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 at least one of a support transverse stiffness and 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 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

an input to the gear system, 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.

2. The gas turbine engine as recited in claim 1 , wherein the at least one of the support transverse stiffness and the support lateral stiffness is a support transverse stiffness, the at least one of the flexible support transverse stiffness and the flexible support lateral stiffness is a flexible support transverse stiffness, and the at least one of the input transverse stiffness and the input lateral stiffness is an input transverse stiffness.

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

4. The gas turbine engine as recited in claim 2 , wherein the support and the flexible support are mounted to a front center body of the gas turbine engine.

5. The gas turbine engine as recited in claim 2 , wherein the flexible support is mounted to a planet carrier of the gear system.

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

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

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

9. The gas turbine engine as recited in claim 2 , wherein the flexible support is mounted to a ring gear of the gear system.

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

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

12. The gas turbine engine as recited in claim 2 , further comprising a low speed spool for driving the input.

13. The gas turbine engine as recited in claim 2 , wherein the flexible support transverse stiffness and the input transverse stiffness are both less than the support transverse stiffness.

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

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

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

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

18. The gas turbine engine as recited in claim 2 , wherein a turbine provides an input to the gear system.

19. The gas turbine engine as recited in claim 18 , wherein the gear system further drives a compressor rotor at a common speed with the fan shaft.

20. 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.

21. 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 transverse stiffness;

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

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

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

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

24. The gas turbine engine as recited in claim 22 , wherein the input transverse stiffness is less than about 5% of the gear mesh transverse stiffness.

25. The gas turbine engine as recited in claim 21 , wherein a transverse stiffness of a ring gear of the gear system is less than about 20% of the gear mesh transverse stiffness.

26. The gas turbine engine as recited in claim 21 , wherein a transverse stiffness of a ring gear of the gear system is less than about 12% of the gear mesh transverse stiffness.

27. The gas turbine engine as recited in claim 21 , wherein a transverse stiffness of a planet journal bearing which supports a planet gear of the gear system is less than or equal to the gear mesh transverse stiffness.

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

providing a fan shaft;

providing a support which supports the fan shaft, the support defining at least one of a support transverse stiffness and 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 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

providing an input to the gear system, 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.

29. The method as recited in claim 28 , comprising providing 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.