Gas turbine engine with high speed low pressure turbine section and bearing support features
A turbofan engine according to an example of the present disclosure includes, among other things, a fan, a compressor section, a turbine section including a fan drive turbine and a second turbine, an epicyclic gear system with a gear reduction, first and second bearings, and a fan drive shaft interconnecting the gear system and the fan. The fan drive turbine has a first exit area at a first exit point and is rotatable at a first speed. The second turbine has a second exit area at a second exit point and is rotatable at a second speed. A first performance quantity is defined as the product of the first speed squared and the first area. A second performance quantity is defined as the product of the second speed squared and the second area.
1. A turbofan engine comprising:
a fan including a plurality of fan blades, and an outer housing surrounding the fan to define a bypass flow path;
a low fan pressure ratio of less than 1.45 measured across the plurality of fan blades alone at a cruise condition at 0.8 Mach and 35,000 feet;
a compressor section including a first compressor and a second compressor;
a turbine section including a fan drive turbine and a second turbine, wherein the fan drive turbine includes three stages, the second turbine drives the second compressor, and the second turbine includes two stages;
an epicyclic gear system with a gear reduction and a gear reduction ratio of greater than 2.5:1, the fan drive turbine driving the fan through the epicyclic gear system, and the epicyclic gear system is straddle-mounted by first and second bearings on opposite sides of the gear reduction relative to an engine longitudinal axis;
a fan drive shaft interconnecting the epicyclic gear system and the fan;
wherein the fan drive turbine has a first exit area at a first exit point and is rotatable at a first speed, the second turbine has a second exit area at a second exit point and is rotatable at a second speed, the second speed being greater than twice the first speed;
wherein a first performance quantity is defined as the product of the first speed squared and the first exit area, a second performance quantity is defined as the product of the second speed squared and the second exit area, and a performance ratio of the first performance quantity to the second performance quantity is between 0.8 and 1.5;
wherein the gear system includes a sun gear, a plurality of intermediate gears surrounding the sun gear, and a ring gear surrounding the plurality of intermediate gears;
wherein the gear system is a planetary gear system including a carrier that supports the plurality of intermediate gears, the first bearing supports the fan drive shaft, the fan drive shaft interconnects the carrier and the fan, and the second bearing supports an aft portion of the carrier; and
wherein the second bearing is axially aligned with the carrier relative to the engine longitudinal axis, and the second bearing is situated along an outer periphery of the carrier.
2. The turbofan engine as set forth in claim 1 , wherein the first compressor includes a plurality of stages.
3. The turbofan engine as set forth in claim 2 , wherein the performance ratio is between 0.8 and 1.075.
4. The turbofan engine as set forth in claim 2 , wherein the performance ratio is between 1.0 and 1.5.
5. The turbofan engine as set forth in claim 4 , wherein the first compressor includes a greater number of stages than the second turbine.
6. The turbofan engine as set forth in claim 5 , wherein the sun gear is mounted to a flexible input attached to a shaft driven by the fan drive turbine.
7. The turbofan engine as set forth in claim 6 , further comprising:
a frame supporting at least a portion of the fan drive shaft, the frame defining a frame transverse stiffness and a frame lateral stiffness;
a flexible support at least partially supporting the gear system relative to a static structure, the flexible support defining a support transverse stiffness with respect to the frame transverse stiffness and a support lateral stiffness with respect to the frame lateral stiffness; and
wherein the support lateral stiffness is less than the frame lateral stiffness, and the support transverse stiffness is less than the frame transverse stiffness.
8. The turbofan engine as set forth in claim 7 , wherein the support lateral stiffness is less than 80% of the frame lateral stiffness.
9. The turbofan engine as set forth in claim 8 , wherein the support lateral stiffness is less than 80% of the frame lateral stiffness.
10. The turbofan engine as set forth in claim 9 , wherein the fan drive turbine is a 3-stage to 6-stage turbine.
11. The turbofan engine as set forth in claim 10 , wherein:
the turbofan engine is a two-spool engine including a low spool and a high spool;
the low spool includes an inner shaft that interconnects the fan, the first compressor and the fan drive turbine;
the high spool includes an outer shaft that interconnects the second compressor and the second turbine; and
the inner shaft and the outer shaft are concentric and are rotatable about the engine longitudinal axis.
12. The turbofan engine as set forth in claim 11 , wherein the sun gear is mounted to a flexible input attached to the inner shaft driven by the fan drive turbine.
13. The turbofan engine as set forth in claim 12 , wherein the fan drive turbine drives both the first compressor and the sun gear.
14. The turbofan engine as set forth in claim 13 , wherein the support transverse stiffness is less than 20% of the frame transverse stiffness.
15. The turbofan engine as set forth in claim 14 , wherein the support lateral stiffness is less than 20% of the frame lateral stiffness.
