Turbomachine with transfer of power from a free turbine to a gas generator

Abstract

The invention relates to a turbomachine (1) for an aircraft, the turbomachine comprising: - a shaft (8) of a gas generator which generates a gas flow; - a power shaft (11) of a free turbine (10), the free turbine being rotationally driven by the gas flow at a free turbine rotational speed (N2); and - a drive shaft (14) of a power take-off (13), the power take-off being driven via a mechanical reduction gear (15) at a rotational speed (Ns) lower than the rotational speed (N2) of the power shaft (11). According to the invention, the mechanical reduction gear comprises an epicyclic gear train which connects the power shaft (11) of the free turbine to the shaft (8) of the gas generator so as to transfer part of the power of the power turbine to the shaft (8) of the gas generator.

Description

Description TITLE: TURBOMACHINE WITH POWER TRANSFER FROM A FREE TURBINE TO A GAS GENERATOR Technical field of the invention

[0001] The present invention relates to the field of aircraft turbomachinery. It relates in particular to a turbomachine intended to equip, in particular, an aircraft and comprising a gas generator and a free turbine. Technological background

[0002] Prior art includes documents EP1553276B1, US4064690A, WO2023 / 135378A1, and US2009 / 116955.

[0003] A turbomachine, and in particular a helicopter turboshaft engine 01 such as the one shown in longitudinal section in Figure 1, comprises a gas generator 02 which has a gas generator shaft 03 carrying a wheel 04 of a centrifugal compressor 05 and a wheel 06 of a turbine 07. Fresh air entering the turboshaft engine 01 via an air inlet 08 is compressed by the centrifugal compressor 05 and is then mixed with fuel in a combustion chamber 09 of the gas generator. The combustion of the air / fuel mixture generates a stream of burnt gas which drives the rotation of the wheel 06 of the turbine 07, which in turn drives the wheel 04 of the centrifugal compressor 05.

[0004] The turboshaft engine includes, downstream of the gas generator, a free turbine 010 or power turbine which is also driven in rotation by the gas flow generated by the gas generator. The free turbine 010 includes a power shaft 011 which is coaxial with the shaft of the gas generator, and in particular, uncoupled. The power shaft 011 of the free turbine is connected to the helicopter propeller via a mechanical speed reducer 013. In particular, the power shaft 011 includes a pinion 014 which meshes with an input gear 015 of the speed reducer 013. The latter includes an output gear 016 which is coupled to a power take-off of the helicopter 017.

[0005] This conventional speed reducer only allows the rotational speed of the 011 power shaft of the free turbine (which has a so-called high speed and forms the input of the speed reducer) to drive a drive shaft of the power take-off (forming the output of the speed reducer).

[0006] On some turboshaft engines, the efficiency of the free turbine can be higher than that of the gas generator turbine. This is because the clearance-to-blade height ratio of the free turbine wheel is lower than the clearance-to-blade height ratio of the gas generator turbine wheel, and the free turbine operates in a relatively cooler region than the gas generator turbine. Furthermore, the gas generator turbine (O2) would reach certain mechanical limits that would prevent further increases in the gas generator's compression ratio, particularly for small and medium-sized turboshaft engines that do not use cooled blades.

[0007] Therefore, there is a need to resolve all or part of the aforementioned drawbacks. Summary of the invention

[0008] The objective of the present invention is to provide a solution to improve the performance and efficiency of the gas generator while being economical and minimizing the impact on mass.

[0009] We achieve this objective in accordance with the invention by means of an aircraft turbomachine, comprising: - a gas generator shaft which generates a gas flow, - a power shaft of a free turbine, driven in rotation by the gas flow at a free turbine rotational speed, and - a drive shaft for a power take-off, driven via a mechanical gearbox at a rotational speed lower than the rotational speed of the power shaft, the mechanical reducer comprising an epicyclic gear train which connects the power shaft of the free turbine to the shaft of the gas generator so as to transfer part of the power of the free turbine to the shaft of the gas generator.

[0010] Thus, this solution achieves the aforementioned objective. In particular, the epicyclic gear reducer allows the power from the free turbine, which is more efficient than the gas generator turbine, to be distributed to both the power take-off and the generator. This "hybrid" mechanical architecture is passive because it does not involve any external actuators. Equilibrium is achieved solely through the forces acting on the gears of the mechanical reducer. This power distribution from the free turbine can then be used to reduce the load on the gas generator turbine or increase the compressor's compression ratio while maintaining the same load on the gas generator turbine. Furthermore, the proposed architecture is adaptable to turbomachinery and requires minimal structural modifications, resulting in both cost and time savings.

