Control system for a stopped rotor aircraft

1. A system, comprising:

a processor; and

a memory coupled with the processor, wherein the memory is configured to provide the processor with instructions which when executed cause the processor to:

calculate, in real time while an aircraft which includes a stoppable rotor is mid-flight, a braking start point associated with the stoppable rotor, wherein the stoppable rotor includes a first blade and a second blade and the stoppable rotor is configured to rotate about a substantially vertical axis and the braking start point is a point at which to begin applying negative torque using rotor motors; and

start a process to stop the stoppable rotor, while the aircraft which includes the stoppable rotor is mid-flight, when the stoppable rotor reaches the braking start point, wherein the stoppable rotor is stopped with the first blade pointing forward and the second blade pointing backward.

2. The system of claim 1 , wherein the stopping process includes:

applying an initial amount of torque to the stoppable rotor, wherein the magnitude of the initial amount of torque is strictly less than the magnitude of a maximum amount of torque; and

adjusting an amount of torque applied to the stoppable rotor using feedback.

3. The system of claim 1 , wherein the stopping process includes:

applying an initial amount of torque to the stoppable rotor, wherein the magnitude of the initial amount of torque is strictly less than the magnitude of a maximum amount of torque; and

adjusting an amount of torque applied to the stoppable rotor using feedback, including by using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor.

4. The system of claim 1 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor to: while the stoppable rotor is stopped with the first blade pointing forward and the second blade pointing backward, adjust a nominal torque of zero applied to the stoppable rotor using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor.

5. The system of claim 1 , wherein:

the stopping process includes:

applying an initial amount of torque to the stoppable rotor, wherein the magnitude of the initial amount of torque is strictly less than the magnitude of a maximum amount of torque; and

adjusting an amount of torque applied to the stoppable rotor using feedback, including by using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor; and

the memory is further configured to provide the processor with instructions which when executed cause the processor to: while the stoppable rotor is stopped with the first blade pointing forward and the second blade pointing backward, adjust a nominal torque of zero applied to the stoppable rotor using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor.

6. The system of claim 1 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor to: begin to search for the braking start point once the stoppable rotor has reached a searching start point.

7. The system of claim 1 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor to: begin to search for the braking start point once the stoppable rotor has reached a searching start point, wherein a difference between the searching start point and the braking start point is a pre-defined amount.

8. The system of claim 1 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor to adjust a braking start point based at least in part on a crosswind.

9. The system of claim 1 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor to determine that the stoppable rotor is in an unstable position.

10. The system of claim 9 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor to rotate the stoppable rotor at least one more rotation in a regular direction of rotation.

11. The system of claim 9 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor to rotate the stoppable rotor at least one more rotation in a regular direction of rotation until the stoppable rotor is in a stable position.

12. The system of claim 9 , wherein the memory is further configured to provide the processor with instructions which when executed cause the processor, in response to the stoppable rotor being in a stable position, starting the process to stop to stoppable rotor.

13. A method, comprising:

calculating, in real time while an aircraft which includes a stoppable rotor is mid-flight, a braking start point associated with the stoppable rotor, wherein the stoppable rotor includes a first blade and a second blade and the stoppable rotor is configured to rotate about a substantially vertical axis and the braking start point is a point at which to begin applying negative torque using rotor motors; and

starting a process to stop the stoppable rotor, while the aircraft which includes the stoppable rotor is mid-flight, when the stoppable rotor reaches the braking start point, wherein the stoppable rotor is stopped with the first blade pointing forward and the second blade pointing backward.

14. The method of claim 13 , wherein the stopping process includes:

applying an initial amount of torque to the stoppable rotor, wherein the magnitude of the initial amount of torque is strictly less than the magnitude of a maximum amount of torque; and

adjusting an amount of torque applied to the stoppable rotor using feedback.

15. The method of claim 13 , wherein the stopping process includes:

applying an initial amount of torque to the stoppable rotor, wherein the magnitude of the initial amount of torque is strictly less than the magnitude of a maximum amount of torque; and

adjusting an amount of torque applied to the stoppable rotor using feedback, including by using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor.

16. The method of claim 13 , further comprising: while the stoppable rotor is stopped with the first blade pointing forward and the second blade pointing backward, adjusting a nominal torque of zero applied to the stoppable rotor using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor.

17. The method of claim 13 , wherein:

the stopping process includes:

applying an initial amount of torque to the stoppable rotor, wherein the magnitude of the initial amount of torque is strictly less than the magnitude of a maximum amount of torque; and

adjusting an amount of torque applied to the stoppable rotor using feedback, including by using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor; and

the method further includes: while the stoppable rotor is stopped with the first blade pointing forward and the second blade pointing backward, adjusting a nominal torque of zero applied to the stoppable rotor using a proportional-integral-derivative (PID) controller which uses a rotational angle associated with the stoppable rotor and an angular rate associated with the stoppable rotor.

18. The method of claim 13 further comprising: beginning to search for the braking start point once the stoppable rotor has reached a searching start point.

19. The method of claim 13 further comprising: begin to search for the braking start point once the stoppable rotor has reached a searching start point, wherein a difference between the searching start point and the braking start point is a pre-defined amount.

20. A computer program product, the computer program product being embodied in a non-transitory computer readable storage medium and comprising computer instructions for:

calculating, in real time while an aircraft which includes a stoppable rotor is mid-flight, a braking start point associated with the stoppable rotor, wherein the stoppable rotor includes a first blade and a second blade and the stoppable rotor is configured to rotate about a substantially vertical axis and the braking start point is a point at which to begin applying negative torque using rotor motors; and

starting a process to stop the stoppable rotor, while the aircraft which includes the stoppable rotor is mid-flight, when the stoppable rotor reaches the braking start point, wherein the stoppable rotor is stopped with the first blade pointing forward and the second blade pointing backward.