16. The turbofan engine as set forth in claim 15 , wherein:
the flexible input defines an input lateral stiffness with respect to the frame lateral stiffness and an input transverse stiffness with respect to the frame transverse stiffness;
the input transverse stiffness is less than 20% of the frame transverse stiffness; and
the input lateral stiffness is less than 20% of the frame lateral stiffness.
17. The turbofan engine as set forth in claim 16 , further comprising:
a mid-turbine frame positioned intermediate the fan drive turbine and the second turbine, the mid-turbine frame having a bearing supporting the inner shaft, and the mid-turbine frame including a turning guide vane positioned intermediate the fan drive turbine and the second turbine.
18. The turbofan engine as set forth in claim 17 , wherein:
the bearing of the mid-turbine frame supports the inner shaft in an overhung manner;
the first bearing is a thrust bearing; and
the second bearing is a roller bearing.
19. The turbofan engine as set forth in claim 7 , wherein the support transverse stiffness is less than 80% of the frame transverse stiffness.
20. A turbofan engine comprising:
a fan including a plurality of fan blades, and an outer housing surrounding the fan to define a bypass flow path;
a low fan pressure ratio of less than 1.45 measured across the plurality of fan blades alone at a cruise condition at 0.8 Mach and 35,000 feet;
a compressor section including a first compressor and a second compressor;
a turbine section including a fan drive turbine and a second turbine, wherein the fan drive turbine includes three stages, the second turbine drives the second compressor, and the second turbine includes two stages;
an epicyclic gear system with a gear reduction and a gear reduction ratio of greater than 2.5:1, wherein the fan drive turbine drives the fan through the epicyclic gear system, wherein the epicyclic gear system includes a sun gear, a plurality of intermediate gears surrounding the sun gear, a carrier supporting the plurality of intermediate gears, and a ring gear surrounding the plurality of intermediate gears, and wherein the gear reduction is positioned between the fan drive turbine and the first compressor such that the fan and the first compressor are rotatable at a common speed;
a fan drive shaft interconnecting the gear system and the fan, and wherein first and second bearings are axially aligned with the fan drive shaft relative to an engine longitudinal axis, and the first and second bearings support a periphery of the fan drive shaft on opposite sides of the ring gear relative to the engine longitudinal axis;
wherein the fan drive turbine has a first exit area at a first exit point and is rotatable at a first speed, the second turbine has a second exit area at a second exit point and is rotatable at a second speed, the second speed being greater than twice the first speed; and
wherein a first performance quantity is defined as the product of the first speed squared and the first exit area, a second performance quantity is defined as the product of the second speed squared and the second exit area, and a performance ratio of the first performance quantity to the second performance quantity is no more than 1.5.
21. The turbofan engine as set forth in claim 20 , wherein the first compressor includes a greater number of stages than the second turbine.
22. The turbofan engine as set forth in claim 21 , wherein the performance ratio is between 0.5 and 1.5.
23. The turbofan engine as set forth in claim 22 , further comprising:
a frame supporting at least a portion of the fan drive shaft, the frame defining a frame transverse stiffness and a frame lateral stiffness;
a flexible support at least partially supporting the gear system relative to a static structure, the flexible support defining a support transverse stiffness with respect to the frame transverse stiffness and a support lateral stiffness with respect to the frame lateral stiffness; and
wherein the support lateral stiffness is less than the frame lateral stiffness, and the support transverse stiffness is less than the frame transverse stiffness.
24. The turbofan engine as set forth in claim 23 , wherein:
the support transverse stiffness is less than 80% of the frame transverse stiffness; and
the support lateral stiffness is less than 80% of the frame lateral stiffness.
25. The turbofan engine as set forth in claim 24 , wherein the sun gear is mounted to a flexible input attached to a shaft driven by the fan drive turbine.
26. The turbofan engine as set forth in claim 25 , wherein the performance ratio is no more than 1.0.
27. The turbofan engine as set forth in claim 26 , wherein the fan drive turbine is a 3-stage to 6-stage turbine.
28. The turbofan engine as set forth in claim 27 , wherein the sun gear is mounted to a flexible input attached to a shaft driven by the fan drive turbine.
29. The turbofan engine as set forth in claim 28 , wherein the flexible input defines an input lateral stiffness with respect to the frame lateral stiffness and an input transverse stiffness with respect to the frame transverse stiffness, the input transverse stiffness is less than 80% of the frame transverse stiffness, and the input lateral stiffness is less than 80% of the frame lateral stiffness.
30. The turbofan engine as set forth in claim 29 , further comprising:
a mid-turbine frame positioned intermediate the fan drive turbine and the second turbine, the mid-turbine frame including a bearing supporting the inner shaft, and the mid-turbine frame including a turning guide vane positioned intermediate the fan drive turbine and the second turbine; and
wherein the support transverse stiffness is less than 20% of the frame transverse stiffness, and the support lateral stiffness is less than 20% of the frame lateral stiffness.