Aerodynamic characteristics of Australian Rules footballs

Abstract

The aerodynamic forces on 15 Australian Rules footballs were measured in a wind tunnel. Three leading manufacturers were represented with 5 varieties of ball tested. A total of 13 ball orientations were tested in order to encompass a ball rotation range of 180o about the transverse axis, as commonly seen in a ‘drop punt’. The flow speed was 28.5 m/s, corresponding to a Reynolds number (based on ball diameter) of 3.4 × 105. The drag and lift coefficients were measured for each of these orientations. The results show similar trends to a previous study of AFL balls reported in the literature, with similar lift coefficients but significantly larger drag coefficients. The difference is attributed to the low free-stream turbulence intensity in the present case, which leads to earlier separation of the boundary layer, hence higher pressure drag. A second observation, absent from previous literature, is the impact of the laces on the flow. The laces trip the boundary layer on one side of the ball, leading to delayed separation and ultimately lower drag and greater lift over a small range of pitch angles. These observations were confirmed through the use of smoke visualisation. It is found that there are no statistically-significant differences between the lift and drag produced by balls manufactured to Australian Football League (AFL) specifications. However, the balls not manufactured to the AFL regulations produced higher drag and lift coefficients than the standardised balls.