METHOD FOR CONTROLLING A TURNTABLE, CORRESPONDING TURNTABLE AIRCRAFT AND CORRESPONDING COMPUTER PROGRAM

Description

[0001] The present invention relates to a method for controlling a rotorcraft, a rotorcraft and a computer program associated with the control method.

[0002] A rotorcraft has at least one rotor that contributes to its lift, or even its propulsion.

[0003] For example, a rotorcraft may include a main rotor that contributes to its lift and propulsion, this main rotor having blades with collectively and cyclically variable pitch. In addition, the rotorcraft may include a system that contributes to yaw control, such as another main rotor or a tail rotor. The pitch control system can be activated, for example, by the pilot or crew. Alternatively, the system can be activated by an automatic control system, known as an autopilot.

[0004] To set the rotor(s) in motion, the rotorcraft includes a propulsion system, possibly multi-engine, and a power transmission chain running from the engines to one or more rotors.

[0005] For example, two motors are connected to a power transmission unit, which drives the rotor(s). The power transmission unit then has one mechanical input per motor, with the mechanical inputs meshing with a combiner, the combiner driving one mechanical output of the power transmission unit per rotor. The motor inputs, the combiner, and the mechanical outputs can include at least one pinion or gear, at least one shaft, at least one speed reduction stage, etc.

[0006] The engines can be internal combustion engines with an output shaft set in motion by the combustion of fuel.

[0007] For example, at least one engine may be a turboshaft engine equipped with a gas generator and a free-working turbine connected to the output shaft.

[0008] Each engine can be controlled by a control system known by the acronym FADEC and the English expression "Full Authority Digital Engine Control". Such a control system includes an engine computer in communication with sensors measuring operating parameter values ​​of the controlled engine, or even of the rotorcraft, such as the rotational speed of a gas generator or a free turbine, an internal temperature, the rotational speed of a rotor of the rotorcraft.

[0009] This engine control unit is then configured to control a fuel metering system supplying the associated engine in order, for example, to stabilize the output shaft's rotational speed at a set point, regardless of the power consumed by that output shaft. This power varies according to a pilot's actions on flight controls, particularly on controls that collectively and / or cyclically vary the pitch of the main rotor blades on a helicopter. Alternatively, these controls can be operated by an autopilot system.

[0010] Regulation based on the rotational speed (N2) of the turboshaft output shaft then ensures a permanent match between the power produced by the output shafts of the engines and the power consumed by the rotor(s), so that this or these rotors rotate at a nominal rotational speed compatible with their function of generating lift for the rotorcraft.

[0011] However, the engine control unit (ECU) is configured to prevent the power developed by an engine from exceeding a mechanical limit acceptable to the engine or the transmission. Typically, various operating modes, each with its own limits and durations, are defined. For each operating mode, these limits might include, for example, a torque limit for the engine's output, an internal engine temperature limit, a rotational speed limit for an engine shaft, or a torque limit for at least one component of the transmission.

[0012] On a multi-engine aircraft, there are two operating modes known as "AEO" (All Engines Operative), which can be used for certain periods when all engines are functioning normally, and two operating modes known as "OEI" (One Engine Inoperative), which can be used for certain periods when one of the engines is out of service. The use of certain operating modes may necessitate maintenance on the engines, or even on the power transmission system.

[0013] From then on, an engine control unit (ECU) manages the fuel metering system to modulate the power of the associated engine. One type of regulation involves adjusting the output shaft speed towards a setpoint, while preventing it from exceeding a limit. This type of regulation is called N2 regulation because it depends on the output shaft speed of the associated engine.

[0014] Another type of regulation involves delivering a predefined power to the motor, for example to reach an optimal operating point in terms of thermodynamic efficiency.

[0015] However, in the case of a rotorcraft, it is advantageous for at least one of the engines to be regulated according to the rotational speed N2 of its output shaft. Indeed, such regulation, known as "N2 regulation," prevents a regular decrease in the rotational speed NR of the lift rotor, which is itself directly linked to the rotational speed N2.

[0016] When a limit is reached, the engine control unit restricts the fuel flow to the engine, and consequently, the power delivered by its output shaft becomes less than the required power. The pilot must then use the flight controls to reduce the power needed for flight.

[0017] To this end, the rotorcraft may include various indicators allowing a pilot to assess the situation. These indicators may include an indicator showing the rotational speed of the main rotor and the associated limits, engine indicators showing the current values ​​of the engine monitoring parameters (temperature, speed, torque) and the associated limits, indicators showing the current values ​​of the power transmission chain monitoring parameters and the associated limits, and a first limitation indicator displaying information relating to the parameter closest to one of these limits, among several monitored parameters.

