Flexible support structure for a geared architecture gas turbine engine

1. A gas turbine engine comprising:

a fan shaft;

a support which supports the fan shaft, the support defines a support lateral stiffness;

a gear system to drive the fan shaft;

a flexible support which at least partially supports the gear system, the flexible support defines a flexible support lateral stiffness with respect to the support lateral stiffness; and

an input to the gear system, the input defining an input lateral stiffness with respect to the support lateral stiffness.

2. The gas turbine engine as recited in claim 1 , wherein the support and the flexible support are mounted to a static structure.

3. The gas turbine engine as recited in claim 1 , wherein the support and the flexible support are mounted to a front center body of a gas turbine engine.

4. The gas turbine engine as recited in claim 1 , wherein the flexible support is mounted to a planet carrier of the gear system.

5. The gas turbine engine as recited in claim 4 , wherein the input is mounted to a sun gear of the gear system.

6. The gas turbine engine as recited in claim 5 , wherein the fan shaft is mounted to a ring gear of the gear system.

7. The gas turbine engine as recited in claim 6 , wherein the gear system is a star system.

8. The gas turbine engine as recited in claim 1 , wherein the flexible support is mounted to a ring gear of the gear system.

9. The gas turbine engine as recited in claim 8 , wherein the input is mounted to a sun gear of the gear system.

10. The gas turbine engine as recited in claim 9 , wherein the fan shaft is mounted to a planet carrier of the gear system.

11. The gas turbine engine as recited in claim 10 , wherein the gear system is a planet system.

12. The gas turbine engine as recited in claim 1 , further comprising a low speed spool for driving the input.

13. The gas turbine engine as recited in claim 1 , wherein the flexible support lateral stiffness and the input lateral stiffness are both less than the support lateral stiffness.

14. The gas turbine engine as recited in claim 13 , wherein at least one of the flexible support lateral stiffness and the input lateral stiffness is less than about 20% of the support lateral stiffness.

15. The gas turbine engine as recited in claim 14 , wherein the flexible support lateral stiffness and the input lateral stiffness are each less than about 20% of the support lateral stiffness.

16. The gas turbine engine as recited in claim 13 , wherein at least one of the flexible support lateral stiffness and the input lateral stiffness is less than about 11% of the support lateral stiffness.

17. The gas turbine engine as recited in claim 16 , wherein the flexible support lateral stiffness and the input lateral stiffness are each less than about 11% of the support lateral stiffness.

18. The gas turbine engine as recited in claim 1 , wherein a turbine provides an input to the gear system, and the gear system further drives a compressor rotor at a common speed with the fan shaft.

19. The gas turbine engine as recited in claim 1 , comprising a turbine section to drive the gear system and at least two compressor rotors, wherein the turbine section includes a fan drive turbine to drive the gear system and at least two other turbines to drive the at least two compressor rotors.

20. A gas turbine engine comprising:

a fan shaft;

a support which supports the fan shaft;

a gear system to drive the fan shaft, the gear system includes a gear mesh that defines a gear mesh lateral stiffness;

a flexible support which at least partially supports the gear system, the flexible support defining a flexible support lateral stiffness with respect to the gear mesh lateral stiffness; and

an input to the gear system, the input defining an input lateral stiffness with respect to the gear mesh lateral stiffness.

21. The gas turbine engine as recited in claim 20 , wherein both the flexible support lateral stiffness and the input lateral stiffness are less than the gear mesh lateral stiffness.

22. The gas turbine engine as recited in claim 21 , wherein the flexible support lateral stiffness is less than about 8% of the gear mesh lateral stiffness.

23. The gas turbine engine as recited in claim 21 , wherein the input lateral stiffness is less than about 5% of the gear mesh lateral stiffness.

24. The gas turbine engine as recited in claim 20 , wherein a lateral stiffness of a ring gear of the gear system is less than about 20% of the gear mesh lateral stiffness.

25. The gas turbine engine as recited in claim 20 , wherein a lateral stiffness of a ring gear of the gear system is less than about 12% of the gear mesh lateral stiffness.

26. The gas turbine engine as recited in claim 20 , wherein a lateral stiffness of a planet journal bearing which supports a planet gear of the gear system is less than or equal to the gear mesh lateral stiffness.

27. A method of designing a gas turbine engine comprising:

providing a fan shaft;

providing a support which supports the fan shaft, the support defining a support lateral stiffness;

providing a gear system to drive the fan shaft;

providing a flexible support which at least partially supports the gear system, the flexible support defining a flexible support lateral stiffness with respect to the support lateral stiffness; and

providing an input to the gear system, the input defining an input lateral stiffness with respect to the support lateral stiffness.

28. The method as recited in claim 27 , comprising a turbine section to drive the gear system and at least two compressor rotors, wherein the turbine section includes a fan drive turbine to drive the gear system and at least two other turbines to drive the at least two compressor rotors.

29. A method of designing a gear system for a gas turbine engine comprising:

designing an output shaft;

designing a support which supports the output shaft, the support defining at least one of a support transverse stiffness and a support lateral stiffness;

designing a gear assembly to drive the output shaft;

designing a flexible support which at least partially supports the gear assembly, the flexible support defining at least one of a flexible support transverse stiffness and a flexible support lateral stiffness with respect to the at least one of the support transverse stiffness and the support lateral stiffness; and

designing an input to the gear assembly, the input defining at least one of an input transverse stiffness and an input lateral stiffness with respect to the at least one of the support transverse stiffness and the support lateral stiffness.