Aircraft propulsion system, method for controlling a starter and a turbocharger, and corresponding computer program

Abstract

The aircraft propulsion system comprises: - a turboshaft engine; - a starter; - a freewheel clutch system; - a control system designed to, with the turboshaft engine at idle: - receive a standby request (Rv); and - in response to the standby request (Rv), shut down the turboshaft engine. The control system is designed to, in response to the standby request (Rv): - before the turboshaft engine shuts down, command the starter so that the starter speed (ND) becomes non-zero; and - after shutdown, command the starter to maintain the starter speed (ND) non-zero while the gas generator speed (NGG) decreases to the standby speed (NGGv). Figure for the abbreviation: Fig. 3

Description

Title of the invention: AIRCRAFT PROPULSION SYSTEM, METHOD FOR CONTROLLING A STARTER AND A TURBOENGER AND CORRESPONDING COMPUTER PROGRAM Technical field of the invention

[0001] The present invention relates to an aircraft propulsion system, a method for controlling a starter and a turbomotor and a corresponding computer program. Technological background

[0002] A propulsion system for an aircraft is known from the prior art, comprising: - a turboshaft engine equipped with a gas generator designed to rotate at a speed; - a starter designed to rotate at a certain speed; - a freewheel clutch system between the starter and the gas generator, the clutch system being designed to: • Disengage the gas generator starter when the starter speed is lower than the gas generator speed, and • Engage the starter to the gas generator when the speed of the starter is equal to the speed of the gas generator; - a starter and gas generator control system, the control system being designed so that the turboshaft engine is in an idle speed in which the speed of the gas generator is equal to an idle speed and the speed of the starter is zero: • receive a sleep mode request to enter a standby mode in which the turbocharger is switched off and the starter drives the gas generator at a standby speed lower than the idle speed; and - in response to the standby request, shut down the turbomotor so that the speed of the gas generator decreases to the standby speed.

[0003] In this prior art, the starter remains off until the gas generator speed approaches the standby speed. The control system then commands the starter to accelerate to the standby speed and engage the gas generator. The gas generator's speed is then maintained at standby speed by the starter.

[0004] In the event of receiving a reactivation request during the reduction of the speed of the gas generator, the starter is still kept off and only on when the gas generator is close to the standby speed.

[0005] However, the reduction in the speed of the gas generator by autorotation (i.e., without being driven by the turboshaft engine) can be lengthy. In the case of a standby mode where a combustion chamber of the turboshaft engine is shut down, the autorotation time between the moment when the turboshaft engine is idling and the moment when the speed of the gas generator reaches the standby speed can reach several tens of seconds (for example, from 20 to 50 seconds).

[0006] Thus, with the state-of-the-art control system, it is not possible to respond quickly to a reactivation request received during the reduction in speed of the gas generator.

[0007] It may therefore be desirable to provide a propulsion system that overcomes at least some of the aforementioned problems and constraints. Summary of the invention

[0008] An aircraft propulsion system is therefore proposed, comprising: - a turboshaft engine equipped with a gas generator designed to rotate at a speed; - a starter designed to rotate at a certain speed; - a freewheel clutch system between the starter and the gas generator, the clutch system being designed to: • Disengage the gas generator starter when the starter speed is lower than the gas generator speed, and • Engage the starter to the gas generator when the speed of the starter is equal to the speed of the gas generator; - a starter and gas generator control system, the control system being designed so that the turboshaft engine is in an idle speed in which the speed of the gas generator is equal to an idle speed and the speed of the starter is zero: • receive a sleep mode request to enter a standby mode in which the turbocharger is switched off and the starter drives the gas generator at a standby speed lower than the idle speed; and • in response to the standby request, shut down the turbomotor so that the speed of the gas generator decreases to the standby speed;

[0009] characterized in that the control system is designed to, in response to the standby request: - before the turbocharger shuts down, activate the starter motor so that its speed becomes non-zero; and - after the turbocharger has switched off, command the starter to maintain the starter speed not zero while the gas generator speed is reduced to standby speed.