[0011] The turbomachine also includes one or more of the following features, taken alone or in combination: - the mechanical reducer includes a solar gear coupled to the power shaft, satellite gears meshing with the solar gear, and a satellite carrier, holding the satellite gears, which is coupled to the gas generator shaft. - the satellite gears mesh with a drive wheel connected to the drive shaft of the power take-off. - each satellite pinion includes a first set of teeth which meshes with the solar and a second set of teeth which meshes with the drive wheel of the mechanical reducer. - the mechanical reducer has a center distance between the axis of the solar and the axis of the first toothing which is equal to a center distance between the axis of the drive wheel and an axis of the second toothing. - the power transferred to the gas generator shaft is between 0% and 60%. - the transfer of power from the power shaft to the gas generator shaft is achieved when a ratio between the torque of the power take-off and the torque supplied by the power shaft is constant. - the blocking of rotation of one of the shafts among the drive shaft, the gas generator shaft and the power shaft does not hinder the rotation of the two other rotating shafts coupled to the mechanical reducer. - The mechanical reducer is arranged upstream of the gas generator shaft. - The drive shaft is connected to a helicopter main rotor. - - that the constant ratio between the torque of the drive shaft and the torque of the power shaft is a function of the ratio between the radii of the satellite gears, the drive wheel, and the solar. - - each satellite gear includes a first set of teeth and a second set of teeth which rotate at the same speed.- - the mechanical speed reducer is configured so as to distribute the torque of the power shaft between the gas generator shaft and the drive shaft 14 of the power take-off. - - The ratio between the torque of the drive shaft and the torque of the power shaft is defined by the following formula: Cs > R321.R33 C2 ~ R322.R31 With : - R321, which corresponds to the radius of the first tooth, - R322, which corresponds to the radius of the second tooth, - R33, which corresponds to the radius of the drive wheel, - R31 which corresponds to the radius of the sun.

[0012] The invention also relates to an aircraft comprising a turbomachine as described above. Brief description of the figures

[0013] The invention will be better understood, and other objects, details, features and advantages thereof will become more apparent upon reading the detailed explanatory description that follows, of embodiments of the invention given by way of purely illustrative and non-limiting examples, with reference to the accompanying schematic drawings in which: - Figure 1 is an axial cross-sectional view of an example of a prior art turbomachine; - Figure 2 is an axial cross-sectional view of an example of a turbomachine comprising a gas generator, a free turbine and a mechanical speed reducer according to the invention; - Figure 3 schematically represents a mechanical reducer coupled to a gas generator shaft, a power shaft and a drive shaft of a power take-off according to the invention; - Figure 4 is a detailed and schematic view of an example of different shafts of the turbomachine coupled together by means of gears according to the invention; - Figure 5 is an example of a graph representing the percentage of power from the power shaft transferred to the gas generator for given speed ranges according to the invention; - Figure 6 illustrates a graph representing, for a given speed mechanical reducer configuration, the evolution of the power from the power shaft transferred to a gas generator for different operating points of the gas generator and an output speed fixed at a value according to the invention; and - Figure 7 illustrates a graph representing the state of the evolution of the power of the power shaft transferred to a gas generator, as a function of three operating points of the gas generator and for a variation of the output speed Ns. Detailed description of the invention

[0014] Figure 1 has already been described previously.

[0015] Figure 2 shows a turbomachine 1, preferably a turboshaft engine, intended for use on an aircraft. Preferably, the aircraft 2 is a helicopter, for example, a single-engine aircraft. Of course, the turbomachine 1 could be a turbojet engine intended for use on an aircraft such as an airplane.

[0016] Advantageously, the helicopter includes a main rotor 3 which provides propulsion and lift. It may also include a secondary rotor (not shown), generally located at the tail of the helicopter (not shown).

[0017] The turbomachine 1 comprises a gas generator 4 which includes a compressor 5, a combustion chamber 6, and a turbine 7. Advantageously, but not exclusively, the turbine 7 of the gas generator is a high-pressure turbine. The compressor 5 includes a rotor coupled to the rotor of the high-pressure turbine. The combustion chamber 6 is located downstream of the compressor 5 and preferably radially around the turbine 7 of the gas generator 4. The high-pressure turbine may comprise one or more stages.

