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

Abstract

The invention relates to an aircraft propulsion system (100), which comprises a turbine engine (102A, 102B) provided with a rotary portion (104A, 104B), a starter (122A, 122B), a freewheel clutch system (124A, 124B) and a control system (126) designed, when the turboshaft engine is operating at idling speed, to receive a standby request (Rv), and in response to the standby request (Rv), to turn off the turbine engine (102A, 102B). The control system (126) is designed, in response to the standby request (Rv), before the turbine engine is switched off, to control the starter so that the speed (ND) of the starter becomes non-zero, and after the turbine engine is switched off, to control the starter to maintain the non-zero speed (ND) of the starter during the decrease in the speed (NGG) of the rotary portion to the standby speed (NGGv).

Description

Description TITLE: 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 turboshaft engine, and a corresponding computer program. Technological background

[0002] We know from the prior art, for example from the French patent application published under number FR 3138412 A1, an aircraft propulsion system, comprising: - a turboshaft engine equipped with a gas generator designed to rotate at a speed; - a starter designed to rotate at a speed; - a freewheel clutch system between the starter and the gas generator, the clutch system being designed to: • disengage the starter from the gas generator when the speed of the starter is less than the speed of the gas generator, and • engage the starter with 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, with the turboshaft engine 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: • receive a standby request to enter a standby regime in which the turboshaft engine is 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 turboshaft engine 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 standby speed. The control system then commands the starter to accelerate to standby speed and engage the gas generator. The starter then maintains the gas generator's speed at standby speed.

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

[0005] However, the reduction in the gas generator's speed through autorotation (i.e., without being driven by the turbocharger) can be lengthy. In the case of standby mode, where one of the turbocharger's combustion chambers is shut down, the autorotation time between the turbocharger idling and the gas generator's speed reaching standby speed can be several tens of seconds (for example, 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 speed reduction of the gas generator, or more generally of a rotating part of a turbomachine.

[0007] It may therefore be desirable to design a propulsion system that overcomes at least some of the aforementioned problems and constraints. The prior art also includes the European patent published under number EP 3123017 B1 and the French patent application published under number FR 3019215 A1. Summary of the invention

[0008] An aircraft propulsion system is therefore proposed according to claim 1.

[0009] Thanks to the invention, the starter motor rotates at a non-zero speed throughout the entire deceleration of the rotating part to its standby speed. In some embodiments, the starter motor can even be engaged with the rotating part during this entire deceleration period.

[0010] Thus, the invention allows the starter to quickly reach the speed of the rotating part to engage it, or even to already be engaged.

[0011] The invention may further include one or more of the optional features introduced in claims 2 to 9, according to any technically possible combination.

[0012] A control method according to claim 10 is also proposed.

[0013] A computer program is also proposed according to claim 11. Brief description of the figures

[0014] 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, - Figure 3 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 method of Figure 2, - Figure 4 is a block diagram of a second example of a control method according to the invention, - Figure 5 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 method of Figure 4, - Figure 6 is a block diagram of a third example of a control method according to the invention.- Figure 7 illustrates the evolution over time of the speed of a starter and a gas generator of the propulsion system of Figure 1, during the implementation of the method of Figure 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 gas generator of the propulsion system of Figure 1, during the implementation of the method of Figure 8, and, Figure 10 is a simplified diagram of a control system according to the invention. Detailed description of the invention

[0015] With reference to Figure 1, a propulsion system 100 according to the invention for an aircraft will now be described. The aircraft is, for example, a helicopter.

[0016] The propulsion system 100 comprises, firstly, first and second turboshaft engines 102A, 102B. Each turboshaft engine 102A, 102B includes a gas generator 104A, 104B designed to rotate at a speed denoted NGG and a free turbine 106A, 106B powered by the gas generator 104A, 104B. The gas generator 104A, 104B includes 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 that provide 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.

[0017] 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.

[0018] The propulsion system 100 further includes an overall freewheel clutch system 120 between the output shafts 116A, 116B of the turboshaft engines 102A, 102B and the main output shaft 118.

[0019] The propulsion system 100 further comprises, for each turboshaft engine 102A, 102B, a starter 122A, 122B designed to rotate at a speed denoted ND 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.

