METHOD FOR SPEED CONTROL OF A DRIVE FOR A HYBRID DRIVE IN THE CASE OF FAILURE OF THE MAIN CONTROL SYSTEM OF THE THERMAL DRIVE OF THE HYBRID DRIVE

Description

TECHNICAL FIELD

[0001] The present invention relates to parallel hybrid propulsion systems for aircraft. The invention is particularly applicable to propulsion and lift systems for fixed-wing or rotary-wing aircraft (helicopters), or vertical take-off and landing (VTOL) aircraft. The invention can also be applied to multi-propeller architectures. PREVIOUS STATE OF THE ART

[0002] In a parallel hybrid propulsion system, the means of generating and supplying propulsive power are redundant to allow the aircraft to land in satisfactory safety conditions in the event of a partial failure of a power chain.

[0003] A characteristic application example is a parallel hybrid helicopter propulsion unit, consisting of a turboshaft engine and an electric motor both driving the main and tail rotors.

[0004] An example of such a parallel hybrid propulsion system is illustrated in the figure 1 It includes: a main transmission box 3 (hereinafter referred to as MTB); a propulsion unit 4, connected to the MTB; this can, for example, be a rotor, a propeller, a fan, etc.; a main thermal engine (named engine 18), connected to the MTB and arranged to deliver mechanical power PM1 to the MTB; a secondary electric motor (named engine 2), connected to the MTB and arranged to deliver mechanical power PM2 to the MTB; a control system 5 for engine 18 capable of regulating the rotational speed of engine 1 by means of the control PM1*; a control system 6 for engine 2 capable of regulating the rotational speed of engine 2 by means of the control PM2*; an aircraft control system 7 capable of communicating with the control systems of both engines to send them high-level commands, such as the rotational speed setpoint NR* of the propulsion unit to be driven.

[0005] In fact, each motor control system is capable of regulating the mechanical power delivered by the motor to which it is connected, via the PM1 control*, in order to control the motor's rotational speed to a speed setpoint NR*.

[0006] Each of the motors is connected by a shaft 10 to the main transmission box 3, which will transmit the power of the motor(s) to the propulsion unit 4.

[0007] The parameters of each motor (Motor Parameters 18 and Motor Parameters 2) are sent to their respective control systems.

[0008] The parallel hybrid architecture as schematically represented on the figure 1This system primarily allows for compensating for the failure of the main internal combustion engine (engine 18) and enabling an emergency maneuver and / or landing using the power delivered by the auxiliary electric engine (engine 2). In this type of failure, the main engine (engine 18) malfunctions, preventing it from supplying power to the main transmission. A typical example of this type of failure is the main engine shutting down in flight, with no possibility of restarting it. The control system 5 of the failed engine 18 therefore becomes inoperative. The power delivered to the main transmission then comes solely from the power delivered by the auxiliary engine (engine 2), this power being controlled by the control system 6 of engine 2, under the authority of the aircraft control system 7.

[0009] However, other types of failure can occur. In the context of the present invention, we are interested in failures that affect the main motor's control system 5 and prevent it from controlling the power delivered by the motor 18. In this case, the motor 18 is generally still running and capable of delivering power, but this power can no longer be regulated by the motor 18's control system 5. This is generally referred to as a total control failure.

[0010] A well-known strategy in the world of helicopter turbines is to freeze the fuel flow injected into engine 18, in order to keep the engine running and freeze the power it delivers. The malfunction is then indicated to the aircraft control system 7 and the pilot, so that they can perform an appropriate maneuver and an emergency landing.

[0011] With a prior art parallel hybrid propulsion group as previously described, the auxiliary engine 2 can modulate the additional power it delivers to the thruster 4 and thus maintain the thruster's rotational speed within an acceptable range.

[0012] However, if the engine 18 control failure occurs while it is delivering high power, engine 2 will be unable to reduce the power delivered to the main engine, and the pilot will have to deliberately shut down the main engine to stabilize the helicopter and land. The pilot is therefore forced to perform a maneuver, followed by an emergency landing, thus foregoing the remaining power available from main engine 18.