[0018] Advantageously, but not limitingly, the turbomachine 1 includes a gas generator shaft 8 which mechanically connects the compressor rotor 5 and turbine rotor 7 of the gas generator 4. The gas generator shaft 8 extends along a longitudinal axis X.

[0019] The turbomachine 1 includes a free turbine 10 or power turbine which is independent and separate from the gas generator 4. The free turbine 10 is advantageously arranged downstream of the gas generator 4 along the longitudinal axis X, which allows it to be driven by the gas flow generated by the gas generator 4.

[0020] The free turbine 10 is advantageously, but not exclusively, a low-pressure turbine. The free turbine 10 comprises, for example, a power shaft 11 and a free turbine rotor 12.

[0021] The low-pressure turbine may also comprise one or more stages. The power shaft 11, for example, passes through the gas generator shaft 8 4, which is preferably hollow. The power shaft 11 advantageously rotates at a speed N2 that differs from the speed N1 of the gas generator shaft 8 8. The power shaft 11 advantageously transmits a predetermined torque C2. The gas generator shaft transmits a predetermined torque C1. The speed N1 of the gas generator shaft 8 4 is typically determined by the fuel flow rate entering the combustion chamber 6. The speed N2 of the power shaft 11 can be on the order of 20,000 rpm.

[0022] The free turbine 10 is advantageously, but not exclusively, connected to the main rotor 3 of the helicopter.

[0023] In particular, the turbomachine 1 includes a power take-off 13 which is coupled to the main rotor 3 to drive the latter and possibly the secondary tail rotor. The power take-off 13 may include a power transmission gearbox known by the acronym BTP.

[0024] Advantageously, but not exclusively, the power take-off 13 includes a drive shaft 14. This shaft is, for example, coupled to the main rotor 3. The drive shaft 14 rotates, for example, at a predetermined rotational speed Ns. Optionally, the rotational speed N2 of the power shaft 11 is greater than the rotational speed Ns of the drive shaft 14.

[0025] Optionally, the power shaft 11 extends between a first end 11a and a second end 11b. The first end 11a is, for example, upstream of the gas generator 4, following the gas flow in the turbomachine 1, and is configured to be coupled to the power take-off 13. The second end 11b carries, for example, the rotor of the free turbine 10.

[0026] In Figure 3, the turbomachine 1 is equipped with a mechanical speed reducer 15. Advantageously, but not limitingly, the mechanical reducer 15 is arranged between the power take-off 13 and the power shaft 11. Advantageously, but not limitingly, the mechanical reducer 15 is arranged upstream of the gas generator 4 (and more precisely of the gas generator shaft 8).

[0027] The mechanical reducer 15 is of the epicyclic gear train type.

[0028] Advantageously, the mechanical reducer 15 includes a sun gear 18 (or internal planet gear), which is a rotating pinion. In this example, the sun gear 18 is connected to the power shaft 11. In particular, the sun gear 18 is coupled to the first end 11a of the power shaft 11. The sun gear 18 has an axis that advantageously coincides with the axis of the power shaft 11 of the free turbine 10. The power shaft 11 is coupled to the sun gear 18, for example, by means of splines, so that the power shaft 11 is rotationally fixed to the sun gear 18. The power shaft 11 thus drives the sun gear 18 in rotation.

[0029] The mechanical reducer 15 includes planet gears 19 that cooperate with the sun gear 18. Each planet gear 19 has a first set of teeth 20a that meshes with the sun gear 18. The planet gears 19 are advantageously, but not exclusively, rotatable. The planet gears 19 are, for example, regularly spaced around the axis of the sun gear 18. Each planet gear 19 has an axis that is parallel to the axis of the sun gear 18. There are, for example, between three and five planet gears 19. Each planet gear 19 rotates freely about its own axis (for example, by means of plain bearings (not shown)) and about the axis of the sun gear 18.

[0030] The first gear 20a and the solar 18 form a first stage of the mechanical reducer 15 for example.

