Propulsion unit for an aircraft comprising a turbine engine and a reduction gear

Abstract

The invention relates to a propulsion unit (2) for an aircraft (1) comprising: - a turbine engine (6) comprising an output shaft (7); - a reduction gear (8) arranged axially at the front of the turbine engine (6) and comprising an output freewheel (9) which is rotated by the output shaft (7), the output freewheel (9) comprising an inner shaft (11); the reduction gear (8) comprising a casing (13) which is directly fixed to a casing (14) of a main gearbox (4) of the aircraft (1), the inner shaft (11) of the freewheel (9) being rotatably connected to an input shaft (15) of the main gearbox (4) via a splined sleeve (16), the inner shaft (11) of the freewheel (9) being hollow and comprising front and rear ends which are internally open, to allow the sleeve (16) to be introduced and removed via the rear end of the inner shaft (11).

Description

[0001] DESCRIPTION

[0002] TITLE: PROPEL UNIT FOR AN AIRCRAFT COMPRISING A TURBOENGER AND A REDUCTION GEAR

[0003] Technical field of the invention

[0004] The present invention relates to a propulsion unit for an aircraft comprising a turboshaft engine and a reduction gear.

[0005] Technical background

[0006] A turboshaft engine typically comprises, from upstream to downstream following the direction of gas flow in the turboshaft engine, an air inlet, a compressor, a combustion chamber, an expansion turbine (high-pressure turbine), a power turbine (low-pressure turbine), and finally an exhaust nozzle.

[0007] Air entering through the air intake is compressed by the compressor, then injected into the combustion chamber to be mixed with fuel. The air / fuel mixture is burned and expanded in the expansion turbine and then in the power turbine before being expelled from the turbocharger through the exhaust nozzle.

[0008] The compressor, combustion chamber and expansion turbine form a gas generator in which the compressor rotor is mechanically driven by the expansion turbine rotor via a drive shaft.

[0009] The power turbine is independent of the gas generator and includes an output shaft (or power shaft) designed to drive the aircraft's propulsion system(s). For example, in a helicopter, the turboshaft engine's output shaft drives both the main rotor and the tail rotor (also called the tail rotor or anti-torque rotor). Typically, the turboshaft engine is mounted on the aircraft's structural mechanical floor. The turboshaft engine's output shaft is rotationally connected, either directly or indirectly (via a reduction gear), to the input shaft of the main transmission by means of a flexible shaft (equipped with flexible elements) which has a significant axial length (approximately 400 mm). This flexible shaft allows for misalignment between the turboshaft engine's output shaft and the input shaft of the main transmission.The turbocharger mounting can be completed with a front attachment that connects the turbocharger and the main transmission.

[0010] Such a turbocharger mounting presents several disadvantages.

[0011] Indeed, firstly, the structural mechanical floor represents a significant mass which penalizes the overall performance of the aircraft.

[0012] Secondly, the large axial dimension of the flexible shaft means that the turboshaft engine is located far from the main gearbox. This distance is particularly detrimental to the helicopter's center of gravity, as it shifts its center of gravity away from the axis of its main rotor, negatively impacting its maneuverability, stability, and payload distribution.

[0013] To remedy at least some of the aforementioned disadvantages, it is known from document EP3289189B1 in the name of the applicant to fix the turbocharger directly onto the main transmission gearbox.

[0014] To achieve this, the front end of the turbocharger's output shaft is fitted with a drive gear (or driving sprocket). This drive gear protrudes from the turbocharger housing and is connected to various guide bearings. When mounting the turbocharger onto the main transmission, the drive gear is recessed into the transmission housing so that it meshes with a driven gear (or driven sprocket).

[0015] This method of mounting the turbocharger offers advantages but still has room for improvement. Firstly, such a fixed mounting of the drive gear inevitably creates a high risk of shocks (or impacts) during installation, which is undesirable. This high risk of shocks also applies to the guide bearings associated with the drive gear. This risk is significantly reduced if the pinion shafts are vertical and the entire assembly is lowered vertically.

[0016] Secondly, such a close integration inevitably implies increased interdependence between the turbocharger and the main transmission, with, for example, additional common interfaces (particularly at the guide bearings). This increased interdependence proves detrimental from a maintenance perspective when it is necessary to identify the source of a problem, and in other words, to determine whether the problem originates in the turbocharger or the main transmission.

[0017] Furthermore, the remainder of this application will focus on an engine architecture incorporating a gearbox that is axially positioned at the front of the turboshaft engine. The gearbox includes an output freewheel that is driven in rotation by the turboshaft engine's output shaft via one or more gear trains within the gearbox.

[0018] The objective of the present invention is therefore to provide a simple, effective, and economical solution to the aforementioned problem. Prior art also includes documents FR3008679A1, FR3131755A1, EP3129619B1, WO2024 / 018137A1, and FR3138167A1.

[0019] Summary of the invention

[0020] The invention thus proposes a propulsion system for an aircraft comprising:

[0021] - a turboshaft engine with a longitudinal axis X comprising an output shaft;

[0022] - a reducer arranged axially at the front of the turboshaft engine and comprising an output freewheel which is driven in rotation by the output shaft of the turboshaft engine via a gear train of the reducer, the output freewheel comprising an internal shaft and an external gear mechanically linked to each other; characterized in that the reducer comprises a housing which is configured to be directly fixed to a housing of a main gearbox of the aircraft, the internal shaft of the freewheel being configured to be rotationally linked with an input shaft of the main gearbox via a splined sleeve, the internal shaft of the freewheel being hollow and comprising front and rear ends which are internally open, to allow the insertion and removal of the sleeve via the rear end of the internal shaft.