[0013] It would therefore be advantageous, in the event of a failure of the main engine 18's control system 5, to be able to modulate the power delivered by the main engine in order to adapt it to the aircraft's needs, while regulating the propeller's rotational speed using the auxiliary engine 2. In other words, it would be advantageous to be able to continue regulating the propeller's rotational speed using both engines, even in the event of a failure of the control system of engine 18.

[0014] US2016 / 357994A1 specifies a method for rotating the rotor of an aircraft equipped with two internal combustion engines and an electric motor adapted to rotate the rotor. The rotor is driven using the internal combustion engines together. Authorization is generated only during a predetermined phase of flight, permitting the use of the electric motor to rotate the rotor. While the authorization is valid, if one of the internal combustion engines fails, an operating command is generated requiring the electric motor to operate. As long as the operating command is valid, the rotor is driven by each non-failing internal combustion engine, in conjunction with the electric motor. DESCRIPTION OF THE INVENTION

[0015] To this end, the invention relates to a method for regulating the rotational speed Np of a thruster in a hybrid aircraft propulsion system, in the event of a failure of the main control system of the internal combustion engine of the hybrid propulsion system, the hybrid propulsion system comprising: the propulsion system and a main transmission, capable of driving the propulsion system; the internal combustion engine and at least one electric motor, mounted in parallel on the main transmission, the internal combustion engine being equipped with a fuel circuit capable of supplying fuel to a combustion chamber of the internal combustion engine; a main control system for the internal combustion engine, capable of regulating the rotational speed of the internal combustion engine; a backup control system for the internal combustion engine, capable of regulating the rotational speed of the internal combustion engine when the main control system is in failure; a control system for the electric motor, capable of regulating the rotational speed of the electric motor; an aircraft control system, capable of sending a speed or power command to each of the control means for the internal combustion engine and the electric motor; the process comprising, when the main control system of the internal combustion engine is faulty and said system is stuck on a fuel flow command QCarbP*, the steps of: send a speed command N M2ref to the electric motor control system, so that the electric motor control system sends a power command P M2 * to the electric motor, thereby obtaining an instantaneous power P M2m of the electric motor; simultaneously, send a rotation speed or power command to the backup control system of the internal combustion engine, so that the backup control system sends a fuel flow command QCarbAux* to the fuel circuit of the internal combustion engine, the QCarbAux* command being chosen so as to vary the fuel flow QCarb injected into the combustion chamber of the internal combustion engine according to whether one wishes to increase or decrease the power P M1 of the internal combustion engine.

[0016] The fuel flow command QCarbP* corresponds to the last power command P M1 * obtained before the total failure of the main control system.

[0017] According to one embodiment of the invention, a reference power setpoint of the electric motor P M2ref * being sent to the backup control system of the internal combustion engine, the fuel flow control QCarbAux* is chosen by comparing the instantaneous power P M2m of the electric motor to the reference power P M2ref and if P M2m <p m2réf , on commande un débit de carburant auxiliaire qcarbaux* tel qu'il réduit le qcarb injecté dans la chambre combustion du moteur thermique, diminuant ainsi puissance p m1 thermique ; si m2m>P M2ref, we command an auxiliary fuel flow QCarbAux* such that it increases the fuel flow QCarb injected into the combustion chamber of the heat engine, thus increasing the power P M1 of the heat engine; if P M2m =P M2ref, we keep the QCarbAux* flow control constant.

[0018] The reference power P M2ref is chosen to modulate the power delivered by the internal combustion engine with a sufficient control margin.

[0019] The term "control margin" refers to the maximum authority of the electric motor to vary the total power delivered to the propulsion system around the average power delivered by the internal combustion engine. In other words, the electric motor and its power chain (power electronics, battery if powered by this means) are limited by design within a bounded power range. This range can be entirely positive or extend into a negative range if the electric motor's power chain is capable of drawing power from the main transmission (for example, by recharging the battery). The choice of P M2ref therefore allows adjustment of the average power at which the electric motor will operate within its operational range.An example of a sensible choice is to position PM2ref in the middle of the electric motor's range so that, in the event of a rapid change (positive or negative) in the thruster's power demand, the electric motor has maximum authority to adapt to that demand. This maximizes the thruster's speed regulation performance.

