IMPROVED DRIVE ARRANGEMENT FOR A MULTI-ENGINE HYBRID AIRCRAFT

Description

Technical Field

[0001] The present invention relates to the field of hybrid aircraft, comprising at least two engines such as turboshaft or turboprop engines, for flying machines such as helicopters or twin-engine airplanes. In particular, the invention relates to a propulsion system for a multi-engine hybrid aircraft, especially a twin-engine one, a hybrid aircraft comprising such a propulsion system, and a method using such a propulsion system. Previous technique

[0002] As is known, a turbomachine, for example a turbomotor, especially for a helicopter, comprises a gas turbine having a gas generator and a free turbine driven in rotation by the gas flow generated by the gas generator.

[0003] Traditionally, a gas generator consists of at least one compressor and one turbine coupled in rotation. The operating principle is as follows: fresh air entering the gas turbine is compressed by the compressor's rotation before being sent to a combustion chamber where it is mixed with fuel. The exhaust gases from combustion are then expelled at high speed. A first expansion occurs in the gas generator's turbine, during which it extracts the energy necessary to drive the compressor. The gas generator's turbine does not absorb all the kinetic energy of the exhaust gases; the excess kinetic energy corresponds to the gas flow generated by the gas generator.The latter therefore provides kinetic energy to the free turbine so that a second expansion occurs in the free turbine which transforms this kinetic energy into mechanical energy in order to drive a receiving organ, such as the rotor of the helicopter.

[0004] Some aircraft have two or more turbomachines, each comprising a gas turbine as described above. This is particularly true of twin-engine or multi-engine helicopters. Such aircraft allow for operation in SEO (Single Engine Operative) mode. SEO mode is an operating mode for a twin-engine configuration in which one of the gas turbines is intentionally shut down, while the other provides all the power. This mode optimizes specific fuel consumption, which decreases with the power delivered by a turbomachine. Indeed, since the specific fuel consumption of a turbine decreases with the power delivered, it is preferable to provide 100% of the power with one turbine, rather than 50% with each of them.

[0005] One of the critical aspects of SEO mode lies in the ability to restart a stopped turbine in the event of a power loss in the operating turbine. To ensure the fastest possible restart, it is possible to keep the stopped turbine in standby mode (or "super idle"), meaning that the gas generator continues to run solely using an electric motor, without any fuel input. The generator is then maintained within its "ignition window" (typically 10-30% of the gas generator's nominal rotational speed) to allow for immediate fuel ignition. Alternatively, in "super idle" mode, combustion occurs at a low speed threshold, assisted by mechanical power input from an electric motor, thus benefiting from a combustion chamber that is already ignited but with a controlled internal temperature.FR 3 115 825 A1, WO 95 / 02120 A1, US 2007 / 151258 A1 and WO 2022 / 101586 A1 disclose examples of turbomachinery including electrical machines.

[0006] Twin-engine applications involve numerous functions, such as starting the gas generator, generating onboard electricity, and providing mechanical power to the main rotor. Existing solutions for some of these functions are not entirely satisfactory. In particular, they do not provide the necessary redundancy for starting the engine in standby mode, nor do they limit the impact of electrical load on the engine when the helicopter is not operating in standby mode. Therefore, there is a need for a propulsion system for twin-engine or multi-engine aircraft with an architecture that at least partially addresses the aforementioned drawbacks. Description of the invention

[0007] This presentation concerns a propulsion system for a hybrid aircraft, in particular a multi-engine helicopter, comprising: at least one first engine and a second engine each having a gas generator and a free turbine driven in rotation by a gas flow generated by the gas generator, a main rotor coupled to the free turbine of the first and second engines, the first engine comprising a first electric machine and a second electric machine of lower power than the first electric machine, one of the first or second electric machines being capable of being coupled to the gas generator and of rotating the gas generator during an engine start-up phase, and being further capable of being coupled to the free turbine in order to generate electrical power after the start-up phase, the other of the first or second electric machine being coupled to the gas generator only.

[0008] The main rotor can be coupled to the free turbine of the first and second motors via a first and second main coupling means, respectively. Furthermore, a shaft of the free turbine of each of the first and second motors can be directly connected to the main rotor, or via a mechanical gearbox.

[0009] It is understood that, according to the present description, the first engine at least is equipped with a first electric machine and a second electric machine. In some embodiments, the second engine includes a third electric machine, preferably with a lower power output than the first electric machine, capable of driving the gas generator of the second engine during a starting phase, and of being driven by said gas generator in order to generate electrical energy after the starting phase.