[0023] Securing the gearbox housing directly to the main gearbox housing allows for precise alignment between the internal shaft of the freewheel and the input shaft of the main gearbox, which allows the use of a simple splined sleeve (without flexible elements) to couple them.

[0024] This type of gearbox mounting also eliminates the need for a structural floor or additional connection between the propulsion unit and the aircraft structure, thus minimizing the aircraft's mass and improving overall performance. The absence of a structural floor also isolates the aircraft cabin from propulsion unit vibrations.

[0025] This type of gearbox mounting also minimizes the distance between the helicopter's center of gravity and the axis of its main rotor, which benefits its size, maneuverability, stability and payload distribution.

[0026] Using a sleeve allows for the separation of the mounting and coupling processes. This enables the engine to be mounted to the main transmission housing first, followed by the coupling of the transmission components. This separation significantly reduces the risk of impacts or misalignment between parts when mounting the reduction gear to the main transmission. The sleeve also allows for the distinct separation of the reduction gear components from the main transmission components, making it quick and easy to identify, from a maintenance perspective, whether a problem originates in the powertrain or the main transmission.

[0027] Finally, the sleeve is inserted (or removed) quickly and easily by sliding it from the rear end of the freewheel's internal shaft. The propulsion unit according to the invention may comprise one or more of the following features, taken individually or in combination:

[0028] - the sleeve includes front external splines which are configured to be rotationally linked with internal splines of the input shaft of the main transmission, and rear external splines which are rotationally linked with internal splines of the internal shaft of the freewheel;

[0029] - the internal shaft of the freewheel is housed entirely within the gearbox casing;

[0030] - the sleeve is configured to be stopped axially at the front by a shoulder formed in the input shaft of the main transmission;

[0031] - the sleeve is stopped axially at the rear by an element which is clamped onto the internal shaft of the freewheel;

[0032] - the element is a flange or tail rotor shaft that is configured to drive a tail rotor of the aircraft into rotation;

[0033] - the gearbox housing is configured to be directly fixed to at least one flange of the main gearbox housing via at least one upper screw located in an upper part of the flange and at least one lower screw located in a lower part of the flange, the upper screw being inserted from front to back from the side of the main gearbox, and preferably the lower screw being inserted from back to front from the side of the gearbox to simplify assembly / disassembly operations; - the upper screw includes a head located on the side of the main gearbox and the lower screw includes a head located on the side of the gearbox;

[0034] - the upper screw includes a rod which on one hand passes through a hole opening from the flange of the main transmission housing and on the other hand cooperates with a tapped orifice formed in the housing of the reducer, the lower screw includes a rod which on one hand passes through a hole opening from a lower flange of the housing of the reducer and on the other hand cooperates with a tapped orifice formed in the flange of the main transmission housing;

[0035] - the front external splines of the sleeve and the internal splines of the input shaft form a front splined joint, the rear external splines of the sleeve and the internal splines of the freewheel internal shaft form a rear splined joint, the front splined joint including at least one visual indicator configured to quantify wear of the front splined joint and / or the rear splined joint including at least one visual indicator configured to quantify wear of the rear splined joint;

[0036] - the front external splines of the sleeve and the internal splines of the input shaft are lubricated via a front oil circuit, the front oil circuit comprising at least one path which is defined axially between a shoulder of the input shaft and the front end of the sleeve, the rear external splines of the sleeve and the internal splines of the inner shaft being lubricated via a rear oil circuit, the rear oil circuit comprising at least one axial passage formed internally in the sleeve, a cavity which is delimited externally by the inner shaft, and an opening which connects the passage and the cavity.

[0037] The present invention further relates to a method of mounting a propulsion unit as described above on a main gearbox of an aircraft, the method comprising chronologically the steps of: a) fixing the gearbox housing directly onto a housing of the main gearbox; b) introducing the splined sleeve via the rear end of the internal shaft, to link in rotation the internal shaft of the freewheel with an input shaft of the main gearbox.

[0038] The present invention also relates to an assembly for an aircraft comprising a main gearbox and a propulsion unit as described above, the gearbox housing being directly fixed to a housing of the main gearbox, the internal shaft of the freewheel being rotationally linked with an input shaft of the main gearbox via a splined sleeve of the assembly.

[0039] The present invention finally relates to an aircraft comprising an assembly as described above or a propulsion group as described above.

[0040] Brief description of the figures

[0041] The invention will be better understood and other details, features and advantages of the invention will become more apparent upon reading the following description, given by way of non-limiting example and with reference to the accompanying drawings in which:

[0042] [Fig.1] Figure 1 is a schematic view of a helicopter comprising a propulsion unit according to the invention;

[0043] [Fig.2] Figure 2 is a detailed view in longitudinal half-section of the link between the propulsion unit and the main transmission box of the helicopter, according to a first example of embodiment;

[0044] [Fig.3] Figure 3 is a view similar to Figure 2 according to a second embodiment example;

[0045] [Fig.4] Figure 4 is a view similar to Figures 2 and 3 according to a third embodiment; [Fig.5] Figure 5 is a schematic cross-sectional view of the front splined joint between a sleeve and an input shaft, when the front splined joint is unworn;

[0046] [Fig.6] Figure 6 is a schematic detail and perspective view of the front and rear splined links, when the splined links are unworn;

[0047] [Fig.7] Figure 7 is a detail view of Figure 5, when the front grooved linkage is worn;

[0048] [Fig.8] Figure 8 is a view similar to Figure 6, when the front and rear splined links are worn.