[0020] According to another embodiment of the invention, a rotational speed setpoint NM1ref being sent to the backup control system of the internal combustion engine, the instantaneous speed NM1m of the internal combustion engine is measured and compared to the rotational speed setpoint NM1ref: if N M1ref <N M1m , on commande un débit de carburant auxiliaire QCarbAux* tel qu'il réduit le débit carburant QCarb injecté dans la chambre de combustion du moteur thermique, diminuant ainsi la puissance P M1 du moteur thermique ; si N M1réf > NM1m, an auxiliary fuel flow rate QCarbAux* is commanded such that it increases the fuel flow rate QCarb injected into the combustion chamber of the internal combustion engine, thus increasing the power PM1 of the internal combustion engine; and if NM1ref = NM1m, the QCarbAux* flow rate command is kept constant; and simultaneously, the instantaneous power PM2m of the electric motor is compared to the reference power PM2ref, and if PM2m <p m2réf , la consigne de vitesse rotation du moteur thermique n m1réf est réduite ; si p m2m>P M2ref, the rotation speed setpoint of the internal combustion engine N M1ref is raised.

[0021] The invention also relates to a device for regulating the rotational speed (Np) of a thruster in a hybrid aircraft propulsion system, in the event of a failure of the main control system of the internal combustion engine of the hybrid propulsion system, the hybrid propulsion system comprising: the propulsion system and a main transmission, capable of driving the propulsion system; the internal combustion engine and at least one electric motor, mounted in parallel on the main transmission, the internal combustion engine being equipped with a fuel circuit capable of supplying fuel to a combustion chamber of the internal combustion engine; a main control system for the internal combustion engine, capable of regulating the rotational speed of the internal combustion engine; a backup control system for the internal combustion engine, capable of regulating the rotational speed of the internal combustion engine when the main control system is in failure; a control system for the electric motor, capable of regulating the rotational speed of the electric motor; an aircraft control system, capable of sending a speed or power command to each of the control means for the internal combustion engine and the electric motor; the device further comprising, when the main control system of the internal combustion engine is in failure and said system is stuck on a fuel flow command QCarbP*, means configured to: send a speed command N M2ref to the electric motor control system, so that the electric motor control system sends a power command P M2 * to the electric motor, thereby obtaining an instantaneous power P M2m of the electric motor; simultaneously, send a rotation speed or power command to the backup control system of the internal combustion engine, so that the backup control system sends a fuel flow command QCarbAux* to the fuel circuit of the internal combustion engine, the QCarbAux* command being chosen so as to vary the fuel flow QCarb injected into the combustion chamber of the internal combustion engine according to whether one wishes to increase or decrease the power P M1 of the internal combustion engine.

[0022] According to one variant, a reference power setpoint of the electric motor PM2ref* is sent to the backup control system of the internal combustion engine, the fuel flow control QCarbAux* is selected by comparing the instantaneous power PM2m of the electric motor to the reference power PM2ref, and wherein the device includes means configured for: if P M2m <p m2réf , commander un débit de carburant auxiliaire qcarbaux* tel qu'il réduit le qcarb injecté dans la chambre combustion du moteur thermique, diminuant ainsi puissance p m1 thermique ; si m2m>P M2ref, command an auxiliary fuel flow QCarbAux* such that it increases the fuel flow QCarb injected into the combustion chamber of the internal combustion engine, thus increasing the power P M1 of the internal combustion engine; if P M2m =P M2ref, maintain the QCarbAux* flow control constant.

