Start system for gas turbine engines

1. A gas turbine engine comprising:

a high spool along an engine axis, said high spool operable to communicate a core airflow through a core flow path;

a fan section operable to communicate a fan airflow into a secondary flow path and said core flow path;

a low spool along said engine axis to drive said fan section;

a starter/generator connected to said low spool, said starter/generator having a start mode in which the starter/generator rotationally drives said low spool and a generate mode in which said low spool drives said starter/generator to produce electric power;

a flow control mechanism operable to direct at least a portion of said fan airflow from said secondary flow path into said core flow path, said flow control mechanism including a static ring and a moveable ring relative to said static ring, said static ring including a plurality of first circumferentially-spaced portions in said secondary flow path and said moveable ring including a corresponding plurality of second circumferentially-spaced portions in said secondary flow path, said static ring and said moveable ring having an aligned position and a staggered position with respect to one another, wherein in said aligned position each of said first circumferentially-spaced portions and each of said corresponding second circumferentially-spaced portions form an aerodynamic shape and in said staggered position each of said first circumferentially-spaced portions is circumferentially offset from each of said corresponding second circumferentially-spaced portions; and

a controller configured with an engine starter mode and an engine flight mode, wherein responsive to said engine starter mode said static ring and said moveable ring are in said staggered position blocking said secondary flow path and said starter/generator is in said start mode, and responsive to changing from said engine starter mode to said engine flight mode said controller changes said static ring and said moveable ring to be in said aligned position opening said secondary flow path and changes said starter/generator to be in said generate mode.

2. The gas turbine engine as recited in claim 1 , wherein said starter/generator is forward of said low spool.

3. The gas turbine engine as recited in claim 1 , wherein said starter/generator is aft of said low spool.

4. The gas turbine engine as recited in claim 1 , wherein said starter/generator is driven by said low spool through a towershaft.

5. The gas turbine engine as recited in claim 1 , wherein said high spool along said engine axis with a high pressure compressor section and a high pressure turbine section, said high pressure compressor section includes a variable guide vane.

6. The gas turbine engine as recited in claim 1 , wherein said high spool along said engine axis with a high pressure compressor section and a high pressure turbine section, said high pressure turbine section includes a variable guide vane.

7. The gas turbine engine as recited in claim 1 , wherein said fan section includes a variable pitch fan blade.

8. The gas turbine engine as recited in claim 1 , wherein said fan section includes a variable pitch vane.

9. The gas turbine engine as recited in claim 1 , wherein said high spool along said engine axis with a high pressure compressor section and a high pressure turbine section, a start bleed in selective communication with said high pressure compressor section.

10. The gas turbine engine as recited in claim 1 , wherein said low spool drives said fan section through a geared architecture.

11. A method of operating a gas turbine engine comprising:

driving a low spool to generate a fan airflow from a fan section connected to the low spool; and

modulating a flow control mechanism and a starter/generator according to an engine starter mode and an engine flight mode to selectively direct at least a portion of the fan airflow from a secondary flow path into a core flow path to drive a high spool, wherein the starter/generator is connected to the low spool, the starter/generator having a start mode in which the starter/generator rotationally drives the low spool and a generate mode in which the low spool drives the starter/generator to produce electric power,

the flow control mechanism including a static ring and a moveable ring relative to the static ring, the static ring including a plurality of first circumferentially-spaced portions in the secondary flow path and the moveable ring including a corresponding plurality of second circumferentially-spaced portions in the secondary flow path, the static ring and the moveable ring having an aligned position and a staggered position with respect to one another, wherein in the aligned position each of the first circumferentially-spaced portions and each of the corresponding second circumferentially-spaced portions form an aerodynamic shape and in the staggered position each of the first circumferentially-spaced portions is circumferentially offset from each of the corresponding second circumferentially-spaced portions, and

wherein the modulating includes,

the static ring and the moveable ring being in the staggered position blocking the secondary flow path and the starter/generator being in the start mode responsive to the engine starter mode, and

responsive to changing from the engine starter mode to the engine flight mode the static ring and the moveable ring move to the aligned position and the starter/generator changes to the generate mode.

12. The method as recited in claim 11 , wherein no accessory gearbox is driven by the high spool.

13. The method as recited in claim 11 , further comprising:

rotating the high spool in a direction opposite that of the low spool.

14. The method as recited in claim 11 , further comprising:

driving the fan section through a geared architecture driven by the low spool.

15. The method as recited in claim 11 , further comprising:

driving a constant speed transmission with the low spool.

16. The gas turbine engine as recited in claim 1 , wherein in said staggered position, said plurality of first circumferentially-spaced portions and said plurality of second circumferentially-spaced portions fully block said secondary flow path.

17. The gas turbine engine as recited in claim 1 , wherein in said staggered position, said plurality of first circumferentially-spaced portions and said plurality of second circumferentially-spaced portions partially block said secondary flow path.

18. The gas turbine engine as recited in claim 1 , further comprising a nozzle section circumscribing exits of, respectively, said core flow path and said secondary flow path, wherein said nozzle section is a convergent/divergent nozzle.

19. The gas turbine engine as recited in claim 1 , wherein, responsive to said engine starter mode, said controller is configured to move said moveable ring from said aligned position to said staggered position blocking said secondary flow path such that said fan airflow is communicated into said coreflow path to drive said high spool, followed by activating said starter/generator to accelerate said low spool.

20. The gas turbine engine as recited in claim 19 , wherein, after a self-sustaining speed of said high spool is achieved, said controller is configured to move said moveable ring from said staggered position to said aligned position opening said secondary flow path such that said fan airflow is communicated into said secondary flow path, followed by changing said starter/generator from said start mode to said generate mode.