The article describes an experiment of the movement of a butterfly wing model in glycerin to obtain a relationship between rotation functions and aerodynamic forces.Measurements were taken during the movement of an artificial wing, modelled on a real one, in a dense viscous medium with proportional reduction of the movement frequency. Reducing the frequency, i.e. slowing down the wing, makes the observation easier and produces more reliable results. The tests started with the movement observed in the living butterfly and then, in each subsequent step, the angles of inclination were modified in such a way as to describe it in terms of simple functions, without losing their physical properties, i.e. due to aerodynamic forces. According to the literature, the insect flight can be divided into three phases of stroke in flapping flight. In each, aerodynamic forces are generated or aerodynamic drag is minimised. The experimental tests were filmed, and the pressure differences were measured. Based on the data analysis, the functions of wing inclination angles in time for the characteristic flight moments – acceleration, hover, and take-off – were specified.