Rotor collective pitch VS Mu to control flapping and mast/rotor tilt to control rotor RPM

1. A method of operating a rotor aircraft, comprising:

(a) measuring the true airspeed of the aircraft;

(b) measuring a rotational speed of the rotor;

(c) determining a Mu of the rotor based on the true airspeed of the aircraft and the rotational speed of the rotor;

(d) varying a collective pitch in relation to the Mu; and

wherein step (d) comprises determining a Mu vs collective pitch curve, programming a controller with the curve, then with the controller, varying the collective pitch in accordance with at least a portion of the curve.

2. The method according to claim 1 , wherein step (d) further comprises:

tilting a mast of the rotor in fore and aft directions to maintain a desired rotational speed of the rotor.

3. The method according to claim 1 , wherein step (d) further comprises:

at take-off of the aircraft, maintaining the collective pitch at a substantially constant take-off collective pitch; comparing the take-off collective pitch to the increase in actual Mu as the aircraft leaves the ground, then

when the actual Mu exceeds the Mu that corresponds to the take-off collective pitch as observed on the curve, decreasing the collective pitch in accordance with at least a portion of the curve.

4. The method according to claim 1 , wherein step (d) further comprises decreasing the collective pitch as the Mu increases up to a selected upper level of Mu, then maintaining the collective pitch substantially constant as Mu increases beyond the selected upper level.

5. The method according to claim 4 , further comprising:

above the selected upper level, tilting the rotor fore and aft to maintain a selected minimum rotor RPM level, whereby tilting the rotor aft increases the rotor RPM and tilting the rotor forward decreases the rotor RPM.

6. The method according to claim 1 , wherein the aircraft has wings that provide substantially all of the lift required for the aircraft when reaching cruise conditions, and wherein the method further comprises:

prior to reaching cruise conditions and when the rotor is still providing a significant portion of lift for the aircraft, automatically tilting a mast of the rotor to maintain a desired angle of attack for the wings; and

when at cruise conditions, automatically tilting a mast of the rotor in fore and aft directions to slow the rotor and maintain the rotor at a selected RPM.

7. The method according to claim 1 , wherein step (d) further comprises:

at take-off of the aircraft, maintaining the collective pitch at a substantially constant take-off level collective pitch; and

when the Mu of the rotor exceeds the take-off level collective pitch corresponding to the Mu vs collective pitch curve, decreasing the collective pitch in a selected relationship with Mu as the Mu continues to increase; then

at a selected upper level of Mu, maintaining the collective pitch substantially constant as Mu increases more than said upper level.

8. A method of operating a rotor aircraft, comprising:

(a) measuring the true airspeed of the aircraft;

(b) measuring a rotational speed of the rotor;

(c) determining a Mu of the rotor based on the true airspeed of the aircraft and the rotational speed of the rotor;

(d) varying a collective pitch in relation to the Mu; and wherein:

step (d) comprises determining a Mu vs collective pitch curve that limits flapping to a selected range for each Mu, programming a controller with the curve, selecting a desired take-off level collective pitch, then with the controller:

determining a higher Mu level on the curve that corresponds to the take-off collective pitch,

at take-off of the aircraft, maintaining the collective pitch at a substantially constant take-off level collective pitch until reaching this higher Mu level, then

decreasing the collective pitch in accordance with the curve as the Mu increases beyond this higher Mu level to a selected even higher Mu level; then

maintaining the collective pitch substantially constant as Mu increases more than the even higher Mu level; and

controlling the tilt of the rotor fore and aft to maintain a desired minimum rotational speed of the rotor.

9. A method of operating an aircraft having a rotor, a set of wings, and a propulsion source, comprising:

(a) determining a Mu vs collective pitch curve for the aircraft and programming a controller with the curve;

(b) pre-rotating the rotor, operating the propulsion source, and increasing a collective pitch of the rotor to cause the aircraft to lift off and commence forward flight;

(c) during the forward flight, tilting the rotor aft to cause the rotor to auto-rotate;

(d) measuring a true airspeed of the aircraft;

(e) measuring a rotational speed of the rotor;

(f) continuously calculating a Mu of the rotor based the airspeed of the aircraft and the rotational speed of the rotor; and

(g) with the controller varying the collective pitch according to at least part of the Mu vs collective pitch curve.

10. The method according to claim 9 , wherein the curve of step (a) reaches a substantially constant level of collective pitch after which the controller maintains the collective pitch substantially constant as the Mu increases.

11. The method according to claim 9 , wherein:

the Mu vs. collective pitch curve of step (a) has a maximum collective pitch at take-off;

step (b) comprises at take-off selecting a take-off collective pitch less than the maximum collective pitch defined by the Mu vs pitch curve, the take-off collective pitch corresponding to a particular Mu on the Mu vs. collective pitch curve; and

step (g) comprises maintaining the selected take-off collective pitch until the actual Mu reaches the particular Mu on the Mu vs. collective pitch curve, then varying the collective pitch according to at least a portion of the curve as the Mu increases beyond the particular Mu.

12. The method according to claim 9 , wherein:

step (c) comprises controlling the amount of tilt of the rotor with the controller to maintain a selected minimum auto-rotational speed of the rotor.

13. The method according to claim 9 , wherein the curve of step (a) is based on adjusting the collective pitch at a level versus Mu that limits flapping of the rotor to a selected range.

14. The method according to claim 9 , further comprising:

disabling step (g) during forward flight and manually controlling the collective pitch.

15. The method according to claim 9 , wherein steps (c) and (g) are coordinated to substantially unload the rotor.

16. The method according to claim 9 , wherein:

the Mu vs. collective pitch curve of step (a) has a maximum collective pitch at take-off and a substantially constant minimum collective pitch that occurs at and beyond an upper level Mu;

step (b) comprises at take-off selecting a take-off collective pitch less than the maximum collective pitch defined by the Mu vs pitch curve, the take-off collective pitch having a corresponding higher level Mu on the Mu vs. collective pitch curve;

step (g) comprises maintaining the selected take-off collective pitch until the actual Mu reaches this higher level Mu, then varying the collective pitch according to the curve as the actual Mu exceeds this higher level Mu; and

step (g) comprises maintaining the collective pitch at the minimum collective pitch after the actual Mu reaches and exceeds an even higher level Mu.

17. The method according to claim 9 , further comprising, during a short roll or zero airspeed landing, with the controller controlling an increase in collective pitch in order to maintain the required lift for the aircraft.