Hybrid turbofan with differential electrical and mechanical power transfer

1. A turbofan engine comprising:

a fan section including a plurality of fan blades;

a first spool including a first turbine section;

a second spool including a second turbine section;

a first electric machine;

a second electric machine; and

a gear system coupled between the first spool, the second spool, the first electric machine, the second electric machine and the fan section for driving the fan section, wherein the gear system comprises a differential gear system including a plurality of spider gears supported in a differential carrier, a fan output gear driven by the plurality of spider gears, a first input shaft configured to drive the differential carrier and an input gear coupled to drive the plurality of spider gears.

2. The turbofan engine as recited in claim 1 , wherein the first input shaft is coupled to the first spool and the input gear is coupled to the second spool.

3. The turbofan engine as recited in claim 2 , wherein the gear system includes an epicyclic gear box including a sun gear driven by a first spool, the sun gear coupled to drive a plurality of planet gears supported within a carrier and a ring gear circumscribing the plurality of planet gears and an output of the epicyclic gear box coupled to drive the differential carrier.

4. The turbofan engine as recited in claim 3 , wherein the ring gear is fixed to a static engine structure and the carrier provides the output to drive the differential carrier.

5. The turbofan engine as recited in claim 3 , wherein the carrier is fixed to a static engine structure and the ring gear provides the output to drive the differential carrier.

6. The turbofan engine as recited in claim 4 , wherein the fan section includes a fan shaft coupled to the fan output gear and the first electric machine.

7. The turbofan engine as recited in claim 5 , wherein the second electric machine is coupled to the first spool.

8. The turbofan engine as recited in claim 7 , wherein the first electric machine is an electric motor configured to drive the fan shaft and the second electric machine is an electric motor configured to drive the first spool.

9. The turbofan engine as recited in claim 8 , including an electrical bus electrically coupled to the first electric machine and the second electric machine.

10. The turbofan engine as recited in claim 9 , wherein the electrical bus includes an AC Bus and a DC Bus.

11. A turbofan engine comprising:

a fan section including a fan shaft and a plurality of fan blades;

a high speed spool including a high pressure turbine section;

a low speed spool including a low pressure turbine section;

a differential gearbox including a plurality of spider gears supported in a differential carrier, the fan shaft driven by the plurality of spider gears and an input gear driven by the low speed spool;

an epicyclic gearbox including a sun gear driven by the high speed spool, the sun gear driving a plurality of intermediate gears supported within a carrier and a ring gear circumscribing the plurality of intermediate gears, wherein the epicyclic gearbox includes an output configured to drive the differential carrier;

a first electric machine coupled to the fan shaft; and

a second electric machine coupled to the high speed spool.

12. The turbofan engine as recited in claim 11 , wherein the ring gear is fixed to an engine static structure and the carrier is coupled to drive the differential carrier.

13. The turbofan engine as recited in claim 11 , wherein the carrier is fixed to an engine static structure and the ring gear is coupled to drive the differential carrier.

14. The turbofan engine as recited in claim 11 , including a low pressure compressor coupled to fan shaft to rotate at a speed common with the fan section.

15. The turbofan engine as recited in claim 11 , including a first tower shaft coupling the first electric machine to the fan shaft and a second tower shaft coupling the second electric machine to the high speed spool.

16. The turbofan engine as recited in claim 15 , including a first clutch for selectively decoupling the first electric machine from the fan shaft and a second clutch for selectively decoupling the second electric machine from the high speed spool.

17. A method of operating a turbofan engine comprising:

coupling a fan shaft gear to a plurality of spider gears of a differential gearbox;

supporting the plurality of spider gears with a rotatable differential carrier;

coupling a first spool to drive the rotatable differential carrier and a second spool to an input shaft configured to drive the plurality of spider gears; and

driving a fan section through the fan shaft gear with a combination of a first rotational input from the first spool into the differential carrier and a second rotational input from the second spool to the input shaft, wherein the fan section is driven at a speed different than a speed of the first spool and the second spool responsive to the first rotational input and the second rotational input.

18. The method as recited in claim 17 , including coupling a first electric motor to the fan shaft and providing a rotational input into the fan shaft independent of the first spool and the second spool.

19. The method as recited in claim 18 , including coupling a second electric motor to the first spool and providing a rotational input into the first spool.

20. The method as recited in claim 19 , including providing a third rotational input with the first electric motor to drive the fan shaft in the absence of the first rotational input from the first spool and the second rotational input from the second spool.

21. The method as recited in claim 19 , including controlling a speed of the fan by varying a rotational input from one or both of the first electric motor and the second electric motor.