[0049] Detailed description of the invention

[0050] Figure 1 schematically represents an aircraft 1, which here is a helicopter 1. Alternatively, aircraft 1 could be, for example, a gyrodyne or another type of rotorcraft.

[0051] The helicopter 1 includes a propulsion unit 2 which drives a main rotor 3 via a main gearbox 4, and a tail rotor 5 (also called tail rotor or anti-torque rotor).

[0052] More specifically, propulsion group 2 comprises:

[0053] - a turboshaft engine 6 with longitudinal axis X comprising an output shaft 7 (or a power shaft);

[0054] - a reduction gear 8 arranged axially at the front of the turboshaft engine 6 and comprising an output freewheel 9 which is driven in rotation by the output shaft 7 of the turboshaft engine 6 via a gear train 10 (or gear mechanism) of the reduction gear 8, the output freewheel 9 comprising an internal shaft 11 and an external gear 12 mechanically linked to each other. According to the invention, the reduction gear 8 comprises a housing 13 which is configured to be directly fixed to a housing 14 of the main gearbox 4 of the aircraft 1. The internal shaft 11 of the freewheel 9 is configured to be rotationally linked with an input shaft 15 of the main transmission gearbox 4 via a splined sleeve 16. The internal shaft 11 of the freewheel 9 is hollow and includes front and rear ends 17, 18 which are internally through-holes, to permit the insertion and removal of the sleeve 16 via the rear end 18 of the internal shaft 11.

[0055] The fixing of the housing 13 of the reducer 8 directly onto the housing 14 of the main transmission 4 allows for precise alignment between the internal shaft 11 of the free wheel 9 and the input shaft 15 of the main transmission 4, which allows the use of a simple splined sleeve 16 (without flexible elements) to couple them.

[0056] This method of attaching the reduction gear 8 also eliminates the need for a structural floor or additional connection between the propulsion unit 2 and the aircraft structure 1, thereby minimizing the mass of aircraft 1 and improving its overall performance. The absence of a structural floor also isolates the cabin of aircraft 1 from vibrations of the propulsion unit 2.

[0057] Such a fixing of the reduction gear 8 also makes it possible to minimize the distance between the center of gravity of the helicopter 1 and the axis of its main rotor 3, to the benefit in particular of its size, its maneuverability, its stability and the distribution of its payload.

[0058] The use of a sleeve 16 allows the fastening and coupling processes to be separated, and consequently, the fastening can be carried out first and the coupling second. This separation significantly reduces the risk of impacts or misalignment between parts when the reducer 8 is attached to the main transmission 4.

[0059] Such a sleeve 16 also allows the elements of the reduction gear 8 and the elements of the main transmission 4 to be separated distinctly, and consequently, from a maintenance point of view, it is quick and easy to identify whether the origin of a problem comes from the propulsion unit 2 or the main transmission 4. Finally, the introduction (or removal) of the sleeve 16 is done quickly and easily by sliding from the rear end 18 of the internal shaft 11 of the free wheel 9.

[0060] The turboshaft engine 6 is defined along the longitudinal axis X which corresponds to the axis of rotation of the compressor 20 and the turbines 22, 23 of the turboshaft engine 6.

[0061] For the purposes of this application, "axial" or "axially" means any direction parallel to the X-axis of the turboshaft engine 6, and "radial" or "radially" means any direction perpendicular to the X-axis of the turboshaft engine 6.

[0062] Similarly, the terms "internal" and "external" are defined radially with respect to the X-axis of the turboshaft engine 6 or an axis parallel to the X-axis.

[0063] Finally, the terms "front" and "rear" define the axial positions of the elements relative to each other.

[0064] As illustrated in Figure 1, the turboshaft engine 6 is mounted horizontally on the housing 13 of the reducer 8, and in other words the longitudinal axis X of the turboshaft engine 6 is horizontal and perpendicular to the axis of rotation of the main rotor 3 of the helicopter 1.

[0065] The turboshaft engine 6 conventionally comprises, from upstream to downstream following the direction of gas flow in the turboshaft engine 6, an air inlet 19, a compressor 20, a combustion chamber 21, an expansion turbine 22 (or high-pressure turbine), a power turbine 23 (or low-pressure turbine), and an exhaust nozzle 24. The compressor 20, the combustion chamber 21 and the expansion turbine 22 form the gas generator of the turboshaft engine 6.

[0066] More specifically, the compressor 20 is supplied with air via the air inlet 19 and comprises one or more axial and / or centrifugal compression stages. The rotor(s) of the compressor 20 are rotationally linked to the rotor(s) of the expansion turbine 22 via a drive shaft 25. The rotors of the compressor 20 and the expansion turbine 22 are free to rotate about the X-axis.

[0067] The combustion chamber 21 is supplied with compressed air via the compressor 20 and with fuel via one or more injectors. The air / fuel mixture is burned by one or more ignition devices. The exhaust gases from the combustion chamber 21 are expanded in the expansion turbine 22 and then in a power turbine 23, before being expelled into the external environment via the exhaust nozzle 24.

[0068] The expansion turbine 22 comprises one or more axial and / or centripetal expansion stages. As indicated above, the rotor(s) of the expansion turbine 22 are rotationally linked to the rotor(s) of the compressor 20 via the drive shaft 25.

[0069] The power turbine 23 is independent of the gas generator and comprises one or more axial and / or centripetal expansion stages. The rotor(s) of the power turbine 23 are rotationally connected to the output shaft 7 of the turboshaft engine 6. The output shaft 7 is rotationally connected to the gear train 10 of the gearbox 8 so as to drive the output freewheel 9 of the gearbox 8.