[0031] Following this embodiment, the planetary gears 19 advantageously, but not exclusively, include a second set of teeth 20b which meshes with a toothed drive wheel 23 of the mechanical reducer 15. The drive wheel 23 is advantageously rotatable. The drive wheel 23 is advantageously coupled to the drive shaft 14. The coupling is achieved, for example, by means of splines so that the drive shaft 14 and the drive wheel 23 are rotationally fixed and the drive wheel 23 transmits an output torque Cs to the drive shaft 14.

[0032] The second toothing 20b and the drive wheel 23 form a second stage of the mechanical reducer 15 for example.

[0033] Each planetary gear 19 comprises two teeth 20a, 20b. This configuration contributes to the epicyclic gear train architecture, enabling power distribution at least between the sun gear 18 and the drive wheel 23. The arrangement of the second tooth 20b meshing with the drive wheel 23 closely replicates the meshing of a planetary gear and an external ring gear in another type of mechanical speed reducer with an external ring gear centered on the axis of the sun gear. However, a mechanical speed reducer with an external ring gear would not allow for the compatibility of torque speeds.

[0034] Furthermore, this double pinion allows for a wider range of transmission ratios than with a satellite pinion having only one tooth.

[0035] Of course, the first gear set 20a and the second gear set 20b could each be mounted on two separate gears connected, for example, by a connecting shaft. In this way, the torque and speed from one of the first and second gear sets is transmitted to the other.

[0036] Advantageously, but not limitingly, the mechanical speed reducer 15 comprises the first gear 20a (engaging with the solar 18) and the second gear 20b (engaging with the drive wheel 23) which rotate at the same speed.

[0037] The drive wheel 23 has an axis parallel to the axis of the solar element 18. The axes of the drive wheel 23 and the solar element 18 are preferably coaxial. In this configuration, the satellite carrier 21, for example, describes a circular trajectory, and the train's architecture dictates coaxial axes for the drive shafts 14 and the power shaft 11. This gear set reduces the rotational speed Ns of the drive shaft 14 of the power take-off 13 relative to the rotational speed N2 of the power shaft 11. The rotational speed N2 of the power shaft 11 is indeed greater than the rotational speed Ns of the drive shaft 14, as explained previously. The main rotor 3 will thus have a suitable rotational speed.Advantageously, but not exclusively, the rotational speed Ns of the drive shaft 14 is almost constant (during operation, the speed may vary according to the aircraft's instructions, for example). The rotational speed Ns of the drive shaft 14 is typically on the order of 6000 rpm.

[0038] According to an advantageous feature, the power shaft 11 is connected to the gas generator shaft 8 via the mechanical reducer 15. For this purpose, the mechanical reducer 15 includes, for example, a planet carrier 21 which holds the planet gears 19. The axes of the planet gears 19 are mounted, for example, at a first end 21a of the planet carrier 21. The planet gears 19 advantageously drive the planet carrier 21 in rotation about the longitudinal axis X. The planet carrier 21 is coupled, at a second end 21b, to the gas generator shaft 8.

[0039] Referring to Figure 4, the coupling of the planet carrier 21 to the gas generator shaft 8 is achieved, for example, by means of a gear system. As a non-limiting example, the gear system comprises a gear 24 located at the second end 21b of the planet carrier 21. The gear 24 meshes, for example, with a first row of teeth 25a connected to the gas generator shaft 8. The gear 24 also optionally meshes with a gear 28 attached to a stator 17, for example, a housing of the turbomachine 1.

[0040] Advantageously, but not exclusively, the turbomachine 1 includes a double-toothed wheel 26 carrying a first row of teeth 25a and a second row of teeth 25b, which meshes with a gear 27 carried by the gas generator shaft 8. In this embodiment, the wheel 26 is traversed by the power shaft 11. This allows the speeds of the various shafts to be adjusted, particularly that of the drive shaft 14 of the power take-off 13. Alternatively, the first row of teeth 25a and the second row of teeth 25b are carried by separate, distinct wheels.

[0041] Following this architecture, the sizing can be adapted to the direction of rotation of the gas generator. The range can, for example, be between 20,000 rpm and 45,000 rpm. During the rotation of the power shaft 11, some of its power is injected or transferred to the shaft 8 of the gas generator 4 via the mechanical reducer 15 (and in particular through the gears and wheels of the mechanical reducer 15). This helps to supply power (at the same speed) to the gas generator 4 and thus reduce its fuel consumption. This also advantageously increases the efficiency of the gas generator. In other words, the work of the free turbine 10, which has the highest efficiency, is prioritized and utilized.

