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, and an outer housing surrounding the fan to define a bypass flow path, a compressor, a turbine section including a fan drive turbine and a second turbine, an epicyclic gear system with a gear reduction, the fan drive turbine driving the fan through the gear system, and the gear system straddle-mounted by first and second bearings. 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 fan blades alone at a cruise condition at 0.8 Mach and 35,000 feet;
a compressor section including a low pressure compressor and a high pressure compressor, and the low pressure compressor including 3 stages;
a turbine section including a fan drive turbine and a second turbine, wherein the fan drive turbine is a 3-stage to 6-stage turbine, and the second turbine is a 2-stage turbine driving the high pressure compressor;
an epicyclic gear system with a gear reduction, the fan drive turbine driving the fan through the gear system, and the gear system 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 gear system and the fan;
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 an engine 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 transverse stiffness is between 50% and 80% of the frame transverse stiffness, and the support lateral stiffness is at least 20% but less than 80% of the frame lateral stiffness;
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 1.0 and 1.075;
wherein the turbofan engine is a two-spool engine including a low spool and a high spool, wherein the low spool includes an inner shaft that interconnects the fan, the low pressure compressor and the fan drive turbine, wherein the high spool includes an outer shaft that interconnects the high pressure compressor and the second turbine, and wherein the inner shaft and the outer shaft are concentric and are rotatable about the engine longitudinal axis;
wherein the gear system includes a sun gear, a plurality of intermediate gears surrounding the sun gear, and a ring gear surrounding the intermediate gears;
wherein the gear system is a planetary gear system including a carrier that supports the 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 , further comprising a mid-turbine frame positioned intermediate the fan drive turbine and the second turbine, and the mid-turbine frame has a bearing supporting the inner shaft rotatable with the fan drive turbine.
3. The turbofan engine as set forth in claim 2 , wherein the fan drive turbine and the second turbine are rotatable in opposed directions, the mid-turbine frame includes a guide vane positioned intermediate the fan drive turbine and the second turbine, and the guide vane being an air turning guide vane.
4. The turbofan engine as set forth in claim 3 , wherein the support transverse stiffness is less than 65% of the frame transverse stiffness.
5. The turbofan engine as set forth in claim 4 , wherein the support lateral stiffness is less than 50% of the frame lateral stiffness.
6. The turbofan engine as set forth in claim 5 , wherein the gear reduction is positioned between the fan drive turbine and the low pressure compressor such that the fan and the low pressure compressor are rotatable at a common speed.
7. The turbofan engine as set forth in claim 6 , wherein the bearing of the mid-turbine frame supports the inner shaft in an overhung manner.
8. The turbofan engine as set forth in claim 6 , wherein the sun gear is mounted to a flexible input attached to the inner shaft driven by the fan drive turbine.
9. The turbofan engine as set forth in claim 8 , wherein the first bearing is a thrust bearing, the second bearing is a roller bearing.
10. The turbofan engine as set forth in claim 1 , wherein the gear system includes a carrier that supports the intermediate gears, the carrier is attached to an engine static structure, the first bearing supports the fan drive shaft, and the fan drive shaft interconnects the ring gear and the fan.
11. The turbofan engine as set forth in claim 10 , further comprising a mid-turbine frame positioned intermediate the fan drive turbine and the second turbine, and the mid-turbine frame has a bearing supporting the inner shaft rotatable with the fan drive turbine.
12. The turbofan engine as set forth in claim 11 , wherein the support lateral stiffness is less than 50% of the frame lateral stiffness.
13. The turbofan engine as set forth in claim 12 , wherein the fan drive turbine and second turbine are rotatable in opposed directions.
14. The turbofan engine as set forth in claim 13 , wherein the gear reduction is positioned between the fan drive turbine and the low pressure compressor such that the fan and the low pressure compressor are rotatable at a common speed.
15. The turbofan engine as set forth in claim 14 , wherein the sun gear is mounted to a flexible input attached to the inner shaft driven by the fan drive turbine, the first bearing is a thrust bearing, the second bearing is a roller bearing.
16. The turbofan engine as set forth in claim 14 , wherein the fan drive turbine is a 3-stage turbine, and the low pressure compressor is a three-stage compressor.
17. The turbofan engine as set forth in claim 14 , wherein the low pressure compressor has a greater number of stages than the fan drive turbine.
18. The turbofan engine as set forth in claim 11 , wherein the fan drive turbine is a 3-stage turbine.
19. The turbofan engine as set forth in claim 18 , wherein the low pressure compressor is a three-stage compressor.
20. The turbofan engine as set forth in claim 11 , wherein the low pressure compressor has a greater number of stages than the fan drive turbine.