Winning the game with a football spiral

There's a minute and a half to go in the game. Dallas is behind by 6, but is on the Washington 45 yard line. Quarterback Troy Aikman takes the snap from center, rolls back, and gets ready to throw a long one. Just before he releases the ball he wonders, "Why do I want the football to spiral as it moves through the air?" His moment of hesitation leads to a mild concussion courtesy of the Redskins' defensive line. While Troy regains consciousness, an SwRI Whizard will answer his question.

"There are two main scientific reasons why Troy wants to put a spiral on the football. The first is that spiraling the football turns it into sort of a spinning gyroscope. The gyroscopic 'stiffening' keeps the ball going in crosswinds in the direction that Troy launched it a lot better than if it didn't spin (just like the gyroscopic autopilots on airplanes keep the planes on course). The second reason is that spiraling the football reduces the friction between the football and the air, so Troy is able to throw the ball farther downfield before it loses its speed and falls to the ground or is intercepted.

"As the football flies through the air above the Redskins defenders, it drags some air along with it, and this causes friction. The spiraling keeps mixing this air with the still air the ball is flying through, so both the amount of air being dragged along and the friction it causes are minimized (this is what fluid dynamicists call 'tripping the boundary layer'). The laces, seams, and dimples on the football cause more of this mixing than if the ball were perfectly smooth. Anyway, it is practically impossible to throw a football without spiraling it, so why not take advantage of it?"

Thanks to this month's Whizard, Institute Engineer Dr. Frank Dodge of the Mechanical and Materials Engineering Division. A specialist in fluid mechanics, Dodge is involved in studying the dynamical effects of liquid motions in a low-gravity environment.

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March 25, 2013