Geared turbofan gas turbine engine architecture

1. A gas turbine engine comprising:

a fan section including a fan rotor having less than 18 fan blades, with a low fan pressure ratio of less than 1.45 measured across said fan blades alone at a cruise condition;

a gear train driving said fan rotor;

a compressor section;

a turbine section including a fan drive turbine and a second turbine, said fan drive turbine driving said fan rotor through said gear train wherein:

a power density greater than 1.5 and less than 5.5 lbf/in 3 , and defined as thrust in lbf measured by a volume of said turbine section in in 3 measured between an inlet of said turbine section to an exit of said turbine section; and

said fan drive turbine has a number of fan drive turbine stages, and a ratio between a number of fan blades and said number of fan drive turbine stages is between 2.5 and 8.5.

2. The gas turbine engine set forth in claim 1 , wherein said power density is greater than or equal to 3.0 lbf/in 3 .

3. The gas turbine engine set forth in claim 2 , wherein said power density is greater than or equal to 4.0 lbf/in 3 .

4. The gas turbine engine as set forth in claim 3 , wherein said fan drive turbine has a first exit area and is configured to rotate at a first speed, said second turbine has a second exit area and is configured to rotate at a second speed, which is faster than said first speed, said first and second speeds being redline speeds, wherein a first performance quantity is defined as a product of said first speed squared and said first exit area, a second performance quantity is defined as a product of said second speed squared and said second exit area, and a performance quantity ratio of said first performance quantity to said second performance quantity is between 0.5 and 1.5, and wherein said second turbine is a two stage turbine.

5. The gas turbine engine set forth in claim 4 , further comprising a mid-turbine frame between said fan drive turbine and said second turbine, said mid-turbine frame including at least one vane in a flow path.

6. The gas turbine engine as set forth in claim 4 , wherein said second speed is more than twice said first speed.

7. The gas turbine engine as set forth in claim 4 , wherein said second speed is more than twice said first speed.

8. The gas turbine engine set forth in claim 3 , wherein said second turbine is a two stage turbine.

9. The gas turbine engine set forth in claim 1 , further comprising a mid-turbine frame between said fan drive turbine and said second turbine, said mid-turbine frame including at least one airfoil in a flow path.

10. The gas turbine engine set forth in claim 1 , wherein said second turbine is a two stage turbine.

11. A gas turbine engine comprising:

a gear train for driving a fan rotor;

a compressor section;

a turbine section including a fan drive turbine driving said fan rotor through said gear train and a second turbine, wherein:

a power density greater than 1.5 and less than 5.5 lbf/in 3 , and defined as thrust in lbf measured by a volume of said turbine section in in 3 measured between an inlet of said turbine section to an exit of said turbine section;

said fan drive turbine has a number of fan drive turbine stages, and a ratio between a number of fan blades and said number of fan drive turbine stages is between 2.5 and 8.5.

12. The gas turbine engine set forth in claim 11 , wherein said second turbine is a two stage turbine.

13. The gas turbine engine set forth in claim 12 , wherein said power density is greater than or equal to 4.0 lbf/in 3 .

14. The gas turbine engine set forth in claim 13 , wherein said number of fan blades is less than 18 fan blades.

15. The gas turbine engine as set forth in claim 14 , wherein said fan drive turbine has a first exit area and is configured to rotate at a first speed, said second turbine has a second exit area and is configured to rotate at a second speed, which is faster than said first speed, said first and second speeds being redline speeds, wherein a first performance quantity is defined as a product of said first speed squared and said first exit area, a second performance quantity is defined as a product of said second speed squared and said second exit area, and a performance quantity ratio of said first performance quantity to said second performance quantity is between 0.5 and 1.5.

16. The gas turbine engine set forth in claim 15 , wherein said performance quantity ratio is greater than or equal to 0.8.

17. The gas turbine engine as set forth in claim 16 , wherein said second speed is greater than twice said first speed.

18. The gas turbine engine set forth in claim 17 , wherein said fan drive turbine includes at least one rotor having a live rim radius (r) and a bore width (W), and a ratio of r/W is between 4.65 and 5.55.

19. The gas turbine engine as set forth in claim 15 , wherein said second speed is greater than twice said first speed.

20. The gas turbine engine set forth in claim 11 , wherein said power density is greater than or equal to 4.0 lbf/in 3 .

21. The gas turbine engine set forth in claim 11 , wherein said power density is greater than or equal to 4.0 lbf/in 3 .

22. A gas turbine engine comprising:

a fan section including a fan rotor having less than 18 fan blades;

a gear train for driving said fan rotor;

a compressor section; and

a turbine section including a fan drive turbine and a second turbine, said fan drive turbine driving a fan rotor through said gear train wherein:

a power density greater than 1.5 and less than 5.5 lbf/in 3 , and defined as thrust in lbf measured by a volume of said turbine section in in 3 measured between an inlet of said turbine section to an exit of said turbine section,

said fan drive turbine has a number of fan drive turbine stages, and a ratio between a number of fan blades and said number of fan drive turbine stages is between 2.5and 8.5.

23. The gas turbine engine as set forth in claim 22 , wherein said fan drive turbine has a first exit area and is configured to rotate at a first speed, said second turbine is a two stage second turbine and has a second exit area and is configured to rotate at a second speed, which is faster than said first speed, said first and second speeds being redline speeds, wherein a first performance quantity is defined as a product of said first speed squared and said first exit area, a second performance quantity is defined as a product of said second speed squared and said second exit area, and a performance quantity ratio of said first performance quantity to said second performance quantity is between 0.5 and 1.5.

24. The gas turbine engine as set forth in claim 23 , wherein said second speed is greater than twice said first speed.

25. The gas turbine engine set forth in claim 24 , wherein said gear train is a planetary gear system.

26. The gas turbine engine set forth in claim 22 , wherein said fan drive turbine includes at least one rotor having a bore width (W) and a live rim radius (r), and a ratio of r/W is between 4.65 and 5.55.

27. The gas turbine engine set forth in claim 26 , wherein said fan drive turbine includes at least one rotor having a bore radius (R) and a live rim radius (r), and a ratio of r/R is between 2.00 and 2.30.

28. The gas turbine engine as set forth in claim 22 , wherein said fan drive turbine has a first exit area and is configured to rotate at a first speed, said second turbine is a two stage second turbine and has a second exit area and is configured to rotate at a second speed, which is faster than said first speed, said first and second speeds being redline speeds, wherein a first performance quantity is defined as a product of said first speed squared and said first exit area, a second performance quantity is defined as a product of said second speed squared and said second exit area, and a performance quantity ratio of said first performance quantity to said second performance quantity is between 0.5 and 1.5.

29. The gas turbine engine as set forth in claim 28 , wherein said second speed is greater than twice said first speed.