[0020] Each 122A, 122B starter, for example, includes an electric motor.

[0021] Each local freewheel clutch system 124A, 124B is designed to disengage the starter 122A, 122B from the gas generator 104A, 104B when the speed ND of the starter 122A, 122B is less than the speed NGG of the gas generator 104A, 104B.

[0022] Each local freewheel clutch system 124A, 124B is designed to engage the starter 122A, 122B with the gas generator 104A, 104B when the speed ND of the starter 122A, 122B becomes equal to the speed NGG of the gas generator 104A, 104B.

[0023] The propulsion system 100 also 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. The control system 126 is specifically designed to receive the speed ND of the starter 122A, 122B, for example provided by a suitable sensor (not shown).

[0024] Below, various control methods will be described, specifically for controlling the 102A turboshaft engine and its associated 122A starter. These methods can, of course, also be used to control the 102B turboshaft engine and the 122A starter.

[0025] With reference to Figure 2 and Figure 3, a first control method 200 that can be implemented by the control system 126 will now be described.

[0026] Initially, during step 202 (before time t1 in Figure 3), the turboshaft engine 102A is in normal operation and the starter 122A is stopped. Thus, the speed NGG evolves above an idle speed NGGr, while the speed ND is zero.

[0027] During a step 204 (time t1 in Figure 3), the control system 126 receives a standby request Rv asking the control system 126 to put the turboshaft engines 102A, 102B into standby.

[0028] In response, during a step 206 (between time t1 and time t2 in Figure 3), the control system 126 commands the turboshaft engine 102A to decrease the speed NGG to the idle speed NGGr.

[0029] During a step 208 (time t2 in figure 3), the speed NGG reaches the idle speed NGGr.

[0030] In response, during a step 210 (starting at time t2 in Figure 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.

[0031] During a step 214 (from time t3 in Figure 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.

[0032] 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 Figure 3), between 90% and 95% of the idle speed NGGr, implements 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 based on its inertia and resistive torque. This torque command is preferably much lower than the starting torques. This acceleration control N'D makes it possible to limit the acceleration N'D below a predefined threshold. Thus, the shock during clutch engagement is limited.

[0033] During a step 216 (time t5 in Figure 3), the starter 124A engages the gas generator 104A, that is to say the speed ND becomes equal to the idle speed NGGr at which the gas generator 104A is located.

[0034] During step 218 (from time t5 in Figure 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, called assist torque, for example between 1 Nm and 5 Nm.

[0035] During a step 220 (time t6 in Figure 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.

[0036] During step 222 (starting at time t6 in Figure 3), the control system 126 commands the starter 124 to decrease its speed ND 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 an assist torque command. Indeed, excessive torque will tend to significantly increase the descent time and even, in an extreme case, prevent the standby speed NGGv from being reached. Alternatively, the control system 126 can command the starter 124A by providing it with a speed gradient command.

[0037] The following steps are carried out in the absence of a reactivation request received by the 126 control system.

[0038] During a step 224 (time t7 in figure 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.

[0039] During a step 226 (from time t7 in Figure 3), the control system 126 commands the starter 124A to maintain the speed ND of the starter 124A, and therefore also that NGG of the gas generator 104A, at the standby speed NGGv.

[0040] The following steps are carried out if the 126 control system receives a reactivation request.

[0041] Thus, during a step 228 (time t'1 in figure 3), the control system 126 receives a reactivation request Rr.

[0042] In response, during a step 230 (from time t'1 in Figure 3), the control system 126 commands the ignition of the turboshaft engine 102A, then, for example in response to ignition detection, commands the starter 124A to increase its speed ND, and therefore also the speed NGG of the generator gas 104A engaged. For example, the control system 126 commands the starter 124A by providing it with a torque command to follow.

[0043] During a step 232 (time t'2 in Figure 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.

[0044] In response, during step 236 (starting at time t'2 in Figure 3), the control system 126 commands the starter 124A to decrease its speed ND until it reaches zero (time t'3 in Figure 3). Simultaneously, the control system 126 can control the turboshaft engine 102A as needed. In particular, if the reactivation requirement does not necessitate direct power-up, the turboshaft engine 102A can be temporarily set to idle speed NGGr.

