Propulsion systems for low observable aircraft

1. A propulsion system for an aircraft having a body, the aircraft operable to transition between thrust-borne lift in a VTOL orientation and wing-borne lift in a forward flight orientation, the propulsion system comprising:

a turboshaft engine disposed within the body, the engine having a turboshaft mode and a turbofan mode, the engine configured to generate engine exhaust;

a lift fan system disposed within the body, the lift fan system including at least one ducted fan;

a forced air bypass system disposed within the body, the forced air bypass system configured to generate bypass air;

a transmission disposed between the engine and the forced air bypass system, the transmission engaging the engine with the forced air bypass system when the engine is in the turbofan mode and disengaging the engine from the forced air bypass system when the engine is in the turboshaft mode; and

an exhaust system coupled to the engine and the forced air bypass system;

wherein, in the VTOL orientation, the engine is in the turboshaft mode coupled to the lift fan system such that the engine provides rotational energy to the at least one ducted fan generating the thrust-borne lift; and

wherein, in the forward flight orientation, the engine is in the turbofan mode coupled to the forced air bypass system such that the bypass air combines with the engine exhaust in the exhaust system to provide forward thrust generating the wing-borne lift.

2. The propulsion system as recited in claim 1 wherein the lift fan system further comprises first and second ducted fans in a tandem lateral orientation.

3. The propulsion system as recited in claim 1 wherein the lift fan system further comprises first and second ducted fans in a tandem longitudinal orientation.

4. The propulsion system as recited in claim 1 wherein the lift fan system further comprises first, second and third ducted fans in a tandem lateral and forward orientation.

5. The propulsion system as recited in claim 1 wherein the at least one ducted fan further comprises a rotor assembly having a plurality of variable pitch rotor blades.

6. The propulsion system as recited in claim 1 wherein the at least one ducted fan further comprises a coaxial rotor system having counter-rotating rotor assemblies.

7. The propulsion system as recited in claim 1 wherein the forced air bypass system further comprises an inlet, a bypass fan disposed within the body downstream of the inlet and a bypass duct disposed within the body downstream of the bypass fan, the bypass duct coupled to the exhaust system.

8. The propulsion system as recited in claim 1 wherein the forced air bypass system further comprises:

first and second inlets;

first and second bypass fans disposed within the body downstream of the first and second inlets, respectively; and

first and second bypass ducts disposed within the body downstream of the first and second bypass fans, respectively, the first and second bypass ducts coupled to the exhaust system.

9. The propulsion system as recited in claim 1 wherein the exhaust system further comprises a mixing nozzle.

10. The propulsion system as recited in claim 1 wherein the exhaust system further comprises a film cooled nozzle.

11. The propulsion system as recited in claim 1 wherein the transmission engages the engine with the lift fan system when the engine is in the turboshaft mode and disengages the engine from the lift fan system when the engine is in the turbofan mode.

12. The propulsion system as recited in claim 1 wherein, in the forward flight orientation, the forced air bypass system is configured to generate a bypass ratio of between 8 to 1 and 12 to 1.

13. The propulsion system as recited in claim 1 wherein, in the forward flight orientation, the forced air bypass system is configured to generate a pressure ratio of 1.08 to 1.12.

14. The propulsion system as recited in claim 1 further comprising an ancillary component coupled to the engine, the ancillary component configured to extract shaft power from the engine when the engine in is the turboshaft mode and the turbofan mode.

15. The propulsion system as recited in claim 14 wherein the ancillary component further comprises a compressor.

16. An aircraft operable to transition between thrust-borne lift in a VTOL orientation and wing-borne lift in a forward flight orientation, the aircraft comprising:

a body;

a turboshaft engine disposed within the body, the engine having a turboshaft mode and a turbofan mode, the engine configured to generate engine exhaust;

a lift fan system disposed within the body, the lift fan system including at least one ducted fan;

a forced air bypass system disposed within the body, the forced air bypass system configured to generate bypass air; and

a transmission disposed between the engine and the forced air bypass system, the transmission engaging the engine with the forced air bypass system when the engine is in the turbofan mode and disengaging the engine from the forced air bypass system when the engine is in the turboshaft mode;

an exhaust system coupled to the engine and the forced air bypass system;

wherein, in the VTOL orientation, the engine is in the turboshaft mode coupled to the lift fan system such that the engine provides rotational energy to the at least one ducted fan generating the thrust-borne lift; and

wherein, in the forward flight orientation, the engine is in the turbofan mode coupled to the forced air bypass system such that the bypass air combines with the engine exhaust in the exhaust system to provide forward thrust generating the wing-borne lift.

17. The aircraft as recited in claim 16 wherein the aircraft further comprises a low observable aircraft.

18. The aircraft as recited in claim 16 wherein the aircraft further comprises a fan-in-wing aircraft.

19. The aircraft as recited in claim 16 wherein the aircraft further comprises a fixed wing aircraft.