[0010] Alternatively, the second motor could be equipped with the first and second electric machines, and the first motor could then be equipped with the third electric machine, or each of the first and second motors could be equipped with two electric machines symmetrically.

[0011] While one or the other of the first or second electric machines can be coupled to the gas generator and / or the free turbine depending on the operating phases and engine architecture, the other electric machine is coupled only to the gas generator. Furthermore, the first electric machine is sized to provide more power than the second electric machine, particularly during rapid start-up phases. The first electric machine is therefore high-power, on the order of one or several hundred kilowatts, whereas the second electric machine (and the third electric machine of the second engine, if applicable) is low-power, on the order of 10 kW, such as a generator / starter typically used in aircraft engines.

[0012] According to this configuration, one of the first or second electric machines can be coupled to the free turbine, at least after the turboshaft engine's start-up phase, to generate electrical power. In other words, during flight, the free turbine advantageously drives the rotation of the first or second electric machine, which operates as an electrical generator, such that the kinetic energy intended for conversion into electrical energy is advantageously drawn from the free turbine, and not from the gas generator. This makes it possible to supply electricity while limiting the impact on engine efficiency. As a result, the turboshaft engine according to the invention advantageously provides electricity without significantly compromising its efficiency.

[0013] It should be noted that in flight, when electrical generation is carried out by the said first or second electric machine driven by the free turbine of the first engine, the third electric machine, if necessary, driven by the gas generator of the second engine, may be in redundancy or in supply of additional electrical power.

[0014] Furthermore, during SEO operation, when the second motor alone drives the main rotor, the first or second electric machine of the first motor, operating in standby mode, can be used to keep that motor's gas generator rotating at low speed. The first high-power electric machine can also be used for a rapid restart of the motor from standby mode, particularly in emergencies. It is also possible to use both electric machines simultaneously to increase the power supplied to the gas generator during the rapid restart phase, thus reducing the restart time. Additionally, the first or second electric machine can be used independently to perform a normal restart, thereby providing redundancy.

[0015] Therefore, the multi-engine architecture described herein offers the advantage of simplicity by limiting the number of components and connections. In particular, it improves the reliability of engine start-up in standby mode within the SEO (Self-Operating Engine) configuration, and optimizes engine performance by drawing power from the free turbine via one of the electric machines after the start-up phase in conventional twin-engine operation. This allows for streamlining and optimizing the number of electric machines and the overall propulsion system architecture, while enabling a high number of functions and reducing the system's mass.

[0016] In some embodiments, the first electric machine is coupled to a shaft of the gas generator via first switchable coupling means configured to be activated during the start-up phase, and to be deactivated after the start-up phase.

[0017] By "switchable coupling means" it is understood that the coupling means can be in an activated position in which the components connected to said coupling means are coupled, or in a deactivated position in which said components are decoupled, it being understood that "component" means the electrical machines, the main rotor and the gas generator.

[0018] According to the invention, the first electric machine is preferably reversible and operates as an electric motor when the first coupling means are activated, so as to drive the gas generator during the engine start-up phase. Correspondingly, the first reversible electric machine operates as an electric generator so as to produce electricity by extracting kinetic energy from the free turbine, after the engine start-up phase, i.e., essentially in flight. During this post-engine start phase, the first coupling means are deactivated so that the gas generator cannot drive the first electric machine.

[0019] It is understood that, according to this embodiment, the second electric machine is coupled only to the gas generator. However, it should be noted that a configuration in which the second and first electric machines are reversed is also possible.

[0020] In some embodiments, the first electric machine is coupled to a shaft of the free turbine by being in direct contact with it.

[0021] By "direct drive", we understand that the first electric machine is coupled to the shaft of the free turbine by a simple shaft, or possibly via gears, but that no means of disabling coupling (for example a free wheel) is arranged between the first electric machine and the shaft of the free turbine.

[0022] In other words, while the first electric machine is coupled to the gas generator shaft via switchable couplings, which can be deactivated after the start-up phase, the second electric machine is directly coupled to the free turbine shaft, such that the electric machine also drives the free turbine during engine start-up and remains permanently connected to it. This architecture reduces the overall mass of the propulsion system.

[0023] In some embodiments, the first electric machine is coupled to a shaft of the free turbine via second switchable coupling means, the first and second switchable coupling means being configured so as not to be activated simultaneously.

