a and a trailing edge 14b (see for example FIGS. 2a and 2b). The leading edge 14a is configured to extend facing the flow of gases entering the fan rotor 9. It corresponds to the anterior portion of an aerodynamic profile which faces the air stream and which divides the air flow into an intrados flow and an extrados flow. The trailing edge 28b corresponds to the posterior portion of the aerodynamic profile, where the intrados and extrados flows meet. It is noted here that, when the blades 14 comprise a leading edge and/or trailing edge shield, the leading edge 14a (respectively the trailing edge 14b) of the blades 14 corresponds to the anterior portion of the profile of the shield which reconstitutes the leading edge (respectively the posterior portion of the profile of the shield which reconstitutes the trailing edge 14b) and whose function is to divide the flow into an intrados flow and an extrados flow (respectively to join the flows).

[0070] The fan rotor 9 moreover comprises a series of platforms each extending between two adjacent blades 14 and configured to delimit radially inside the air stream F passing through the rotor 9.

[0071] The fan blade 14 further has a chord at the blade tip c₁ and a chord at the blade base c₂.

[0072] The chord at the blade tip c₁ corresponds to the straight line segment which connects an upstream intersection point P between the leading edge 14a and the top 21 of a blade 14 and a downstream intersection point between the trailing edge 14b and the top 21 of the blade 14.

[0073] The chord at the blade base c₂ corresponds to the straight line segment parallel to the axis of rotation X which connects a second downstream intersection point between the trailing edge 14b and the surface that delimits radially inside the flowpath in the fan rotor 9 (and corresponds to the connection point of the trailing edge 14b with the aerodynamic surface of a platform of the fan rotor 9) and a second upstream intersection point P between the leading edge 14a and a plane circumferential to the axis X which comprises the second downstream intersection point. The second upstream and downstream intersection points are therefore at an iso-distance from the axis X (same radius). The second upstream intersection point P moreover extends at a distance from the aerodynamic surface of the platform, as can be seen from FIG. 1 given as a purely illustrative example.

[0074] The reduction mechanism 19 can comprise a reduction mechanism 19 with an epicyclic gear train, for example of the “epicyclic” or “planetary” type according to the terminology sometimes encountered by those skilled in the art, single-staged or two-staged. According to a first variant, the reduction mechanism 19 can be of the planetary (star) type (FIG. 3) and comprise a sun pinion 19a (inlet of the reduction mechanism 19), centered on an axis of rotation X of the reduction mechanism 19 (generally coincident with the longitudinal axis X) and configured to be driven in rotation by the low-pressure shaft 11, a ring gear 19b (outlet of the reduction mechanism 19) coaxial with the sun pinion 19a and configured to drive in rotation the fan shaft 20 about the axis of rotation X, and a series of planet gears 19c circumferentially distributed about the axis of rotation X between the sun pinion 19a and the ring gear 19b, each planet gear 19c being internally meshed with the sun pinion 19a and externally with the ring gear 19b. The series of planet gears 19c is mounted on a planet gear carrier 19d which is fixed relative to a stator portion 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 can be of the epicyclic (planetary) type (FIG. 4), in which case the ring gear 19b is fixedly mounted on the stator portion 19e of the propulsion system 1 and the fan shaft 20 is driven in rotation by the planet gear carrier 19d (which is therefore movable in rotation relative to a stator portion 19e of the propulsion system 1, for example relative to a casing of the compressor section 4, 5).

[0075] Whatever the configuration of the reduction mechanism 19, the diameter of the ring gear 19b and of the planet gear carrier 19d are greater than the diameter of the sun pinion 19a, so that the rotational speed of the fan rotor 9 is lower than the rotational speed of the low-pressure shaft 11.

[0076] 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 greater than or equal to 2.7 and less than or equal to 6.0, for example around 3.0.

[0077] The double-spool propulsion system 1 can 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.

