According to quasi-steady-state aerodynamic theory, slow-flying vertebrates (e.g., insects) should not be able to generate enough lift to remain aloft . Therefore, unsteady aerodynamic mechanisms to enhance lift production have been proposed. Muijres et al.  showed that unsteady aerodynamic mechanisms are used not only by insects but also by larger and heavier fliers. Hubel and Tropea  verified Muijres et al.  findings by showing that the unsteady effects are not negligible for a goose-sized flapping model. Thus far, the main purpose of investigating unsteady aerodynamic mechanisms has been to understand their ability to enhance lift generation. However, it remains relatively unknown how unsteady aerodynamics participate in the drag and thrust balance of flapping flight.
Therefore, the drag is composed of two terms: steady (second term) and unsteady (first term). The steady drag per unit span, referred to as the velocity deficit drag in classical aerodynamics, can be derived from the second term in Eq. (9) and expressed as 2b1af7f3a8