[0024] When the second coupling means are activated, the first coupling means are deactivated; that is, the first electric machine is coupled to the free turbine while being decoupled from the gas generator. Conversely, when the first coupling means are activated, the second coupling means are deactivated; that is, the first reversible electric machine is coupled to the gas generator while being decoupled from the free turbine. Without departing from the scope of the invention, an intermediate position can also be provided in which both the first and second coupling means are deactivated simultaneously.

[0025] The first electric machine operates as an electric motor when the first coupling means are activated, so as to rotate the gas generator during the engine start-up phase. Correspondingly, the first reversible electric machine operates as an electric generator when the second coupling means are activated, so as to produce electricity by extracting kinetic energy from the free turbine, and this occurs after the engine start-up phase, that is, essentially during flight. In other words, the first electric machine is only coupled to the free turbine after the start-up phase.

[0026] Because the first and second coupling means cannot be activated simultaneously, the undesirable situation in which the free turbine drives the gas generator in rotation is avoided.

[0027] In some embodiments, the first switchable coupling means include a first freewheel, the second switchable coupling means include a second freewheel, and the first and second freewheels are mounted in opposition.

[0028] One advantage of a freewheel is that it does not require electronic or mechanical control by an external operator. Such a freewheel typically consists of a hub and a peripheral ring mounted to rotate on the hub. The hub can rotate the peripheral ring, but not the other way around. Therefore, the hub can only drive the ring when it rotates in a predetermined direction, referred to as the "direction of engagement." Otherwise, the hub and the peripheral ring rotate freely relative to each other. In this case, the disengageable coupling means are activated when the freewheel hub rotates the peripheral ring, and conversely, the disengageable coupling means are deactivated when the freewheel hub does not rotate the peripheral ring.

[0029] By "opposite mountings", we mean that the first free wheel can transmit a rotational torque from the first electric machine, while the second free wheel can transmit a rotational torque towards the first electric machine.

[0030] In some embodiments, the first switchable coupling means comprise a first reducer having a first reduction coefficient, while the second switchable coupling means comprise a second reducer having a second reduction coefficient, and the ratio of the first and second reduction coefficients is less than a limit value.

[0031] A reduction gear is defined as one or more reduction stages, comprising, for example, gear trains. Such reduction gears are known elsewhere. Since the gas generator and the free turbine generally rotate significantly faster than the first electrical machine, the reduction gear allows the rotational speed of the electrical machine to be adapted to the speeds of the gas generator and the free turbine.

[0032] Preferably, the limit value is chosen so that the first and second freewheels are not engaged simultaneously. Preferably, this limit value is proportional to the ratio of the rated speed of the gas generator to the rated speed of the freewheel turbine.

[0033] In some embodiments, when the second engine alone drives the main rotor, the gas generator of the first engine is kept in standby mode, via the first or second electric machine.

[0034] When the aircraft is operating in SEO mode, the second engine, for example, provides all the power, while the first engine is intentionally shut down, or preferably placed in standby mode, to optimize specific fuel consumption. Standby mode keeps the first engine's gas generator within 5 to 30% of its rated speed, allowing for a quick restart if necessary, particularly if the second engine shuts down unintentionally.

[0035] It should be noted that when the first electric machine is directly coupled to the shaft of the free turbine and used to keep the gas generator of the first engine in standby mode, it inevitably drives the free turbine as well. This configuration is made possible by the fact that the free turbine of the first engine rotates at a lower speed than the main rotor, the first main coupling means being deactivated.

[0036] In some embodiments, the second electric machine is coupled to the gas generator only, via a freewheel.

[0037] Therefore, the second electric machine can drive the gas generator during startup, but conversely, the gas generator cannot drive the second electric machine. The latter thus remains idle when not in use. According to this configuration, only the first electric machine can generate electrical energy.

[0038] This presentation also relates to a hybrid aircraft comprising a propulsion system according to any of the preceding embodiments, the hybrid aircraft being a multi-engine helicopter, in particular a twin-engine one.

[0039] The term "hybrid aircraft" refers to an aircraft comprising a thermal engine capable of driving a main rotor in rotation, and at least one electric machine capable of providing power to the thermal engine.

[0040] The present exposition also relates to a method for optimizing the operation of a multi-engine aircraft using a propulsion system according to any of the preceding embodiments, in which, during a start-up phase of the first engine, the first electric machine and / or the second electric machine drive the gas generator of said first engine, and after the start-up phase, the free turbine of said first engine drives one of the first or second electric machines in order to generate electrical power.