[0045] With reference to Figure 4 and Figure 5, a second control method 400 which can be implemented by the control system 126 will now be described.

[0046] The steps common with the first ordering process 200 will be designated by the same references and will not be described again.

[0047] Thus, the second control process 400 first includes the steps 202, 204, 206, 208, 210 described above.

[0048] Following step 210 of maintaining the speed NGG at the idle speed NGGr, during a step 402 (from time t3 in Figure 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.

[0049] During a step 404 (time t4 in Figure 5), the control system 126 detects that the speed ND of the starter 122A reaches the speed ND1.

[0050] In response, during a step 406 (from time t4 in Figure 5), the control system 126 commands the starter 122A to maintain the speed ND of the starter 122A at speed ND1.

[0051] Step 406 is followed by step 224 (time t6 in figure 5) of shutdown of the turboshaft engine 102A.

[0052] During a step 408 (from time t6 in Figure 5), the gas generator 104A is in autorotation, so that its NGG velocity decreases.

[0053] During a step 410 (time t8 in Figure 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.

[0054] Step 410 is then followed by step 222 (from time t8 in Figure 5) of decreasing the speed ND of starter 124A with the speed NGG, while keeping starter 124A and gas generator 104 engaged.

[0055] The second control process 400 then includes steps 224 to 236, as for the first control process 200.

[0056] With reference to Figure 6 and Figure 7, a third control method 600 which can be implemented by the control system 126 will now be described.

[0057] The steps common with the first and second ordering processes 200, 400 will be designated by the same references and will not be described again.

[0058] Thus, the third control process 600 first includes the steps 202, 204, 206, 208, 210, 402, 404, 406 and 408 described above.

[0059] Following step 406, which shuts down the turboshaft engine 102A, during step 602 (starting at time t6 in Figure 7), the control system 126 commands the starter 124A to decrease the speed ND to 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.

[0060] The deceleration of the gas generator 104A is very strong at the beginning of the autorotation phase and is uncontrolled. Therefore, to prevent a sudden and unexpected 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 it reaches the standby speed NGGv. For example, this difference E is defined as relative to the NGG speed of the gas generator 104A. In the latter case, the deviation E could go from 40% to 1% of the NGG speed of the gas generator 104A, as illustrated in figure 7.

[0061] The following steps are carried out in the absence of a reactivation request received by the 126 control system.

[0062] During a step 604 (time t9 in Figure 7), the control system 126 detects that the speed ND of the starter 124A reaches the standby speed NGGv.

[0063] In response, during a step 606 (from time t9 in Figure 7), the control system 126 commands the starter 124A to maintain the speed ND of the starter 124A at the standby speed NGGv.

[0064] During a step 608 (at time t10 in Figure 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 gas generator 104A engages the starter 124A.

[0065] The third control method 600 then includes step 226 (from time t10 in figure 7), maintaining the speed ND of the starter 122A, and therefore also the speed NGG of the engaged gas generator 104A, at the standby speed NGGv.

[0066] The following steps are carried out if the 126 control system receives a reactivation request.

[0067] The third 600 control process thus includes step 228 (time t'1 in figure 7) of receiving the reactivation request Rr.

[0068] In response, during a step 612 (from time t'1 in Figure 7), the control system 126 commands an ignition of the turbomotor 102A, then, for example in response to an ignition detection, commands the starter 124A to increase its speed ND.

[0069] 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 Figure 3), between 90% and 95% of the speed NGG, implements acceleration control N'D via a torque command (around 5 Nm), which is significantly higher. The torque is lower than that of starting torques, in order to limit the latter under a predefined acceleration. This reduces the shock during clutch engagement.

[0070] During a step 614 (time t'4 in figure 7), the speed ND reaches the speed NGG, so that the starter 124A engages the gas generator 104A.

[0071] The third control method 600 then includes the step 230 (from time t'4 in figure 7) of increasing the speed ND, and therefore also the speed NGG.

[0072] The third control process 600 then includes steps 232 to 236 described above, as for the first control process 200.

