Long pass: pigskin in wobbly flight.

It's the last minute of the game. Quarterback John Elway of the Denver Broncos fires the football down the field. Wobbling slightly as it soars through the air, the spinning pigskin travels in a long arc into the outstretched arms of a receiver. