The Surprising Science behind Tom Brady's Perfect Spiral

Tonight, the Atlanta Falcons face the New England Patriots in Super Bowl LI in what will be a quarterback duel for the ages.  Matt Ryan and Tom Brady have led their teams to the Super Bowl by posting the league’s highest quarterback ratings since 2011.  We decided to dig into the aerodynamics at play behind their success. 

In 2009, ESPN’s Sports Science aired a segment on the throwing accuracy of Drew Brees, one of the most prolific passers of all time.  Drew’s passes were consistently released at 52 miles per hour at a rotation rate of 600 revolutions per minutes. Almost unbelievably, Brees landed 10 out of 10 bullseyes from 20 yards; better than Olympic archers can do from the same distance!  The segment went way off the rails in attempting to explain the aerodynamics of such accuracy, but at least the slow-motion videos were cool. 

Football spin rate is critical for the same reason that modern rifles have grooved barrels and why archers fletch their arrows in a helical pattern; spin increases stability. The more stable a projectile is, the more it will resist external forces that would otherwise act to change its orientation.  In engineering this is known as gyroscopic stability and is a result of Newton’s law of conservation of momentum.  A spiraling throw helps the football maintain its orientation and prevents end-over-end tumbling, or the “wounded duck” pass. 

If gyroscopic stability resists reorientation, why does a football continuously pitch to stay in-line with its path of flight when it should remain at its original angle of rotation?  The two concepts are shown below.

Computational Fluid Dynamics (CFD) provides a modern approach to answering these questions and more.  To demonstrate, we built a 3-d virtual model of a football and a virtual wind tunnel for experimentation.  We set the ball in motion per the speed and rotation rate mentioned previously and allowed the football to pitch freely about its center of mass in the off-axis directions.  We gave the football a slight nudge, or perturbation, in the off-axis direction to mimic the behavior of the initial throw angle becoming out of line with the flight trajectory as the football rises.  What we found is that this perturbation is quickly damped by the aerodynamic forces on the spinning football and that the football returns to alignment with its direction of travel while the non-spinning football the perturbation continued to grow.  This is an example of the weather-vane effect in which broadsided objects seek the orientation that will produce the least drag, or energy losses.  The video below is a visualization of results produced with the computer simulation.

Interested in seeing how your spiral compares to the pros, check out the new Wilson's X Connected football with embedded sensors measuring velocity, spinrate and other stats.