[0042] Advantageously, the power transferable to the gas generator shaft 8 is between 0% and 60%. Preferably, the transferable power is a function of the configuration of the mechanical speed reducer 15.

[0043] Figure 5 shows an L10 curve which represents the percentage of power transferred between the power shaft 11 of the free turbine 10 and the shaft 8 of the gas generator 4 for different given configurations.

[0044] The portion C1 of curve L10 located in zone Z1 C1 advantageously represents all operating points where power is transferred from the power shaft 11 to the gas generator shaft for given speeds of the gas generator shaft, the transmission shaft, and the drive shaft. The value of 60% of the transferred power is represented, for example, on the line X1.

[0045] The portion C2 of curve L10 located in the zone Z2 advantageously represents all the operating points where power is taken from the shaft of the gas generator with a mechanical speed reducer 15 configured other than for a transfer.

[0046] Note that for a satellite carrier rotation speed N21 of zero, there is no power transfer to the gas generator. For a satellite carrier rotation speed N21 greater than 0, the gas generator 4 supplies torque to the mechanical gearbox 15. For a satellite carrier rotation speed 21 less than 0, the mechanical gearbox 15 supplies torque to the gas generator 4.

[0047] According to a remarkable feature, the architecture of the mechanical reducer 15 with epicyclic gear train connects three shaft lines, namely, the power shaft 11, the gas generator shaft 4 8 and the drive shaft 14 of the power take-off 13. In other words, the power shaft 11 is connected to the drive shaft 14 via the mechanical reducer 15.

[0048] According to one operating mode, the rotation of the power shaft 11 causes the rotation of the planetary gears 19 around their axes, which in turn drive the planet carrier 21. The latter drives the shaft 8 of the gas generator 4. The planetary gears 19 also drive the power take-off 13. Advantageously, the planetary gears are in equilibrium under the effect of the torque supplied by the power shaft 11 and the torques transmitted to the shaft 8 of the gas generator and the drive shaft 14. The dimensions of the gears are also compatible with the speeds of these different shafts.

[0049] The mechanical reducer 15 thus comprises one input (formed by the power shaft 11) and two outputs (formed by the drive shaft 14 and the generator shaft 8).

[0050] Such an architecture also allows the torque supplied by the free turbine 10 to be distributed passively between the gas generator 8 and the power take-off 13 while remaining compatible with the technological limits of the mechanical speed reducer 15 (speed, rotation, forces, teeth).

[0051] To enable the transmission of power from the free turbine 10 to the gas generator 4, the mechanical reducer 15 has predetermined dimensions. These predetermined dimensions (which depend on the shaft speeds) for torque transfer may be based on a predetermined number of teeth on the gears and pinions, a center distance between gear stages, or a predetermined radius of the gears and pinions, etc.

[0052] For this purpose, the power shaft 11 provides a torque C2 which differs from the torque C11 of the gas generator shaft 8. The drive shaft 14 of the power take-off 13 provides a torque Cs to the main rotor 3 which is also different from the torque C21 of the satellite carrier and the torque C2 of the power shaft 11.

[0053] Advantageously, the ratio R1 between the torque Cs and the torque C2 is constant, for a given configuration (because it depends only on the characteristics of the gears of the mechanical reducer 15 as described by the formula described below).

[0054] The constant R1 ratio allows power to be transferred to the gas generator 4. In particular, the R1 ratio is defined by the following formula: Cs > R321.R33 C2 ~ R322.R31 With : - R321 which corresponds to the radius of the first tooth 20a, - R322 which corresponds to the radius of the second tooth 20b, - R33, which corresponds to the radius of the drive wheel 23, - R31 which corresponds to the solar radius 18.

[0055] With reference to figures 3 and 4, the mechanical reducer 15 has a center distance E1 between the axis of the sun gear 18 and the axis of the first toothing 20a (carried by the planetary pinion 19) which is equal to a center distance E2 between the axis of the drive wheel 23 (carried by the drive shaft 14) of the power take-off 13 and an axis of the second toothing 20b (carried by the planetary pinion 19).

[0056] The mechanical reducer 15 could allow power to be drawn from the gas generator according to another predetermined dimensioning.