[0041] In some embodiments, the second motor is capable of operating alone, the first motor then operating in a standby mode by being driven at idle speed by the first or second electric machine, the first motor operating in standby mode being restarted by the first electric machine at least during a rapid restart phase.

[0042] Since the power of the first electric machine is on the order of several tens to a few hundred kilowatts, it is possible to start the gas generator much more quickly by using the first electric machine at least during a rapid restart phase.

[0043] In some embodiments, during a start-up phase of the first motor, the first electric machine and / or the second electric machine drive the gas generator of said first motor without driving the free turbine, when the first electric machine is coupled to the shaft of the free turbine by means of the second switchable coupling means comprising the second free wheel.

[0044] In a normal ground start or when exiting SEO mode, given the presence of the second switchable coupling means, neither the first nor the second electric machine can drive the free turbine shaft. Therefore, either or both electric machines can be used in SEO mode to keep the first engine's gas generator in standby without driving the free turbine. Similarly, during a normal ground start or a normal in-flight restart—that is, outside of an emergency situation—either or both electric machines can be used without driving the free turbine.

[0045] In some embodiments, the second motor is capable of operating on its own, and, when the first electric machine is coupled to the shaft of the free turbine of the first motor by being directly connected to it, the first motor, then operating in a standby mode, is driven at idle speed by the second electric machine coupled to the gas generator only, or by the first electric machine, the main rotor being coupled to the free turbine of the first motor by means of a main coupling means, the main rotor driven by the second motor rotating at a higher speed than the shaft of the free turbine of the first motor such that the main coupling means is deactivated.

[0046] In this SEO (Self-Employed) operating configuration, given the absence of switchable coupling means between the first electric machine and the free turbine, the first electric machine is permanently coupled to the free turbine and therefore drives the free turbine shaft even when it is used to keep the first motor in standby mode at idle. This configuration is made possible by the fact that the free turbine of the first motor rotates at a lower speed than the main rotor, with the main coupling means being deactivated. Furthermore, driving the gas generator at idle with the first electric machine, thereby driving both the free turbine and the gas generator, allows for faster reconnection of the free turbine shaft to the main rotor in case of a rapid start-up.

[0047] Alternatively, the second electric machine coupled to the gas generator only can be used to drive the gas generator of the first engine at idle, without driving the free turbine.

[0048] Moreover, in this scenario as well, the first motor operating in standby mode can be restarted by the first electrical machine at least, during a rapid restart phase.

[0049] In some embodiments, when the first electric machine is coupled to the shaft of the free turbine of the first motor by being in direct contact with it, the first motor is started by that of the first or second electric machine coupled to the gas generator only when the second motor is stopped, or by the first and / or second electric machine when the second motor has been previously started.

[0050] In the case of an initial ground start, with the main rotor stopped, the first electric machine directly connected to the free turbine shaft cannot be used to drive the gas generator and start the first engine, as the inertia and resistive torque of the main rotor are too great. Therefore, the second electric machine, coupled only to the gas generator, is used to start the first engine. Conversely, when the second engine has already started, driving the main rotor, the first electric machine can be used to start the gas generator of the first engine, while simultaneously driving the free turbine. The first engine then starts with its turbines linked. Given this architecture and the presence of the two electric machines, it is possible to optimize the operation of the multi-engine aircraft and its starting process according to various scenarios. Brief description of the drawings

[0051] The invention and its advantages will be better understood upon reading the detailed description below of various embodiments of the invention, given by way of non-limiting examples. This description refers to the accompanying figure pages, on which: [ Fig. 1 ] There figure 1 represents a cross-sectional view of a propulsion system for a twin-engine aircraft according to a first embodiment, [ Fig. 2 ] There figure 2 represents a functional diagram of the propulsion system of the figure 1 , [ Fig. 3 ] There figure 3 represents a cross-sectional view of a propulsion system for a twin-engine aircraft according to a second embodiment, [ Fig. 4 ] There figure 4 represents a functional diagram of the propulsion system of the figure 3 . Description of the implementation methods

[0052] An architecture of a propulsion assembly 100 according to a first embodiment of the invention will be described in the remainder of the description, with reference to figures 1 And 2 .

[0053] It should be noted that, for the sake of clarity, the figures schematically represent a simplified, functional architecture of the device, without showing all the details of the components constituting the turbomachines and the various power transmission elements. In particular, the gears P that drive the shafts 13 and 14 with the electric machines, and vice versa where applicable, are shown schematically, and any speed ratios are not shown.

