Turbine section of high bypass turbofan

1. A turbofan engine comprising:

an engine case;

a gaspath through the engine case;

a fan having a circumferential array of fan blades;

a compressor in fluid communication with the fan;

a combustor in fluid communication with the compressor;

a turbine in fluid communication with the combustor, the turbine having a low pressure turbine section having 3 to 6 blade stages; and

a speed reduction mechanism coupling the low pressure turbine section to the fan, wherein:

a bypass area ratio is between about 8.0 and about 20.0;

a ration of maximum gaspath radius along the low pressure turbine section to maximum radius of the fan is less than about 0.50; and

a ratio of an low pressure turbine section airfoil count to the bypass area ratio is between about 10 and about 170, said low pressure turbine section airfoil count being the total number of blade airfoils and vane airfoils of the low pressure turbine section.

2. The engine of claim 1 further comprising:

a fan case encircling the fan blades radially outboard of the engine case.

3. The engine of claim 1 wherein:

a hub-to-tip ratio (R I :R O ) of the low pressure turbine section is between about 0.4 and about 0.5 measured at the maximum R O axial location in the low pressure turbine section.

4. The engine of claim 3 wherein:

the low pressure turbine section has 3 to 5 blade stages.

5. The engine of claim 3 wherein:

an airfoil count of the low pressure turbine section is below about 1600.

6. The engine of claim 1 wherein:

the compressor comprises:

a low pressure compressor section; and

a high pressure compressor section.

7. The engine of claim 6 wherein:

the turbine has a high pressure turbine section coupled to drive the high pressure compressor section.

8. The engine of claim 7 wherein:

there are no additional compressor or turbine sections.

9. The engine of claim 6 wherein:

blades of the low pressure compressor section and low pressure turbine section share a shaft; and

the speed reduction mechanism comprises an epicyclic transmission that couples the shaft to a fan shaft to drive the fan with a speed reduction.

10. The engine of claim 1 wherein:

the speed reduction mechanism comprises an epicyclic transmission.

11. The engine of claim 1 wherein:

the low pressure turbine section has 3 to 4 blade stages.

12. The engine of claim 1 wherein:

the low pressure turbine section has 3 blade stages.

13. The engine of claim 1 wherein:

said low pressure turbine airfoil count is below 1600.

14. The engine of claim 1 in combination with a mounting arrangement wherein an aft mount reacts at least a thrust load.

15. The engine of claim 1 wherein the 3 to 6 blade stages are interspersed with vane stages.

16. The engine of claim 1 wherein the 3-6 blade stages are interspersed with vane stages.

17. A turbofan engine comprising:

a fan case, and a gas generator including a core cowl, wherein the fan case and core cowl are configured so that a flow-path bypass ratio therebetween is between about 8.0 and about 20.0; and

wherein the gas generator includes a fan drive turbine having at least three blade stages and configured so that a ratio of a turbine airfoil count to the bypass ratio between about 10 and about 170, said low pressure turbine section airfoil count being the total number of blade airfoils and vane airfoils of the low pressure turbine section and a ratio of maximum gaspath radius along the low pressure turbine section to maximum radius of the fan is less than about 0.50.

18. The engine of claim 17 wherein the at least three blade stages are interspersed with vane stages.

19. The engine of claim 17 wherein there is a single stage of said fan blades.

20. The engine of claim 1 wherein there is a single stage of said fan blades.