[0010] Thanks to the invention, during the entire period of the gas generator speed reduction to standby speed, the starter rotates at a non-zero speed. In certain embodiments, it may even be provided that the starter is engaged with the gas generator for this entire period.

[0011] Thus, the invention allows the starter to quickly reach the speed of the gas generator to engage it, or even to be already engaged.

[0012] The invention may further include one or more of the following optional features, according to any technically possible combination.

[0013] Optionally, the starter speed is maintained greater than or equal to the standby speed, during the reduction of the gas generator speed to the standby speed.

[0014] Optionally, before the turbocharger shuts down, the starter speed is increased to idle speed so that the starter engages the gas generator, and the control system is designed so that, after the turbocharger shuts down: - control the starter to reduce the starter speed while keeping the starter and gas generator engaged, down to standby speed.

[0015] Optionally, the starter speed is also kept strictly below the idle speed, so that, after the turbocharger shuts down, the gas generator speed decreases until it reaches the starter speed so that the gas generator engages the starter, and the control system is designed so that, after the gas generator engages with the starter: - control the starter to reduce the starter speed while keeping the starter and gas generator engaged, down to standby speed.

[0016] Optionally, the starter speed is also kept strictly lower than the gas generator speed, so that the starter remains disengaged from the gas generator, throughout the reduction of the gas generator speed to standby speed.

[0017] Optionally also, the starter speed is maintained with a deviation from the gas generator speed decreasing during the reduction of the gas generator speed to standby speed.

[0018] Optionally, the starter speed is also maintained at the standby speed during the reduction of the gas generator speed to the standby speed.

[0019] Optionally, the control system is also designed to, during the reduction of the gas generator speed, in response to a reactivation request: - turn on the turbomachine; and - If the starter and gas generator are disengaged, command the starter in response to: • Increase the starter speed until it matches the speed of the gas generator so that the starter engages the gas generator, then • increase the starter speed to increase the speed of the engaged gas generator at least up to idle speed; - If the starter and gas generator are engaged, command the starter to respond by: • Increase the starter speed to increase the speed of the engaged gas generator.

[0020] A method for controlling a propulsion system comprising: is also proposed. - a turboshaft engine equipped with a gas generator designed to rotate at a speed; - a starter designed to rotate at a certain speed; - a freewheel clutch system between the starter and the gas generator, the clutch system being designed to: • Disengage the gas generator starter when the starter speed is lower than the gas generator speed, and • Engage the starter to the gas generator when the speed of the starter is equal to the speed of the gas generator;

[0021] the method comprising, the turboshaft engine being in an idle regime in which the speed of the gas generator is equal to an idle speed and the speed of the starter is zero: - receiving a request to enter a standby mode in which the turbocharger is switched off and the starter drives the gas generator at a standby speed lower than the idle speed; and - in response to the standby request, a shutdown of the turbomotor so that the speed of the gas generator decreases to the standby speed;

[0022] characterized in that it further comprises, in response to the sleep request: - before the turbocharger shuts down, a starter motor command is sent so that the starter motor speed becomes non-zero; and - after the turbocharger is switched off, a starter control is used to maintain the starter speed from zero while the gas generator speed is reduced to standby speed.

[0023] Also proposed is a computer program downloadable from a communication network and / or recorded on a computer-readable medium, characterized in that it includes instructions for the execution of the steps of a process according to the invention, when said program is executed on a computer. Brief description of the figures

[0024] The invention will be better understood with the aid of the following description, given solely by way of example and made with reference to the accompanying drawings in which: - Figure 1 is a simplified view of an example of a propulsion system according to the invention. - Figure 2 is a block diagram of a first example of a control method according to the invention. - [Fig.3] illustrates the evolution over time of the speed of a starter and the speed of a gas generator of the propulsion system of [Fig.1], during the implementation of the process of [Fig.2], - Figure 4 is a block diagram of a second example of a control method according to the invention. - [Fig.5] illustrates the evolution over time of the speed of a starter and the speed of a gas generator of the propulsion system of [Fig.1], during the implementation of the process of [Fig.4], - Figure 6 is a block diagram of a third example of a control method according to the invention. - [Fig.7] illustrates the evolution over time of the speed of a starter and the speed of a gas generator of the propulsion system of [Fig.1], during the implementation of the process of [Fig.6], - Figure 8 is a block diagram of a fourth example of a control method according to the invention. - Figure 9 illustrates the evolution over time of the speed of a starter and a speed of a gas generator of the propulsion system of Figure 1, during the implementation of the process of Figure 8, and - [Fig. 10] is a simplified diagram of a control system according to the invention. Detailed description of the invention