[0054] There figure 1 This schematically represents a propulsion system 100 of a twin-engine aircraft, comprising a first engine, in this example a first turbomachine 1, and a second engine, in this example a second turbomachine 2, driving in rotation the transmission components 60 of a helicopter carrying a propeller or a main rotor 62. The turbomachines may be turboshaft engines or turboprop engines. Although the propulsion system described below comprises two turbomachines, this example is not limiting, as the invention also applies to propulsion systems of multi-engine aircraft comprising more than two engines.

[0055] The first turbomachine 1 and the second turbomachine 2 are largely identical and share common characteristics. Therefore, the description below refers to both the first and second turbomachines 1 and 2 for the characteristics they have in common.

[0056] The first turbomachine 1 and the second turbomachine 2 respectively comprise a gas turbine 10, 20 having a gas generator 12, 22 and a free turbine 11, 21 capable of being driven into rotation by a gas flow generated by the gas generator 12, 22. The free turbine 11, 21 is mounted on a shaft 13, 23 which transmits the rotational motion to a receiving element such as a main rotor 62 of the helicopter via the transmission elements 60. According to this example, the gas turbine 10, 20 shown in the figure 1 is of the front-drive type with coaxial shaft drive. Without departing from the scope of the present invention, one could also consider a free-turbine gas turbine of the front-drive type with internal or external shaft drive, or a free-turbine turbomachine of the rear-drive type. Similarly, the turbine can be directly driven or incorporate a speed reducer 51, 52 without altering the principle of the invention.

[0057] The gas generator 12, 22 comprises a rotating shaft 14, 24 on which are mounted a compressor 15, 25 and a turbine 16, 26, as well as a combustion chamber 17, 27 arranged axially between the compressor 15, 25 and the turbine 16, 26 when the gas generator 12, 22 is considered along the axial direction of the rotating shaft 14, 24. The gas turbine 10, 20 has a casing 18, 28 equipped with an air inlet 19, 29 through which fresh air enters the gas generator 12, 22. After its admission into the chamber of the gas generator 12, 22, the fresh air is compressed by the compressor 15, 25 which forces it towards the inlet of the combustion chamber 17, 27 where it is mixed with fuel. The combustion which takes place in the combustion chamber 17, 27 causes the burnt gases to be evacuated at high speed towards the turbine 16, 26, which in turn causes the shaft 14, 24 of the gas generator 12, 22 to rotate and, consequently, the compressor 16, 26.The rotational speed of the shaft 14, 24 of the gas generator 12, 22 is determined by the fuel flow entering the combustion chamber 17, 27.

[0058] Despite the extraction of kinetic energy by turbine 16, 26, the gas flow exiting the gas generator possesses significant kinetic energy. As can be understood from the figure 1 , the gas flow F is directed towards the free turbine 11, 21 which has the effect of causing an expansion in the free turbine 11, 21 leading to the rotation of the turbine wheel and the shaft 13, 23.

[0059] The main rotor 62 is coupled, via the transmission elements 60, to the shaft 13 of the free turbine 11 of the first gas turbine 10 by means of a first main coupling means 510 disposed between the speed reducer 51 and the transmission elements 60. The main rotor 62 is also coupled, via the transmission elements 60, to the shaft 23 of the free turbine 21 of the second gas turbine 20 by means of a second main coupling means 520 disposed between the speed reducer 52 and the transmission elements 60.

[0060] Preferably, the first and second main coupling means 510, 520 include a freewheel mounted such that the rotation of the shaft 13, 23 can drive the main rotor 62, but conversely, the rotation of the main rotor 62 cannot drive the shaft 13, 23 of the free turbine 11, 21. In other words, the freewheel of the first and second main coupling means 510, 520 can only transfer rotational torque from the free turbine 11, 21 to the main rotor 62, but not the other way around. On a helicopter, this freewheel is commonly called a "motor freewheel." It should be noted that the use of a freewheel for the main coupling means 510, 520 is not restrictive; the freewheel can be replaced by any dog ​​clutch or clutch system.

[0061] The turbomachine 1 further comprises a first electric machine 30, preferably reversible and including an electric motor capable of reversibly operating as an electric generator. The first electric machine 30 is mechanically coupled to the shaft 14 of the gas generator 12 by means of first switchable coupling means 310.

[0062] The first switchable coupling means 310 include a first freewheel 312 and, preferably, a first speed adaptation reducer 314 disposed between the gas generator 12 and the first freewheel 312.