[0070] As illustrated in Figure 1, the gear train 10 of the gearbox 8 transmits the mechanical power delivered by the output shaft 7 of the turboshaft engine 6 to the output freewheel 9. The gear train 10 of the gearbox 8 comprises a plurality of gears located inside the housing 13 and meshed with each other. The gear train 10 has a transmission ratio of less than 1, the rotational speed of the freewheel 9 being thus lower than that of the output shaft 7.

[0071] The main transmission box 4 is located at the front of the propulsion unit 2 and more specifically at the front of the reduction gear 8. The main transmission box 4 also includes a mechanism 26 for transmitting the mechanical power received on the input shaft 15 to the main rotor 3 of the helicopter 1.

[0072] The main rotor 3 of the helicopter 1 acts as both lift and propulsion, and for this purpose comprises a rotating wing 27 with a vertical axis of rotation. The tail rotor 5 of the helicopter 1 is driven in rotation by the freewheel 9 of the reduction gear 8, notably via a tail rotor shaft 28. Alternatively, the tail rotor 5 could be driven in rotation by the main transmission 4, particularly in the case of a twin-engine aircraft 1 (or helicopter). The tail rotor shaft 28 is associated with a brake 29. The brake 29 is fixed at the rear to the housing 13 of the reduction gear 8.

[0073] As indicated above, according to the invention, the housing 13 of the reducer 8 is directly fixed to the housing 14 of the main transmission box 4. The internal shaft 11 of the free wheel 9 is rotationally linked with the input shaft 15 of the main transmission box 4 via a splined sleeve 16. The internal shaft 11 of the free wheel 9 is hollow and includes front and rear ends 17, 18 which are internally open, to allow the insertion and removal of the sleeve 16 via the rear end 18 of the internal shaft 11.

[0074] Advantageously, the sleeve 16 includes front external splines 30 which are configured to be rotationally linked with internal splines 31 of the input shaft 15 of the main transmission gearbox 4, and rear external splines 32 which are rotationally linked with internal splines 33 of the internal shaft 11 of the freewheel 9.

[0075] Advantageously, the 30-33 splines are self-engaging with pointed chamfers to ensure a shock-free blind coupling while eliminating the need to manually turn the rotors, the rotation and alignment of the 30-33 splines being induced by the progressive docking of the 30-33 splines.

[0076] Advantageously, the internal shaft 11 of the freewheel 9 is entirely housed within the housing 13 of the reduction gear 8, and in other words, no portion of the internal shaft 11 protrudes from the housing 13 of the reduction gear 8. Such an arrangement of the internal shaft 11 prevents it from coming into contact with any external element when the reduction gear 8 is mounted on the main transmission 4. Advantageously, in the same way, the input shaft 15 of the main transmission 4 is entirely housed within the housing 14 of the main transmission 4, and in other words, no portion of the input shaft 15 protrudes from the housing 14 of the main transmission 4.

[0077] The sleeve 16 can be stopped axially at the front by a shoulder 34 formed in the input shaft 15 of the main transmission box 4. Alternatively, the sleeve 16 could be stopped axially by a ring attached to the input shaft 15.

[0078] Advantageously, the sleeve 16 is stopped axially at the rear by an element 28, 35, 36 which is flanged on the internal shaft 11 of the free wheel 9. The element 28, 35, 36 can be a flange 35, 36 (or a cap or a cover) or the rear rotor shaft 28.

[0079] Advantageously, the housing 13 of the reduction gear 8 is directly fixed to at least one flange 37 of the housing 14 of the main transmission 4 via at least one upper screw 39 located in an upper part of the flange 37 and at least one lower screw 40 located in a lower part of the flange 37. The upper screw 39 is inserted from front to back from the side of the main transmission 4. Preferably, the lower screw 40 is inserted from back to front from the side of the reduction gear 8, to simplify assembly / disassembly. Alternatively, depending on access conditions, the lower screw 40 can be inserted from front to back from the side of the main transmission 4.

[0080] This insertion of the upper screw 39 simplifies its assembly / disassembly since access is available above the main transmission box 4. Inserting the upper screw 39 in the other direction (i.e. from back to front) is not possible due to the presence of the reducer 8.

[0081] The housing 13 of the reducer 8 can obviously be fixed to the main transmission box 4 via several upper screws 39 and / or several lower screws 40. Advantageously, the upper screw(s) 39 each include a head 39a located on the side of the main transmission box 4.

[0082] Advantageously, the lower screw(s) 40 each include a head 40a located on the side of the reducer 8.

[0083] Each upper screw 39 may include a rod 39b which on one hand passes through a hole through the flange 37 of the housing 14 of the main transmission 4 and on the other hand cooperates with a tapped hole formed in the housing 13 of the reducer 8. Alternatively, each upper screw 39 could be associated with a nut.

[0084] Each lower screw 40 may include a rod 40b which on one hand passes through a hole through a lower flange 41 of the housing 13 of the reducer 8 and on the other hand cooperates with a tapped hole formed in the flange 37 of the housing 14 of the main transmission 4. Alternatively, each lower screw 40 could be associated with a nut.

[0085] Advantageously, the reducer 8 is positioned on the main transmission box 4 through a short centering and / or one or more positioning pins (also known by the English term "locatings").

[0086] The external splines 30 of the sleeve 16 and the internal splines 31 of the input shaft 15 form a front splined joint 42. The front splined joint 42 may include at least one visual indicator 43 configured to quantify wear on the front splined joint 42. Such a visual indicator 43 allows the wear of the front splined joint 42 to be monitored without disassembly. The visual indicator 43 can, for example, be inspected using an endoscope.