[0025] With reference to [Fig. 1], a propulsion system 100 according to the invention for an aircraft will now be described. The aircraft is, for example, a helicopter.

[0026] The propulsion system 100 comprises, firstly, first and second turboshaft engines 102A, 102B. Each turboshaft engine 102A, 102B comprises a gas generator 104A, 104B and a free turbine 106A, 106B supplied by the gas generator 104A, 104B. The gas generator 104A, 104B comprises an air compressor 108A, 108B and a combustion chamber 110A, 110B connected to each other. In the combustion chamber 110A, 110B, fuel is intended to be burned with air compressed by the air compressor 108A, 108B to deliver gases providing kinetic energy. Each gas generator 104A, 104B further comprises a turbine 112A, 112B for the partial expansion of these gases, connected to the air compressor 108A, 108B by a drive shaft 114A, 114B, in order to rotate the air compressor 108A, 108B. The gases are also designed to rotate the free turbine 106A, 106B.Each turboshaft engine 102A, 102B further comprises an output shaft 116A, 116B connected to the free turbine 106A, 106B in order to be driven in rotation by the latter, at a speed denoted n2A, respectively n2B. Each speed n2A, n2B is generally expressed as a percentage of a fixed rated speed.

[0027] The propulsion system 100 further includes a main output shaft 118 designed to be connected to a mechanical load (not shown), for example a main rotor of the helicopter, and to rotate at a speed denoted nr to drive this mechanical load into rotation.

[0028] The propulsion system 100 further comprises an overall freewheel clutch system 120 between the output shafts 116A, 116B of the turboshaft engines 102A, 102B and the main output shaft 118.

[0029] The propulsion system 100 further comprises, for each turboshaft engine 102A, 102B, a starter 122A, 122B and a local freewheel clutch system 124A, 124B between the starter 122A, 122B and the gas generator 104A, 104B of the turboshaft engine 102A, 102B considered.

[0030] Each starter 122A, 122B includes, for example, an electric motor.

[0031] Each local freewheel clutch system 124A, 124B is designed to disengage the starter 122A, 122B from the gas generator 104A, 104B when the speed of the starter 122A, 122B is less than the speed of the gas generator 104A, 104B.

[0032] Each local freewheel clutch system 124A, 124B is designed to engage the starter 122A, 122B with the gas generator 104A, 104B when the speed of the starter 122A, 122B becomes equal to the speed of the gas generator 104A, 104B.

[0033] The propulsion system 100 further includes a control system 126 for the turboshaft engines 102A, 102B and the starters 122A, 122B. The term "system" is generic and covers the case of several engine control units communicating with each other and linked to the control electronics of the starters 122A, 122B.

[0034] Below, various control methods will be described for the control of the turboshaft engine 102A and the associated starter 122A. Of course, these methods can also be used to control the turboshaft engine 102B and the starter 122A.

[0035] With reference to [Fig.2] and [Fig.3], a first control method 200 which can be implemented by the control system 126, will now be described.

[0036] Initially, during a step 202 (before time t1 in [Fig.3]), the turboshaft engine 102A is in normal operation and the starter 122A is stopped. Thus, the speed NGG evolves above the idle speed NGGr, while the speed ND is zero.

[0037] During a step 204 (time tl on the [Fig.3]), the control system 126 receives a standby request Rv asking the control system 126 to put the turboshaft engines 102A, 102B into standby.

[0038] In response, during a step 206 (between time t1 and time t2 on the [Fig.3]), the control system 126 commands the turbomotor 102A to decrease the speed NGG to the idle speed NGGr.

