Stowable lift rotors for VTOL aircraft

1. A stowable lift rotor coupled to an airframe of an aircraft, the aircraft convertible between a vertical takeoff and landing flight mode and a forward flight mode, the stowable lift rotor comprising:

a lift arm having proximal and distal ends, the proximal end coupled to the airframe of the aircraft;

a four-bar linkage extending through the lift arm; and

a rotor assembly including a plurality of rotor blades coupled to the distal end of the lift arm;

wherein, the lift arm is movable between a plurality of positions including an extended position in the vertical takeoff and landing flight mode, a stowed position in the forward flight mode and intermediate positions therebetween such that a distance between the rotor assembly and the airframe is greater in the extended position than in the stowed position.

2. The stowable lift rotor as recited in claim 1 wherein the airframe forms a recess to receive the lift arm in the stowed position.

3. The stowable lift rotor as recited in claim 1 wherein the proximal end of the lift arm comprises a forward end rotatably coupled to the airframe of the aircraft and the distal end of the lift arm comprises an aft end rotatably coupled to the rotor assembly.

4. The stowable lift rotor as recited in claim 3 further comprising a motor coupled to the airframe of the aircraft adjacent to the proximal end of the lift arm;

wherein, the lift arm comprises a driveshaft transmitting rotational energy from the motor at the proximal end of the lift arm to the rotor assembly at the distal end of the lift arm, the driveshaft forming a link of the four-bar linkage.

5. The stowable lift rotor as recited in claim 4 further comprising a first plurality of bevel gears transmitting rotational energy from the motor to a proximal end of the driveshaft and a second plurality of bevel gears transmitting rotational energy from a distal end of the driveshaft to the rotor assembly.

6. The stowable lift rotor as recited in claim 4 wherein the rotor assembly comprises a rotor hub housing and the lift arm comprises a linkage having proximal and distal ends, the proximal end of the linkage rotatably coupled to the airframe of the aircraft and the distal end of the linkage rotatably coupled to the rotor hub housing, each of the rotor hub housing and the linkage forming a link of the four-bar linkage.

7. The stowable lift rotor as recited in claim 3 further comprising an actuator assembly configured to move the lift arm between the extended position and the stowed position.

8. The stowable lift rotor as recited in claim 7 wherein the actuator assembly comprises a ball screw actuator assembly and a strut, a first end of the strut coupled to the ball screw actuator assembly and a second end of the strut coupled to the lift arm.

9. The stowable lift rotor as recited in claim 1 wherein the plurality of rotor blades comprise first and second rotor blades; and

wherein, the rotor assembly is lockable in the stowed position such that the first and second rotor blades are parallel to a longitudinal axis of the aircraft, thereby reducing drag by minimizing a frontal profile area of the rotor assembly exposed to freestream airflow in the forward flight mode.

10. An aircraft convertible between a vertical takeoff and landing flight mode and a forward flight mode comprising:

a fuselage;

a tailboom coupled to the fuselage; and

a stowable lift rotor coupled to the tailboom, the stowable lift rotor comprising:

a lift arm having proximal and distal ends, the proximal end coupled to the tailboom;

a four-bar linkage extending through the lift arm; and

a rotor assembly including a plurality of rotor blades coupled to the distal end of the lift arm;

wherein, the lift arm is movable between a plurality of positions including an extended position in the vertical takeoff and landing flight mode, a stowed position in the forward flight mode and intermediate positions therebetween such that a distance between the rotor assembly and the tailboom is greater in the extended position than in the stowed position.

11. The aircraft as recited in claim 10 further comprising a wing coupled to the fuselage;

wherein, the tailboom comprises left and right tailbooms coupled to the fuselage via the wing; and

wherein, the stowable lift rotor comprises left and right stowable lift rotors, the left stowable lift rotor coupled to the left tailboom and the right stowable lift rotor coupled to the right tailboom.

12. The aircraft as recited in claim 10 wherein the plurality of rotor blades are rotatable in a first rotational plane in the extended position;

wherein, the plurality of rotor blades lie in a second plane in the stowed position; and

wherein, the first rotational plane is substantially parallel to the second plane.