Multi-ratio rotorcraft drive system and a method of changing gear ratios thereof

A method of reducing the speed of a rotor system driven by one or more engines from a high speed to a low speed, the method comprising:

• lowering the speed of the rotor system from the high speed to a transitional speed;
• shifting a first multi-ratio transmission coupled to a first engine of the one or more engines from a high gear train integrating a high speed clutch to a low gear train integrating a low speed clutch while the speed of the rotor system is maintained at or near the transitional speed;
• shifting a second multi-ratio transmission coupled to a second engine of the one or more engines from a high gear train integrating a high speed clutch to a low gear train integrating a low speed clutch while the speed of the rotor system is maintained at or near the transitional speed;
• and
• restoring the speed of the first engine and second engine to an optimum engine speed, causing the speed of the rotor system to be reduced to the low speed.

A method of increasing the speed of a rotor system driven by one or more engines from a low speed to a high speed, the method comprising:

• increasing the speed of the rotor system from the low speed to a transitional speed;
• shilling a first multi-ratio transmission coupled to a first engine of the one or more engines from a low gear train integrating a low speed clutch to a high gear train integrating a high speed clutch while the speed of the rotor system is maintained at or near the transitional speed;
• shifting a second multi-ratio transmission coupled to a second engine of the one or more engines from a low gear train integrating a low speed clutch to a high gear train integrating a high speed clutch while the speed of the rotor system is maintained at or near the transitional speed;
• and
• restoring the speed of the first engine and second engine to an optimum engine speed, causing the speed of the rotor system to be increased to the high speed.