[0087] The grooved front link 42 can obviously include several visual indicators 43.

[0088] Each visual indicator 43 can include a first reference (for example a line) formed on the external surface of the input shaft 15 at one of the internal splines 31 and a second reference (for example a line) formed on the external surface of the sleeve 16 at the corresponding tooth 60. To quantify the wear of the front splined joint 42, the splines 30, 31 are brought into contact, the circumferential (or tangential) gap of the references from each other representing the wear of the front splined joint 42.

[0089] The rear external splines 32 of the sleeve 16 and the internal splines 33 of the internal shaft 11 of the freewheel 9 form a rear splined joint 44. The rear splined joint 44 may include at least one visual indicator 45 configured to quantify the wear of the rear splined joint 44. Such a visual indicator 45 allows the wear of the rear splined joint 44 to be monitored without the need for disassembly. The visual indicator 45 can, for example, be inspected using an endoscope.

[0090] The rear grooved link 44 can obviously include several visual indicators 45.

[0091] Each visual indicator 45 can include a first reference (for example a line) formed on the external surface of the internal shaft 11 at the level of one of the internal splines 33 and a second reference (for example a line) formed on the external surface of the sleeve 16 at the level of the corresponding tooth 62. To quantify the wear of the rear splined joint 44, the splines 32, 33 are brought into contact, the circumferential (or tangential) gap of the references from each other representing the wear of the rear splined joint 44.

[0092] When the front and rear splined links 42, 44 have the same characteristics, only one of the front and rear splined links 42, 44 can be equipped with one or more visual indicators 43, 45. Indeed, in this specific case, the wear of the front and rear splined links 42, 44 is the same.

[0093] Wear of the 30-33 splines can also be quantified via magnetic plugs, spectrometric oil analyses and particle counters.

[0094] Advantageously, the external front splines 30 of the sleeve 16 and the internal splines 31 of the input shaft 15 are lubricated via a front oil circuit 46. The front oil circuit 46 includes at least one path 48 which is defined axially between a shoulder 34 of the input shaft 15 and the front end of the sleeve 16.

[0095] Such an oil circuit before 46 allows the lubrication of the splines 30, 31 to be forced, to the benefit of their lifespan.

[0096] Advantageously, the rear external splines 32 of the sleeve 16 and the internal splines 33 of the internal shaft 11 are lubricated via a rear oil circuit 49. The rear oil circuit 49 comprises at least one axial passage 51 formed internally in the sleeve 16, a cavity 52 which is externally delimited by the internal shaft 11, and an opening 53 which connects the passage 51 and the cavity 52.

[0097] Such a rear oil circuit 49 allows the lubrication of the splines 32, 33 to be forced, to the benefit of their lifespan.

[0098] The front circuit 46 can be supplied with oil by one or more injection nozzles 47 which are specific to the front circuit 46. Similarly, the rear circuit 49 can be supplied with oil by one or more injection nozzles 50 which are specific to the rear circuit 49. Alternatively, the front and rear circuits 46, 49 can be supplied with oil by one or more injection nozzles which are common to the front and rear circuits 46, 49, this common injection nozzle or these common injection nozzles being, for example, associated with a flow distributor located in the center of the sleeve 16.

[0099] Advantageously, the oil system 71 of the reducer 8 is separate from the oil system 72 of the main transmission 4. Such a separation of the oil systems 71, 72 allows the oil viscosity to be adapted as needed, and from a maintenance point of view to quickly and easily identify whether the origin of a hydraulic problem comes from the propulsion unit 2 or from the main transmission 4.

[0100] The propulsion unit 2 typically includes various accessories (or equipment) for the operation of the propulsion unit 2 and the aircraft 1. These accessories may include, for example, one or more electric machines, one or more pumps (oil and fuel), one or more air / oil heat exchangers, etc. The various accessories are driven in rotation by the rotating part of the gas generator via a gear train (not shown) which is separate from the gear train 10. This gear train is also located inside the housing 13 of the gearbox 8.

[0101] To have good accessibility to the accessories (particularly for assembly and maintenance), some or all of them can be attached to the rear face 54 of the housing 13 of the reducer 8.

[0102] As indicated above, according to the invention, the housing 13 of the reducer 8 is directly fixed to the housing 14 of the main transmission box 4.

[0103] More specifically, as illustrated in the figures and in particular Figure 1, the housing 13 of the reducer 8 is directly fixed to a flange 37 of the housing 14 of the main transmission 4 via upper screws 39 located in the upper part of the flange 37 and lower screws 40 located in the lower part of the flange 37. The upper screws 39 are each inserted from front to back from the side of the main transmission 4, to take advantage of the available mounting space above the main transmission 4. The lower screws 40 are each inserted from back to front from the side of the reducer 8, to take advantage of the available mounting space below the reducer 8.

[0104] Each upper screw 39 comprises a head 39a located on the side of the main transmission box 4, and a shank 39b which on one hand passes through a hole opening from the flange 37 of the housing 14 of the main transmission box 4 and on the other hand cooperates with a tapped hole formed in the housing 13 of the reducer 8.

