Dual rotor propulsion systems for tiltrotor aircraft

1. A tiltrotor aircraft having a vertical takeoff and landing flight mode and a forward flight mode, the aircraft comprising:

a longitudinally extending fuselage;

a first wing extending laterally from the fuselage, the first wing having first and second outboard ends;

a second wing extending laterally from the fuselage aft of the first wing, the second wing having third and fourth outboard ends;

first and second dual rotor propulsion systems respectively coupled to the first wing between the fuselage and the first and second outboard ends, the first and second dual rotor propulsion systems reversibly tiltable relative to the first wing between vertical lift and forward thrust orientations;

third and fourth dual rotor propulsion systems respectively coupled to the second wing between the fuselage and the third and fourth outboard ends, the third and fourth dual rotor propulsion systems reversibly tiltable relative to the second wing between vertical lift and forward thrust orientations, each dual rotor propulsion system including:

an engine operable to provide an input torque;

a transmission coupled to the engine and operable to receive the input torque and generate an output torque;

a first output shaft coupled to the transmission and operable to receive a first portion of the output torque;

a first rotor assembly coupled to and rotatable with the first output shaft;

a second output shaft coupled to the transmission and operable to receive a second portion of the output torque; and

a second rotor assembly coupled to and rotatable with the second output shaft;

wherein, the first and second rotor assemblies rotate coaxially;

wherein, the first rotor assembly has a different diameter than the second rotor assembly; and

wherein, the first rotor assembly is stoppable and foldable in the forward flight mode.

2. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the transmission further comprises a clutch system operable to selectively engage and disengage torque paths to the first and second output shafts.

3. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first rotor assembly has a larger diameter than the second rotor assembly.

4. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first rotor assembly is aft of the second rotor assembly in the forward flight mode.

5. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first and second rotor assemblies co-rotate.

6. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first and second rotor assemblies counter-rotate.

7. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the second rotor assembly further comprises a rigid rotor.

8. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the second rotor assembly further comprises collective pitch control.

9. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first rotor assembly further comprises a gimbaled rotor.

10. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first rotor assembly further comprises collective pitch control and cyclic pitch control.

11. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first rotor assembly further comprises a rigid rotor having collective pitch control.

12. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the first rotor assembly is aerodynamically stoppable responsive to rotor blade feathering.

13. The tiltrotor aircraft as recited in claim 1 wherein each dual rotor propulsion system further comprises a brake unit operably associated with the first output shaft that is configured to stop the rotation of the first rotor assembly.

14. The tiltrotor aircraft as recited in claim 1 wherein each dual rotor propulsion system further comprises a lock assembly operably associated with the first output shaft that is configured to prevent rotation of the first rotor assembly.

15. The tiltrotor aircraft as recited in claim 1 wherein each dual rotor propulsion system further comprises a brake unit operably associated with the second output shaft that is configured to stop the rotation of the second rotor assembly.

16. The tiltrotor aircraft as recited in claim 1 wherein each dual rotor propulsion system further comprises a lock assembly operably associated with the second output shaft that is configured to prevent rotation of the second rotor assembly.

17. The tiltrotor aircraft as recited in claim 1 wherein, for each dual rotor propulsion system, the transmission further comprises a differential transmission operable to split torque paths to the first and second output shafts.

18. The tiltrotor aircraft as recited in claim 17 wherein, for each dual rotor propulsion system, the differential transmission further comprises a ring gear, a spider gear set coupled to the ring gear, a first output gear coupled between the spider gear set and the first output shaft and a second output gear coupled between the spider gear set and the second output shaft;

wherein a rotational speed of the ring gear is equal to half the sum of a rotational speed of the first output shaft and a rotational speed of the second output shaft; and

wherein at a constant engine operating speed, reduction in a rotational speed of the first rotor assembly causes an increase in a rotational speed of the second rotor assembly up to a maximum rotational speed of the second rotor assembly when rotation of the first rotor assembly is stopped.