Apparatus and method for roll moment equalization at high advance ratios for rotary wing aircraft
A method for equalizing rolling moments at high advance ratios is disclosed including impelling an aircraft in a forward direction at an airspeed by means of a thrust source and rotating a rotor of the aircraft at an angular velocity with respect to the airspeed effective to cause a positive total lift on each blade due to air flow over the blades in the retreating direction when the blade is moving in the retreating direction. The rotor includes an even number of blades placed at equal angular intervals around the rotor hub. One or both of cyclic pitch and rotor angle of attack are adjusted such that a rolling moment of the retreating blade due to reverse air flow is between 0.3 and 0.7 times a rolling moment on the advancing blade due to lift.
1. A method for operating a rotary wing aircraft comprising:
impelling the aircraft in a forward direction at an airspeed by means of a thrust source, the aircraft having secured thereto a rotor comprising an even number of blades secured at equal angular intervals around a rotor hub rotationally mounted to the aircraft, the rotor operable to rotate about an axis of rotation; and
rotating the rotor at an angular velocity with respect to the aircraft such that each blade is alternately moving in a retreating direction and an advancing direction with respect to the aircraft, the retreating direction having a component opposite the forward direction and the advancing direction having a component in the forward direction;
wherein the angular velocity and airspeed are effective to cause a positive total lift on each blade due to air flow over the blades in the retreating direction, wherein each blade defines a pitch angle relative to a direction of blade movement, the method further comprising:
cyclically modulating the pitch angle effective to cause a bending moment induced on the blades when moving in the retreating direction due to lift to be equal to between 0.3 and 0.7 times a bending moment induced on the blades when moving in the advancing direction due to lift when the blade is moving in the retreating direction;
determining that a rotational frequency of the rotor is less than a minimum rotational frequency;
in response to determining that the rotational frequency of the rotor is less than the minimum rotational frequency, suspending cyclically modulating the pitch angle effective to cause the bending moment induced on the blades when moving in the retreating direction due to lift to be equal to between 0.3 and 0.7 times the bending moment induced on the blades when moving in the advancing direction due to lift when the blade is moving in the retreating direction.
2. The method of claim 1 , wherein the number of blades is four.
3. The method of claim 1 , wherein rotation of the rotor defines a rotor disc and wherein the rotor disc defines an angle of attack above the forward direction; and
wherein the angle of attack is effective to cause the bending moment on the blades when moving in the retreating direction due to lift to be equal to from about 0.3 to about 0.7 times the bending moment on the blades when moving in the advancing direction due to lift.
4. The method of claim 1 , wherein the blades are rigidly connected to the rotor hub.
5. The method of claim 1 , wherein the aircraft comprises fixed wings, the fixed wings providing lift sufficient to support at least 50 percent of a weight of the aircraft for airspeeds greater than 200 miles per hour.
6. The method of claim 1 , wherein the aircraft comprises fixed wings, the fixed wings providing lift sufficient to support at least 90 percent of a weight of the aircraft for airspeeds greater than 200 miles per hour.
7. The method of claim 1 , wherein the airspeed is greater than 200 miles per hour.
8. The method of claim 1 , wherein tips of the blades have a tip speed and wherein the airspeed of the aircraft divided by the tip speed is greater than from about 0.7 to about 2.5.
9. The method of claim 1 , wherein:
jets are secured to tips of the blades;
the blades each define an air channel; and
the jets are in fluid communication with a compressed air source through the air channel.
10. The method of claim 9 , wherein:
the thrust source is a jet engine;
the compressed air source comprises a bypass turbine of the jet engine; and
the method further comprises cyclically increasing the drag on the blades when the blades are moving in the retreating direction.
11. An aircraft comprising:
a fuselage;
a thrust source coupled to the fuselage to urge the fuselage in a forward direction at an airspeed;
a rotor including an even number of blades secured at equal angular intervals around a rotor hub rotatably mounted to the fuselage, the rotor operable to rotate relative to the fuselage at an angular velocity such that each blade is alternately moving in a retreating direction and an advancing direction with respect to the aircraft, the retreating direction having a component opposite the forward direction and the advancing direction having a component in the forward direction;
the rotor, wherein each blade defines a pitch angle relative to a direction of blade movement and wherein an actuated flap is secured at a distal end of each blade;
a flight control system coupled to the actuated flaps and programmed to control the angular velocity effective to cause a positive total lift on each blade due to air flow in the retreating direction when the blade is moving in the retreating direction; and
the flight control system further programmed to cyclically modulate the pitch angle effective to cause a bending moment induced on the blades when moving in the retreating direction due to lift to be equal to between 0.3 and 0.7 times a bending moment induced on the blades when moving in the advancing direction due to lift;
the flight control system further programmed to open the actuated flap of each blade when moving in the retreating direction and close the actuated flap of each blade when moving in the advancing direction.
12. The aircraft of claim 11 , wherein rotation of the rotor defines a rotor disc and wherein the rotor disc defines an angle of attack above the forward direction; and wherein the flight control system is programmed to adjust the angle of attack effective to cause the bending moment on the blades when moving in the retreating direction due to lift to be equal to between 0.3 and 0.7 times the bending moment on the blades when moving in the advancing direction due to lift.
13. The aircraft of claim 11 , wherein the blades are rigidly connected to the rotor hub.
14. The aircraft of claim 11 , wherein the aircraft further comprises fixed wings, the fixed wings providing lift sufficient to support at least 50 percent of a weight of the aircraft for airspeeds greater than 200 miles per hour.
15. The aircraft of claim 11 , further comprising:
a compressed air source coupled to the fuselage, the blades comprising ducts in fluid communication with the compressed air source; and
jets secured to tips of the blades in fluid communication with the ducts.
16. The aircraft of claim 15 , wherein the thrust source is a jet engine and wherein the compressed air source comprises a bypass turbine of the jet engine.