[0018] Furthermore, in the event of a failure or malfunction of an engine's fuel control system, the fuel flow to the affected engine is fixed at the last flow rate used. A pilot is notified and can then either shut down the affected engine for safety reasons or continue the flight as is, i.e., with an engine not regulated at N2.

[0019] If the flight continues in its current state, the pilot must ensure that no limits of the mechanical transmission chain are exceeded. On a twin-engine rotorcraft, if a power demand is applied by a pilot to perform a maneuver, the sum of the power delivered by the engine regulated at N2 and the engine not regulated at N2 could exceed the permissible value for the combiner of a power transmission, for example.

[0020] More generally, managing a lack of regulation, which can be voluntary or accidental, of one of the engines of a rotorcraft can require an additional workload from the crew in order not to exceed a limit of the mechanical transmission chain.

[0021] In addition, documents US 2019 / 382124, US 2014 / 283527, US 2016 / 221685, EP 3251955 and US 2013 / 054053 relate to other known rotorcraft control methods.

[0022] The present invention aims to provide a method for limiting the workload of a crew in the event of a failure of an engine control system on a multi-engine rotorcraft, or in the absence of control of one of the engines. Furthermore, this method allows a pilot to perform piloting maneuvers safely, without risking damage to the rotorcraft's mechanical drive system.

[0023] The invention therefore relates to a method of controlling a rotorcraft, the rotorcraft comprising at least two combustion engines, at least one regulation system and a transmission system, said at least two engines being mechanically connected to the transmission system respectively by at least two couplings.

[0024] According to the invention, such a control method is remarkable in that it comprises at least the following steps: identification of at least one N2-regulated engine among said at least two engines, said at least one N2-regulated engine being identified as a fuel-fed engine with a fuel flow regulated according to the rotational speed N2 of at least one first output shaft of said at least one N2-regulated engine and supplied by at least one first fuel metering unit controlled according to a flow setpoint transmitted by at least one first control system dependent on the rotational speed N2 of said at least one first output shaft, identification of a so-called non-N2'-regulated engine among said at least two engines, the non-N2'-regulated engine being identified as a fuel-fed engine with a fuel flow not regulated according to the rotational speed N2' of a second output shaft of the non-N2'-regulated engine and supplied by a second fuel metering unit being either non-regulated,be controlled according to a flow rate setpoint transmitted by at least one second control system independent of the rotational speed N2' of the second output shaft, determination of at least one first permissible input torque limit for at least one first coupling of the transmission system connected to said at least one first output shaft, determination of a second permissible input torque limit for a second coupling of the transmission system connected to the second output shaft, determination of a first current operating torque transmitted by said second output shaft to the second coupling of the transmission system, determination of at least one permissible residual combined torque limit for at least one component of a combiner of the transmission system, said at least one residual combined torque limit being calculated according to the formula: , CR 1 = Cadm − C 1 ∗ Z where Cadm is an permissible combined torque limit for said at least one component of the combiner, C1 is the first current operating torque transmitted by the second output shaft to the second coupling of the transmission system, and Z is a reduction coefficient between a rotational speed of the second coupling and a rotational speed of said at least one component of the combiner, identification of a critical torque limit as being the smallest of the values ​​among said at least a first input torque limit, second input torque limit and at least one residual combined torque limit, generation of a control setpoint limit for said at least a first control system, the control setpoint limit being generated as a function of said critical torque limit.

[0025] The step of identifying an engine not regulated in N2' can be implemented in different ways. For example, on an aircraft with identical engines that must operate in the same way, this step may involve detecting asymmetric engine operation by detecting if a predetermined difference is exceeded between a first fuel flow rate supplied by a first fuel metering unit supplying fuel to one engine and a second fuel flow rate supplied by a second fuel metering unit supplying fuel to another engine.

[0026] According to another example, an engine not regulated in N2' can be identified when the fuel flow admitted by its fuel metering device is constant over a time interval or varies according to a control parameter different from the rotational speed N2' of the second output shaft.

[0027] According to another example, an engine not regulated in N2' can be identified when the fuel flow admitted by its fuel metering device, and therefore the position of the metering device, does not correspond to a setpoint for the flow rate or position of the metering device that it receives.

[0028] Such a step of identifying an engine not regulated in N2' can be implemented using a control system connected, either wired or wirelessly, to one or more control systems comprising one or more engine control units (ECUs), such as FADECs, managing the fuel flow setpoints for each engine. A controller within the control system can thus identify engines regulated in N2 and engines not regulated in N2'.

[0029] In addition, each N2 regulated engine can be identified when the corresponding fuel flow is variable within a predetermined time interval by following a control parameter dependent on the N2 rotational speed of each first output shaft.

