An aircraft rotor assembly (12) comprises first and second rotors (42A, 42B) and a drive assembly (46). The first and second rotors are rotatable about a common axis (A) synchronously and with respect to each other. The first rotor has a first blade (44A) extending radially from the common axis; the second rotor has a second blade (44B) extending radially from the common axis, longitudinally offset from the first blade along the common axis. The drive assembly is coupled to the first and second rotors and configured to controllably vary, during continuous rotation of the first and second rotors, a differential phase angle (φ) separating the first and second blades as projected onto a plane (P A , P B ) perpendicular to the common axis.