Method and a device for limiting torque on a rotorcraft having at least three motors with non-equal power distribution

1. A method of limiting mechanical torque on a rotorcraft, the rotorcraft comprising an installation having a rotor, the installation including at least three motors driving a power transmission gearbox, the power transmission gearbox including a rotor mast for moving the rotor, wherein the method includes a step of determining an engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3) for each of the at least three motors, each engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3) being established by a control computer of the aircraft by taking into consideration a distribution among the at least three motors of the total power (Ptot) delivered by the at least three motors for enabling the rotorcraft to operate,

the step of determining a engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3) for each of the three motors comprising the following steps:

determining (STP 1 ) with the control computer (STP 2 ) for each of the at least three motors a respective distribution coefficient (coef, alpha, beta, gamma), the distribution coefficient (coef, alpha, beta, gamma) of each of the three motors giving the percentage of the total power (Ptot) to be delivered by that motor;

determining with the control computer (STP 2 . 1 ) for each of the at least three motors the associated engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3) by taking into consideration the associated distribution coefficient (coef, alpha, beta, gamma) with the following steps:

determining with the control computer (STP 2 . 2 ) for each of the at least three motors an intermediate limit (intlim) as a function of a stored common rotor torque limit (rotorlim) and the associated distribution coefficient (coef, alpha, beta, gamma); and

determining with the control computer the engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3) for each of the at least three motors, a respective gearbox inlet torque limit (GBtorquelim1, GBtorquelim2, GBtorquelim3) being stored for each of the at least three motors, the engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3) associated with each of the at least three motors being equal to the minimum from among the gearbox inlet torque limit (GBtorquelim1, GBtorquelim2, GBtorquelim3) allocated to the motor and the intermediate limit (intlim) allocated to the motor.

2. The method according to claim 1 , wherein for each of the at least three motors, the intermediate limit (intlim) is obtained by applying the following equation:

intlim=[coef*(rotorlim* Nr )/( N 2)]

where “intlim” represents the intermediate limit allocated to a motor, “coef” represents the distribution coefficient allocated to the motor, “rotorlim” represents the rotor torque limit, “Nr” represents the speed of rotation of the rotor, “N2” represents a speed of rotation of an outlet shaft of the motor, “*” represents the multiplication sign, and “/” represents the division sign.

3. The method according to claim 1 , wherein the method includes a step of displaying (STP 3 . 1 ) an indication on an instrument relating at least to one engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3).

4. The method according to claim 1 , wherein the method includes, for each of the three motors, the following steps:

determining a current engine torque allocated to the motor;

regulating with a motor computer the motor as a function of the current engine torque and of the engine torque limit (enginetorquelim1, enginetorquelim2, enginetorquelim3) allocated to the motor.

5. The method according to claim 1 , wherein the current engine torque allocated to a motor is measured on an outlet shaft of the motor.

6. A rotorcraft, the rotorcraft comprising an installation having a rotor, the installation having at least three motors driving a power transmission gearbox, the power transmission gearbox having a rotor mast for driving the rotor, wherein the rotorcraft includes a control computer configured to apply the method according to claim 1 .