[0030] Furthermore, the steps for determining the first and second input torque limits can be implemented through calculations, tests, or simulations prior to a rotorcraft flight. The first and second input torque limits are then transmitted and stored by the rotorcraft's control system.

[0031] The step of determining a first current torque can be implemented by the control system, for example, by a torque measurement carried out on the input shafts of the transmission system using sensors such as torque meters or from power and rotational speed information transmitted by engine computers of each of the N2 regulated and unregulated N2' motors.

[0032] In addition, the permissible torque limit values ​​Cadm and the reduction coefficient Z can be determined by ground tests, flight tests or simulations.

[0033] The permissible torque limit values ​​Cadm and the reduction coefficient Z can thus be stored prior to a mission of the rotorcraft in at least one memory on board the rotorcraft.

[0034] The step of determining at least one residual combined torque limit is implemented in flight after identifying an unregulated N2' engine. Indeed, the control system controller can calculate the residual combined torque limit(s) based on, firstly, at least one permissible combined torque limit of the component(s) of a combiner in the transmission system, and secondly, the first current operating torque transmitted by the unregulated N2' engine.

[0035] The permissible combined torque limit for a given component of the combiner can be determined through calculations, tests, or simulations prior to a rotorcraft flight. The permissible combined torque limit(s) are therefore predetermined values ​​transmitted and stored in the rotorcraft's control system memory.

[0036] The control system controller then identifies the critical torque limit by comparing the values ​​of at least one first input torque limit, second input torque limit and at least one residual combined torque limit.

[0037] Based on this critical torque limit, the step of generating a control setpoint limit for at least one first control system allows for the control, without pilot intervention, of, or even the limitation, of the total power transmitted jointly by the engines, regulated in N2 and unregulated in N2', to the transmission system to prevent exceeding an input torque limit or a combined torque limit. This limitation of the power transmitted to the transmission system can be performed manually by a rotorcraft pilot or automatically by the autopilot system to comply with the control setpoint limit of the first control system(s).

[0038] Such a control process thus makes it possible to protect the components of a transmission system, and in particular by limiting the torque transmitted by motors to a power transmission chain, in the event of a lack of regulation of one of the motors.

[0039] Furthermore, each of the first and second input torque limits can be defined for a maximum duration of use corresponding to a particular operating regime. Several first input torque limits for a first coupling can then form a first set of input torque limits, and several second input torque limits for a second coupling can then form a second set of input torque limits.

[0040] Consequently, each residual combined torque limit can also be defined for a maximum duration of use corresponding to a particular operating regime. Several residual combined torque limits for a component of the combiner can then form a set of residual combined torque limits.

[0041] In addition, the residual combined torque limit of an assembly corresponding to an operating regime is compared with the input torque limits of the first and second assemblies corresponding to the same operating regime.

[0042] In other words, the control process can implement simultaneously or sequentially several identifications of a critical torque limit for several operating regimes.

[0043] In practice, prior to the identification of an unregulated N2' engine, the unregulated N2' engine may initially be an N2' regulated engine which is supplied with fuel with a regulated fuel flow provided by the second fuel metering unit controlled according to a flow setpoint transmitted by a second regulation system.

[0044] In other words, the identification of an unregulated engine in N2' can be carried out in flight and result from a command voluntarily issued by the pilot, for example, for flight training purposes. The identification of an unregulated engine in N2' can also be carried out in flight as a result of a technical problem.

[0045] In practice, identifying an unregulated engine in N2' can be an identification of a failure of the second regulation system.

[0046] Such a failure could, for example, consist of a malfunction in one of the electronic boards of the secondary control system and can be detected by identifying an inconsistency between input data. As previously mentioned, this failure of the secondary control system can indeed be identified when the fuel flow rate admitted by a fuel metering device does not attempt to approach the received flow rate setpoint.

[0047] According to another advantageous aspect, the control setpoint limit can be generated so that a second current operating torque transmitted by at least one first output shaft to said at least one first coupling of the transmission system is maintained equal to or less than the critical torque limit.

[0048] Furthermore, compliance with the control setpoint limit can be achieved by reducing the control setpoint of one or more control systems or by reducing the collective pitch of the blades of a lift rotor using a collective pitch lever. Indeed, such a reduction in the collective blade pitch reduces the power required to drive the rotor to a predetermined rotational speed based on the current operating conditions.

[0049] Furthermore, when the critical torque limit is one of said at least a first input torque limit and second input torque limit, said at least a first control system can regulate the fuel flow supplied by said at least a first fuel metering device as a function of a first rotation speed setpoint Nrref1 of at least one lift rotor.

