The aim of the present study is to understand the mechanism of propulsion in biological flyers, initially starting from rest, towards development of micro-air vehicles employing the same concept using a computational analysis of the plunging motion of a rigid wing. A two-dimensional numerical model has been developed to probe the fluid mechanics associated with the vertical plunging motion of a flat plate analogous to the biological flight of birds and insects. Recent experiments have shown that propulsion can be achieved in still air by a rigid wing if it is flapped above a critical Reynolds number (based on the flapping frequency). The influence of parameters such as density ratio ρ* and flapping Reynolds number Ref on the propulsion of a vertically heaving wing unconstrained to move in horizontal direction has been investigated. Flow fields generated at different time instants indicate that the interaction with the previous shed vortices creates asymmetry that pushes the wing into locomotion. The vortical wake patterns are scrutinized at steady translation which varied depending upon the Strouhal number. In addition, the behavior of input/output power and efficiency has been quantified.