Erratic Behaviour of Knuckleball

Erratic behaviour of knuckleball is studied by quasi- steady aerodynamic theory. On TV, We observed ball trajectory, spin rate and its axes of knuckleball pitched by Tim Wakefield who belongs to Pittsburg Pirates of national league. The dependence of Reynolds number for aerodynamic forces of drag, lift and side force are measured with change of seam position by wind tunnel testing. Three dimensional equations for the force equilibrium are shown and solved under quasi-steady approximation of aerodynamic forces acting on the ball. Various types of knuckleball trajectory which were commented by major league batters with fuzzy expressions are shown on the computer screen. Flutter experiments were conducted using wind tunnel. Oscillation amplitude of knuckleball by flutter analysis corresponded well with the results of flutter experiment. We conclude that the erratic behaviour of knuckleball is a flutter phenomenon such as unstable one of bluff body immersed in a fluid flow.