Rotor position control system

1 . A rotor position control system (RPCS), comprising:

a rotor blade;

an electric motor configured to selectively rotate the rotor blade to provide thrust;

a target marker kinematically associated with the rotor blade;

a sensor configured to sense a position of the target marker; and

a flight control computer configured to selectively control the electric motor in a normal mode of operation in which the rotor blade provides thrust and a phase lock mode of operation in which the electric motor is configured to, without using mechanical connections not used when the electric motor provides thrust, maintain the rotor blade in a predetermined indexed position by applying power to the electric motor while the rotor blade is stopped in the predetermined indexed position to prevent movement of the rotor blade from the predetermined indexed position;

wherein in response to the rotor blade being perturbed from the predetermined indexed position during the phase lock mode of operation, power is applied to the electric motor as a function of at least one of (1) passage of a predetermined amount of time and (2) differentially as a function of an amount of angular offset of the rotor blade from the predetermined indexed position.

2 . The RPCS of claim 1 , wherein the electric motor is a stepper motor.

3 . The RPCS of claim 1 , wherein the target marker is carried by a rotor shaft that drives the rotor blade.

4 . The RPCS of claim 1 , wherein the target marker is carried by a rotor gear that drives the rotor blade.

5 . The RPCS of claim 1 , wherein the target marker is carried by a motor gear that drives the rotor blade.

6 . The RPCS of claim 1 , comprising a second electric motor.

7 . The RPCS of claim 1 , wherein the predetermined indexed position of the rotor blade is selected as a position of least aerodynamic drag.

8 . A tiltrotor aircraft, comprising:

a rotor position control system (RPCS), comprising:

a rotor blade;

an electric motor configured to selectively rotate the rotor blade to provide thrust when the tiltrotor aircraft is operating in a helicopter mode of operation;

a target marker kinematically associated with the rotor blade;

a sensor configured to sense a position of the target marker; and

a flight control computer configured to selectively control the electric motor in a normal mode of operation in which the rotor blade provides thrust and a phase lock mode of operation in which the electric motor is configured to, without using mechanical connections not used when the electric motor provides thrust, maintain the rotor blade in a predetermined indexed position by applying power to the electric motor while the rotor blade is stopped in the predetermined indexed position to prevent movement of the rotor blade from the predetermined indexed position;

wherein in response to the rotor blade being perturbed from the predetermined indexed position during the phase lock mode of operation, power is applied to the electric motor as a function of at least one of (1) passage of a predetermined amount of time and (2) differentially as a function of an amount of angular offset of the rotor blade from the predetermined indexed position.

9 . The tiltrotor aircraft of claim 8 , wherein the predetermined indexed position is a position in which the rotor blade is aligned in a substantially fore-aft direction.

10 . A method of operating a tiltrotor aircraft, comprising:

providing a rotor system comprising a rotor blade;

providing an electric motor configured to selectively rotate the rotor blade about an axis of rotation to provide thrust when the tiltrotor aircraft is in a helicopter mode of operation;

switching operation of the electric motor from operating in a normal mode to drive the rotor blade to generate thrust to a phase lock mode of operation that stops rotation of the rotor blade in a predetermined indexed position;

wherein when the tiltrotor aircraft is in an airplane mode of operation and while the rotor blade is stopped in the predetermined indexed position and not providing thrust, applying power to the electric motor to, without using mechanical connections not used when the electric motor provides thrust, maintain the rotor blade in the predetermined indexed position so as to prevent movement of the rotor blade from the predetermined indexed position; and

during the phase lock mode of operation, responding to the rotor blade being perturbed from the predetermined indexed position by applying power to the electric motor as a function of at least one of (1) passage of a predetermined amount of time and (2) differentially as a function of an amount of angular offset of the rotor blade from the predetermined indexed position.

11 . The method of claim 10 , wherein the power applied to the electric motor to keep the rotor blade in the predetermined indexed position is further selected as a function of a sensed perturbation of the rotor blade from the predetermined indexed position.

12 . The method of claim 10 , wherein the electric motor is switched from the normal mode to the phase lock mode in response to changing operation of the tiltrotor aircraft from the helicopter mode to the airplane mode.

13 . The method of claim 10 , wherein the electric motor is switched from the normal mode to the phase lock mode in response to changing operation of the tiltrotor aircraft from the helicopter mode to a transition mode.

14 . The method of claim 10 , wherein the electric motor is switched from the phase lock mode to the normal mode in response to changing operation of the tiltrotor aircraft from the airplane mode to the helicopter mode or to a transition mode.

15 . The method of claim 10 , wherein the electric motor is switched from the normal mode to the phase lock mode in response to a predetermined forward airspeed being achieved or exceeded.

16 . The method of claim 10 , wherein the electric motor is switched to the normal mode from the phase lock mode in response to a predetermined forward airspeed no longer being achieved or exceeded.