[0023] According to another variant, a rotational speed setpoint NM1ref is sent to the backup control system of the internal combustion engine, the instantaneous speed NM1m of the internal combustion engine is measured and compared to the rotational speed setpoint NM1ref, and in which the device includes means configured for: if N M1ref <N M1m , commander un débit de carburant auxiliaire QCarbAux* tel qu'il réduit le débit carburant QCarb injecté dans la chambre de combustion du moteur thermique, diminuant ainsi la puissance P M1 du moteur thermique ; si N M1réf > NM1m, control an auxiliary fuel flow rate QCarbAux* such that it increases the fuel flow rate QCarb injected into the combustion chamber of the internal combustion engine, thus increasing the power PM1 of the internal combustion engine; and if NM1ref = NM1m, maintain the QCarbAux* flow rate control constant; and simultaneously, compare the instantaneous power PM2m of the electric motor to the reference power PM2ref and if PM2m <p m2réf , réduire la consigne de vitesse rotation du moteur thermique n m1réf ; si p m2m>P M2ref, raise the rotation speed setpoint of the internal combustion engine N M1ref.

[0024] The invention also relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to implement the process as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other aspects, objectives, advantages, and features of the invention will become clearer upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which: there figure 1 represents an example of a parallel hybrid architecture according to earlier art; the figure 2 represents an example of a parallel hybrid architecture used by the process according to the invention; the figure 3 represents a detail of the parallel hybrid architecture used by the process according to the invention in one embodiment; the figure 4 represents a detail of the parallel hybrid architecture used by the process according to the invention in another embodiment. DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION

[0026] The solution proposed by the invention is a speed control architecture for a thruster, which allows the power delivered by the two engines to continue to be modulated, despite the failure of the main regulation system of the internal combustion engine.

[0027] This solution can be applied to any parallel hybrid propulsion system where at least one electric motor and at least one internal combustion engine (for example, a gas turbine) drive, in parallel, a gearbox to a propulsion unit, which could be, for example, a rotor, a turboprop propeller, or a turbojet fan. This architecture can, for example, be used to control the rotational speed of a helicopter rotor.

[0028] The method of modulating the power of motor 1 according to the invention is not limiting in terms of the physical implementation of the different control systems.

[0029] A particularly advantageous, but not limiting, implementation of the invention is described in the figure 2 .

[0030] In the figure 2 , the parallel hybrid propulsion group includes, as in the figure 1 , a main transmission 3, a propulsion unit 4, a main thermal engine (engine 1), a secondary electric motor (engine 2), a control system 6 for engine 2, and an aircraft control system 7.

[0031] Unlike the figure 1 In the prior art, the engine 1 control system 5 is here replaced by a main engine 1 control system 51 capable of regulating the engine 1's rotational speed by modulating the power delivered by the engine 1 through the main fuel flow control QCarbP* sent to the fuel circuit 8 of the engine 1. This main fuel flow control QCarbP* varies the fuel flow rate QCarb that is injected into the combustion chamber of the engine 1. It should be noted that the reference numerals 1 (internal combustion engine) and 8 (fuel circuit) of the figure 2 correspond, with a more precise level of detail, to reference 18 of the figure 1 (the internal combustion engine 18 of the figure 1 including a fuel circuit which is not shown).

[0032] There is also an emergency control system 52 for engine 1 capable of modulating the power PM1 delivered by engine 1 by controlling an auxiliary fuel flow QCarbAux*. The emergency control system 52 for engine 1 is independent of the main control system 51, so that a failure of the main system 51 does not simultaneously affect the emergency system 52.

[0033] The auxiliary fuel flow control QCarbAux* can indifferently increase or decrease the fuel flow QCarb, which is delivered by the fuel circuit 8 of the engine 1 and which is injected into the combustion chamber of the engine 1. This auxiliary fuel flow is therefore added to or subtracted from the fuel flow QCarbP controlled by the main control system 51.