[0050] In practice, this initial rotational speed setpoint Nrref1 can be equal to a predetermined reference value or calculated from a predetermined reference value, and for example, be equal to 105% of a predetermined reference value. Such an initial rotational speed setpoint can correspond to one or more initial operating regimes of the rotorcraft in which one engine among all the engines is unable to transmit engine torque to the rotor.

[0051] In addition, the first rotation speed setpoint Nrref1 can be determined by ground tests, flight tests or simulations.

[0052] This first rotation speed instruction Nrref1 can thus be stored prior to a mission of the rotorcraft in at least one memory on board the rotorcraft.

[0053] On the other hand, when the critical torque limit is said to be at least a residual combined torque limit, said at least a first control system can regulate the fuel flow supplied by said at least a first fuel metering device as a function of a second rotational speed setpoint Nrref2 of said at least one lift rotor, the second rotational speed setpoint Nrref2 being lower than the first rotational speed setpoint Nrref1.

[0054] In this case, this second rotational speed setpoint Nrref2 can be equal to a predetermined reference value or calculated from a predetermined reference value, and for example, be equal to 100% of a predetermined reference value. This second rotational speed setpoint can correspond to one or more second operating regimes of the rotorcraft in which all engines are capable of transmitting engine torque to the lift rotor.

[0055] In addition, this second rotation speed setpoint Nrref2 can also be determined by ground tests, flight tests or simulations.

[0056] This second rotation speed instruction Nrref2 can thus be stored prior to a mission of the rotorcraft in at least one memory on board the rotorcraft.

[0057] In addition, the control method may include a display of information useful for piloting on a display arranged in or outside the rotorcraft.

[0058] According to a first alternative, when the critical torque limit is one of at least a first input torque limit and a second input torque limit, the process may include the following steps: generation of information representative of a first operating regime, and display on a display of at least one first limit indicator corresponding to the first operating regime.

[0059] Such a first limit indicator can, for example, include an index opposite a graduated scale. The index then represents a current value of engine torque transmitted by the engines. Such an index can thus move relative to the scale, for example, depending on flight conditions and maneuvers performed by the rotorcraft.

[0060] In practice, a value of the critical torque limit can be assigned to a first limit indicator among said at least one first limit indicator.

[0061] Thus, a marker on the graduated scale of the first OEI limit indicator can represent this current value of at least one critical torque limit. Such a marker then constitutes a threshold value that must not be exceeded to maintain a predetermined stress level on the engine(s) in operation for a maximum flight time.

[0062] Furthermore, several markers on the graduated scale can be displayed simultaneously and can represent different threshold values ​​that must not be exceeded to maintain different stress levels according to this initial operating regime. Each predetermined stress level can then correspond to a predetermined maximum usage time for the corresponding limit.

[0063] According to a second alternative, when the critical torque limit is the residual combined torque limit, the process may include the following steps: generation of information representative of a second operating regime, and display on a display of at least one second limit indicator corresponding to the second operating regime.

[0064] As in the first operating mode, such a second limit indicator can then, for example, include an index and a graduated scale. The index then represents a current torque value transmitted by the motors.

[0065] In practice, the value(s) of said at least one critical torque limit are in this case not affected by said at least one second limit indicator.

[0066] Advantageously, the process can include the following steps: preliminary determination of threshold values ​​of said at least a first limit indicator at least a second limit indicator, and storage of the threshold values ​​in at least one memory.

[0067] In other words, the threshold values ​​of said at least one first limit indicator and at least one second limit indicator can be determined upstream of a gyroplane mission, for example by bench tests, flight tests or simulation.

[0068] The control system then includes a memory allowing the storage of the different threshold values ​​of said at least a first limit indicator and at least a second limit indicator.

[0069] Such a memory is then advantageously carried on board the rotary-wing aircraft.

[0070] According to another advantageous embodiment, the rotorcraft comprising an autopilot system and at least one lift rotor equipped with blades, the method may include a calculation of a collective pitch margin applicable to the blades and a control of a collective pitch of the blades to automatically pilot the rotorcraft while respecting said at least one critical torque limit.

[0071] In other words, the control system can be linked to a rotorcraft autopilot system, allowing it to be piloted along four axes: roll, pitch, yaw, and collective pitch. The control system's controller can then generate and transmit to the autopilot system one or more power margins, or even directly collective pitch margins applicable to the rotor blades. The control system can also transmit to the autopilot system either information representing the first operating mode of the at least two engines, or information representing the second operating mode of the same two engines.

[0072] The present invention also relates to a rotorcraft comprising at least two combustion engines, at least one control system and a transmission system, said at least two engines being mechanically connected to the transmission system respectively by at least two couplings.

[0073] Such a rotorcraft is remarkable in that it includes a control system configured to implement the aforementioned control process.

