Patent ID: 8201772

Claim:
A method for suppressing an external acoustic signature of an aircraft having at least one pair of non-intermeshing multiple-blade rotors, the method comprising: providing a tiltrotor having: a fuselage; a wing coupled to the fuselage; and a nacelle rotatably coupled to the wing and configured to pivot the at least one pair of intermeshing multiple-blade rotors in relation to the fuselage; determining the indexing angle for the at least one pair of non-intermeshing multiple-blade rotors, the rotors being indexed according to the number of blades, for achieving a phasing of generated forces exhibited in a plane parallel to that of the rotation of the blades, the forces generated from the reduction of rotor speed as the rotors pivot about the wing in relation to the fuselage; selecting a phasing angle configured to be selected in relation to the phasing of the generated force; and indexing each non-intermeshing multiple-blade rotor prior to flight, so that the blades of one rotor are selectively indexed in relation to the blades of the other rotor, the indexing between each pair of rotors remains consistently out of phase during flight such that the revolutions per minute of each rotor remains constant with respect to the other rotor; wherein the selected amount in degrees of asymmetrical indexing suppresses the external acoustic signature of the aircraft.