Abstract: A propeller for an aircraft or fan comprises an even number of propeller blades, at least six in number, which are unsymmetrically arranged around the axis of roation of the propeller in two circumferentially equidistantly spaced groups of at least three blades each. Each blade of each group is paired and longitudinally aligned with a respective blade of another group at the opposite side of the axis of rotation of the propeller, and each two next adjacent blades in each group are offset relative to each other by a spacing angle of between about 15.degree. and 40.degree.. For a selected speed of rotation of the propeller corresponding to a particular blade-tip Mach number, the magnitude of the spacing angle is inversely related to the magnitude of that blade-tip Mach number, for effecting a reduction of the level of the rotational sound harmonics by interference.

Abstract: A propeller for an aircraft or a blower or fan, wherein the propeller blades are intended for operation at speeds characterized by blade tip Mach numbers in the range of about 0.4 to 0.8, comprises at least two pairs of diametrically opposed blades, the blade pairs being arranged at a spacing angle .epsilon. of between about 15.degree. and 50.degree. between them, and the magnitude of the spacing angle being inversely related to the magnitude of the blade tip Mach numbers, thereby to effect a reduction in the level of the rotational sound harmonics by interference. The pairs of blades may be axially spaced from one another, i.e., arranged in respective spaced parallel planes of rotation, shifted, with the rear one of two adjacent blades leading in the direction of rotation.