US Patent 10036392 Axial fan for industrial use

An axial fan comprises a hub and a plurality of blades extending from the hub; wherein each blade comprises a main blade portion and a secondary blade portion and the secondary blade portion has a leading edge adjacent to a leading edge of the main blade portion and forms a flap for the main blade portion; wherein a fluid passage is defined between the leading edge of the main blade portion and the leading edge of the secondary blade portion; wherein the main blade portion has a main chord and the secondary blade portion has a secondary chord; and wherein the main chord and the secondary chord form a relative attack angle comprised between 5° and 35°.