This paper presents a longitudinal dynamic model to be used in the control of new animal flight bio-inspired UAVs designed to achieve better performance in terms of energy consumption, flight endurance, and safety when comparing with conventional multi-rotors. In order to control these UAVs, simple models are needed to predict its dynamics in real time by the on-board autopilots, which are very limited in term of computational resources. To that end, the model presented considers transitional aerodynamic unsteady effects, which change significantly the evolution of the system. The physical relevance of these aerodynamic unsteady terms in gliding flight is validated by comparing with results when these new terms are neglected. Finally, an analysis of dynamic stability is proposed in order to characterize the transitional phases of gliding flight.