[0039] During a step 208 (time t2 on the [Fig.3]), the speed NGG reaches the idle speed NGGr.

[0040] In response, during a step 210 (from time t2 in [Fig. 3]), the control system 126 commands the turboshaft engine 102A to maintain the speed NGG at the idle speed NGGr. While the turboshaft engine 102A is running at the idle speed NGGr, the control system 126 can, for example, perform checks.

[0041] During a step 214 (from time t3 on the [Fig.3]), for example once the planned checks have been carried out, the control system 126 commands the starter 124A to increase the speed ND of the starter 122A up to the idle speed NGGr so that the starter 122A engages the gas generator 104A.

[0042] To this end, the control system 126 first implements, for example, a speed control ND, then, in response to a detection that the speed ND reaches a predefined threshold lower than the idle speed NGGr (time t4 in [Fig. 3]), between 90% and 95% of the idle speed NGGr, implements an acceleration control N'D, for example by means of a torque command (for example between 4 Nm and 6 Nm, for example 5 Nm) supplied to the starter 124A, which will result in acceleration as a function of its inertia and resistive torque. This torque command is preferably much lower than the starting torque. This N'D acceleration control allows the N'D acceleration to be limited below a predefined threshold. Thus, the shock during clutch engagement is limited.

[0043] During a step 216 (time t5 on the [Fig.3]), the starter 124A engages the gas generator 104A, that is to say that the speed ND becomes equal to the idle speed NGGr at which the gas generator 104A is located.

[0044] During a step 218 (from time t5 in [Fig. 3]), the control system 126 commands the starter 124 to maintain the speed ND at the idle speed NGGr, in order to keep the starter 124A and the gas generator 104A aligned (ND equal to NGGr). For example, the control system 126 commands the starter 124 to maintain a low torque, referred to as assist torque, for example between 1 Nm and 5 Nm

[0045] During a step 220 (time t6 on the [Fig.3]), the control system 126 commands the shutdown of the turboshaft engine 102A so that the gas generator 104A is no longer driven by the turboshaft engine 102A.

[0046] During a step 222 (from time t6 in [Fig. 3]), the control system 126 commands the starter 124 so that its speed ND decreases with the speed NGG of the gas generator 104, while keeping the starter 124A and the gas generator 104A engaged. For example, the control system 126 commands the starter 124A by providing it with a torque command. This command is sufficiently low to limit the time it takes for the speed to decrease to a standby speed NGGv, for example, by providing it with the assist torque command. Indeed, too high a torque will tend to significantly increase the descent time and even, in an extreme case, prevent it from reaching the standby speed NGGv. Alternatively, the control system 126 can command the starter 124A by providing it with a speed gradient command.

[0047] The following steps are carried out in the absence of receipt by the control system 126 of a reactivation request.

[0048] During a step 224 (time t7 on the [Fig.3]), the control system 126 detects that the speed ND of the starter 124A, and therefore also that NGG of the gas generator 104A, reaches the standby speed NGGv.

[0049] During a step 226 (from time t7 on the [Fig.3]), the control system 126 maintains the speed ND of the starter 124A, and therefore also that NGG of the gas generator 104A, at the standby speed NGGv.

[0050] The following steps are carried out if the control system 126 receives a reactivation request.

[0051] Thus, during a step 228 (time t' 1 on the [Fig.3]), the control system 126 receives a reactivation request Rr.

[0052] In response, during a step 230 (starting at time t'1 in [Fig. 3]), the control system 126 ignites the turboshaft engine 102A, and then, for example in response to ignition detection, commands the starter 124A to increase its speed ND, and thus also the speed NGG of the engaged gas generator 104A. For example, the control system 126 commands the starter 124A by providing it with a torque command to follow.

[0053] During a step 232 (time t'2 in [Fig.3]), the control system 126 detects that the speed ND of the starter 122A, and therefore also the speed NGG of the engaged gas generator 104A, reaches a predefined cut-off speed of the starter 124A. This cut-off speed may be lower, equal to, or higher than the idle speed NGGr, and may also be different from a cut-off speed used during a conventional assisted start of the starter 124A.

