UB Professor's Theory Explains Why A Football Does Not Fly Like A Guided Missile

BUFFALO. N.Y. -- Contrary to what sports fans might think, a football launched by an NFL quarterback does not fly like a missile or a bullet, according to a University at Buffalo professor who has used the science of ballistics to study a pigskin's flight.

William Rae, Ph.D., UB professor of mechanical and aerospace engineering, has proposed a theory that addresses why, contrary to aerodynamic principles, a football on a forward pass doesn't swerve to the right or the left.

It is the first attempt to explain the flight of a football that takes into account the effect of aerodynamic torques, the twisting forces that allow airplanes to maneuver.

Rae plans to test his idea in a wind tunnel, using a football fitted with sensors.

"It turns out that the flight of a football is almost as complicated as the flight of an airplane," he adds.

Rae began studying the flight of a football when he used it as a way to get students in his flight dynamics class interested in fundamental properties of aerodynamic flight.

To study the problem, he developed computer simulations of a football's flight, as well as frame-by-frame analyses of a long forward pass from a videotaped football game.

According to Rae, the wobbly forward pass football fans often see in slow-motion replay demonstrates a type of motion familiar to ballistics engineers as the atmospheric flight of a spinning missile.

"If the missile carries no fins or wings to control it, then it usually is unstable and it will tumble end over end," said Rae.

"One way to stabilize it is to spin it, but once the missile is spun, it experiences gyroscopic torques, just like those seen in the child's toy."

These torques, he explained, compete with other torques caused by the air rushing over the missile. The net result is that, like the toy, the missile may sometimes swerve to the right or left of its intended course.

"Anyone working in ballistics knows that artillery shells tend to swerve, too," said Rae. "And like a bullet, a football is nothing but a gyroscope with air loads."

The answer, he discovered, has to do with the Magnus effect, named for a nineteenth-century physicist, G. Magnus, who published research on this subject in 1852. The Magnus effect impacts the amount of sideways drift in a flying object.

With the football, the Magnus effect provides a torque that opposes the other two torques involved in the football's flight: the gyroscopic torque and the aerodynamic torque.

"When the nose of the football is facing up, the wind pushes it even higher," said Rae, "that's the aerodynamic torque. The gyroscopic torque is what would make the nose of the football try to turn to the left as the football is falling back to earth."

However, Rae explained, computer simulations show that the football does not turn to the left because the aerodynamic torque is stronger than the gyroscopic torque.

"In simulations where I put in aerodynamic and gyroscopic torques, but leave out the Magnus effect, the football swerves to the right," Rae said. "I propose that in reality, the Magnus effect essentially cancels out the other two torques and allows the football to fly without swerving."

Rae said that his simulations confirm the results of Peter Brancazio, a physicist at Brooklyn College, who was the first to examine the dynamics of a football.

Now, Rae hopes to test his theory in a wind tunnel, using a football fitted with sensors.

These experiments will enable Rae to measure air loads around the football and calculate precisely how much lift and how much torque it takes to loft a football.