Turbofan engine bearing and gearbox arrangement

1. A turbofan engine comprising:

a fan delivering air into a bypass duct and into a core engine;

a fan drive gear system having a gear carrier;

a fan shaft coupling the fan drive gear system to the fan;

the core engine including a low spool comprising:

a low pressure turbine;

a low pressure shaft; and

a high spool comprising:

a high pressure turbine;

a compressor; and

a high pressure shaft coupling the high pressure turbine to the compressor;

a case, and wherein the fan drive gear system is coupled to the case via a compliant flexure; and

wherein a first bearing is forward of the fan drive gear system, and supporting said fan shaft and said fan drive gear system, and a second bearing is aft of the fan drive gear system, and supporting the fan drive gear system.

2. The engine as recited in claim 1 , wherein the fan gear drive system includes a ring gear encircling the sun gear, and includes a plurality of intermediate gears positioned between and enmeshing with the sun gear and the ring gear, with the plurality of intermediate gears carried by the gear carrier, and wherein only a single bearing is along the fan shaft forward of the fan drive gear system, the single bearing being the first bearing.

3. The engine as recited in claim 2 , wherein:

the fan is a single-stage fan;

the fan drive gear system defines a speed reduction ratio between 2:1 and 13:1; and

the low pressure turbine has three or more stages.

4. The engine as recited in claim 3 , wherein:

the second bearing supports the fan shaft relative to a rear support of a static structure, the rear support extending inward from a forward frame of the static structure behind the fan drive gear system relative to a longitudinal axis of the core engine.

5. The engine as recited in claim 4 , further comprising a third bearing engaging the low shaft and the fan shaft.

6. The engine as recited in claim 5 , wherein the low pressure shaft has a flexible forward portion.

7. The engine as recited in claim 6 , wherein each of the first bearing and the third bearing is a thrust bearing, and the third bearing is nested within the second bearing relative to the longitudinal axis.

8. The engine as recited in claim 5 , wherein the fan shaft is mounted to rotate with the gear carrier as a unit, and the unit engages the first bearing forward of the gear carrier and engages the second bearing aft of the gear carrier.

9. The engine as recited in claim 8 , further comprising:

a fourth bearing engaging the low pressure shaft and a fixed frame, the fixed frame extending across a core flow path of the core engine, and the fixed frame aft of the high pressure turbine with respect to a longitudinal axis of the core engine; and

wherein the high pressure turbine has two stages.

10. The engine as recited in claim 5 , wherein a centerplane of the third bearing is behind a centerplane of the second bearing by a distance no more than 10% of a radius of the second bearing with respect to the longitudinal axis of the core engine.

11. The engine as recited in claim 1 , wherein the second bearing supports the fan shaft relative to a rear support of a static structure, the rear support extending inward from a forward frame of the static structure behind the fan drive gear system relative to a longitudinal axis of the core engine.

12. The engine as recited in claim 11 , wherein one of the first and second bearings is a thrust bearing, and another one of the first and second bearings is a non-thrust bearing.

13. The engine as recited in claim 12 , wherein the first and second bearings prevent radial excursions and overturning moments produced by the fan during operation, the low pressure turbine has three or more stages, and the fan drive gear system defines a speed reduction ratio between 2:1 and 13:1.

14. The engine as recited in claim 13 , wherein the first bearing is a thrust bearing, and the second bearing being a roller bearing.

15. The engine as recited in claim 14 , wherein only a single bearing is along the fan shaft forward of the fan drive gear system, the single bearing being the first bearing.

16. The engine as recited in claim 11 , wherein the low pressure turbine has three or more stages, and the fan drive gear system defines a speed reduction ratio between 2:1 and 13:1.

17. The engine as recited in claim 16 , wherein the low pressure shaft has a flexible forward portion.

18. The engine as recited in claim 17 , wherein the low pressure shaft has a splined outer diameter surface and the sun gear has a splined inner diameter surface, the splined outer diameter surface of the low pressure shaft is interfitting with the splined inner diameter surface of the sun gear, the high pressure turbine has two stages, and the first bearing is a bidirectional tapered roller bearing.

19. A turbofan engine comprising:

a fan including a plurality of fan blades;

a fan drive gear system including a sun gear encircled by a ring gear, a plurality of intermediate gears positioned between and enmeshing with said sun gear and said ring gear, with said plurality of intermediate gears carried by a gear carrier;

a fan shaft coupling the fan drive gear system to the fan;

a low spool extending along a longitudinal axis, the low spool including a low pressure turbine driving the fan drive gear system through a low shaft;

a core spool including a high pressure turbine driving a high pressure compressor through a core shaft;

a first bearing engaging the fan shaft;

a second bearing engaging the fan shaft on an opposite side of the fan drive gear system from the first bearing, one of the first and second bearings being a thrust bearing, and another one of the first and second bearings being a roller bearing; and

a third bearing engaging the low shaft and the fan shaft.

20. The engine as recited in claim 19 , further comprising a case, and wherein the fan drive gear system is coupled to the case via a compliant flexure.

21. The engine as recited in claim 20 , wherein the low shaft has a flexible forward portion.

22. The engine as recited in claim 21 , wherein the fan shaft is mounted to rotate with the gear carrier as a unit, the unit engages the first bearing axially forward of the gear carrier and engages the second bearing axially aft of the gear carrier with respect to the longitudinal axis, the low pressure turbine has three or more stages, the high pressure turbine has two stages, and the fan drive gear system defines a speed reduction ratio between 2:1 and 13:1.

23. The engine as recited in claim 22 , further comprising:

a fourth bearing engaging the low shaft and a fixed frame;

a fifth bearing engaging the core shaft and a case;

a sixth bearing engaging the core shaft and the case; and

wherein the second bearing is a roller bearing.

24. The engine as recited in claim 23 , wherein:

the low shaft having a spline connection to the sun gear;

the third bearing is a thrust bearing;

the fourth bearing is a roller bearing;

the fifth bearing is a thrust bearing; and

the fixed frame is axially aft of the low pressure turbine with respect to the longitudinal axis.

25. The engine as recited in claim 24 , wherein:

a centerplane of the third bearing is behind a centerplane of the second bearing by a distance no more than 10% of a radius of the second bearing with respect to the longitudinal axis.

26. The engine as recited in claim 21 , wherein the fan shaft is mounted to rotate with the gear carrier as a unit, the unit engages the first bearing axially forward of the gear carrier and engages the second bearing axially aft of the gear carrier with respect to the longitudinal axis, and the low shaft engages the third bearing, and also engages two additional bearings axially aft of the third bearing with respect to the longitudinal axis.

27. The engine as recited in claim 26 , wherein the first bearing is a tapered roller bearing oriented to resist at least forward movement of the fan shaft with respect to the longitudinal axis, and the third bearing is a thrust bearing.

28. The engine as recited in claim 27 , wherein:

the third bearing is nested within the second bearing with respect to the longitudinal axis.