[0073] With reference to Figure 8 and Figure 9, a fourth control method 800 which can be implemented by the control system 126 will now be described.

[0074] The steps common with the first, second and third ordering processes 200, 400, 600 will be designated by the same references and will not be described again.

[0075] 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.

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

[0077] The fourth control process 800 thus includes step 228 (time t'1 in figure 7) of receiving the reactivation request Rr.

[0078] In response, during step 612 (from time t'1 in Figure 7), the control system 126 commands an ignition of the turbomotor 102A, then, for example in response to a detection of the ignition, commands the starter 124A to increase its speed ND, so that the starter 122A engages the gas generator 104A.

[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 Figure 3), between 90% and 95% of the speed NGG, implements acceleration control N'D via a torque command (around 5 Nm), which is significantly higher. The torque is lower than that of starting torques, in order to limit the latter under a predefined acceleration. This reduces the shock during clutch engagement.

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

[0081] The third control process 600 then comprises steps 230 to 236.

[0082] With reference to Figure 10, the control system 126 is, for example, a 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 computer program 1008 is executed by the processing unit 1002 of the computer system. Specifically, each time the control system 126 commands a certain element to perform a certain action, this implies that the control system 126 provides control data for that element, designed so that the element performs the function.Thus, the instructions of computer program 1008 are designed, when said computer program 1008 is executed by processing unit 1002, to provide this control data.

[0083] Alternatively, all or part of these modules could be implemented as hardware modules, i.e. as an electronic circuit, for example micro-wired, not involving a computer program.

[0084] In conclusion, it is clear that a propulsion system such as the one described above allows for rapid reactivation, even during the phase of reducing the speed of the gas generator towards standby speed.

[0085] It should also be noted that the invention is not limited to the embodiments described above. Indeed, it will 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.

