Patent ID: 9205919
Filing Date: 2015-12-08
Classification: B64C

Abstract:
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.