The spin of the football used in the 2006 World Cup, has been newly analysed by a University of Sheffield academic, providing valuable information for professional sports engineers and scientists as well as football designers and manufacturers.
Dr Matt Carré from the University´s Department of Mechanical Engineering, along with Associate Professor of Physics, John Eric Goff from Lynchburg College, Virginia, performed experiments in which the Adidas Teamgeist ball was launched from a machine while two high-speed cameras recorded portions of the trajectory (the flight path of the ball).
When in flight, spinning balls typically experience a force perpendicular to the plan of spin (for example, causing a ball with top spin to dip). Using the trajectory data and published drag coefficients, Professor Goff and Dr Carré extracted the aerodynamic coefficients linked to this perpendicular force for a wide range of spin parameters, including several spin parameters that have not been obtained by today´s wind tunnels. Here, the "spin parameter" refers to the amount of spin relative to ball speed, so high spin parameters are achieved by having a low-speed ball with a large amount of spin. Flight behaviour of balls with different orientations, launch speeds and spins were examined by the researchers and the results compared with existing data from wind tunnel studies.The findings, published in the European Journal of Physics, show that the coefficients related to the amount of perpendicular force begin to level off at high spin parameters. In other words, as the spin is increased it begins to have less effect. The researchers suggested reasons for this could be related to the different types of airflow that may be experienced on either side of the ball (e.g. laminar of turbulent behaviour). Dr Carré and Professor Goff also compared the aerodynamic coefficients of the Teamgeist ball (made up of with thermally bonded panels) to that of a more traditional, stitched 32-panel ball.
While trajectory analysis work has been performed in other sports like tennis and baseball, prior to this study, researchers investigated football aerodynamics using wind tunnels and computer models. Besides acting as a technique complementary to wind-tunnel experiments, trajectory analysis allows for the study of the aerodynamics of projectiles for researchers without access to expensive wind tunnel machines. It also offers a valuable tool for examining aerodynamic coefficients in parameter ranges not currently studied by modern wind tunnels.The findings add to the knowledge of flight ball behaviour for professional sports scientists and the trajectory data can now be used to create a computer model. This would enable footballers and coaches to virtually score a goal, such as from a free-kick, to study the best strategy to beat a goal keeper and defensive wall.
The research follows mixed reviews over the current World Cup ball `Jabulani,´ which some managers and players, including England´s manager Fabio Capello, have claimed is a difficult ball to play with due to problems in anticipating its trajectory.Dr Matt Carré, from the University of Sheffield´s Department of Mechanical Engineering, said: "This work has helped to plug a gap in some of the sets of aerodynamic data currently available for footballs. It also helps us to understand the limits of what can be achieved by kicking balls with increasing amounts of spin. We hope that this will be of use to researchers in this field, as well as football manufacturers and coaches."