[0054] In response, during a step 236 (starting at time t'2 in [Fig. 3]), the control system 126 commands the starter 124A to decrease its speed ND until it reaches zero (time t'3 in [Fig. 3]). In parallel, the control system 126 can control the turboshaft engine 102A as required. In particular, if the need for reactivation does not require direct power-up, the turboshaft engine 102A can be temporarily set to idle speed NGGr.

[0055] With reference to [Fig.4] and [Fig.5], a second control method 400 which can be implemented by the control system 126, will now be described.

[0056] The steps common with the first control method 200 will be designated by the same references and will not be described again.

[0057] Thus, the second control method 400 first includes the steps 202, 204, 206, 208, 210 described above.

[0058] Following step 210 of maintaining the speed NGG at the idle speed NGGr, during a step 402 (from time t3 on the [Fig.5]), the control system 126 commands the starter 124A to increase the speed ND of the starter 122A to a speed ND1 lower than the idle speed NGGr.

[0059] During a step 404 (time t4 on the [Fig.5]), the control system 126 detects that the speed ND of the starter 122A reaches the speed ND1.

[0060] In response, during a step 406 (from time t4 on the [Fig.5]), the control system 126 commands the starter 122A to maintain the speed ND of the starter 122A at speed ND1.

[0061] Step 406 is followed by step 224 (time t6 on the [Fig.5]) of shutdown of the turboshaft engine 102A.

[0062] During a step 408 (from time t6 on the [Fig.5]), the gas generator 104A is in autorotation, so that its NGG speed decreases.

[0063] During a step 410 (time t8 on the [Fig.5]), the speed NGG of the gas generator 104A reaches the speed ND of the starter 124A and the gas generator 104A therefore engages the starter 124A.

[0064] Step 410 is then followed by step 222 (from time t8 on [Fig.5]) of decreasing the speed ND of the starter 124A with the speed NGG, while keeping the starter 124A and the gas generator 104 engaged.

[0065] The second control method 400 then includes steps 224 to 236, as for the first control method 200.

[0066] With reference to [Fig.6] and [Fig.7], a third control method 600 which can be implemented by the control system 126, will now be described.

[0067] The steps common with the first and second control processes 200, 400 will be designated by the same references and will not be described again.

[0068] Thus, the third control method 600 first includes the steps 202, 204, 206, 208, 210, 402, 404, 406 and 408 described above.

[0069] Following step 406 of shutting down the turboshaft engine 102A, during step 602 (starting at time t6 in [Fig. 7]), the control system 126 commands the starter 124A to decrease the speed ND with the speed NGG of the gas generator 104A. More precisely, the speed ND is kept strictly lower than the speed NGG of the gas generator 104A. Thus, the gas generator 104A and the starter 124A are kept disengaged from each other.

[0070] The deceleration of the gas generator 104A is very strong at the beginning of the autorotation phase and is uncontrolled. Thus, to avoid an untimely and abrupt collision between the starter 124A and the gas generator 104A, the difference E between the speed ND of the starter 122A and the speed NGG of the gas generator 104A decreases over time as the speed NGG of the gas generator 104A decreases until the standby speed NGGv is reached. For example, this difference E is defined relative to the speed NGG of the gas generator 104A. In this latter case, the difference E could decrease from 40% to 1% of the speed NGG of the gas generator 104A, as illustrated in [Fig. 7].

[0071] The following steps are carried out in the absence of receipt by the control system 126 of a reactivation request.

[0072] During a step 604 (time t9 on the [Fig.7]), the control system 126 detects that the speed ND of the starter 124A reaches the standby speed NGGv.

[0073] In response, during a step 606 (from time t9 on the [Fig.7]), the control system 126 maintains the speed ND of the starter 124A at the standby speed NGGv.

[0074] During a step 608 (at time t10 on [Fig.7]), the speed NGG of the gas generator 104A reaches the speed ND of the starter 124A (i.e. the standby speed NGGv), so that the 104A gas generator engages the 124A starter.

[0075] The third control method 600 then includes the step 226 (from the instant tlO on the [Fig.7]), of maintaining the speed ND of the starter 122A, and therefore also of the speed NGG of the engaged gas generator 104A, at the standby speed NGGv.