[0034] The modulation of the fuel flow rate QCarb, increasing or decreasing it relative to the fixed value QCarbP*, can be achieved by an electro-hydraulic device located in the fuel circuit 8 of engine 1, called the "auxiliary metering valve." An electro-hydraulic device ("main metering valve"), also located in the fuel circuit 8 of engine 1, also modulates the fuel flow rate QCarb, except when the engine 1 control system fails and the QCarbP* command is blocked. The circuit architecture and the presence of the auxiliary metering valve allow the flow rate initially fixed by the main metering valve to be increased or decreased. Therefore, there are two separate hydraulic devices, one for the main metering and the other for the auxiliary metering.

[0035] The emergency control system 52 of engine 1 is independent of the main control system 51, so that a failure of the main control system 51 does not simultaneously affect the other system.

[0036] In a preferred, but not exhaustive, manner (and as represented in the figure 2 The main control system 51 of motor 1 can exchange information with the control system 6 of motor 2 in order to coordinate the power delivered by each motor when they operate simultaneously. According to this particular embodiment, the main control system 51 of motor 1 can act on the power delivered by motor 2.

[0037] According to the architecture depicted in the figure 2 In the event of a total failure of the main control system 51 of engine 1, the main fuel flow control QCarbP* is frozen, but the aircraft control system 7 can continue to regulate the propeller rotation speed, regardless of the power displayed by engine 1 at the time of the failure, thanks to: to engine 2 and its dedicated control system 6; and to the backup control system 52 of engine 1.

[0038] It is assumed that, due to the failure of the main control system 51 of engine 1, no further information processed by the main control system 51 is available. In particular, the measurement of the power delivered by engine 1 PM1, at the moment when the main fuel flow rate QCarbP was frozen, is not available.

[0039] This backup regulation of the engine 1 by means of the backup regulation system 52 may offer degraded performance compared to the main regulation system 51, in particular with regard to power regulation dynamics.

[0040] Therefore, in this failure situation, the control of the propeller's rotational speed can be carried out in a preferential manner according to the following embodiment.

[0041] The emergency control system 52 of engine 1 is configured to provide a slow modulation of the power delivered by engine 1. To do this, the emergency control system 52 modulates the auxiliary fuel flow control QCarbAux* so that engine 1 delivers an appropriate power P M1, which may be less than the power requirements of the propulsion system. The emergency control system 52 thus roughly adapts the power delivered by engine 1 to the aircraft's flight situation (for example, in the case of a helicopter, climb, cruise, descent, etc.).

[0042] In parallel, and simultaneously, the control system 6 of engine 2 rapidly and precisely adjusts the power PM2 delivered by engine 2 by finely regulating the thruster's rotational speed. The power delivered by engine 2 PM2 is then added to that delivered by engine 1 PM1 to meet the thruster's requirements. The total power delivered to the thruster thus benefits from the rapid dynamics of engine 2 to respond to the thruster's instantaneous power variations.

[0043] The "slow" nature of the real-time adaptation of the QCarbAux* control allows motor 2 (faster than motor 1) to perfectly compensate for the additional power supplied to the rotor by the PM2* control. The fact that the variations of motor 1 are "slow" avoids the risk of dynamic interference between the regulation of the two motors, which would generate power and / or speed oscillations of thruster 4, detrimental to piloting.

[0044] Slow modulation of the power delivered by engine 1PM1 can be achieved by determining the aircraft's power requirements. This determination can be based on one or more of the elements listed below, which are not exhaustive: a collective pitch command or measurement of the aircraft; an anticipation of power information from the aircraft; the power delivered by the 2PM2 engine, averaged over a certain period; any other information allowing estimation of the average power requirement level of the propulsion system.

[0045] An advantageous solution for modulating the power of motor 1PM1 is described in the figure 3 It consists of regulating the rotation speed of the propeller through the regulation of the rotation speed N M2 of the engine 2. This regulation can be "fast", in order to efficiently vary the power delivered to the propeller around an average power.

