To improve power extraction efficiency of a flapping foil in reversed D-shaped mode, the present paper proposes a new type of expandable frontal flap (EFF). The EFF expands only in the second quarter of the flapping cycle which increases drag and reduces the power consumed by horizontal motion \({P}_{x}\left(t\right).\) For a Reynold number Re = 1100, the effect of flap length and its expansion angle were investigated numerically via a 2D analysis and for a specific rang of reduce frequency \({f}^{*}\) . As a primary result, it was found that the frontal flap increases the output power by 4.7%. This result is employed to propose a novel concept of expandable Gurney flap (EGF) that expands sequentially throughout the rest of the motion cycle. It was found that this design is capable of increasing the efficiency by 17% relative to the conventional reversed D-shaped trajectory and by 20% as compared to vertical path.