[0105] Each lower screw 40 comprises a head 40a located on the side of the gearbox 8, and a shaft 40b which, on the one hand, passes through a hole opening in a lower flange 41 of the gearbox housing 13 and, on the other hand, cooperates with a threaded hole formed in the flange 37 of the main gearbox housing 14. According to the embodiments illustrated in Figures 2 to 4, the output freewheel 9 of the gearbox 8 is free to rotate about an axis X1 which is parallel to the axis X of the turboshaft engine 6. The external gear 12 of the freewheel 9 is guided in rotation relative to the gearbox housing 13 via two front roller bearings 55 and one rear roller bearing 56. The internal shaft 11 of the freewheel 9 is guided in rotation relative to the gear 12 of the freewheel 9 via a front roller bearing 57 and a rear bearing support 58.The inner shaft 11 and the outer gear 12 of the freewheel 9 are mechanically linked to each other via a mechanism 59 configured to allow the rotation of the inner shaft 11 by the outer gear 12 and to prevent the rotation of the outer gear 12 by the inner shaft 11. The inner shaft 11 of the freewheel 9 is fully enclosed within the housing 13 of the reducer 8. The inner shaft 11 of the freewheel 9 is hollow and annular around the axis X1. The inner shaft 11 has front and rear ends 17, 18 that are internally open to allow the insertion and removal of the sleeve 16 via the rear end 18 of the inner shaft 11.

[0106] As illustrated in Figures 2 to 4, the input shaft 15 of the main transmission box 4 is housed entirely within the casing 14 of the main transmission box 4. The input shaft 15 is hollow and annular around the axis X1.

[0107] As indicated above, according to the invention, the internal shaft 11 of the freewheel 9 is rotationally linked with the input shaft 15 of the main transmission box 4 via the splined sleeve 16.

[0108] As illustrated in Figures 2 to 4, the sleeve 16 is annular and free to rotate about the axis X1. The sleeve 16 includes front external splines 30 which are rotationally linked with internal splines 31 of the input shaft 15 of the main transmission 4, and rear external splines 32 which are rotationally linked with internal splines 33 of the internal shaft 11 of the freewheel 9. The front splines 30 of the sleeve 16 form front teeth 60 which mesh with the teeth 61 formed by the splines 31 of the input shaft 15. Similarly, the rear splines 32 of the sleeve 16 form rear teeth 62 which mesh with the teeth 63 formed by the splines 33 of the internal shaft 11 of the freewheel 9.

[0109] More specifically, according to the first and second embodiments illustrated in Figures 2 and 3, the front external splines 30 of the sleeve 16 are arranged axially at a front end of the sleeve 16. The rear external splines 32 of the sleeve 16 are arranged axially between the front external splines 30 and the rear end of the sleeve 16. According to the third embodiment illustrated in Figure 4, the front external splines 30 and the rear external splines 32 of the sleeve 16 are respectively arranged axially at a front end and a rear end of the sleeve 16.

[0110] As illustrated in Figures 2 to 4, the sleeve 16 is stopped axially at the front by a shoulder 34 formed in the input shaft 15 of the main transmission box 4. More precisely, the shoulder 34 protrudes radially inside the input shaft 15.

[0111] As illustrated in figures 2 to 4, the sleeve 16 is stopped axially at the rear by an element 28, 35, 36 which is clamped on the internal shaft 11 of the free wheel 9.

[0112] More specifically, according to the first embodiment illustrated in Figure 2, the sleeve 16 is axially stopped at the rear by a flange 35 (or cover) which is jointly clamped to the internal shaft 11 with the tail rotor shaft 28, the flange 35 being located axially between the internal shaft 11 and the tail rotor shaft 28. Jointly clamping the flange 35 and the tail rotor shaft 28 allows for quick and easy identification of any omission in the mounting of the flange 35. The flange 35 is hollow and annular around the axis X1. The flange 35 includes a radial flange 64 and an axial projection 65 projecting forward, the axial projection 65 forming the axial stop of the sleeve 16. Such an embodiment is particularly suitable for a single-engine aircraft 1 (or helicopter). According to the second embodiment illustrated in Figure 3, the sleeve 16 is stopped axially at the rear by the rear rotor shaft 28.The tail rotor shaft 28 is hollow and annular around the axis X1. The tail rotor shaft 28 comprises a body 66 extending rearward from a front flange 67 and an axial tail 68 projecting forward from a front end of the body 66, the axial tail 68 forming the axial stop for the sleeve 16. The integration of the axial tail 68 into the tail rotor shaft 28 ensures the axial stop of the sleeve 16 at the rear. Such an embodiment is also particularly suitable for a single-engine aircraft 1 (or helicopter).

[0113] According to the third embodiment illustrated in Figure 4, the sleeve 16 is axially stopped at the rear by a flange 36 (or cover). The flange 36 is solid and of revolution about the axis X1. The flange 36 comprises a radial flange 69 and an axial rod 70 projecting forward, the axial rod 70 forming the axial stop of the sleeve 16. Such an embodiment is particularly suitable for a twin-engine aircraft 1 (or helicopter) (possibly without a tail rotor).

[0114] As illustrated in Figures 5 to 8, the front splined joint 42 includes an annular row of visual indicators 43, each visual indicator 43 allowing the wear of the front splined joint 42 to be quantified.

[0115] More specifically, each visual indicator 43 includes a first reference (here a dotted line) formed on the external surface of the input shaft 15 at the level of one of the internal splines 31 and a second reference (here a dotted line) formed on the external surface of the sleeve 16 at the level of the corresponding tooth 60.

[0116] As illustrated in Figures 5 and 6, when the front splined joint 42 is unworn, the reference points of each of the visual indicators 43 are aligned. Conversely, as illustrated in Figures 7 and 8, when the front splined joint 42 is worn, the reference points of each of the visual indicators 43 are circumferentially (or tangentially) offset from one another. To quantify the wear of the front splined joint 42, the splines 30 and 31 are brought into contact, the circumferential (or tangential) offset of the reference points from one another representing the wear of the front splined joint 42. The circumferential offset can be measured using an endoscope.