[0074] Such a system The control system is then integrated into the rotorcraft and, as such, constitutes equipment of the rotorcraft. The control system can then be connected to a rotorcraft flight management system and an autopilot system.

[0075] Such a control system may include a computer and a memory.

[0076] The present invention also relates to a computer program comprising instructions which, when the program is executed by the control system of the previously described rotorcraft, lead to the implementation of the aforementioned control method.

[0077] The computer program is, for example, executed by a computer or calculator, comprising at least one processor, at least one integrated circuit, at least one programmable system, at least one logic circuit, and a memory, these examples not limiting the scope given to the expression "computer" or "calculator".

[0078] The memory allows the computer program to be stored as well as various information used by the computer program, namely the control instructions to be transmitted to said actuators, the current value of a first current operating torque transmitted by the second output shaft to the second coupling of the transmission system, the first and second permissible input torque limits for at least one first and one second coupling of the transmission system, the permissible residual combined torque limit(s) for at least one component of a combiner of the transmission system and the critical torque limit.

[0079] Such a memory also allows the storage of information representative of the first operating mode or information representative of the second operating mode.

[0080] The invention and its advantages will become apparent in more detail in the following description, with illustrative examples given by reference to the attached figures which represent: there figure 1 , a side view diagram of a rotorcraft according to the invention, the figure 2 , a flowchart illustrating a control method according to the invention, the figure 3 , a flowchart illustrating a first variant of the control method according to the invention, and the figure 4 , a flowchart illustrating a second variant of the control method according to the invention.

[0081] Elements present in several separate figures are assigned a single reference.

[0082] As already mentioned, the invention relates to a method for controlling a rotorcraft.

[0083] As depicted in the figure 1, such a rotary-wing aircraft 1 comprises at least two combustion engines 2, 3, of which a first engine 2 and a second engine 3. The engines 2, 3 are capable of transmitting engine torque, via a transmission system 4, to at least one lift rotor 16 ensuring at least one lift in the air of the rotary-wing aircraft 1. The engines 2, 3 are thus connected to the transmission system 4 which is connected to at least one lift rotor 16 equipped with blades 17.

[0084] The transmission system 4 includes at least one first coupling 25 connected with at least one first output shaft 5 of the first motor 2 and a second coupling 26 connected with a second output shaft 6 of the second motor 3 and at least one output shaft 18 connected to the lift rotor 16.

[0085] The transmission system 4 includes several internal components such as gears, speed reduction stages and at least one combiner 7 for example.

[0086] In addition, such a rotorcraft 1 also includes at least one first control system 12 for regulating the fuel intake of the first engine 2. Optionally, the rotorcraft 1 may include a second control system 13 for modifying the fuel intake of the second engine 3.

[0087] Furthermore, the first engine 2 is supplied with fuel by a first fuel metering unit 22 controlled by the first control system 12. The first engine 2 is thus hereafter referred to as an N2-regulated engine corresponding to the rotational speed N2 of the first output shaft 5. The first fuel metering unit 22 is then capable of modifying a fuel flow transmitted to the N2-regulated engine 2 according to a position or flow rate setpoint transmitted by the first control system 12.

[0088] Furthermore, the second engine 3 can be fueled by a second fuel metering unit 23 controlled by a second control system 13. The control of this system is not based on the N2' speed corresponding to the rotational speed N2' of the second output shaft 6, or it may be controlled without the second control system 13 when the latter malfunctions or seizes. Alternatively, the second engine 3 may not be regulated at N2'. The second engine 3 can thus be referred to hereafter as an engine not regulated at N2' within the context of the method of the invention.

[0089] The control method according to the invention is then implemented by a control system 10 connected, either wired or wirelessly, at least to the first control system 12, and possibly also to the second control system 13 when the latter is present. Such a control system 10 may include, for example, a controller 11 comprising one or more sensors, one or more computers and at least one memory 14, at least one integrated circuit, at least one programmable system, at least one logic circuit; these examples do not limit the scope given to the expression "control system." The term "computer" may refer to a processor, a central processing unit (CPU), a graphics processing unit (GPU), a digital signal processor (DSP), a microcontroller, etc.

[0090] In addition, memory 14 can be used to store a computer program implementing the control method according to the invention.

[0091] Furthermore, the controller 11 of the control system 10 is configured to determine whether a motor is an N2 regulated motor or an N2' unregulated motor.

[0092] As represented at figures 2 to 4 , the control process 30, 40 then includes an identification 31, 41, by the controller 11, of a motor regulated in N2 2 among the at least two motors 2, 3 and an identification 32, 42, by the controller 11, of a motor not regulated in N2' 3 among the at least two motors 2, 3.