[0078] The speed limit (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 EASA European certification specification CS-E 740 (or according to the American certification specification 14-CFR Part 33.87)), is comprised between 8,500 revolutions per minute and 12,000 revolutions per minute, for example between 9,000 revolutions per minute and 11,000 revolutions per minute, when the propulsion system 1 is stationary, uninstalled, in take-off rating in a standard atmosphere (as defined by the International Civil Aviation Organization (ICAO) Manual, Doc 7488/3, 3^rd edition) and at sea level. The speed limit corresponds to the maximum rotational speed when the propulsion system 1 is sound (and potentially at the end of its service life). It is therefore likely to be reached by the low-pressure shaft 11 in flight condition. This speed limit forms part of the data declared in the engine certificate (type certificate data sheet). Indeed, this rotational speed is generally used as a reference speed for the dimensioning and manufacturing of the propulsion systems 1 and in some certification tests (such as blade loss or rotor integrity tests, typically the certification CS-E-800—collision with a bird and ingestion).

[0079] In order to optimize the performance of the propulsion system 1, the fan rotor 9 includes at least seventeen blades 14 and at most twenty blades 14. Moreover, a pressure ratio of the fan section 2 is greater than or equal to 1.05 and less than or equal to 1.5, a solidity of the fan rotor 9 is strictly less than 1.0, where the solidity is equal to a ratio between a chord at the blade tip and an inter-blade pitch 23, and a peripheral speed at the blade tip is greater than or equal to 260 m/s and 400 m/s, for example greater than or equal to 270 m/s and less than or equal to 380 m/s. it is noted that the pressure ratio of the fan section 2 and the peripheral speed are measured here at cruising speed to the extent that this is the flight phase in which the maximum efficiency of the fan rotor 9 is to be obtained.

[0080] The solidity is equal to the ratio between the chord at the blade tip c₁ and an inter-blade pitch 23. The inter-blade pitch 23 corresponds to the angular distance between the upstream intersection points P of two adjacent blades 14; the inter-blade pitch 23 is therefore equal to the external radius R_e of the fan rotor 9 (half-diameter) multiplied by the angle between a first straight line D₁, comprised in a plane normal to the axis X which comes from the upstream intersection point P (see FIGS. 2a and 2b) of a first blade 14 and intersects the axis X and a second straight line D₂, comprised in the plane normal to the axis X which comes from the upstream intersection point P of a second blade 14 immediately adjacent to the first blade 14 and intersects the axis X. The solidity being a ratio of distances, it is measured when the propulsion system 1 is a cold system (under the aforementioned conditions) (see FIG. 2b).

[0081] The pressure ratio of the fan section 2 corresponds to the ratio between the average pressure at the outlet of the fan stator 17 and the average pressure at the inlet of the fan rotor 9. For example, the pressure ratio is greater than or equal to 1.1 and less than or equal to 1.45. The average pressures are measured here on the flowpath (from the surface that radially delimits inside the flowpath at the inlet of the fan rotor 9 to the fan casing 12.

[0082] The selection of a number of fan blades 14 between seventeen and twenty blades 14 makes it possible to obtain an optimized fan section 2 participating in the reduction of the specific consumption and the mass of the propulsion system 1.

[0083] In addition, a solidity of the fan rotor 9 strictly less than 1.0 is specifically adapted to a rotor 9 comprising between seventeen and twenty blades 14. Indeed, the chord at the blade tip c₁ is then reduced compared to the available space (high inter-blade pitch 23), which makes it possible to reduce the mass of the blade 14 and to modify its frequency placement, because the modification of the chord at the blade tip c₁ has a significant impact on the first mode of deformation of the blade 14 (at iso-operating range). Indeed, when the mode of deformation of the blades 14 occurs in a steady state of the fan rotor 9 (in particular, the cruising speed), a solution may consist in increasing the thickness of the blade base in order to improve the margins and move this deformation mode. This increase in the thickness of the blades 14 at the blade base however implies increasing the bulk of the base, which influences the circumferential dimension of the cells that receive the base and therefore the design to manage the mechanical resistance of the disk. The increase in the thickness of the blades 14 is therefore likely to increase the hub-tip ratio of the fan rotor 9. However, the increase in the hub-tip ratio is detrimental to the aerodynamic efficiency of the fan rotor 9. On the contrary, the dimensioning of the fan rotor 9 so that its solidity is less than 1.0 makes it possible to reduce the chord at the blade tip c₁ and to move the first mode of deformation of the blades 14 into a non-stabilized operating range or outside the operating range of the fan rotor, without thickening the blades 14. It is therefore not necessary to increase the hub-tip ratio.