[0057] Figure 6 illustrates three curves, established with the speed of the drive shaft 14 Ns being constant for different operating points. These curves show the variation in the power of the power shaft 11 injected into the gas generator by the free turbine 10 as a percentage on the ordinate axis (right side of Figure 6) as a function of the speed variations of the power shafts and the gas generator 11, 8 coupled to the mechanical speed reducer 15 on the ordinate axis (left side of Figure 6) at different operating points (the speed range of the gas generator 4 is illustrated by curve L2) on the abscissa. Curve L1 represents the variation in rotational speed N2 of the power shaft 11 in revolutions per minute (rpm). Curve L2 represents the variation in rotational speed N1 of the gas generator shaft 8 in revolutions per minute.Curve L3 represents the percentage variation of the power from the power shaft 11 injected or transferred to the gas generator shaft 8. In this embodiment, the rotational speed N2 of the power shaft 11 depends on the rotational speed Ns of the drive shaft 14, which is fixed and constant, for example at 6000 rpm, and the rotational speed N1 of the generator shaft 8 is imposed by the gas generator 4.

[0058] For a given first operating point, for example, when the rotational speed N1 of the gas generator shaft 8 is at its maximum, for example -40,000 rpm (left side of the graph), the power shaft 11 (which rotates at approximately 20,000 rpm) delivers about 11% of its power to the gas generator shaft 8. For a given second operating point, for example, when the rotational speed N1 of the gas generator shaft 8 decreases, for example to -26,000 rpm (right side of the graph), the power shaft 11 (which rotates at approximately 20,000 rpm) delivers about 7% of its power to the gas generator shaft 8. It should be noted that the variation in the rotational speed N2 of the free turbine 10 is small and almost linear. In other words, the variation is small and close to its operation in a conventional engine.

[0059] Figure 7 illustrates a graph in which the power transferred to the gas generator 4 varies according to the rotational speed Ns of the drive shaft 4 and / or the gas generator shaft 8. The rotational speed N2 of the power shaft 11 is not shown in this graph. Specifically, this graph shows three lines representing the advantageously constant rotational speed N1 of the gas generator shaft 8, which, for three selected operating points (curves L5, L6, and L7) of the gas generator 4, illustrate the percentage of power C2 from the power shaft 11 transferred to the shaft 8 of the gas generator 4 (shown on the y-axis) as a function of the rotational speed Ns of the drive shaft 14 (shown on the x-axis).

[0060] Curve L5, for example, represents the rotational speed N1 of a gas generator shaft 8 at 28,173 rpm. Curve L6 represents the rotational speed N1 of a gas generator shaft 8 at 36,751 rpm. Curve L7 represents the rotational speed N1 of a gas generator shaft 8 at 40,375 rpm. For curve L5, when the rotational speed Ns of the drive shaft 14 is 5,600 rpm, the power transferred is approximately 8.5%. Conversely, when the rotational speed Ns of the drive shaft 14 is 6,400 rpm, the power transferred is less than 8%. For curve L6, when the rotational speed Ns of the drive shaft 14 is 5,600 rpm, the power transferred is approximately 11%. Conversely, when the rotational speed Ns of the drive shaft 14 is 6400 rpm, the power transferred is around 9.8%.For curve L7, when the rotational speed Ns of the drive shaft 14 is 5600 rpm, the power transferred is approximately 12%. Conversely, when the rotational speed Ns of the drive shaft 14 is 6400 rpm, the power transferred is around 10.5%. In the proposed configuration, varying the rotational speed Ns of the drive shaft 14 within the standard range has a 1.2% impact on the power returned to the gas generator 4. The lower the rotational speed Ns, the greater the power return, which could be used to minimize fuel consumption during level flight, where the rotor speed (equivalent to the rotational speed Ns of the drive shaft 14) tends to be reduced by operators.

[0061] The configuration of the mechanical speed reducer 15 and the power shaft 11, the gas generator shaft 8 4, and the drive shaft 14 13, apart from power transfer, prevents, for example, the free turbine 10 from locking up in the event of a main rotor 3 blockage. Indeed, if the main rotor and the drive shaft 14 are blocked, the free turbine 10 continues to rotate and drive the gas generator shaft 8 4. With the drive shaft 14 blocked, the planet gears 19 rotate around the drive wheel 23, and the gas generator shaft 8 (connected to the planet carrier 21 which carries the axes of the planet gears 19) is driven in a specific direction, preferably the normal direction of rotation. In other words, the blocking of one of the shafts does not impede the rotation of the other shafts. The 15-speed mechanical reducer includes rotating gears and / or wheels which are connected to other shafts.