[0063] The first freewheel 312 is mounted such that the rotation of the first electric machine 30, operating in motor mode, can drive the gas generator 12 in rotation, but conversely, the rotation of the gas generator 12 cannot drive the first electric machine 30. In other words, the freewheel 312 of the first switchable coupling means 310 can only transfer a rotational torque in the direction from the first electric machine 30 to the gas generator 12.

[0064] The first electric machine 30 is further mechanically coupled to the shaft 13 of the free turbine 11 by means of second switchable coupling means 320.

[0065] The second switchable coupling means 320 include a second free wheel 322 and, preferably, a second speed adaptation reducer 324 disposed between the free turbine 11 and the second free wheel 322.

[0066] The second free wheel 322 is mounted such that the rotation of the free turbine 11 can drive the first electric machine 30 in rotation, which is then operating in generator mode, but conversely, the rotation of the first electric machine 30 cannot drive the free turbine 11. In other words, the free wheel 322 of the second switchable coupling means 320 can only transfer a rotational torque in the direction from the free turbine 11 to the first electric machine 30.

[0067] The first and second freewheels 312 and 322 are mounted in opposition. In this case, they have opposite directions of engagement. Thus, when the first reversible electric machine 30 operates in motor mode to drive the shaft 14 of the gas generator 12 in rotation (first freewheel 312 engaged, i.e. first coupling means 310 activated), the second freewheel 322 does not transmit the rotational torque of the first electric machine 30 to the shaft 13 of the free turbine 11 (second coupling means 320 deactivated). Conversely, when the shaft 13 of the free turbine 11 drives the first electric machine 30 operating as an electric generator (second free wheel 322 engaged, i.e. second coupling means 320 activated), the first free wheel 312 does not transmit the rotational torque of the first electric machine 30 to the shaft 14 of the gas generator 12 (first coupling means 310 deactivated).

[0068] The first speed matching reducer 314 has a first reduction coefficient K1 preferably chosen so that the speed of the first electric machine 30 is adapted to the speed range required for starting the gas generator 12. The second speed matching reducer 324 has a second reduction coefficient K2 preferably chosen so that the speed of the first electric machine 30 is adapted to the speed range required to allow the supply of electricity.

[0069] To prevent the free turbine 11 from rotating the shaft 14 of the gas generator 12, the first freewheel 312 must not be engaged. To achieve this, the reduction ratios K1 and K2 of the first gearbox 314 and the second gearbox 324 are chosen appropriately. For example, but not necessarily, the reduction ratios K1 and K2 follow the ratios described in document FR2929324.

[0070] The turbomachine 1 also includes a second electric machine 32 mechanically coupled to the shaft 14 of the gas generator 12 only, preferably via a third free wheel 33 allowing the second electric machine 32 not to be driven by the gas generator when not in use.

[0071] In this example, while the first electrical machine 30 is a high-power electric machine, specifically one with a capacity of several hundred kilowatts, the second electrical machine 32 can be a commonly used starter, with a power rating of around 10 kW. It should be noted, however, that this configuration is not limiting; the first and second electrical machines can be reversed without departing from the scope of the invention.

[0072] In addition, the turbomachine 2 can be equipped with a third electric machine 40, preferably also of lower power than the first electric machine 30, for example a starter equivalent to the second electric machine 32. The third electric machine 40 is coupled only to the gas generator 22, and is capable of driving the gas generator 22 during a starting phase, and of being driven by said gas generator 22 after the starting phase in order to generate electrical energy.

[0073] This architecture of the propulsion assembly 100 allows different modes of operation described in the rest of the description, and allows the operation of the propulsion assembly 100 to be optimized.

[0074] The presence of the two turbomachines 1 and 2 allows for operation in SEO (Single Engine Operative) mode, in which only the second turbomachine 2 is running, providing all the power to the main rotor 62, while the first turbomachine 1 is intentionally shut down. When SEO mode is engaged, the gas generator 12 is put into standby (or assisted super-idle), meaning it no longer supplies power. However, to start as quickly as possible, the gas generator 12 is driven into the ignition window (within a range of 5 to 30% of its nominal rotational speed) by the first electric machine 30 or by the second electric machine 32.

[0075] Based on this SEO (Self-Effective Operation) mode, it is possible to restart the first turbomachine 1 during a normal restart, i.e., outside of an emergency situation requiring a rapid restart, by optimizing the aircraft's operation. This optimization process initially involves, during the startup phase of the first turbomachine 1, driving the gas generator 12 with the first electric machine 30 and / or the second electric machine 32, without driving the free turbine 11. Indeed, given the presence of the second free wheel 322, mounted in opposition to the first free wheel 312, the first electric machine 30 cannot drive the free turbine 11. Furthermore, the second electric machine 32 is coupled only to the gas generator 12 and therefore cannot drive the free turbine 11.