[0084] On the contrary, the number of blades 14 being comprised between seventeen and twenty blades 14 makes it possible to optimize (and particularly reduce) the centrifugal forces to be supported by the hub 13 of the fan rotor 9, and therefore to reduce its radius R_i. The hub-tip ratio of the fan rotor can therefore be reduced while the section of passage of the fan rotor 9 and therefore the aerodynamic performance of the fan rotor 9, increase. For example, the fan rotor 9 can have a hub-tip ratio comprised between 0.22 and 0.32, for example. In the case of a fan rotor 9 with fixed pitch angle, the hub-tip ratio can be comprised between 0.22 and 0.30, for example between 0.235 and 0.27. The hub-tip ratio corresponds to the ratio between the inner radius R_i and the outer radius R_e of the fan rotor 9. The inner radius R_i corresponds to the distance between the axis of rotation X and the intersection point between the leading edge 22 and the surface that delimits radially inside the flowpath 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 the platform of the fan rotor 9). The outer radius R_e is equal to half the diameter D of the fan.

[0085] In a propulsion system 1 with a reduction mechanism 19, the rotational speed of the fan rotor 9 is reduced, here comprised between 260 m/s and 400 m/s. Its pressure ratio can also be reduced and is here comprised between 1.05 and 1.5, for example between 1.1 and 1.45, so that the speed difference between the outlet of the fan rotor 9 and the inlet of the fan rotor 9 is reduced while optimizing the efficiency of the fan section 2. The flow at the blade tip through the fan rotor 9 is then supersonic. Particularly, a supersonic shock is generated at the level of the fan blades 14. However, this supersonic shock cannot be eliminated, in particular at cruising speed: it must therefore be monitored to prevent it from deteriorating the efficiency of the fan section 2. By dimensioning and manufacturing the fan rotor 9 with seventeen to twenty blades so that its solidity is strictly less than 1.0, the inter-blade pitch 23 and the chord at the blade tip c₁ are such that the supersonic shock is stable at cruising speed and moves only in the divergent portion of the inter-blade channel 24. The inter-blade channel 24 corresponds to the passage between two adjacent blades 14 which extends between an inlet plane 25, which is normal to the air stream F entering the fan rotor 9 and passes through the leading edge 14a of the first blade 14, and an outlet plane 26 which is parallel to the inlet plane and passes through the trailing edge 14b of the second blade 14 (see for example FIG. 2a). This channel 24 has, from upstream to downstream through the fan rotor 9, a convergent portion which extends from the inlet plane 25 to an intermediate plane 27 corresponding to the minimum section of the channel 24 (neck), and a divergent portion which extends from the intermediate plane 27 to the outlet plane 26. However, the Applicant has observed that, when the supersonic shock reached the convergent portion of the inter-blade channel 24, it became unstable and had the effect of reducing the efficiency of the fan section 2. Conversely, when the supersonic shock remains in the divergent portion, by being for example in the vicinity of the intermediate plane 27, the supersonic shock is stable at cruising speed. The longer the inter-blade channel 24, that is to say the greater the distance between the inlet 25 and outlet 26 planes, the easier it is to design a convergent-divergent channel and therefore to maintain the supersonic shock in the divergent portion of the channel 24. In addition, the lower the hub-tip ratio, the longer the inter-blade channel 24 without penalizing the chord at the blade tip c₁. The decrease in the hub-tip ratio indeed makes it possible to increase the section of passage of the fan rotor 9 and to reduce the axial speed of the air stream F. The blades 14 can therefore have a more open pitch angle (so that the chord at the blade base c₂ is closer to the air stream F entering the fan rotor 9), which lengthens the inter-blade channel 24 and therefore makes it possible to better manage the supersonic shock, knowing that the pitch angle of a blade 1 is considered “open” when the orientation of the chord is closer to that of the axis than for a “closed” pitch angle. Thus, the selection of a solidity less than 1.0 combined with a number of blades 14 comprised between seventeen and twenty makes it possible to monitor the supersonic shock in the fan rotor 9, thanks to a length of the adapted inter-blade channel 24, and to obtain an efficient fan section 2.

