A rapid pitch-up and pitch-down maneuver of a chord-wise flexible wing in a steady free stream is studied by using a lattice Boltzmann flexible particle method in a three-dimensional space at a chord based Reynolds number of 100. The pitching rates, flexibility, and wing density are systematically varied, and their effects on aerodynamic forces are investigated. It is demonstrated that the flexibility can be utilized to significantly improve lift forces. The flexible wing has a larger angular momentum due to elasticity and inertia and generates a larger leading edge vortex as compared with a rigid wing. Such lift enhancement occurs mainly during the pitch-down motion while a large stall angle is produced during the pitch-up motion. At a low pitch rate, the flexibility cannot improve lift. (C) 2014 AIP Publishing LLC.;A rapid pitch-up and pitch-down maneuver of a chord-wise flexible wing in a steady free stream is studied by using a lattice Boltzmann flexible particle method in a three-dimensional space at a chord based Reynolds number of 100. The pitching rates, flexibility, and wing density are systematically varied, and their effects on aerodynamic forces are investigated. It is demonstrated that the flexibility can be utilized to significantly improve lift forces. The flexible wing has a larger angular momentum due to elasticity and inertia and generates a larger leading edge vortex as compared with a rigid wing. Such lift enhancement occurs mainly during the pitch-down motion while a large stall angle is produced during the pitch-up motion. At a low pitch rate, the flexibility cannot improve lift. (C) 2014 AIP Publishing LLC.