Aircraft hybrid propulsion system

1. An aircraft hybrid propulsion system comprising;

an internal combustion engine;

an electric motor;

a propulsor;

a combining gearbox, the internal combustion engine being coupled to a first input of the combining gearbox, the electric motor being coupled to a second input of the combining gearbox, and the propulsor being coupled to an output of the combining gearbox, such that the propulsor is driveable in use by either or both of the internal combustion engine and the electric motor; wherein

each of the internal combustion engine and the electric motor is coupled to its respective input by a respective clutch;

the combining gearbox comprises a multi-stage gearbox with a first stage comprising a parallel-axis gearbox, and a second stage comprising an epicyclic gearbox;

the parallel-axis gearbox comprising first and second input gears, and an output gear, wherein teeth of the first and second input gears mesh with teeth of the output gear; and

the output gear of the parallel-axis gearbox is coupled to a sun gear of the epicyclic gearbox, and one of a planet carrier and a ring gear of the epicyclic gearbox is coupled to the propulsor.

2. An aircraft hybrid propulsion system according to claim 1 , wherein one or both of the clutches comprises any of an overrunning clutch, and an activated clutch such as a friction clutch or fluidic clutch.

3. An aircraft hybrid propulsion system according to claim 1 , wherein the electric motor comprises one of a permanent magnet motor and an induction motor.

4. An aircraft hybrid propulsion system according to claim 1 , wherein the combining gearbox comprises a reduction gearbox, wherein at least one of the inputs is configured to turn at a higher speed than the output in use.

5. An aircraft hybrid propulsion system according to claim 1 , wherein the aircraft hybrid propulsion system comprises one or more of an electric energy storage device and a generator configured to provide electrical power to the electric motor.

6. An aircraft hybrid propulsion system according to claim 1 , wherein the internal combustion engine comprises a gas turbine engine.

7. An aircraft hybrid propulsion system according to claim 6 , wherein the gas turbine engine comprises a compressor coupled to a first turbine, and comprises a second turbine which may be de-coupled from an engine compressor.

8. An aircraft hybrid propulsion system according to claim 7 , wherein the first and/or second turbine is coupled to one or both of a generator and the combining gearbox.

9. An aircraft hybrid propulsion system comprising;

an internal combustion engine drivably coupled to a sun gear of an epicyclic gearbox with a first input shaft, a first clutch disposed on the first input shaft between the internal combustion engine and the sun gear;

an electric motor drivably coupled to a planet gear of the epicyclic gearbox with a second input shaft, a second clutch disposed on the second input shaft between the electric motor and the planet gear, the planet gear located radially outward of the sun gear and configured to mesh with the sun gear;

a planet carrier fixed relative to the internal combustion engine and configured to rotably mount the planet gear;

a ring gear located radially outward of the planet gear and configured to mesh with the planet gear;

a propulsor drivably coupled to the ring gear, the propulsor being driveable in use by either or both of the internal combustion engine and the electric motor.

10. An aircraft hybrid propulsion system according to claim 9 , wherein one or both of the clutches comprises any of an overrunning clutch, and an activated clutch such as a friction clutch or fluidic clutch.

11. An aircraft hybrid propulsion system according to claim 9 , further comprising a second gearbox stage disposed between the propulsor and the ring gear, the second gearbox stage comprising one of a parallel-axis gearbox, and a second stage epicyclic gearbox.

12. An aircraft hybrid propulsion system according to claim 9 , wherein the aircraft hybrid propulsion system comprises one or more of an electric energy storage device and a generator configured to provide electrical power to the electric motor.

13. An aircraft hybrid propulsion system according to claim 9 , wherein the internal combustion engine comprises a gas turbine engine.

14. An aircraft hybrid propulsion system according to claim 13 , wherein the gas turbine engine comprises a compressor coupled to a first turbine, and comprises a second turbine which may be de-coupled from an engine compressor.

15. An aircraft hybrid propulsion system according to claim 14 , wherein the first and/or second turbine is coupled to one or both of the generator and the combining gearbox.

16. An aircraft hybrid propulsion system comprising;

an internal combustion engine drivably coupled to a first sun gear of a first stage epicyclic gearbox with a first input shaft, a first clutch disposed on the first input shaft between the internal combustion engine and the first sun gear;

at least one first planet gear of the first stage epicyclic gearbox located radially outward of the first sun gear and configured to mesh with the first sun gear;

a first ring gear fixed relative to the internal combustion engine located radially outward of the at least one first planet gear and configured to mesh with the at least one first planet gear;

a first planet carrier configured to rotably mount the at least one first planet gear, the first planet carrier comprising a toothed outer annulus;

an electric motor drivably coupled to a motor output gear with a second input shaft, a second clutch disposed on the second input shaft between the electric motor and the motor output gear, the motor output gear located radially outward of the first planet carrier and configured to mesh with the toothed outer annulus of the first planet carrier;

a second sun gear of a second stage epicyclic gearbox drivably coupled to the first planet carrier, the second sun gear being driveable in use by either or both of the internal combustion engine and the at least one electric motor;

at least one second planet gear of the second stage epicyclic gearbox located radially outward of the second sun gear and configured to mesh with the second sun gear;

a second ring gear fixed relative to the internal combustion engine located radially outward of the at least one second planet gear and configured to mesh with the at least one second planet gear;

a second planet carrier configured to rotably mount the at least one second planet gear; and

a propulsor drivably coupled to the second planet carrier.

17. The aircraft hybrid propulsion system of claim 16 , further comprising at least one additional electric motor drivably coupled to an additional motor output gear with an additional input shaft, an additional clutch disposed on the additional input shaft between the additional electric motor and the additional motor output gear, the additional motor output gear located radially outward of the first planet carrier and configured to mesh with the toothed outer annulus of the first planet carrier.