[0086] In one embodiment, the solidity of the fan rotor remains greater than 0.6 in order to be able to sufficiently open the pitch angle of the blades 14 and to guarantee that the chord at the blade tip c₁ remains sufficiently long to ensure the presence of a sufficiently long inter-blade channel 24 and monitor the supersonic shock.

[0087] The diameter D of the fan rotor can then be comprised between 80 inches (203.2 cm) and 185 inches (469.9 cm) inclusive. The diameter D is for example comprised between 85 inches (215.9 cm) and 120 inches (304.8 cm) inclusive, for example of the order of 90 inches (228.6 cm), which allows the integration of the propulsion system 1 in a conventional manner, particularly under the wing of an aircraft 100. The diameter of the fan rotor 9 is measured here in a plane normal to the axis of rotation X at the level of the upstream intersection point P (between the top 21 and the leading edge 14a of the blades 14 of the fan rotor 9) and is expressed in meters (m). It is noted that FIG. 1 being a partial view, the diameter D is only partially visible.

[0088] The number of vanes 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 of the fan rotor 9. In one embodiment, the number of vanes 16 in the fan stator 17 is at least equal to 38 and at most equal to 48, for example exactly equal to 40.

[0089] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$
However, the decrease in the hub-tip ratio of the fan rotor 9 implies an increase in the mechanical load of the hub 13 of the fan rotor 9. The dimensioning and manufacturing of the fan rotor 9 so that its hub-tip ratio is comprised between 0.22 and 0.32 particularly makes it possible to obtain an optimized thrust density per fan blade 14. Particularly, the thrust density per fan rotor 9 blade 14 may be greater than or equal to 0.5×10⁴ and less than or equal to 3.0×10⁴ N/m² where the thrust density per blade 14 is defined by the following formula:

[0090] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$

[0091] $\mathrm{Thrust}\mathrm{density}=\frac{FN}{n*D^2}*100$
and where: FN is the thrust generated by the propulsion system 1 and is expressed in Newton (N) and is measured when the propulsion system 1 is at cruising speed (10,668 m altitude, 0.8 Mach and in ISA conditions);

• • [0092] n is the number of blades 14 in the fan rotor 9 and is at least equal to seventeen blades 14 and at most equal to twenty blades 14; and
  • [0093] D is the diameter of the fan rotor 9.

[0094] The Applicant noticed the fact that, when the thrust density per blade at cruising speed is less than 0.5×10⁴, it is difficult to integrate the propulsion system 1 because it is too bulky, has a too significant mass and generates excessive drag. Moreover, when the thrust density is greater than 3.0×10⁴ N/m², the performance of the propulsion system 1 in terms of specific consumption is degraded. The dimensioning and manufacturing of the propulsion system 1, so that the thrust density per blade 14 of the fan rotor 9 is comprised between 0.5×10⁴ and 3.0×10⁴ N/m² at cruising speed, therefore makes it possible to obtain a compromise between the integration and the performance of the propulsion system 1 when the propulsion system 1 comprises a reduction mechanism 19 and has a high bypass ratio. Such an interval of thrust density per blade 14 is further compatible with a fan pressure ratio of less than 1.45, which makes it possible to optimize the propulsive efficiency of the propulsion system 1.

