The ability to understand and predict the flight trajectory of a sports ball is important in many fields of sports engineering. The aerodynamic coefficients of sports balls have been studied in detail and yet aerodynamic models generally assume that spin rates are constant during flight. This study examines the spin decay of sports balls in flight in order to improve the accuracy of aerodynamic models. Several types of sports ball (tennis ball, worn tennis ball, football, oversize tennis ball) were launched in a sports hall using artificial bowlers at various speeds and spin rates. The Start and end of the trajectories were filmed using two high speed video cameras and the resulting measurements were used to assess the spin decay of each ball as well as the factors affecting spin decay. The data shows a strong linear relationship between spin decay and the product of initial spin and speed for all the different sports balls tested. However, the constant in this linear relationship was found to be different for each of the different balls and was assumed to be dependent on the physical attributes of the different balls. Dimensionless parameters were derived such that the differences between the sports ball spin characteristics could be more easily identified. It was found that the rate of spin decay is largely dependent on the balls moment of inertia; however, its surface roughness and its boundary layer flow regime are also of importance.
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