[0046] The instantaneous power of motor 2 P M2m, required to maintain the propeller's rotational speed at its setpoint, is used by the backup control system 52 of motor 1 to compare it to a reference power P M2ref chosen by design to guarantee a satisfactory control margin. Thus: if the instantaneous power delivered by engine 2 P M2m is less than the reference power P M2ref, the emergency control system 52 of engine 1 commands an auxiliary fuel flow QCarbAux* such that it reduces the fuel flow QCarb injected into the combustion chamber of engine 1; conversely, if the instantaneous power delivered by engine 2 P M2m is greater than the reference power P M2ref, the emergency control system 52 commands an auxiliary fuel flow QCarbAux* such that it increases the fuel flow QCarb injected into the combustion chamber of engine 1, thus increasing the power of engine 1.

[0047] QCarbAux* is an auxiliary fuel flow control for engine 1 that is adapted to the average requirements of the propulsion system. This QCarbAux* control can be negative, in order to reduce the power of engine 1 below the power it was displaying at the time of the failure of the main control system 51, or positive.

[0048] It is noted that this method of slow modulation of the power delivered by motor 1 according to this embodiment does not use any information on the state of motor 1. This embodiment therefore does not require a measurement means dedicated to the backup system.

[0049] This slow modulation of the power delivered by engine 1 PM1 aims to maintain the average power delivered by engine 2 at a chosen level, for example, in the middle of engine 2's power range. This choice maximizes the amplitude of rapid power variation of the thruster. The power regulation of engine 1 must be sufficiently slow to both: be compatible with the operation of the backup control system of motor 1; and that the modulation of the power of motor 1 does not disrupt the speed regulation of motor 2.

[0050] The main advantages of this method of slow modulation of the power delivered by motor 1 proposed within the framework of the invention are: to be able to keep the main engine (here, engine 1) running, regardless of the power level it is at when the main control system fails; to offer the pilot the maximum power margin to complete the flight until landing, thus ensuring maximum safety; to guarantee the stability of the propeller rotation speed regulation through frequency decoupling between the two engines, namely the slow modulation of the power of engine 1 and the rapid regulation of the rotation speed by engine 2; to minimize the power required by engine 2, which also minimizes the power drawn from the battery that powers it and thus increases its range.This aspect has two advantages: firstly, it allows the pilot more time, and therefore a greater distance to cover, to carry out an emergency landing (safety advantage); and secondly, the autonomy offered can also allow the pilot to complete his flight to reach his intended destination, or to return to land at the departure base in order to carry out the necessary maintenance operations (operational advantage).

[0051] As previously stated, the method of modulating the power of motor 1 as described in the figure 3 This is a particularly advantageous, but not limiting, implementation and can take different forms. Indeed, the proposed invention is not limiting in terms of the physical implementation of the various control systems mentioned above.

[0052] Thus, the emergency control system 52 of engine 1 can, for example, be part of engine 1 itself or be integrated into the aircraft control system 7.

[0053] The regulation of the propeller rotation speed in the event of failure of the main control system 51 of engine 1 can, for example, be implemented in the control system of engine 2 or in the aircraft control system.

[0054] The engine 2 control system 6 can itself be part (partially or totally) of the aircraft control system 7.

[0055] Among the other forms of implementation mentioned above, we can present the implementation method described in the figure 4 Here, the backup control system 52 of engine 1 regulates the rotational speed of engine 1 according to the setpoint NM1ref. To do this, it uses a rotational speed measurement chain 9 of the free turbine of engine 1, which is independent of any measurement means of the main control system 51 and which measures the rotational speed of engine 1 NM1. According to another embodiment not shown, a rotational speed measurement chain of the propeller 4 can also be used.

[0056] Unlike conventional turboshaft engines (without parallel hybridization), the rotational speed setpoint NM1ref is, in this embodiment, regulated according to the instantaneous power delivered by the engine 2PM2m, in order to maintain it at a desired level, according to the same selection criteria as the embodiment described in the figure 3 .

[0057] So : when the instantaneous power of motor 2 P M2m is greater than the desired level (reference power P M2ref), the rotation speed setpoint of motor 1 N M1ref is raised so that motor 1 delivers more power; and conversely, when the instantaneous power of motor 2 P M2m is too low, the rotation speed setpoint of motor 1 N M1ref is reduced, so that motor 1 delivers less power.

