‘Spin’ doctors study the aerodynamics of football
21 June 2010
How did experienced goalkeepers, England’s Robert Green and Algeria’s Fawzi Chaouchi manage to miss seemingly easy saves in the current World Cup tournament? The more lurid newspaper headlines and on-field displays of emotion aside, there’s a controversy looming and it doesn’t concern player skills – or, more precisely, the lack of them.
Now, I have a confession to make at this point: the Beautiful Game doesn’t exactly float my boat and my knowledge of the sport is very, very limited, so enough from me about the levels of manual dexterity displayed by goalkeepers on the South African pitches of late.
The controversy I refer to surrounds the adidas Jabulani ball, which commentators, team coaches and players alike are claiming has a mind of its own when it comes to trajectory and bounce. The physics of football and, for that matter, any other game involving the motion of small spherical objects, has exercised the minds of many researchers, recent among them being two academics from the University of Sheffield and Lynchburg College, Virginia.
Sheffield’s Matt Carré and Lynchburg’s John Eric Goff have just published the results* of a series of experiments in which the adidas Teamgeist ball, used in the 2006 World Cup competition, was launched and tracked by two high-speed cameras that recorded portions of its trajectory.
When in flight, spinning balls typically experience a force perpendicular to the plane of spin (causing a ball with top spin to dip, for example). 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 that have not been obtained from modern wind tunnel observations.
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 (laminar or turbulent, for example). Dr Carré and Professor Goff also compared the aerodynamic coefficients of the Teamgeist ball (constructed from thermally bonded panels) to that of a more traditional, stitched 32-panel ball.
According to the researchers, this form of trajectory analysis enables the aerodynamics of projectiles to be studied without having to gain 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. Moreover, the trajectory data can be used to create a computer model that footballers and coaches can use in a virtual goal scoring situation, such as from a free-kick, to study the best strategy to beat a goalkeeper and a defensive wall.
The research coincides with the current furore over the official 2010 World Cup ball. Jabulani (a Bantu word meaning ‘to celebrate’) is, however, far from being a cause for celebration among some of the managers and players, including England’s Fabio Capello, who claim it is a difficult ball to play due to problems anticipating its trajectory.
Loughborough University was adidas’ exclusive research partner for the Jabulani after previously developing the 2008 European Championship and 2006 World Cup footballs for the company. Loughborough says the ball was tested and endorsed by a number of world-class international players, including Frank Lampard, Michael Ballack, Petr Cech and Kaka well before the start of the tournament and adidas partners AC Milan, FC Bayern München, the Orlando Pirates and Ajax Cape Town tested the ball in 2008, contributing to improvements in its surface structure and material composition.
Jabulani comprises eight thermally bonded, 3D spherically formed EVA and TPU panels that are moulded together to envelope an inner carcass. The result, according to Loughborough’s researchers, is a perfectly round, energetic unit. The grooves that create the clearly visible profile on the ball’s surface - so-called ‘Grip’n’Groove’ profiles - circle the entire ball in an optimal aerodynamic way to provide stability and accuracy in flight.
Loughborough is robust in its assertion that Jabulani is the most stable and most accurate ball that adidas has produced to date, its performance having been confirmed not only by the players themselves, but in wind tunnel observations at Loughborough University and during tests at the adidas football laboratory in Scheinfeld, Germany.
Meanwhile Dr Carré believes the work he and Professor Goff have recently published will help to plug a gap in some of the sets of aerodynamic data currently available for footballs, and improve understanding of the limits of what can be achieved by kicking balls with increasing amounts of spin. He hopes it will be of use to researchers in this field, as well as to football manufacturers and coaches.
*The paper, entitled ‘Soccer ball lift coefficients via trajectory analysis’ can be read here
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