[0117] As illustrated in Figures 6 and 8, the rear splined joint 44 includes an annular row of visual indicators 45, each visual indicator 45 allowing the wear of the rear splined joint 44 to be quantified.

[0118] More specifically, each visual indicator 45 includes a first reference (here a dotted line) formed on the external surface of the internal shaft 11 at the level of one of the internal grooves 33 and a second reference (here a dotted line) formed on the external surface of the sleeve 16 at the level of the corresponding tooth 62.

[0119] As illustrated in Figure 6, when the rear splined joint 44 is unworn, the reference points of each of the visual indicators 45 are aligned. Conversely, as illustrated in Figure 8, when the rear splined joint 44 is worn, the reference points of each of the visual indicators 45 are circumferentially (or tangentially) offset from each other.

[0120] To quantify the wear of the rear splined joint 44, the splines 32, 33 are brought into contact, the circumferential (or tangential) gap of the references relative to each other representing the wear of the rear splined joint 44. The circumferential gap can be measured using an endoscope.

[0121] As illustrated in Figures 2 to 4, the reducer 8 includes an oil system 71 which is separate from the oil system 72 of the main transmission 4. Therefore, the oil chamber 73 of the reducer 8 is also separate (or independent) from the oil chamber 74 of the main transmission 4.

[0122] The oil system 72 of the main transmission 4 includes a front oil circuit 46 for lubricating the front splined link 42. The front oil circuit 46 includes one or more radial injection nozzles 47 and a radial path 48 which is defined axially between the shoulder 34 of the input shaft 15 and the front end of the sleeve 16.

[0123] As illustrated by the arrows in figures 2 to 4, the oil injected by the radial injection nozzle(s) 47 flows to the front grooved connection 42 via the path 48.

[0124] The oil system 72 of the main transmission 4 further includes a rear oil circuit 49 for lubricating the rear splined link 44. According to the first and second embodiment examples illustrated in Figures 2 and 3, the rear oil circuit 49 includes one or more axial injection nozzles 50, an axial passage 51, a cavity 52, and one or more openings 53 which connect the passage 51 and the cavity 52.

[0125] More precisely, the passage 51 is formed internally in the sleeve 16. The passage 51 opens at the front but not at the rear, the passage 51 being delimited at the rear by a bottom 75 formed in the sleeve 16. The cavity 52 is delimited radially by the sleeve 16 and the internal shaft 11. The cavity 52 is delimited axially by the rear splined connection 44 and a bead 76 formed in the sleeve 16. The opening(s) 53 extend radially from the passage 51 to the cavity 52.

[0126] As illustrated by the arrows in figures 2 and 3, the oil injected by the axial injection nozzle(s) 50 flows to the rear grooved connection 44, passing successively through the passage 51, the opening(s) 53 and the cavity 52.

[0127] According to the first and second embodiment examples illustrated in figures 2 and 3, the oil chamber 74 of the main transmission box 4 is sealed at the rear by a sealing gasket 77 which is received in a groove formed in the bead 76 of the sleeve 16, the sealing gasket 77 cooperating with the internal surface of the internal shaft 11 to seal the oil chamber 74 of the main transmission box 4 against the external environment.

[0128] According to the third embodiment illustrated in Figure 4, the rear oil circuit 49 includes one or more axial injection nozzles 50, an axial passage 51, a cavity 52, and an opening 53 which connects the passage 51 and the cavity 52.

[0129] More specifically, the passage 51 is formed internally in the sleeve 16. The passage 51 opens at the front and rear. The cavity 52 is radially delimited by the flange 36 and the internal shaft 11. The cavity 52 is axially delimited by the rear splined connection 44 and a bulge 78 formed in the flange 36. The opening 53 is axially defined between the rear end of the sleeve 16 and the front end of the rod 70 of the flange 36.

[0130] As illustrated by the arrows in Figure 4, the oil injected by the axial injection nozzle(s) 50 flows to the rear grooved connection 44, passing successively through the passage 51, the opening 53 and the cavity 52.

[0131] According to the third embodiment illustrated in Figure 4, the oil chamber 74 of the main transmission 4 is sealed at the rear by a seal 79 which is received in a groove formed in the bulge 78 of the flange 36. The seal 79 cooperates with the inner surface of the internal shaft 11 to seal the oil chamber 74 of the main transmission 4 against the external environment. Sealing the oil chamber 74 via the flange 36 allows for the rapid detection of any omission in the installation of the flange 36, as this would result in a significant oil leak at the rear end 18 of the internal shaft 11.

[0132] It is worth noting that the third example of implementation illustrated in the figure

[0133] 4 is particularly suited to a propulsion architecture where the rear rotor

[0134] 5 is driven in rotation by the main transmission box 4 (and not by the output freewheel 9).

[0135] In the following description, a method of mounting the propulsion group 2 on the main gearbox 4 of the aircraft 1 will be presented, the method comprising chronologically the steps of: a) fixing the housing 13 of the reducer 8 directly onto a housing 14 of the main gearbox 4; b) introducing the splined sleeve 16 via the rear end 18 of the internal shaft 11, to link in rotation the internal shaft 11 of the free wheel 9 with an input shaft 15 of the main gearbox 4.

[0136] More specifically, according to the embodiment illustrated in the figures, step a) is carried out by installing the upper screws 39 and the lower screws 40.

[0137] In addition, step b) is carried out by sliding the sleeve 16 into the internal shaft 11 from its rear end 18.

[0138] Advantageously, the process also includes a step c) consisting of axially stopping the sleeve 16 at the rear.