[0076] The following steps are carried out if the control system 126 receives a reactivation request.

[0077] The third control method 600 thus includes the step 228 (time t' 1 on the [Fig.7]) of receiving the reactivation request Rr.

[0078] In response, during a step 612 (from time t' 1 on the [Fig.7]), the control system 126 starts the turbomotor 102A, then, for example in response to an ignition detection, commands the starter 124A to increase its speed ND.

[0079] To achieve this, the control system 126, for example, first implements speed control ND, then, in response to a detection that the speed ND reaches a predefined threshold lower than the idle speed NGGr (time t4 in [Fig. 3]), between 90% and 95% of the speed NGG, implements acceleration control N'D via a torque command (around 5 Nm), significantly lower than the starting torques, in order to limit the latter under a predefined acceleration. Thus, the shock during clutch engagement is limited.

[0080] During a step 614 (time t'4 on [Fig.7]), the speed ND reaches the speed NGG, so that the starter 124A engages the gas generator 104A.

[0081] The third control method 600 then includes the step 230 (from time t'4 on the [Fig.7]) of increasing the speed ND, and therefore also the speed NGG.

[0082] The third control method 600 then includes the steps 232 to 236 described above, as for the first control method 200.

[0083] With reference to [Fig.8] and [Fig.9], a fourth control method 800 which can be implemented by the control system 126, will now be described.

[0084] The steps common with the first, second and third control processes 200, 400, 600 will be designated by the same references and will not be described again.

[0085] Thus, the fourth control method 800 first includes the steps 202, 204, 206, 208, 210, 402, 224, 404, 406, 408, 410, 226 described above, but this time with the speed ND1 equal to the standby speed NGGv.

[0086] The following steps are carried out if the control system 126 receives a reactivation request.

[0087] The fourth control method 800 thus includes the step 228 (time t' 1 on the [Fig.7]) of receiving the reactivation request Rr.

[0088] In response, during step 612 (from time t' 1 on [Fig.7]), the control system 126 starts the turboshaft engine 102A, then, for example in response to an ignition detection, commands the starter 124A to increase its speed ND, so that the starter 122A engages the gas generator 104A.

[0089] To achieve this, the control system 126 first implements speed control ND, then, in response to a detection that the speed ND reaches a predefined threshold lower than the idle speed NGGr (time t4 in [Fig. 3]), between 90% and 95% of the speed NGG, implements acceleration control N'D via a torque command (around 5 Nm), significantly lower than the starting torques, in order to limit the latter under a predefined acceleration. Thus, the shock during clutch engagement is limited.

[0090] During step 614 (time t'4 on [Fig.7]), the speed ND reaches the speed NGG, so that the starter 124A engages the gas generator 104A.

[0091] The third control method 600 then comprises steps 230 to 236.

[0092] With reference to [Fig. 10], the control system 126 is, for example, a system computer system comprising a data processing unit 1002 (such as a microprocessor) and a main memory 1004 (such as RAM, from the English "Random Access Memory") accessible by the processing unit 1002. The computer system further comprises, for example, a network interface and / or a computer-readable medium, such as a local medium (such as a local hard disk 1006) or a remote medium (such as a remote hard disk accessible via the network interface through a communication network) or a removable medium (such as a USB flash drive, from the English "Universal Serial Bus", or a CD, from the English "Compact Disc" or a DVD, from the English "Digital Versatile Disc") readable by means of an appropriate reader of the computer system (such as a USB port or a CD and / or DVD disc drive).A computer program 1008 containing instructions for the processing unit 1002 is stored on the medium 1006 and / or downloadable via the network interface. This computer program 1008 is intended, for example, to be loaded into the main memory 1004, so that the processing unit 1002 can execute its instructions. The computer program 1008 includes, in particular, instructions for executing the steps of processes 200, 400, 600, and 800, when said program is executed by the processing unit 1002 of the computer system.

[0093] Alternatively, all or part of these modules could be implemented as hardware modules, i.e. in the form of an electronic circuit, for example micro-wired, not involving a computer program.