[0076] The first and second electric machines 30, 32 can be controlled by a control unit (not shown). Thus, the first and / or second electric machines 30, 32 drive the gas generator 12 via the freewheels 312, 33 respectively, enabling the gas generator 12 to start. During the start-up of the gas generator 12, the hot gases drive the free turbine 11, the latter being connected to the main rotor 62 via the first freewheel 312, the speed reducer 51 and the freewheel of the first main coupling means 510.

[0077] In a second step, after the start-up phase, the free turbine 11 drives the first electric machine 30 via the second free wheel 322 to generate electrical power. The first electric machine 30 can be electrically connected to the propulsion system 100's electrical network to power various electrical equipment (not shown). It should be noted that the above steps also apply to a normal start-up (and not a restart from SEO mode) of the turbomachine 1, and outside of emergency situations.

[0078] In addition, after the start-up phase and when both gas generators 12, 22 are in operation (for example after the turbomachine 1 is restarted to exit SEO mode), the third electric machine 40 can generate electrical power in redundancy with the first electric machine 30.

[0079] Note that in the example illustrated on the figure 2 Given the presence of the third freewheel 33, the second electric machine 32 cannot be driven to operate in generator mode, and consequently only the first electric machine 30 can generate electrical energy. However, in the absence of the third freewheel 33, the first 30 and second 32 electric machines would each generate electrical energy mutually without departing from the scope of the invention.

[0080] Furthermore, it is also possible to perform a rapid restart of the first turbomachine 1 from standby mode. This rapid restart is identical to the startup or restart step described above, but the first high-power electric machine 30 is necessarily used in this scenario, either alone or possibly supplemented by the second electric machine 32 to optimize reactivation time. Indeed, since the power of the first electric machine 30 is on the order of several tens to a few hundred kilowatts, it is possible to start the gas generator 12 much faster than with a starter with a power of around 10 kW, which is usually used. This provides a particular operational advantage in the case of medical rescue missions, or during rapid restart attempts in flight.

[0081] An architecture of a 100' propulsion system according to a second embodiment of the invention will be described in the remainder of the description, with reference to figures 3 And 4 .

[0082] The characteristics of the propulsion assembly 100' according to this second embodiment are largely identical to those of the propulsion assembly 100 according to the first embodiment, and will not be repeated. The operating modes described above with reference to the first embodiment are also applicable to the second embodiment.

[0083] The propulsion assembly 100' according to the second embodiment differs, however, from the propulsion assembly 100 according to the first embodiment in that it does not include second switchable coupling means. In other words, the first electric machine 30 is directly coupled to the shaft 13 of the free turbine 11, and is therefore permanently connected to it, including during start-up phases.

[0084] Given this architecture, during SEO operation, the gas generator 12 of the turbomachine 1 can be kept at idle speed by the second electric machine 32 so as not to drive the free turbine 11, or by the first electric machine 30. In this second case, the shaft 13 of the free turbine 11 is also driven. However, the main rotor 62 is driven by the turbomachine 2 and rotates at a higher speed than the shaft 13 of the free turbine 11, such that the main coupling means, and in particular the freewheel 510, is deactivated.

[0085] Similarly, during a first ground start, it is possible to start the gas generator 12 using the first electric machine 30, if the turbomachine 2 has been previously started and is already driving the main rotor 62. On the other hand, if the turbomachine 2 has not yet been started, the gas generator 12 of the turbomachine 1 is started using the second electric machine 32 which is coupled to the gas generator 12 only.

[0086] Although the present invention has been described with reference to specific embodiments, it is evident that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. Therefore, the description and drawings should be considered illustrative rather than restrictive.

[0087] It is also evident that all the characteristics described with reference to a process are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a process.

Claims

1. A propulsion assembly (100) for a hybrid aircraft, in particular a multi-engine helicopter, comprising: - at least a first engine (1) and a second engine (2) each having a gas generator (12, 22) and a free turbine (11, 21) driven in rotation by a gas stream generated by the gas generator, - a main rotor (62) coupled to the free turbine (11, 21) of the first and second engines (1, 2), - characterized in that the first engine (1) comprises a first electric machine (30) and a second electric machine (32) of lower power than the first electric machine (30), one of the first or of the second electric machine (30, 32) being able to be coupled to the gas generator (12) and to set the gas generator in rotation during a start phase of the engine, and being further able to be coupled to the free turbine (11) in order to generate electrical energy after the start phase, the other of the first or of the second electric machine being coupled to the gas generator (12) only.

