Rotation

• This year’s event on Sunday – with the added attraction of a romance in the spotlight – promises to attract as many fans.
• In Canada, the most recent Grey Cup final, last November, reached a record audience of 3.7 million viewers who tuned in to watch the Montréal Alouettes’ victory.
• A number of factors affect the ball’s aerodynamics, including the way it is made and its inflation pressure.
• As a professor in the Department of Mechanical Engineering at Québec’s École de technologie supérieure, I am interested in experimental fluid dynamics.

Ball size affects flight stability

• They must be between 11" and 11.25" long.
• The higher the speed of rotation, the more stable the ball will be during its flight.
• Different dimensions can therefore have specific effects on the stability of the ball’s flight.

A larger circumference suggests that more of the ball’s mass is located away from its centre line. This means that it will have a higher moment of inertia (resistance to rotation) and, therefore, that the same force applied to make it rotate will result in a lower speed of rotation.

Two stripes and laces make a difference

• While there are two white stripes on the Canadian ball, as well as laces, American rules don’t mention these.
• The differences between the Canadian and American balls can have an effect on their drag.
• In this case, it is mainly the resistance caused by the air (a fluid), which is called form or pressure drag.

NFL or CFL, which ball is better?

• An aerodynamic force balance could be used to measure the differences in drag between the two balls subjected to the same conditions.
• The movement of these particles could then be captured using a camera to quantify the airspeed at all points around the ball.

Different rotation speeds and flight speeds could be examined, as there is always the possibility of developing flow instabilities, which would lead to a change in its behaviour around the ball. This would help determine whether the NFL or CFL ball is better.

Ball texture influences drag

• The rougher the texture of the ball, the greater the friction drag for the same speed.
• Similarly, a faster ball speed will have a higher friction drag.

And then there’s the weather!

• Cold or hot temperatures can affect the size of the ball by reducing or increasing the air pressure inside it.
• Similarly, temperature can have some effect on the material properties of the ball, with colder temperatures making it stiffer and warmer temperatures making it softer.

Giuseppe Di Labbio ne travaille pas, ne conseille pas, ne possède pas de parts, ne reçoit pas de fonds d'une organisation qui pourrait tirer profit de cet article, et n'a déclaré aucune autre affiliation que son organisme de recherche.

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