[0058] In the figure 4 , the regulation of the engine speed setpoint 1 is done in the aircraft control system 7.

[0059] The comparison between PM2m and PM2ref is made here in block 7 "Modulation of the motor 1 speed setpoint" of the figure 4 The comparison scheme is the same as block 52 of the figure 3 , the difference being that the output is the motor rotation speed setpoint 1 N M1ref , instead of being the auxiliary flow control QCarbAux*.

[0060] The comparison between N M1ref and N M1 is the operation that allows determining the auxiliary flow control QCarbAux*. The embodiment illustrated in the figure 4 is less direct than that of the figure 3 since it involves two nested control loops: a first power loop PM2 which generates the setpoint NM1ref, and a second speed loop to ultimately generate the auxiliary flow control QCarbAux*. This embodiment has the advantage of allowing the use of the backup control system of motor 1 (block 52 of the figure 4 ).

[0061] It should be noted that certain elements that are present in the figure 2 were not represented in the figures 3 And 4 , for example the aircraft control system in the figure 3 , and the main engine control system 1 in the figures 3 And 4 These elements are indeed present in the hybrid propulsion system according to the invention, but have not been shown in order to facilitate reading the figures 3 And 4 .

Claims

1. A method for regulating the rotational speed (Np) of a propulsion device of a hybrid propulsion unit for an aircraft, in the event of failure of the main regulation system of the heat engine of the hybrid propulsion unit, the hybrid propulsion unit comprising: - the propulsion device (4) and a main gearbox (3), capable of driving the propulsion device; - the heat engine (1) and at least one electric motor (2), mounted in parallel on the main gearbox, the heat engine being provided with a fuel circuit (8) capable of supplying fuel into a combustion chamber of the heat engine; - a main regulation system (51) of the heat engine, capable of regulating the rotational speed of the heat engine; - a backup regulation system (52) of the heat engine, capable of regulating the rotational speed of the heat engine when the main regulation system fails; - a regulation system (6) of the electric motor, capable of regulating the rotational speed of the electric motor; - a control system (7) of the aircraft, capable of sending a speed or power setpoint to each of the regulation means of the heat engine and of the electric motor; the method comprising, when the main regulation system of the heat engine fails and said system is locked to a fuel flow command QCarbP*, the step of: - sending a speed setpoint NM2ref to the regulation system (6) of the electric motor, so that the regulation system (6) of the electric motor sends a power command PM2* to the electric motor (2), whereby an instantaneous power PM2m of the electric motor (2) is obtained; the method being characterized in that it further comprises the step of: - simultaneously, sending a rotational speed or power setpoint to the backup regulation system (52) of the heat engine, so that the backup regulation system (52) sends a fuel flow command QCarbAux* to the fuel circuit (8) of the heat engine (1), the command QCarbAux* being selected so as to vary the fuel flow QCarb injected into the combustion chamber of the heat engine depending on whether it is desired to vary the power PM1 of the heat engine.

2. The method according to claim 1, wherein, a reference electric motor power setpoint PM2ref* being sent to the backup regulation system (52) of the heat engine, the fuel flow command QCarbAux* is selected by comparing the instantaneous power PM2m of the electric motor with the reference power PM2ref, and - if PM2m<PM2ref, an auxiliary fuel flow QCarbAux* is controlled such that it reduces the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby reducing the power PM1 of the heat engine; - if PM2m>PM2ref, an auxiliary fuel flow QCarbAux* is controlled such that it increases the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby increasing the power PM1 of the heat engine; - if PM2m=PM2ref, the flow command QCarbAux* is kept constant.

