The spin decay of sports balls in flight (P172)

Abstract

The abdity to understand and predict the flight trajectory of a sportsball is important in many fields of sports engineering. The aerodynamiccoefficients of sports balls have been studied in detail and yetaerodynamic models generally assume that spin rates are constant duringflight. This study examines the spin decay of sports balls in flight inorder to improve the accuracy of aerodynamic models. Several types ofsports ban (tennis ball, worn tennis ball, football, oversize tennisball) were launched in a sports hall using artificial bowlers at variousspeeds and spin rates. The start and end of the trajectories were filmedusing two high speed video cameras and the resulting measurements wereused to assess the spin decay of each ball as well as the factorsaffecting spin decay. The data shows a strong linear relationshipbetween spin decay and the product of initial spin and speed for all thedifferent sports balls tested. However, the constant in this linearrelationship was found to be different for each of the different ballsand was assumed to be dependent oil the physical attributes of thedifferent balls. Dimensionless parameters were derived such that thedifferences between the sports ball spin characteristics could be moreeasily identified. It was found that the rate of spin decay is largelydependent on the balls moment of inertia; however, its surface roughnessand its boundary layer flow regime are also of importance.