Power transmission gearbox presenting a modifiable speed of rotation at its outlet, and a corresponding method of operation

A power transmission gearbox for rotorcraft for transmitting rotary movement from at least two engine members (MG, MD) to a main shaft (1) with determined reduction ratios, wherein the gearbox presents at least two transmission systems (G, D) for imparting rotary movement to the main shaft (1), each of the transmission systems comprising:

• an inlet shaft (3) designed to be driven in rotation by an engine member (MG, MD);
• at least two reduction stages (4, 5) presenting different reduction ratios (k2, k1);
• a first gearwheel (6) connected to the inlet shaft (3) via a declutchable freewheel (7), said gearwheel (6) meshing with a first toothed wheel (8) secured to an intermediate shaft (9), thereby defining at least a first reduction stage (4);
• a second gearwheel (10) connected to the inlet shaft (3) via a simple freewheel (11), said second gearwheel (10) meshing with a second toothed wheel (12) secured to the intermediate shaft (9), thereby defining the second reduction stage (5);
• a main freewheel (13) mounted on the intermediate shaft (9) to rotate a complementary toothed wheel (14) secured to the main shaft (1);
• and
• a clutch mechanism for the declutchable freewheel (7) for the purpose, depending on circumstances, of putting the inlet shaft (3) into engagement with said declutchable freewheel (7) or for drivingly isolating the inlet shaft (3) from said declutchable freewheel (7).