Abstract: A propulsion engine fan incorporates two sets of blades with recessed portions formed along the surface of each blade. One set of blades has a higher resonant frequency than the other set of blades. In an illustrative embodiment of the invention, first and second blade groups each comprise an airfoil portion forming a surface for directing the flow of a working fluid through the engine. Each of the first group of blades has first and second recessed regions formed from the blade surface and has a first resonant frequency. Each of the second group of blades has third and fourth recessed regions formed from the blade surface and has a second resonant frequency less than the first resonant frequency. The first and second group blades are arranged in an alternating pattern along the rotor portion of the fan.