NOVELTY - The apparatus has an airfoil structure including a span (22) extending along spanwise direction between a root (21) and a tip (23), where a part of the span is defined by a set of molded monolithic elements including corrugated portions, which are integrally formed and joined to top flange portions and bottom flange portions. The top and bottom flange portions include stepped interfaces to aid in side-by-side alignment of the molded monolithic elements and to provide distinct planes to glue, bond and affix the molded monolithic elements.    USE - Airfoil apparatus for use in an aircraft wing, a propeller blade, a horizontal-axis-type wind turbine blade and a vertical-axis-type wind turbine blade (all claimed). Can also be used in an aileron, stabilizer, helicopter rotor, fan blade for heating, ventilation and air-conditioning system and an industrial cooling tower.    ADVANTAGE - The airfoil structure integrates portions of top and bottom surfaces of the span and internal support structures that extend between the top and bottom surfaces, thus enhancing structural resistance to torsional loading of an airfoil along the span and reducing need for additional material to stiffen skin of the airfoil, and hence improving strength-to-weight ratio of the airfoil, while avoiding issues of interlaminar shear stress within skin of a composite airfoil. The airfoil structure is assembled in an efficient and cost effective manner.    DETAILED DESCRIPTION - The monolithic elements are constructed from composite material comprising fiber reinforcement material and resin matrix material. The fiber reinforcement material comprises fiber that is selected from a group consisting of glass e.g. E-glass and S-glass, nylon, polyamide, aromatic polyamide, Kevlar (RTM: light strong para-aramid synthetic fiber), polyethylene e.g. ultra-high molecular weight polyethylene, high-modulus polyethylene and high-performance polyethylene, polyethylene terephthalate, carbon and basalt.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a horizontal-axis wind turbine blade including an airfoil structure.    Airfoil section (15)    Root (21)    Span (22)    Tip (23)    Reinforcement stringer (27)