[0094] In conclusion, it is clear that a propulsion system such as the one described above allows for rapid reactivation, even during the phase of decreasing the speed of the gas generator towards standby speed.

[0095] It should also be noted that the invention is not limited to the embodiments described above. It will indeed be apparent to those skilled in the art that various modifications can be made to the embodiments described above, in light of the information just disclosed to them.

[0096] In the detailed presentation of the invention given above, the terms used shall not be interpreted as limiting the invention to the embodiments set forth in this description, but shall be interpreted as including all equivalents which can be foreseen by a person skilled in the art by applying their general knowledge to the implementation of the teaching which has just been disclosed to them.

Claims

1. Demands Aircraft propulsion system (100), comprising: - a turboshaft engine (102A, 102B) equipped with a gas generator (104A, 104B) designed to rotate at a speed (NGG); - a starter (122A, 122B) designed to rotate at a speed (ND); - a freewheel clutch system (124A, 124B) between the starter (122A, 122B) and the gas generator (104A, 104B), the clutch system (124A, 124B) being designed to: • disengage the starter (122A, 122B) from the gas generator (104A, 104B) when the speed (ND) of the starter (122A, 122B) is lower than the speed (NGG) of the gas generator (104A, 104B), and • engage the starter (122A, 1 IB) with the gas generator (104A, 104B) when the speed (ND) of the starter (122A, 122B) is equal to the speed (NGG) of the gas generator (102A, 102B); - a starter (126) control system (126) and a gas generator (104A, 104B) control system, the control system (126) being designed so that, with the turboshaft engine (102A, 102B) in an idle state in which the speed (NGG) of the gas generator (104A, 104B) is equal to an idle speed (NGGr) and the starter speed (ND) is zero: • receive a standby request (Rv) to enter a standby state in which the turboshaft engine (102A, 102B) is off and the starter (122A, 122B) drives the gas generator (104A, 104B) at a standby speed (NGGv) lower than the idle speed (NGGr); and • in response to the standby request (Rv), shut down the turboshaft engine (102A, 102B) so that the speed of the gas generator (104A, 104B) decreases to the standby speed (NGGv); characterized in that the control system (126) is designed to, in response to the standby request (Rv): - before the turboshaft engine (102A, 102B) shuts down, control the starter (122A, 122B) so that the speed (ND) of the starter (122A, 122B) becomes non-zero; and - after the turboshaft engine (102A, 102B) shuts down, control the starter (122A, 122B) to maintain the speed (ND) of the starter (122A, 122B) non-zero during the decrease in the speed (NGG) of the gas generator (104A, 104B) to the standby speed (NGGv).

2. Propulsion system (100) according to claim 1, wherein the speed (ND) of the starter (122A, 122B) is maintained greater than or equal to the standby speed (NGGv), during the reduction of the speed (NGG) of the gas generator (104A, 104B) to the standby speed (NGGv).

3. Propulsion system (100) according to claim 2, wherein, before the turboshaft engine (102A, 102B) is shut down, the speed (ND) of the starter (122A, 122B) is increased up to the idle speed (NGGr) so that the starter (122A, 122B) engages the gas generator (104A, 104B), and wherein the control system (126) is designed to, after the turboshaft engine (102A, 102B) is shut down: - command the starter (122A, 122B) to decrease the speed (ND) of the starter (122A, 122B) while keeping the starter (122A, 122B) and the gas generator (102A, 102B) engaged, up to the standby speed (NGGv).

4. Propulsion system (100) according to claim 2, wherein the speed (ND) of the starter (122A, 122B) is maintained strictly below the idle speed (NGGr), such that, after the turboshaft engine (102A, 102B) shuts down, the speed (NGG) of the gas generator (104A, 104B) decreases until it reaches the speed (ND) of the starter (122A, 122B) so that the gas generator (104A, 104B) engages the starter (122A, 122B), and wherein the control system (126) is designed to, after the gas generator (104A, 104B) engages with the starter (122A, 122B): - control the starter (122A, 122B) to decrease the speed (ND) of the starter (122A, 122B) keeping the starter (122A, 122B) and the gas generator (102A, 102B) engaged, up to standby speed (NGGv).