3. The method according to claim 1, wherein a rotational speed setpoint NM1ref being sent to the backup regulation system (52) of the heat engine, the instantaneous speed NM1m of the heat engine is measured and it is compared with the rotational speed setpoint NM1ref: - if NM1ref<NM1m, an auxiliary fuel flow QCarbAux* is controlled such that it reduces the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby reducing the power PM1 of the heat engine; - if NM1ref>NM1m, an auxiliary fuel flow QCarbAux* is controlled such that it increases the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby increasing the power PM1 of the heat engine; and - if NM1ref=NM1m, the flow command QCarbAux* is kept constant; and simultaneously, the instantaneous power PM2m of the electric motor is compared with the reference power PM2ref, and - if PM2m<PM2ref, the heat engine rotational speed setpoint NM1ref is reduced; - if PM2m>PM2ref, the heat engine rotational speed setpoint NM1ref is raised.

4. A device for regulating the rotational speed (Np) of a propulsion device of a hybrid propulsion unit for an aircraft, in the event of failure of the main regulation system of the heat engine (1) of the hybrid propulsion unit, the hybrid propulsion unit comprising: - the propulsion device (4) and a main gearbox (3), capable of driving the propulsion device; - the heat engine (1) and at least one electric motor (2), mounted in parallel on the main gearbox, the heat engine being provided with a fuel circuit (8) capable of transferring fuel into a combustion chamber of the heat engine; - a main regulation system (51) of the heat engine, capable of regulating the rotational speed of the heat engine; - a backup regulation system (52) of the heat engine, capable of regulating the rotational speed of the heat engine when the main regulation system fails; - a regulation system (6) of the electric motor, capable of regulating the rotational speed of the electric motor; - a control system (7) of the aircraft, capable of sending a speed or power setpoint to each of the regulation means of the heat engine and of the electric motor; the device further comprising, when the main regulation system (51) of the heat engine fails and said system is locked to a fuel flow command QCarbP*, means configured to: - send a speed setpoint NM2ref to the regulation system (6) of the electric motor, so that the regulation system (6) of the electric motor sends a power command PM2* to the electric motor (2), whereby an instantaneous power PM2m of the electric motor is obtained; the device being characterized in that said means are configured to: - simultaneously, send a rotational speed or power setpoint to the backup regulation system (52) of the heat engine, so that the backup regulation system (52) sends a fuel flow command QCarbAux* to the fuel circuit (8) of the heat engine, the command QCarbAux* is selected so as to vary the fuel flow QCarb injected into the combustion chamber of the engine depending on whether it is desired to increase or decrease the power PM1 of the heat engine.

5. The device according to claim 4, wherein, a reference electric motor power setpoint PM2ref* being sent to the backup regulation system (52) of the heat engine, the fuel flow command QCarbAux* is selected by comparing the instantaneous power PM2m of the electric motor with the reference power PM2ref, and wherein the device comprises means configured to: - if PM2m<PM2ref, control an auxiliary fuel flow QCarbAux* such that it reduces the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby reducing the power PM1 of the heat engine; - if PM2m>PM2ref, control an auxiliary fuel flow QCarbAux* such that it increases the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby increasing the power PM1 of the heat engine; - if PM2m=PM2ref, keep the flow command QCarbAux* constant.

6. The device according to claim 4, wherein a rotational speed setpoint NM1ref being sent to the backup regulation system (52) of the heat engine, the instantaneous speed NM1m of the heat engine is measured and it is compared with the rotational speed setpoint NM1ref and wherein the device comprises means configured to: - if NM1ref<NM1m, control an auxiliary fuel flow QCarbAux* such that it reduces the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby reducing the power PM1 of the heat engine; - if NM1ref>NM1m, control an auxiliary fuel flow QCarbAux* such that it increases the fuel flow QCarb injected into the combustion chamber of the heat engine (1), thereby increasing the power PM1 of the heat engine; and - if NM1ref=NM1m, keep the flow command QCarbAux* constant; and simultaneously, compare the instantaneous power PM2m of the electric motor with the reference power PM2ref, and - if PM2m<PM2ref, reduce the heat engine rotational speed setpoint NM1ref; - if PM2m>PM2ref, raise the heat engine rotational speed setpoint NM1ref.

7. A computer program product comprising instructions which, when the program is executed by a device for regulating the rotational speed (Np) according to claim 4, cause the latter to implement the steps of the method according to any one of claims 1 to 3.