Device for monitoring a power transmission system of an aircraft, an aircraft provided with the device, and the method used

1. A method of monitoring a power transmission system of an aircraft, the aircraft including at least one main rotor driven in rotation by the power transmission system, the power transmission system comprising a main gearbox connected to the main rotor by at least one rotor mast, the aircraft including an auxiliary unit with an auxiliary rotor and a mechanical assembly of the power transmission system, the mechanical assembly mechanically connecting the auxiliary rotor to the main gearbox, the aircraft including a power plant comprising at least one engine for driving the main gearbox, the aircraft including control means for controlling the auxiliary rotor,

the method comprising the following steps:

determining a power relationship providing a first power parameter for the auxiliary unit as a function of a position (POSACTU) of the control means, the first power parameter being a power parameter reduced by at least one reduction variable;

setting a setpoint limit (Prot) for a second power parameter for the rotor mast;

determining a current position (POSACTU) of the control means;

determining an “intermediate” value (Vint) of the first power parameter corresponding to the current position (POSACTU) in application of the power relationship;

measuring a “current” value (Vactu) of each reduction variable;

correcting the intermediate value (Vint) at least by using each current value (Vactu) in order to obtain a non-reduced value referred to as the “calculated” value (Vcal); and

determining a “limit” value (Peng) that is not to be exceeded by a monitoring parameter of the power plant, the limit value (Peng) being equal to the sum of at least the setpoint limit (Prot) plus the calculated value (Vcal).

2. A method according to claim 1 , wherein the setpoint limit (Prot) is an authorized power limit on the rotor mast.

3. A method according to claim 1 , wherein the limit value is:

either equal to the sum of the setpoint limit (Prot) plus the calculated value (Vcal);

or else equal to the sum of the setpoint limit (Prot) plus the calculated value (Vcal) plus a predetermined takeoff power (Pext).

4. A method according to claim 1 , wherein the limit value is a power and the limit value (Peng) is used to obtain for each engine a respective limit torque (Tqlim) that is not to be exceeded.

5. A method according to claim 4 , comprising the following steps:

measuring a current torque (Tqactu) developed by each engine; and

comparing the current torque (Tqactu) of an engine with the associated limit torque (Tqlim).

6. A method according to claim 1 , wherein at least one reduction variable is a variable measured by an air data computer of the aircraft.

7. A method according to claim 6 , wherein at least one reduction variable is selected from a list comprising at least one of the following parameters: the relative density (σ) of the air; the temperature and the pressure of the outside air; a speed of advance of the aircraft; and a vertical speed of the aircraft.

8. A method according to claim 1 , wherein the calculated value is equal to the intermediate value (Vint) corrected by each current value.

9. A method according to claim 1 , wherein in order to determine the calculated value (Vcal), the following steps are performed:

determining a current flight stage of the aircraft;

determining an assumed power value (Vforf) as a function of the current flight stage;

determining a non-reduced calculation value equal to the intermediate value (Vint) corrected by each current value; and

if the calculation value is less than the assumed power value (Vforf), the calculated value (Vcal) is equal to the sum of a predetermined accuracy margin (Mrg) plus the calculation value, the calculated value (Vcal) being bounded by the assumed power value (Vforf) in order to be no greater than the assumed power value (Vforf); or

if the calculation value is greater than the assumed power value (Vforf), the calculated value (Vcal) is equal to the difference of the calculation value minus the accuracy margin (Mrg), the calculated value being bounded by the assumed power value (Vforf) so as to be no less than the assumed power value (Vforf).

10. A method according to claim 9 , wherein the assumed power value (Vforf) is equal to a first constant while the aircraft is flying at a speed of advance less than a predetermined speed threshold, and the assumed power value (Vforf) is equal to a second constant while the aircraft is flying at a speed of advance greater than or equal to the predetermined speed threshold.

11. A monitoring device for monitoring a power transmission system of an aircraft, the aircraft including at least one main rotor driven in rotation by the power transmission system, the power transmission system comprising a main gearbox connected to the main rotor by at least one rotor mast, the aircraft including an auxiliary unit with an auxiliary rotor and a mechanical assembly of the power transmission system, the mechanical assembly mechanically connecting the auxiliary rotor to the main gearbox, the aircraft including a power plant comprising at least one engine for driving the main gearbox, the aircraft including control means for controlling the auxiliary rotor, wherein the monitoring device is designed to apply the method according to claim 1 , the monitoring device comprising a processor unit having calculation means and a memory, the memory storing instructions together with the power relationship and the setpoint limit, the monitoring device further comprising a measurement system for measuring a current position (POSACTU) of the control means, the processor unit being connected to the measurement system, and the calculation means executing the instructions in order to apply the above method.

12. A device according to claim 11 , wherein the monitoring device includes an air data computer connected to the processor unit.

13. A device according to claim 11 , wherein the monitoring device includes one torque meter per engine measuring a current torque on a shaft extending between the engine and the main gearbox.

14. A device according to claim 11 , wherein the monitoring device includes a display device for displaying a limit that is a function of the limit value.

15. An aircraft including at least one main rotor driven in rotation by a power transmission system, the power transmission system comprising a main gearbox connected to the main rotor by at least one rotor mast, the aircraft including an auxiliary unit with an auxiliary rotor and a mechanical assembly of the power transmission system, the mechanical assembly mechanically connecting the auxiliary rotor to the main gearbox, the aircraft including a power plant comprising at least one engine for driving the main gearbox, the aircraft including control means for controlling the auxiliary rotor, wherein the aircraft includes a monitoring device according to claim 11 .