Yaw control systems for tailsitting biplane aircraft

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

an airframe including a first wing, a second wing and a fuselage that extends between the first and second wings;

a propulsion assembly coupled to the fuselage, the propulsion assembly including a counter-rotating coaxial rotor system that is tiltable relative to the fuselage to generate a thrust vector;

first and second yaw vanes extending aftwardly from the fuselage; and

a flight control system configured to direct the thrust vector of the coaxial rotor system and control movements of the yaw vanes;

wherein, in the VTOL orientation, the aircraft has a VTOL yaw axis that extends through the fuselage and the propulsion assembly, and differential operation of the yaw vanes provides yaw authority for the aircraft; and

wherein, in the biplane orientation, the aircraft has a biplane yaw axis that extends through the fuselage and the wings, and collective operation of the yaw vanes provides yaw authority for the aircraft.

2. The aircraft as recited in claim 1 wherein the propulsion assembly further comprises a motor assembly and wherein the coaxial rotor system and the motor assembly are tiltable relative to the fuselage to generate the thrust vector.

3. The aircraft as recited in claim 1 wherein the coaxial rotor system is configured to provide thrust in line with the VTOL yaw axis of the aircraft in the VTOL orientation and in line with a roll axis of the aircraft in the biplane orientation.

4. The aircraft as recited in claim 1 wherein, in the VTOL orientation, the first wing is forward of the fuselage and the second wing is aft of the fuselage; and

wherein, in the biplane orientation, the first wing is below the fuselage and the second wing is above the fuselage.

5. The aircraft as recited in claim 1 wherein the first and second yaw vanes are pivotably coupled to the fuselage.

6. The aircraft as recited in claim 1 wherein, in the VTOL orientation, the first and second yaw vanes are symmetrically disposed relative to the VTOL yaw axis.

7. The aircraft as recited in claim 1 wherein, in the biplane orientation, the first and second yaw vanes are aftwardly disposed relative to the biplane yaw axis.

8. The aircraft as recited in claim 1 wherein, in the VTOL orientation, the first yaw vane is actuated in a first direction and the second yaw vane is actuated in a second direction that is opposite of the first direction to provide yaw authority for the aircraft.

9. The aircraft as recited in claim 8 wherein, in the VTOL orientation, downwash generated by the propulsion assembly acting on the actuated first and second yaw vanes creates first and second yaw moments that urge the aircraft to rotate about the VTOL yaw axis.

10. The aircraft as recited in claim 1 wherein, in the biplane orientation, the first and second yaw vanes are both actuated in a first direction to provide yaw authority for the aircraft.

11. The aircraft as recited in claim 10 wherein, in the biplane orientation, airflow generated responsive to the forward airspeed of the aircraft acting on the actuated first and second yaw vanes creates yaw moments that urges the aircraft to rotate about the biplane yaw axis.

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

an airframe including a first wing, a second wing and a fuselage that extends between the first and second wings;

a propulsion assembly coupled to the fuselage, the propulsion assembly including a counter-rotating coaxial rotor system having first and second rotor assemblies, the coaxial rotor system tiltable relative to the fuselage to generate a thrust vector;

first and second yaw vanes extending aftwardly from the fuselage; and

a flight control system configured to direct the thrust vector of the coaxial rotor system, control rotor speeds and collective pitches of the first and second rotor assemblies and control movements of the yaw vanes;

wherein, in the VTOL orientation, the aircraft has a VTOL yaw axis that extends through the fuselage and the propulsion assembly;

wherein, in the biplane orientation, the aircraft has a biplane yaw axis that extends through the fuselage and the wings; and

wherein, in the VTOL orientation, differential operation of the first and second rotor assemblies and differential operation of the yaw vanes provide yaw authority for the aircraft.

13. The aircraft as recited in claim 12 wherein the first and second yaw vanes are pivotably coupled to the fuselage.

14. The aircraft as recited in claim 12 wherein, in the VTOL orientation, the first and second yaw vanes are symmetrically disposed relative to the VTOL yaw axis.

15. The aircraft as recited in claim 12 wherein, in the VTOL orientation, when the first yaw vane is actuated in a first direction and the second yaw vanes is actuated in a second direction that is opposite of the first direction, downwash generated by the propulsion assembly acting on the first and second yaw vanes creates first and second yaw moments that urge the aircraft to rotate about the VTOL yaw axis.

16. The aircraft as recited in claim 12 wherein, in the VTOL orientation, the flight control system is configured to provide yaw authority for the aircraft responsive to actuating the first yaw vane in a first direction, actuating the second yaw vane in a second direction that is opposite of the first direction and differentially operating the first and second rotor assemblies by adjusting the rotor speeds.

17. The aircraft as recited in claim 12 wherein, in the VTOL orientation, the flight control system is configured to provide yaw authority for the aircraft responsive to actuating the first yaw vane in a first direction, actuating the second yaw vane in a second direction that is opposite of the first direction and differentially operating the first and second rotor assemblies by adjusting the collective pitches.

18. The aircraft as recited in claim 12 wherein, in the VTOL orientation, the flight control system is configured to provide yaw authority for the aircraft responsive to actuating the first yaw vane in a first direction, actuating the second yaw vane in a second direction that is opposite of the first direction and differentially operating the first and second rotor assemblies by adjusting the rotor speeds and the collective pitches.