Midshaft Rating For Turbomachine Engines

A turbomachine engine comprising:

• a fan section having a fan shaft supported by a fan bearing;
• a turbomachine including a compressor section, a turbine section that includes a power turbine, and a combustion chamber in flow communication with the compressor section and the turbine section;
• a low-pressure shaft coupled to the power turbine and the fan shaft, the low-pressure shaft being supported by a forward bearing and an aft bearing and characterized by a midshaft rating (MSR) between two hundred (ft/sec)1/2 and three hundred (ft/sec)1/2, wherein the low-pressure shaft has a length L defined between the forward bearing and the aft bearing, an outer diameter D, and a ratio L/D between twelve and thirty-seven, and the turbomachine engine is configured to operate up to a redline speed of the low-pressure shaft without passing through a critical speed associated with a first-order bending mode of the low-pressure shaft;
• a gearbox assembly that couples the fan shaft to the low-pressure shaft;
• an engine static structure having a fan frame supporting the fan bearing;
• a flex coupling that couples the gearbox assembly to the low-pressure shaft;
• and
• a flex mount that couples the gearbox assembly to the engine static structure.