[0093] Furthermore, as previously indicated, the engine not regulated in N2' can be either an engine configured not to be regulated in N2' or an engine initially regulated in N2'. Except in the case of a failure, this engine is therefore supplied with fuel at a regulated N2' rate provided by the second fuel metering unit 23, controlled according to a flow rate setpoint transmitted by the second control system 13. The identification 32 of an engine not regulated in N2' 3 can thus be implemented by identifying a failure of the second control system 13.

[0094] Once the motor(s) regulated in N2 2 and the motor not regulated in N2' 3 have been identified, the control method 30, 40 includes a determination 33, 43 of at least a first permissible input torque limit E1 for the first coupling(s) 25 of the transmission system 4 and a determination 34, 44 of a second permissible input torque limit E2 for the second coupling 26 of the transmission system 4.

[0095] The first permissible input torque limit(s) E1 and the second permissible input torque limit E2 can be determined prior to a routine mission of the rotorcraft 1 through flight tests, bench tests, or simulations. These first and second input torque limits E1, E2 can then be stored in the memory 14 of the control system 10.

[0096] Such a control method 30, 40 then involves determining 35, 45 a first operating current torque C1 transmitted by the second output shaft 6 to the second coupling 26

[0097] Such a first operating current torque C1 can in particular be determined by a calculation carried out by the controller 11 of the control system 10 from a current power delivered by the unregulated motor in N2' 3 and a rotation speed of the second output shaft 6.

[0098] Such a first operating current torque C1 can also be measured by torque sensors. By sensor, we mean here a physical sensor capable of directly measuring the parameter in question, in this case the first operating current torque, but also a system that may include one or more physical sensor(s) as well as signal processing means enabling the provision of an estimate of the parameter in question from the measurements provided by these physical sensors.

[0099] Furthermore, the control method 30, 40 includes a determination 34, 44 of at least one permissible residual combined torque limit CR1 for at least one component of the combiner 7 of the transmission system 4. Such at least one residual combined torque limit CR1 is further calculated by the controller 11 of the control system 10 as a function of the first current operating torque C1 transmitted by the second shaft 6 of the unregulated motor in N2' 3.

[0100] A residual combined torque limit CR1 is thus equal to the difference between an permissible combined torque limit Cadm for a component of the combiner 7 and the product between the first current operating torque C1 and a reduction coefficient Z between a rotation speed of the second coupling 26 and a rotation speed of the component of the combiner 7.

[0101] Furthermore, this permissible torque limit (Cadm) can be determined prior to a routine mission of the rotorcraft 1 through flight tests, bench tests, or simulations. This permissible torque limit (Cadm) can also be stored in memory 14 of the control system 10.

[0102] The control method 30, 40 then includes an identification 37, 47 by the controller 11 of the control system 10 of at least one critical torque limit CC1. Such a critical torque limit CC1 is then identified as the smallest of the values ​​among the first input torque limit(s) E1, the second input torque limit E2 and the residual combined torque limit(s) CR1.

[0103] The control process 30, 40 finally includes the generation 38, 48 of a control setpoint limit COM of the first control system(s) 22 for the motor(s) 2 regulated in N2. Such a control setpoint limit COM is then generated by the controller 11 of the control system 10 as a function of at least one critical torque limit CC1. This generation step 38 of the control setpoint limit COM of the first control system(s) 22 makes it possible to automatically limit the total power transmitted jointly by the motors 2 and 3 to the transmission system 4 in order to prevent exceeding an input torque limit E1, E2 or a residual combined torque limit CR1.

[0104] The control system 10 can thus limit the COM control setpoint in order to respect at least one critical torque limit CC1.

[0105] Furthermore, when the critical torque limit CC1 is one of at least one first input torque limit E1 and second input torque limit E2, said at least one first control system 12 can regulate the fuel flow supplied by the first associated fuel metering unit 22 according to a first rotational speed setpoint Nrref1 of at least one lift rotor 16 of the rotorcraft 1. Such a first rotational speed setpoint Nrref1 may also correspond to a nominal rotational speed of the lift rotor(s) 16 or be calculated from the nominal rotational speed, for example, by being equal to 105% of the nominal rotational speed. This first rotational speed setpoint Nrref1 is also used, for example, when one of the engines 2 or 3 does not transmit engine torque to one of the first and second input shafts 5, 6.

[0106] On the other hand, when the critical torque limit CC1 is said to be at least a residual combined torque limit CR1, said at least a first control system 12 can regulate the fuel flow supplied by said at least a first fuel metering device 22 as a function of a second rotational speed setpoint Nrref2 of the lift rotor(s) 16, the second rotational speed setpoint Nrref2 being lower than the first rotational speed setpoint Nrref1.