5. Propulsion system (100) according to claim 2, wherein the speed (ND) of the starter (122A, 122B) is kept strictly lower than the speed (NGG) of the gas generator (104A, 104B), so that the starter (122A, 122B) remains disengaged from the gas generator (104A, 104B), during the entire decrease in the speed (NGG) of the gas generator (104A, 104B) down to the standby speed (NGGv).

6. Propulsion system (100) according to claim 5, wherein the speed (ND) of the starter (122A, 122B) is maintained with a deviation from the speed (NGG) of the gas generator (104A, 104B) decreasing during the decrease in the speed (NGG) of the gas generator (104A, 104B) down to the standby speed (NGGv).

7. Propulsion system (100) according to claim 2, wherein the speed (ND) of the starter (122A, 122B) is maintained equal to the standby speed (NGGv), during the decrease in the speed (NGG) of the gas generator (104A, 104B) to the standby speed (NGGv).

8. Propulsion system (100) according to any one of claims 1 to 7, wherein the piloting system (126) is designed to, during the speed reduction (NGG) of the gas generator (104A, 104B), in response to a reactivation request (Rr): - start the turboshaft engine (102A, 102B); and - if the starter (122A, 122B) and the gas generator (104A, 104B) are disengaged, command the starter (122A, 122B) in response to: • increase the speed (ND) of the starter (122A, 122B) up to the speed (NGG) of the gas generator (122A, 122B) so that the starter (122A, 122B) engages the gas generator (104A, 104B), then • increase the speed (ND) of the starter (122A, 122B) to increase the speed (NGG) of the gas generator (104A, 104B) engaged at least up to the idle speed (NGGr); - If the starter (122A, 122B) and the gas generator (104A, 104B) are engaged, activate the starter (122A, 122B) in response to:

9. • Increase the speed (ND) of the starter (122A, 122B) to increase the speed (NGG) of the engaged gas generator (104A, 104B). Method for controlling a propulsion system (100) comprising: - a turboshaft engine (102A, 102B) equipped with a gas generator (104A, 104B) designed to rotate at a speed (NGG); - a starter (122A, 122B) designed to rotate at a speed (ND); - a freewheel clutch system (124A, 124B) between the starter (122A, 122B) and the gas generator (104A, 104B), the clutch system (124A, 124B) being designed to: • disengage the starter (122A, 122B) from the gas generator (104A, 104B) when the speed (ND) of the starter (122A, 122B) is lower than the speed (NGG) of the gas generator (104A, 104B), and • engage the starter (122A, 1 IB) with the gas generator (104A, 104B) when the speed (ND) of the starter (122A, 122B) is equal to the speed (NGG) of the gas generator (102A, 102B); the process comprising, the turboshaft engine (102A, 102B) being in an idle regime in which the speed (NGG) of the gas generator (104A, 104B) is equal to an idle speed (NGGr) and the speed of the starter (ND) is zero: • receiving a standby request (Rv) to enter a standby mode in which the turboshaft engine (102A, 102B) is switched off and the starter (122A, 122B) drives the gas generator (104A, 104B) at a standby speed (NGGv) lower than the idle speed (NGGr); and • in response to the standby request (Rv), a shutdown of the turboshaft engine (102A, 102B) so that the speed of the gas generator (104A, 104B) decreases to the standby speed (NGGv); characterized in that it also includes, in response to the sleep request (Rv):

10. - before the turbocharger (102A, 102B) shuts down, a command to the starter (122A, 122B) is sent so that the speed (ND) of the starter (122A, 122B) becomes non-zero; and - after the turboshaft engine (102A, 102B) is switched off, a starter (122A, 122B) control is used to maintain the starter (ND) speed (122A, 122B) not zero during the reduction of the speed (NGG) of the gas generator (104A, 104B) to the standby speed (NGGv). Computer program (1008) downloadable from a communication network and / or stored on a computer-readable medium, characterized in that it includes instructions for executing the steps of a process according to claim 9, when said program is executed on a computer.

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