Pitch change mechanism for shrouded fan with low fan pressure ratio

1. A shrouded fan assembly for use with a gas turbine engine, said shrouded fan assembly comprising:

a central hub;

a plurality of variable-pitch fan blades coupled to, and extending radially outward from, said central hub, each variable-pitch fan blade of said plurality of variable-pitch fan blades comprising a radially outer tip and being rotatable about a longitudinal axis extending therethrough;

a shroud coupled to each said radially outer tip; and

at least one magnetic thrust bearing coupled to said shroud and a fan case, wherein said at least one magnetic thrust bearing comprises a first magnetic thrust bearing coupled to said fan case forward of said shroud and a second magnetic thrust bearing coupled to said fan case aft of said shroud, said first and second magnetic thrust bearings configured to axially suspend said shroud therebetween.

2. The shrouded fan assembly in accordance with claim 1 , wherein said each variable-pitch fan blade further comprises a tip pin extending radially outward from said radially outer tip and concentric with the longitudinal axis, said shroud defining a plurality of holes, each hole of said plurality of holes receiving a respective said tip pin.

3. The shrouded fan assembly in accordance with claim 2 , wherein said shroud further comprises a bearing disposed in said each hole, each said bearing configured to receive a respective said tip pin.

4. The shrouded fan assembly in accordance with claim 1 , wherein said shroud is metallic.

5. The shrouded fan assembly in accordance with claim 4 , wherein said shroud is at least one of titanium and a titanium alloy.

6. The shrouded fan assembly in accordance with claim 1 , wherein said shroud is fabricated from a composite material.

7. A gas turbine engine comprising:

a central hub;

a fan assembly comprising a plurality of variable-pitch fan blades coupled to said central hub, each variable-pitch fan blade of said plurality of variable-pitch fan blades comprising a radially outer tip and being rotatable about a longitudinal axis extending therethrough;

a shroud extending around said fan assembly, said each variable-pitch fan blade coupled to said shroud at said radially outer tip;

a nacelle surrounding said shroud, said nacelle comprising a fan case at least partially extending about said shroud; and

at least one magnetic thrust bearing coupled to said shroud and said fan case, wherein said at least one magnetic thrust bearing comprises a first magnetic thrust bearing coupled to said fan case forward of said shroud and a second magnetic thrust bearing coupled to said fan case aft of said shroud, said first and second magnetic thrust bearings configured to axially suspend said shroud therebetween.

8. The gas turbine engine in accordance with claim 7 , wherein said shroud is spaced from said fan case by a clearance fit.

9. The gas turbine engine in accordance with claim 8 , wherein the clearance fit is in the range between about 0.25 mm (0.010 inches) to about 2.54 mm (0.100 inches).

10. The gas turbine engine in accordance with claim 7 , wherein said magnetic thrust bearing comprises:

a stator electromagnet coupled to a portion of said fan case; and

a rotor portion coupled to said shroud in opposing relationship to said stator electromagnet.

11. The gas turbine engine in accordance with claim 7 , wherein said stator electromagnet is an annular ring fabricated from a magnetically permeable material.

12. The gas turbine engine in accordance with claim 7 , wherein said rotor portion is an annular ring fabricated from a magnetically permeable material.

13. A gas turbine engine comprising:

a fan assembly comprising a plurality of radially outward extending variable-pitch fan blades comprising a radially outer tip, said plurality of radially outward extending fan blades coupled to a central hub;

a shroud at least partially extending about said fan assembly, each fan blade of said plurality of fan blades coupled to said shroud at said radially outer tip;

a nacelle comprising a fan case extending about said shroud;

a power generator coupled to said shroud; and

at least one magnetic thrust bearing coupled to at least one of said shroud and said fan case, wherein said at least one magnetic thrust bearing comprises a first magnetic thrust bearing coupled to said fan case forward of said shroud and a second magnetic thrust bearing coupled to said fan case aft of said shroud, said first and second magnetic thrust bearings configured to axially suspend said shroud therebetween.

14. The gas turbine engine in accordance with claim 13 , wherein said power generator is one of a permanent magnet alternator and a permanent magnet generator.

15. The gas turbine engine in accordance with claim 13 , wherein said power generator comprises:

a stator portion coupled to said fan case, said stator portion disposed radially outward of said shroud; and

a rotor portion coupled to said shroud.

16. The gas turbine engine in accordance with claim 15 , wherein said stator portion comprises a permanent magnet.

17. The gas turbine engine in accordance with claim 15 , wherein said rotor portion comprises a permanent magnet.

18. The gas turbine engine in accordance with claim 13 further comprising a high speed generator and an electronic control, wherein said power generator functions as a motor and said electronic control is a bidirectional controller that facilitates said power generator functioning as a motor, the electronic control coupled to said power generator and to said high speed generator such that power is transferred from said high speed generator to said power generator.

19. The gas turbine engine in accordance with claim 18 wherein said power generator provides rotational power to said fan assembly when said power generator is functioning as a motor.