[0107] Such a second rotational speed setpoint Nrref2 can correspond to a rotational speed equal to 100% of the nominal rotational speed of the lift rotor(s) 16. This second rotational speed setpoint Nrref2 can be used when each of the motors 2 and 3 transmits a motor torque respectively to the first and second input shafts 5, 6.

[0108] The control system 10 can then limit the power delivered by the N2 regulated motor(s) 2 according to the critical torque limit CC1.

[0109] As depicted in the figure 3 , when said of at least one critical torque limit CC1 is one of said at least one first or second input torque limits E1, E2, the method 30 may include a generation 391 of information KOEI representative of an asymmetric operation, said OEI, of said at least two motors 2, 3.

[0110] In this case, the process 30 then includes a display 392 on a display 8 of at least one OEI limit indicator 9 corresponding to the asymmetric operation OEI.

[0111] Such a display 8 is for example formed by a screen equipping a dashboard of a rotorcraft cockpit 1 or by a head-up display system worn by a pilot and arranged for example on a helmet or glasses.

[0112] In this case the display 8 is then connected by wired or wireless means to the control system 10 and can display a value of the critical torque limit CC1 on the limit indicator OEI 9 in accordance with an asymmetric operation OEI of the motors 2, 3.

[0113] According to another embodiment, such a display 8 can also be located outside the gyroplane 1, for example when it is remotely piloted.

[0114] As depicted in the figure 4 , when said of at least one critical torque limit CC1 is said at least one residual combined torque limit CR1, such a method 40 may then include a generation 491 of information KAEO representative of a symmetric operation, said AEO, of said at least two motors 2, 3, and a display 48 on the display 8 of at least one AEO limit indicator 19 corresponding to the symmetric AEO operation.

[0115] Furthermore, as represented in figures 3 And 4 , the control process 30, 40 may include a preliminary determination 300, 400 of SAEO, SOEI threshold values ​​of said at least one AEO limit indicator 19 and at least one OEI limit indicator 9.

[0116] Furthermore, the control process 30, 40 may include storage 301, 401 of the SAEO, SOEI threshold values ​​of said at least one AEO limit indicator 19 and at least one OEI limit indicator 9.

[0117] In addition, the rotorcraft 1 may also include an autopilot system 15, which has the capacity to act on the collective pitch of the blades of the lift rotor(s) 16. Such an autopilot system 15 is then connected by wired or wireless means to the control system 10.

[0118] The process 30, 40 can then include a calculation 393, 493 of the collective pitch margins MPC applicable to said blades 17. The calculation 393, 493 can be implemented by the control system 10 which transmits it to the automatic piloting system 15.

[0119] The process 30, 40 can then include a control 394, 494 of a collective pitch PC of the blades 17 to automatically pilot the rotorcraft 1 while respecting said at least a critical torque limit CC1.

[0120] Naturally, the present invention is subject to numerous variations in its implementation. Although several embodiments have been described, it is understood that it is not possible to exhaustively identify all possible embodiments. It is, of course, conceivable to replace a described means with an equivalent means without departing from the scope of the present invention.

Claims

1. Method for controlling (30, 40) a rotorcraft (1), said rotorcraft (1) comprising at least two combustion engines (2, 3), at least one regulation system (12, 13) and a transmission system (4), said at least two engines (2, 3) being mechanically connected to said transmission system (4) respectively by at least two couplings (25, 26), characterised in that the control method (30, 40) comprises at least the following steps: - identification (31, 41) of at least one so-called N2-regulated engine (2) among said at least two engines (2, 3), said at least one N2-regulated engine (2) being identified as an engine supplied with fuel with a fuel flow rate regulated as a function of the rotational speed N2 of at least one first output shaft (5) of said at least one N2-regulated engine and supplied by at least one first fuel metering device (22) piloted as a function of a flow rate setpoint transmitted by at least one first regulation system (12) dependent on the rotational speed N2 of said at least one first output shaft (5), - identification (32, 42) of a so-called non-regulated N2' engine (3) among said at least two engines (2, 3), said non-regulated N2' engine (3) being identified as an engine supplied with fuel with a non-regulated fuel flow rate as a function of the rotation speed N2' of a second output shaft (6) and provided by a second fuel metering device (23) being either non-regulated or piloted as a function of a flow rate setpoint transmitted by at least one second regulation system (13) independent of the rotation speed N2' of said second output shaft (6), - determination (33, 43) of at least a first permissible input torque limit (E1) for at least one first coupling (25) of said transmission system (4) connected to said at least one first output shaft (5), - determination (34, 44) of a second permissible input torque limit (E2) for a second coupling (26) of said transmission system (4) connected to said second output shaft (6), - determination (35, 45) of a first current operating torque (C1) transmitted by said second output shaft (6) to said second coupling (26) of said transmission system (4), - determination (36, 46) of at least one permissible residual combined torque limit (CR1) for at least one member of a combiner (7) of said transmission system (4), said at least one residual combined torque limit (CR1) being calculated according to the formula: CR 1 = Cadm − C 1 * Z wherein Cadm is a permissible combined torque limit for said at least one member of said combiner (7), C1 is said first current operating torque transmitted by said second output shaft (6) to said second coupling (26) of said transmission system, and Z a reduction coefficient between a rotational speed of said second coupling (26) and a rotational speed of said at least one member of said combiner (7), - identification (37, 47) of a critical torque limit (CC1) as being the smallest of the values among said at least one first input torque limit (E1), second input torque limit (E2) and at least one residual combined torque limit (CR1), - generation (38, 48) of a control setpoint limit (COM) of said at least one first regulation system (12), said control setpoint limit (COM) being generated as a function of said critical torque limit (CC1).

