Coaxial rotor systems for VTOL aircraft

1. A coaxial rotor system for a rotorcraft comprising:

a mast;

a top rotor assembly coupled to a distal end of the mast, the top rotor assembly including a plurality of variable pitch rotor blades;

a bottom rotor assembly including a plurality of variable pitch rotor blades and a motor configured to provide rotational energy to the mast, thereby rotating the top rotor assembly; and

a collective control pitch assembly disposed between the top and bottom rotor assemblies, the collective control pitch assembly including a translational sleeve configured to rotate with the mast, a plurality of pitch arms interposed between the translational sleeve and the rotor blades, a limiter base rotatably coupled to an underside of the translational sleeve and a spring interposed between the limiter base and the bottom rotor assembly configured to bias the translational sleeve toward the top rotor assembly, the collective control pitch assembly configured to vary collective pitch of the rotor blades responsive to rotation of the mast;

wherein the bottom rotor assembly experiences a torque reaction force responsive to the motor rotating the mast such that the top and bottom rotor assemblies counter rotate.

2. The coaxial rotor system as recited in claim 1 wherein the mast forms a mast plate disposed in the bottom rotor assembly, the bottom rotor assembly rotatably coupled to the mast plate via a thrust bearing.

3. The coaxial rotor system as recited in claim 1 wherein the bottom rotor assembly further comprises a bottom rotor hub, the motor coupled to an underside of the bottom rotor hub.

4. The coaxial rotor system as recited in claim 1 wherein the motor is rotatably coupled to the mast via a bearing.

5. The coaxial rotor system as recited in claim 1 wherein the mast further comprises an upper mast hingeably coupled to a mast base, further comprising a directional control assembly configured to longitudinally and laterally tilt the upper mast relative to the mast base.

6. The coaxial rotor system as recited in claim 5 wherein the upper mast is hingeably coupled to the mast base via a cardan joint.

7. The coaxial rotor system as recited in claim 5 wherein the directional control assembly further comprises a rotor tilting subassembly including a horizontal arm coupled to the upper mast and a vertical arm including a tilt actuator coupled to the horizontal arm, the tilt actuator configured to tilt the upper mast via the horizontal arm.

8. The coaxial rotor system as recited in claim 7 wherein the rotor tilting subassembly further comprises a longitudinal rotor tilting subassembly and a lateral rotor tilting subassembly spaced approximately 90 degrees from the longitudinal rotor tilting subassembly about the mast.

9. The coaxial rotor system as recited in claim 1 wherein the collective control pitch assembly further comprises an adjustable collective limiter interposed between the limiter base and the bottom rotor assembly configured to limit downward translation of the translational sleeve to control maximum collective pitch of the rotor blades.

10. The coaxial rotor system as recited in claim 1 wherein distal ends of the pitch arms are coupled to trailing ends of the rotor blades and proximal ends of the pitch arms are rotatably coupled to the translational sleeve.

11. The coaxial rotor system as recited in claim 1 wherein the pitch arms further comprise a plurality of upper pitch arms interposed between the translational sleeve and the rotor blades of the top rotor assembly and a plurality of lower pitch arms interposed between the translational sleeve and the rotor blades of the bottom rotor assembly.

12. The coaxial rotor system as recited in claim 1 wherein the mast further comprises one or more splines and the translational sleeve forms one or more spline grooves to receive the one or more splines, the translational sleeve translating along the one or more splines, the one or more splines constraining rotation of the translational sleeve relative to the mast.

13. The coaxial rotor system as recited in claim 1 wherein the mast is rotatably coupled to the top rotor assembly up to a predetermined number of revolutions via a thrust bearing;

wherein the translational sleeve forms a ball screw nut cavity having internal threads;

wherein the collective control pitch assembly further comprises a ball screw having external threads complementary to the internal threads of the ball screw nut cavity, the ball screw coupled to the top rotor assembly; and

wherein the translational sleeve translates downward and the ball screw translates out of the ball screw nut cavity responsive to relative rotation between the mast and the top rotor assembly.

14. An aircraft comprising:

a fuselage;

a mast rotatably coupled to the fuselage;

a top rotor assembly coupled to a distal end of the mast, the top rotor assembly including a plurality of variable pitch rotor blades;

a bottom rotor assembly including a plurality of variable pitch rotor blades and a motor configured to provide rotational energy to the mast, thereby rotating the top rotor assembly; and

a collective control pitch assembly disposed between the top and bottom rotor assemblies, the collective control pitch assembly including a translational sleeve configured to rotate with the mast, a plurality of pitch arms interposed between the translational sleeve and the rotor blades, a limiter base rotatably coupled to an underside of the translational sleeve and a spring interposed between the limiter base and the bottom rotor assembly configured to bias the translational sleeve toward the top rotor assembly, the collective control pitch assembly configured to vary collective pitch of the rotor blades responsive to rotation of the mast;

wherein the bottom rotor assembly experiences a torque reaction force responsive to the motor rotating the mast such that the top and bottom rotor assemblies counter rotate.

15. The aircraft as recited in claim 14 wherein a base of the mast is rotatably coupled to the fuselage via a thrust bearing.

16. The aircraft as recited in claim 14 further comprising a yaw control system including a tailboom control surface rotatably coupled to an aft end of the fuselage and a vertical fin rotatably coupled to an aft end of the tailboom control surface.

17. The aircraft as recited in claim 16 wherein the yaw control system further comprises a yaw control actuator configured to synchronously rotate the tailboom control surface and the vertical fin.