Method and system for dual-voltage start of an aeronautical turbine engine having a free turbine and a single-spool gas generator

1 . A method for starting an aeronautical free turbine and single-spool gas generator turbomachine of a twin-engine aircraft including two independent electrical networks each comprising a 28V battery selectively powering a starter-generator, wherein to ensure, under the control of a turbomachine regulation computer, the startup of the gas generator turbomachine first at a nominal voltage of 28V by putting the two batteries in parallel and then at a nominal voltage of 56V by putting the two batteries in series while avoiding too rapid acceleration of the gas generator turbomachine, the serialization of the two batteries is ordered only once a combustion chamber of the gas generator turbomachine is ignited and the speed of the gas generator turbomachine is greater than a predetermined speed threshold N 1 for ensuring, through this serialization, a positive acceleration margin at a point of maximum drag of the gas generator.

2 . The starting method according to claim 1 , wherein the speed threshold N 1 of the gas generator turbomachine is comprised between 10 and 25% of a nominal speed NTOP of the gas generator turbomachine.

3 . The starting method according to claim 1 , wherein the serialization of the two batteries is further ordered if, once the combustion chamber has been ignited and before the maximum drag point, the acceleration of the gas generator is smaller than a predetermined acceleration threshold DN 2 , so as to avoid a risk of stagnation of the startup.

4 . The starting method according to claim 3 , wherein the acceleration threshold DN 2 of the gas generator turbomachine is comprised between 1 and 3% of a nominal speed NTOP/s of the gas generator turbomachine.

5 . The starting method according to claim 1 , wherein, in order to avoid a possible overlap of the electrical contactors resulting in short-circuiting one of the two batteries during reconfiguration, the serialization of the two batteries is preceded by a dead time lasting between 150 and 300 ms.

6 . A system for starting an aeronautical free turbine and single-spool gas generator turbomachine of a twin-engine aircraft including two independent electrical networks each comprising a 28V battery selectively powering a starter-generator, a turbomachine regulation computer controlling the startup of the gas generator turbomachine first at a nominal voltage of 28V by putting the two batteries in parallel and then at a nominal voltage of 56V by putting the two batteries in series while avoiding too rapid acceleration of the gas generator turbomachine, wherein the turbomachine regulation computer is configured to order the serialization of the two batteries only once a combustion chamber of the gas generator turbomachine is ignited and the speed of the gas generator turbomachine is greater than a predetermined speed threshold N 1 for ensuring, through this serialization, a positive acceleration margin at the point of maximum drag of the gas generator.

7 . The starting system according to claim 6 , further including a ground connection intended to be connected to a 28V ground power unit.

8 . The starting system according to claim 7 , wherein the turbomachine regulation computer is further configured to power a first starter-generator from the ground power unit and then by putting in series the ground power unit with one of the two batteries, once the combustion chamber is ignited and the speed of the gas generator turbomachine is greater than a predetermined speed threshold N 1 for ensuring, through this serialization, a positive acceleration margin at the point of maximum drag of the gas generator turbomachine.

9 . A rotary-wing or fixed-wing twin-engine aircraft turbomachine comprising a starting system according to claim 6 .

10 . The rotary-wing or fixed-wing twin-engine aircraft comprising two turbomachines according to claim 9 .