[0086] 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

Claims [1] Aircraft propulsion system (100), comprising: - a turbomachine (102A, 102B) equipped with a rotating part (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 rotating part (104A, 104B), the clutch system (124A, 124B) being designed to: • disengage the starter (122A, 122B) from the rotating part (104A, 104B) when the speed (ND) of the starter (122A, 122B) is less than the speed (NGG) of the rotating part (104A, 104B), and • engage the starter (122A, 11B) with the rotating part (104A, 104B) when the speed (ND) of the starter (122A, 122B) is equal to the speed (NGG) of the gas generator (102A, 102B);- a system (126) for controlling the starter (122A, 122B) and the rotating part (104A, 104B), the control system (126) being designed so that, the turboshaft engine (102A, 102B) being in an idle regime in which the speed (NGG) of the rotating part (104A, 104B) is equal to an idle speed (NGGr) and the speed of the starter (ND) is zero: • receive a standby request (Rv) to switch to a standby regime in which the turboshaft engine (102A, 102B) is off and the starter (122A, 122B) drives the rotating part (104A, 104B) at a standby speed (NGGv) lower than the idle speed (NGGr); and • in response to the standby request (Rv), switch off the turbomotor (102A, 102B) so that the speed of the rotating part (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) is switched off, 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) is switched off, control the starter (122A, 122B) to maintain the speed (ND) of the starter (122A, 122B); non-zero during the decrease in speed (NGG) of the rotating part (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 decrease in speed (NGG) of the rotating part (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 rotating part (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) is shut down, the speed (NGG) of the rotating part (104A, 104B) decreases until it reaches the speed (ND) of the starter (122A, 122B) so that the rotating part (104A, 104B) engages the starter (122A, 122B), and wherein the control system (126) is designed to, after engaging the rotating part (104A, 104B) 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 rotating part (104A, 104B), so that the starter (122A, 122B) remains disengaged from the rotating part (104A, 104B), during the entire decrease in the speed (NGG) of the rotating part (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 rotating part (104A, 104B) decreasing during the reduction of the speed (NGG) of the rotating part (104A, 104B) 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 reduction of the speed (NGG) of the rotating part (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 decrease in speed (NGG) of the rotating part (104A, 104B), in response to a reactivation request (Rr): - start the turboshaft engine (102A, 102B);and - if the starter (122A, 122B) and the rotating part (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 rotating part (104A, 104B), then • increase the speed (ND) of the starter (122A, 122B) to increase the speed (NGG) of the rotating part (104A, 104B) engaged at least up to the idle speed (NGGr);- if the starter (122A, 122B) and the rotating part (104A, 104B) are engaged, command the starter (122A, 122B) in response to: • increase the speed (ND) of the starter (122A, 122B) to increase the speed (NGG) of the engaged rotating part (104A, 104B). [9] Propulsion system (100) according to any one of claims 1 to 8, wherein the turbomachine is a turboshaft engine (102A, 102B) and the rotating part is a gas generator (104A, 104B) of the turboshaft engine (102A, 102B). [10] Method for controlling a propulsion system (100) comprising: - a turbomachine (102A, 102B) equipped with a rotating part (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 rotating part (104A, 104B), the clutch system (124A, 124B) being designed to: • disengage the starter (122A, 122B) from the rotating part (104A, 104B) when the speed (ND) of the starter (122A, 122B) is less than the speed (NGG) of the rotating part (104A, 104B), and • engage the starter (122A, 11B) with the rotating part (104A, 104B) when the speed (ND) of the starter (122A, 122B) is equal to the speed (NGG) of the rotating part (102A, 102B);the process comprising, the turboshaft engine (102A, 102B) being in an idle regime in which the speed (NGG) of the rotating part (104A, 104B) is equal to an idle speed (NGGr) and the speed of the starter (ND) is zero: • receiving a standby request (Rv) to switch to a standby regime in which the turboshaft engine (102A, 102B) is off and the starter (122A, 122B) drives the rotating part (104A, 104B) at a standby speed (NGGv) lower than the idle speed (NGGr); and • in response to the standby request (Rv), switching off the turboshaft engine (102A, 102B) so that the speed of the rotating part (104A, 104B) decreases to the standby speed (NGGv); characterized in that it further comprises, in response to the standby request (Rv): - before the turbomotor (102A, 102B) is switched off, a command to the starter (122A, 122B) so that the speed (ND) of the starter (122A, 122B) becomes non-zero;and - after the turbomachine (102A, 102B) is switched off, a starter (122A, 122B) command to maintain the non-zero speed (ND) of the starter (122A, 122B) during the reduction of the speed (NGG) of the rotating part (104A, 104B) to the standby speed (NGGv). [11] 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 method for controlling a propulsion system (100) comprising: - a turbomachine (102A, 102B) equipped with a rotating part (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 rotating part (104A, 104B), the clutch system (124A, 124B) being designed to: • disengage the starter (122A, 122B) from the rotating part (104A, 104B) when the speed (ND) of the starter (122A, 122B) is less than the speed (NGG) of the rotating part (104A, 104B), and • engage the starter (122A, 11B) with the rotating part (104A, 104B) when the speed (ND) of the starter (122A, 122B) is equal to the speed (NGG) of the gas generator (102A, 102B);the method comprising, the turboshaft engine (102A, 102B) being in an idle regime in which the speed (NGG) of the rotating part (104A, 104B) is equal to an idle speed (NGGr) and the speed of the starter (ND) is zero: • receiving a standby request (Rv) to switch to a standby regime in which the turboshaft engine (102A, 102B) is switched off and the starter (122A, 122B) drives the rotating part (104A, 104B) at a standby speed (NGGv) lower than the idle speed (NGGr); and • in response to the standby request (Rv), providing control data to switch off the turboshaft engine (102A, 102B) so that the speed of the rotating part (104A, 104B) decreases to the standby speed (NGGv);characterized in that it further comprises, in response to the standby request (Rv): - before the turboshaft engine (102A, 102B) is switched off, a supply of control data to the starter (122A, 122B) so that the speed (ND) of the starter (122A, 122B) becomes non-zero; and - after the turboshaft engine (102A, 102B) is switched off, a supply of control data to the starter (122A, 122B) to maintain the speed (ND) of the starter (122A, 122B) non-zero during the decrease in speed (NGG) of the rotating part (104A, 104B) to the standby speed (NGGv), when said program is executed on a computer.;

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