Flexible coupling for geared turbine engine

A gas turbine engine comprising:

• a fan;
• a fan shaft coupled with the fan and arranged along an engine central axis;
• a frame supporting the fan shaft, the frame defining a lateral frame stiffness (LFS);
• an epicyclic gear system coupled to the fan shaft;
• and
• a non-rotatable flexible coupling and a rotatable flexible coupling supporting the epicyclic gear system, the non-rotatable flexible coupling and the rotatable flexible coupling being subject to a Motion I of parallel offset guided end motion with respect to the engine central axis,
• the non-rotatable flexible coupling and the rotatable flexible coupling each having a stiffness of a common stiffness type under a common type of motion with respect to the engine central axis, the stiffness being defined with respect to the LFS, the common stiffness type is a Stiffness C and the common type of motion is the Motion I, the Stiffness C of the rotatable flexible coupling being greater than the stiffness of the non-rotatable flexible coupling, and a ratio of LFS/Stiffness C of the non-rotatable flexible coupling is in a range of 1.5-7.

A gas turbine engine comprising:

• a fan;
• a fan shaft coupled with the fan and arranged along an engine central axis;
• a frame supporting the fan shaft, the frame defining a lateral frame stiffness (LFS);
• an epicyclic gear system coupled to the fan shaft;
• and
• a non-rotatable flexible coupling and a rotatable flexible coupling supporting the epicyclic gear system, the non-rotatable flexible coupling and the rotatable flexible coupling being subject to a Motion III of angular misalignment no offset motion with respect to the engine central axis,
• the non-rotatable flexible coupling and the rotatable flexible coupling each having a stiffness of a common stiffness type under a common type of motion with respect to the engine central axis, the stiffness being defined with respect to the LFS, the common stiffness type is a Stiffness E and the common type of motion is the Motion III, the Stiffness E being defined with respect to the LFS, the Stiffness E of the rotatable flexible coupling being greater than the stiffness of the non-rotatable flexible coupling, and a ratio of LFS/Stiffness E of the non-rotatable flexible coupling is in a range of 6-40.