[0139] More specifically, according to the embodiment illustrated in the figures, step c) is carried out by clamping an element 28, 35, 36 onto the internal shaft 11 of the free wheel 9.

[0140] As described above, according to the first embodiment illustrated in Figure 2, the element is the flange 35 which is jointly flanged to the internal shaft 11 with the rear rotor shaft 28. According to the second embodiment illustrated in Figure 3, the element is the rear rotor shaft 28. According to the third embodiment illustrated in Figure 4, the element is the flange 36.

Claims

DEMANDS 1. Propulsion unit (2) for an aircraft (1) comprising: - a turboshaft engine (6) with a longitudinal axis (X) comprising an output shaft (7); - a reducer (8) arranged axially at the front of the turboshaft engine (6) and comprising an output freewheel (9) which is driven in rotation by the output shaft (7) of the turboshaft engine (6) via a gear train (10) of the reducer (8), the output freewheel (9) comprising an internal shaft (11) and an external toothed wheel (12) mechanically linked to each other; characterized in that the reducer (8) comprises a housing (13) which is configured to be directly fixed to a housing (14) of a main gearbox (4) of the aircraft (1), the internal shaft (11) of the freewheel (9) being configured to be rotationally linked with an input shaft (15) of the main gearbox (4) via a splined sleeve (16), the internal shaft (11) of the freewheel (9) being hollow and comprising front and rear ends (17, 18) which are internally open, to allow the insertion and removal of the sleeve (16) via the rear end (18) of the internal shaft (11).

2. Propulsion unit (2) according to claim 1, characterized in that the sleeve (16) comprises front external splines (30) which are configured to be rotationally linked with internal splines (31) of the input shaft (15) of the main transmission box (4), and rear external splines (32) which are rotationally linked with internal splines (33) of the internal shaft (11) of the freewheel (9).

3. Propulsion unit (2) according to any one of the preceding claims, characterized in that the internal shaft (11) of the free wheel (9) is housed entirely in the casing (13) of the reducer (8).

4. Propulsion unit (2) according to any one of the preceding claims, characterized in that the sleeve (16) is configured to be stopped axially at the front by a shoulder (34) formed in the input shaft (15) of the main transmission box (4).

5. Propulsion unit (2) according to any one of the preceding claims, characterized in that the sleeve (16) is stopped axially at the rear by an element (28, 35, 36) which is clamped on the internal shaft (11) of the free wheel (9).

6. Propulsion unit (2) according to the preceding claim, characterized in that the element (28, 35, 36) is a flange (35, 36) or a tail rotor shaft (28) which is configured to drive in rotation a tail rotor (5) of the aircraft (1).

7. Propulsion unit (2) according to any one of the preceding claims, characterized in that the housing (13) of the reducer (8) is configured to be directly fixed to at least one flange (37) of the housing (14) of the main transmission box (4) via at least one upper screw (39) located in an upper part of the flange (37) and at least one lower screw (40) located in a lower part of the flange (37), the upper screw (39) being inserted from front to back from the side of the main transmission box (4), and preferably the lower screw (40) being inserted from back to front from the side of the reducer (8) to simplify the assembly / disassembly operations.

8. Propulsion unit (2) according to the preceding claim, characterized in that the upper screw (39) comprises a head (39a) located on the side of the main transmission box (4) and the lower screw (40) comprises a head (40a) located on the side of the reducer (8). Tl 9. Propulsion unit (2) according to claim 2, characterized in that the front external splines (30) of the sleeve (16) and the internal splines (31) of the input shaft (15) form a front splined joint (42), the rear external splines (32) of the sleeve (16) and the internal splines (33) of the internal shaft (11) of the freewheel (9) form a rear splined joint (44), the front splined joint (42) comprising at least one visual indicator (43) configured to quantify the wear of the front splined joint (42) and / or the rear splined joint (44) comprising at least one visual indicator (45) configured to quantify the wear of the rear splined joint (44).

10. Propulsion unit (2) according to claim 2, characterized in that the forward external splines (30) of the sleeve (16) and the internal splines (31) of the input shaft (15) are lubricated via a forward oil circuit (46), the forward oil circuit (46) comprising at least one path (48) defined axially between a shoulder (34) of the input shaft (15) and the forward end of the sleeve (16), the rear external splines (32) of the sleeve (16) and the internal splines (33) of the inner shaft (11) being lubricated via a rear oil circuit (49), the rear oil circuit (49) comprising at least one axial passage (51) formed internally in the sleeve (16), a cavity (52) delimited externally by the inner shaft (11), and an opening (53) connecting the passage (51) and the cavity (52).

11. Method of mounting a propulsion unit (2) according to any one of the preceding claims on a main gearbox (4) of an aircraft (1), the method comprising chronologically the steps of: a) fixing the housing (13) of the reducer (8) directly onto a housing (14) of the main gearbox (4); b) Introducing the splined sleeve (16) via the rear end (18) of the internal shaft (11), to rotationally link the internal shaft (11) of the freewheel (9) with an input shaft (15) of the main gearbox (4).

12. Assembly for an aircraft (1) comprising a main gearbox (4) and a propulsion unit (2) according to any one of claims 1 to 10, the housing (13) of the reduction gear (8) being directly fixed to a housing (14) of the main gearbox (4), the internal shaft (11) of the freewheel (9) being rotationally linked with an input shaft (15) of the main gearbox (4) via a splined sleeve (16) of the assembly.

13. Aircraft (1) comprising an assembly according to the preceding claim or a propulsion group (2) according to any one of claims 1 to 10.

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