Geared differential speed counter-rotatable low pressure turbine

An aircraft gas turbine engine comprising:

• a single stage fan section having a single fan stage having a single fan blade row,
• a counter-rotatable low pressure turbine drivingly connected to the single fan stage by a low pressure shaft,
• the counter-rotatable low pressure turbine including counter-rotatable annular outer and inner drum rotors,
• the outer drum rotor rotatably connected to the low pressure shaft wherein the low pressure shaft is the sole shaft operable for transmitting torque and power out of the low pressure turbine,
• a plurality of axially spaced apart low pressure outer drum turbine blade rows extending radially inwardly from an outer shell of the outer drum rotor,
• the low pressure inner drum rotor including a plurality of axially spaced apart low pressure inner drum turbine blade rows extending radially outwardly from the low pressure inner drum rotor,
• the low pressure outer drum turbine blade rows being interdigitated with the low pressure inner drum turbine blade rows, and
• the inner drum rotor being geared through an epicyclic gearbox for transmitting torque and power produced by the inner drum rotor to the low pressure shaft.

An aircraft gas turbine engine family comprising:

• at least two different engine models having within the family having different size fans and/or operational design fan speeds and different epicyclic gearboxes with different gear ratios respectively for maintaining the same relative differential speed between outer and inner drum turbine blade rows of a counter-rotatable low pressure turbine of the different engine models;
• each of the engine models includes an aircraft gas turbine engine a single stage fan section having a single fan stage having a single fan blade row and a respective one of the different size fans and/or operational design fan speeds,
• the counter-rotatable low pressure turbine drivingly connected to the single fan stage by a low pressure shaft,
• the outer drum rotor rotatably connected to the low pressure shaft wherein the low pressure shaft is the sole shaft operable for transmitting torque and power out of the low pressure turbine,
• a plurality of axially spaced apart low pressure outer drum turbine blade rows extending radially inwardly from an outer shell of the outer drum rotor,
• the low pressure inner drum rotor including a plurality of axially spaced apart low pressure inner drum turbine blade rows extending radially outwardly from the low pressure inner drum rotor,
• the low pressure outer drum turbine blade rows being interdigitated with the low pressure inner drum turbine blade rows, and
• the inner drum rotor being geared through the epicyclic gearbox for transmitting torque and power produced by the inner drum rotor to the low pressure shaft.