[0095] By way of example, a propulsion system 1 according to the present disclosure comprising a ducted fan rotor and whose thrust density per fan blade 14 is equal to 1.6×10⁴ N/m² at cruising speed has a lower specific consumption of 15% compared to the same propulsion system whose thrust density per fan blade is equal to 4×10⁴ N/m². The thrust density per propulsion system 1 blade is influenced to the first order by the diameter D of the fan rotor and the pressure ratio of the fan section 2. The bypass ratio, the overall compression ratio and the number of stages in the compression and turbine sections generally have no or little impact on the thrust density per blade 14.

[0096] The engine 1 is a two-spool propulsion system comprising a ducted fan section corresponding to the current technical standard (at the filing date of the present application) to be improved.

[0097] The engine 2 is a two-spool propulsion system 1 comprising a ducted fan section in accordance with the teaching of the present application.

[0098]

Dimensioning parameter		Engine 2
(SLS unless otherwise	Engine 1	(in accordance with the
indicated)	(reference)	present disclosure)
Solidity	1.1	0.9
peripheral speed of the fan	346 m/s	343 m/s
blades
Number of fan blades	18	18
Hub-tip ratio	0.30	0.30
Thrust density per fan blade	1.04 × 106	9.14 × 105
(cruise)
Pressure ratio of the fan	1.43	1.37
Thrust of the engine (cruising)	24,250N	20,451N
Diameter D	2.11 m (83 in.)	2.413 m (95 in.)
bypass ratio BPR	11.0	12.0
Speed limit of the fan rotor 9	3,284 rpm	2,842 rpm
(redline)
Speed limit of the high-	19,888 rpm	19,741 rpm
pressure shaft (redline)
Speed limit of the low-pressure	8,962 rpm	10,143 rpm
shaft (N1) (redline)
Compression ratio of the low-	1.85	1.85
pressure compressor
Overall compression ratio	42	50
Reduction ratio	2.73	3.57
Number of stages of the low-	2	2
pressure compressor
Number of stages of the high-	10	10
pressure compressor
Number of stages of the high-	2	2
pressure turbine
Number of stages of the low-	4	3
pressure turbine
Temperature at the inlet of the	1,577° C.	1,627° C.
high-pressure turbine
Temperature at the inlet of low-	1,027° C.	1,024° C.
pressure turbine (redline)
N12S	36.7 × 106	48.6 × 106 (rpm)2 · m2
(where S is the outlet section of	(rpm)2 · m2
the low-pressure turbine)

[0099] The engine 1 has a higher solidity for 18 blades compared to the engine 2. The consequence is that the chord of the fan blades of the engine 2 is reduced over the entire height of the blade compared to the chord of the fan blades of the engine 1, so that the first mode of deformation of the fan blades of the engine 2 is lower than in the engine 1. On the other hand, this reduction in the chord of the fan blades in the engine 2 has the consequence that the mass of the fan and of the fan disk of the engine 2 is smaller than in the reference engine, as is the drag of the nacelle (gain due to the distance from the axial chord). The specific consumption of the engine 2 is therefore smaller than that of the engine 2.

[0100] The fan blades of the two engines can in particular be made of a composite material comprising a fibrous reinforcement embedded in a polymer matrix. If necessary, the fibers of the fibrous reinforcement can be hybridized (use of different fibers in the reinforcement in a localized manner) in order to modify the frequency placement of the modes of deformation of the blades.

[0101] To move from the engine 1 (reference) to the engine 2 (in accordance with the disclosure), the diameter D of the fan and the bypass ratio BPR were increased, which made it possible to improve the propulsive efficiency and maintain a similar thrust given the drop in the pressure ratio of the fan 2. Moreover, the overall compression ratio was increased, 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. Finally, to the extent that the temperature of the low-pressure turbine 8 was kept stable, it was possible to increase its mechanical loading (N₁²S) and therefore to reduce the number of stages.