[0062] This configuration also advantageously assists in starting the turbomachine in cold weather (for example, temperatures below -20°C). Typically, in cold weather, lubricants are solid or very viscous, and bearings are seized, for example, creating a resistive torque on the turbomachine shafts. The exhaust gases generated by the gas generator 4 rotate the power shaft 11, which can in turn deliver power to the gas generator shaft 8 to overcome the resistive torques more quickly.

[0063] Furthermore, if the drive shaft 14 breaks, there will be a loss of power sent to the gas generator 4, which is beneficial for managing the overspeed of the gas generator 4. In particular, the breakage of the drive shaft 14 will create a zero torque condition on one of the outputs of the mechanical speed reducer. Since the mechanical speed reducer is in equilibrium under the effect of the three torques at the three shafts, a zero torque on one of the outputs will also impose a zero torque on the other shafts, which means that no power is returned to the gas generator 4.

Claims

Demands [1] Turbomachine (1) for an aircraft (2), comprising: - a gas generator shaft (8) which generates a gas flow, - a power shaft (11) of a free turbine (10), driven in rotation by the gas flow at a free turbine rotational speed (N2), and - a drive shaft (14) of a power take-off (13), driven via a mechanical reducer (15) at a rotational speed (Ns) lower than the rotational speed (N2) of the power shaft (11), characterized in that the mechanical reducer (15) comprises an epicyclic gear train which connects the power shaft (11) of the free turbine to the shaft (8) of the gas generator so as to transfer a part of the power of the free turbine to the shaft (8) of the gas generator. [2] Turbomachine (1) according to claim 1, characterized in that the mechanical reducer (15) comprises a solar (18) coupled to the power shaft (11), satellite gears (19) meshing with the solar (18), and a satellite carrier (21), holding the satellite gears (19), which is coupled to the gas generator shaft (8). [3] Turbomachine (1) according to claim 2, characterized in that the satellite gears (19) mesh with a drive wheel (23) connected to the drive shaft (14) of the power take-off (13). [4] Turbomachine (1) according to claims 2 and 3, characterized in that each satellite pinion (19) comprises a first toothing (20a) which meshes with the sun gear (18) and a second toothing (20b) which meshes with the drive wheel (23) of the mechanical reducer (15). [5] Turbomachine (1) according to claim 4, characterized in that the mechanical reducer (15) has a center distance (E1) between the axis of the sun gear (18) and the axis of the first gear (20a) which is equal to a center distance (E2) between the axis of the drive wheel (23) and an axis of the second gear (20b). [6] Turbomachine (1) according to claim 1, characterized in that the power transferred to the shaft (8) of the gas generator (4) is between 0% and 60%. [7] Turbomachine (1) according to any one of the preceding claims, characterized in that the power transfer from the power shaft (11) to the shaft (8) of the gas generator (4) is achieved when a ratio between the torque (Cs) of the power take-off (13) and the torque (C2) supplied by the power shaft (11) is constant. [8] Turbomachine (1) according to any one of claims 1 to 7, characterized in that the rotational blocking of one of the shafts among the drive shaft (14), the gas generator shaft (8) (4) and the power shaft (11) does not hinder the rotation of the other two rotating shafts coupled to the mechanical reducer (15). [9] Turbomachine (1) according to any one of claims 1 to 8, characterized in that the mechanical reducer (15) is arranged upstream of the shaft (8) of the gas generator (4). [10] Turbomachine (1) according to any one of claims 1 to 9, characterized in that the drive shaft (14) is connected to a helicopter main rotor. [11] Turbomachine (1) according to any one of claims 2 to 10, characterized in that the constant ratio between the torque of the drive shaft (14) and the torque of the power shaft (11) is a function of the ratio between the radii of the satellite gears (19), the drive wheel (23), and the sun gear (18). [12] Turbomachine (1) according to any one of claims 4 to 11, characterized in that the first toothing and the second toothing of each satellite pinion (19) rotate at the same speed. [13] Turbomachine (1) according to any one of claims 1 to 12, characterized in that the mechanical reducer (15) is configured to distribute the torque of the power shaft (11) between the gas generator shaft and the drive shaft [14] of the power take-off. [14] Aircraft comprising a turbomachine (1) according to any one of the preceding claims.

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