2. Control method according to claim 1, characterised in that, prior to said identification (32) of a non-regulated N2' engine (3), said non-regulated N2' engine (3) is initially a regulated N2' engine which is supplied with fuel with a regulated N2' fuel flow rate provided by said second fuel metering device (23) piloted as a function of a flow rate setpoint transmitted by a second regulation system (13).

3. Control method according to claim 2, characterised in that said identification (32) of a non-regulated N2' engine (3) is an identification of a failure of said second regulation system (13).

4. Control method according to any one of claims 1 to 3, characterised in that said control setpoint limit (COM) is generated such that a second current operating torque (C2) transmitted by said at least one first output shaft (5) to said at least one first coupling (25) of said transmission system (4) is kept equal to or less than said critical torque limit (CC1).

5. Control method according to any one of claims 1 to 4, characterised in that, when said critical torque limit (CC1) is one of said at least one first input torque limit (E1) and second input torque limit (E2), said at least one first regulation system (12) regulates the flow rate of fuel supplied by said at least one first fuel metering device (22) as a function of a first rotation speed setpoint (Nrref1) of at least one lift rotor (16) of said rotorcraft (1).

6. Control method according to claim 5, characterised in that, when said critical torque limit (CC1) is said at least one residual combined torque limit (CR1), said at least one first regulation system (12) regulates the flow rate of fuel supplied by said at least one first fuel metering device (22) as a function of a second rotational speed setpoint (Nrref2) of said at least one lift rotor (16) of said rotorcraft (1), said second rotational speed setpoint (Nrref2) being less than said first rotational speed setpoint (Nrrefl).

7. Control method according to any one of claims 1 to 6, characterised in that, when said critical torque limit (CC1) is one of said at least one first input torque limit (E1) and second input torque limit (E2), said method (30) comprises the following steps: - generation (391) of information (KOEI) representative of a first operating speed (OEI) , and - displaying (392) on a display unit (8) of at least one first limit indicator (91, 92) corresponding to said first operating speed (OEI).

8. Control method according to claim 7, characterised in that a value of said critical torque limit (CC1) is assigned to a first limit indicator (91) among said at least one first limit indicator (91, 92).

9. Control method according to any one of claims 1 to 6, characterised in that, when said critical torque limit (CC1) is said residual combined torque limit (CR1), said method (40) can comprise the following steps: - generation (491) of information (KAEO) representative of a second operating speed (AEO), and - displaying (492) on a display unit (8) of at least one second limit indicator (191, 192) corresponding to said second operating speed (AEO)10. Control method according to claims 7 and 9, characterised in that said method (30, 40) comprises the following steps: - preliminary determination (300, 400) of threshold values (SAEO, SOEI) of said at least one first limit indicator (9) and of at least one second limit indicator (19), and - storing (301, 401) of said threshold values (SAEO, SOEI) in at least one memory (14).

11. Control method according to any one of claims 1 to 10, characterised in that, said rotorcraft (1) comprising an automatic piloting system (15) and at least one lift rotor (16) equipped with blades (17), said method (30, 40) comprises a calculation (393, 493) of a collective pitch margin (MPC) applicable to said blades (17) and a control (394, 494) of a collective pitch (PC) of said blades (17) to automatically pilot said rotorcraft (1) while respecting said at least one critical torque limit (CC1).

12. Rotorcraft (1) comprising at least two combustion engines (2, 3), at least one regulation system (13) and a transmission system (4), said at least two engines (2, 3) being mechanically connected to said transmission system (4) respectively by at least two couplings (25, 26), characterised in that said rotorcraft (1) comprises a control system (10) configured to implement said control method (30, 40) according to any one of claims 1 to 11.

13. Computer program comprising instructions which, when said program is executed by said rotorcraft control system (10) according to claim 12, lead to implementing the control method according to any one of claims 1 to 11.