2. The propulsion assembly (100) according to claim 1, wherein the first electric machine (30) is coupled to a shaft (14) of the gas generator (12) via first deactivatable coupling means (310) configured to be activated during the start phase, and to be deactivated after the start phase.

3. The propulsion assembly (100) according to claim 2, wherein the first electric machine (30) is coupled to a shaft (13) of the free turbine (11) by being in direct engagement therewith.

4. The propulsion assembly (100) according to claim 2, wherein the first electric machine (30) is coupled to a shaft (13) of the free turbine (11) via second deactivatable coupling means (320), the first and second deactivatable coupling means being configured so as not to be activated simultaneously.

5. The propulsion assembly (100) according to claim 4, wherein the first deactivatable coupling means (310) comprise a first free wheel (312), the second deactivatable coupling means (320) comprise a second free wheel (322), and the first and second free wheels are mounted in opposition.

6. The propulsion assembly (100) according to claim 4 or 5, wherein the first deactivatable coupling means (310) comprise a first reduction gear (314) having a first reduction coefficient (K1), while the second deactivatable coupling means (320) comprise a second reduction gear (324) having a second reduction coefficient (K2), and the ratio of the first and second reduction coefficients (K1, K2) is smaller than a limit value.

7. The propulsion assembly (100) according to any one of claims 1 to 6, wherein, when the second engine (2) alone drives the main rotor (62), the gas generator (12) of the first engine (1) is kept in a standby mode, via the first or the second electric machine (30, 32).

8. The propulsion assembly (100) according to any one of claims 1 to 7, wherein the second electric machine (32) is coupled to the gas generator (12) only, via a free wheel (33).

9. The propulsion assembly (100) according to any one of claims 1 to 8, wherein the second engine (2) comprises a third electric machine (40) able to drive the gas generator (22) of the second engine (2) during a start phase, and to be driven by said gas generator (22) after the start phase in order to generate electrical energy.

10. A hybrid aircraft comprising a propulsion assembly (100) according to any one of the preceding claims, the hybrid aircraft being a multi-engine helicopter, in particular a twin-engine helicopter.

11. A method for optimizing the operation of a multi-engine aircraft using a propulsion assembly (100) according to any one of claims 1 to 9, wherein, during a start phase of the first engine (1), the first electric machine (30) and / or the second electric machine (32) drive the gas generator (12) of said first engine (1), and after the start phase, the free turbine (11) of said first engine (1) drives one of the first electric machine (30) or of the second electric machine (32) in order to generate electrical energy.

12. The method according to claim 11, wherein the second engine (2) is able to operate alone, the first engine (1) then operating in a standby mode by being driven at idle by the first or the second electric machine (30, 32), the first engine (1) operating in the standby mode being restarted by the first electric machine (30) at least during a quick restart phase.

13. The method according to claim 11, wherein, during a start phase of the first engine (1), the first electric machine (30) and / or the second electric machine (32) drive the gas generator (12) of said first engine (1) without driving the free turbine (11), when the first electric machine (30) is coupled to the shaft (13) of the free turbine (11) via the second deactivatable coupling means (320) comprising the second free wheel (322).

14. The method according to claim 11, wherein the second engine (2) is able to operate alone, and wherein, when the first electric machine (30) is coupled to the shaft (13) of the free turbine (11) of the first engine (1) by being in direct engagement therewith, the first engine (1) then operating in a standby mode is driven at idle by the second electric machine (30, 32) coupled to the gas generator (12) only, or by the first electric machine (30), the main rotor (62) being coupled to the free turbine (11) of the first engine (1) via a main coupling means (510), the main rotor (62) driven by the second engine (2) rotating at a higher speed than the shaft (13) of the free turbine (11) of the first engine (1) such that the main coupling means (510) is deactivated.

15. The method according to claim 11, wherein, when the first electric machine (30) is coupled to the shaft (13) of the free turbine (11) of the first engine (1) by being in direct engagement therewith, the first engine (1) is started by that of the first or of the second electric machine (30, 32) coupled to the gas generator (12) only when the second engine (2) is stopped, or by the first and / or the second electric machine (30, 32) when the second engine (2) has been previously started.