Hybrid tiltrotor drive system

1. A hybrid tiltrotor drive system, comprising:

a first propulsion system comprising a first engine fixed relative to a wing and a first pylon rotatable relative to the wing and the first engine, the first pylon comprising a first proprotor and a first proprotor gearbox, wherein the first engine comprises a first engine output shaft coupled to a first fixed gearbox and the first fixed gearbox is coupled by a first fixed gearbox output shaft to the first proprotor gearbox to apply a first torque from the first engine to the first proprotor;

a second propulsion system comprising a second engine fixed relative to the wing and a second pylon rotatable relative to the wing and the second engine, the second pylon comprising a second proprotor and a second proprotor gearbox, wherein the second engine comprises a second engine output shaft coupled to a second fixed gearbox and the second fixed gearbox is coupled by a second fixed gearbox output shaft to the second proprotor gearbox to apply a second torque from the second engine to the second proprotor; and

a first supplemental driver coupled directly to the first proprotor gearbox to apply a first transient energy to the first proprotor simultaneous with the first torque to maintain the first proprotor and the second proprotor at an equivalent rotational speed.

2. The hybrid tiltrotor drive system of claim 1 , wherein the first engine is a combustion-driven turboshaft engine and the first supplemental driver is an electric motor or a brake.

3. The hybrid tiltrotor drive system of claim 1 , wherein the first supplemental driver is an electric motor.

4. The hybrid tiltrotor drive system of claim 1 , wherein the first engine is a combustion-driven turboshaft engine; and

the first supplemental driver is an electric motor.

5. The hybrid tiltrotor drive system of claim 1 , wherein the first proprotor and the second proprotor are not interconnected by a drive shaft.

6. The hybrid tiltrotor drive system of claim 1 , further comprising a second supplemental driver coupled directly to the second proprotor gearbox to apply a second transient energy to the second proprotor simultaneous with the second torque to maintain the first proprotor and the second proprotor at the equivalent rotational speed.

7. The hybrid tiltrotor drive system of claim 6 , wherein the first proprotor and the second proprotor are not interconnected by a driveshaft.

8. The hybrid tiltrotor drive system of claim 6 , wherein the first and second supplemental drivers are electric motors.

9. The hybrid tiltrotor drive system of claim 8 , wherein the first proprotor and the second proprotor are not interconnected by a drive shaft.

10. The hybrid tiltrotor drive system of claim 1 , further comprising:

sensors coupled with the first and the second proprotors to determine rotational speed; and

an electronic control system operationally coupled to the first supplemental driver and the sensors, the electronic control system configured to selectively operate the first supplemental driver to maintain the equivalent rotational speed of the first and second proprotors.

11. The hybrid tiltrotor drive system of claim 1 , wherein the first supplemental driver is coupled to an accessory drive of the first proprotor gearbox and the first fixed gearbox output shaft is coupled to an input gear of the first proprotor gearbox.

12. A method, comprising:

flying a tiltrotor aircraft, wherein the tiltrotor aircraft comprises:

a first propulsion system comprising a first engine fixed relative to a wing and a first pylon rotatable relative to the wing and the first engine, the first pylon comprising a first proprotor and a first proprotor gearbox, wherein the first engine comprises a first engine output shaft coupled to a first fixed gearbox and the first fixed gearbox is coupled by a first fixed gearbox output shaft to the first proprotor gearbox to apply a first torque from the first engine to the first proprotor;

a second propulsion system comprising a second engine fixed relative to the wing and a second pylon rotatable relative to the wing and the second engine, the second pylon comprising a second proprotor and a second proprotor gearbox, wherein the second engine comprises a second engine output shaft coupled to a second fixed gearbox and the second fixed gearbox is coupled by a second fixed gearbox output shaft to the second proprotor gearbox to apply a second torque from the second engine to the second proprotor;

a first supplemental driver coupled directly to the first proprotor gearbox to apply a first transient energy to the first proprotor simultaneous with the first torque to maintain the first proprotor and the second proprotor at an equivalent rotational speed; and

a second supplemental driver coupled directly to the second proprotor gearbox to apply a second transient energy to the second proprotor simultaneous with the second torque to maintain the first proprotor and the second proprotor at the equivalent rotational speed, wherein the flying comprises:

rotating the first proprotor at a first rotational speed in response to operation of the first engine;

rotating the second proprotor at a second rotational speed in response to operation of the second engine; and

actuating the first supplemental driver to apply energy to the first proprotor to maintain the first proprotor and the second proprotor at the equivalent rotational speed.

13. The method of claim 12 , wherein the first propulsion system and the second propulsion system are not interconnected by a driveshaft.

14. The method of claim 12 , wherein the first and second engines are combustion-driven turboshaft engines; and

the first and second supplemental drivers comprise an electric motor.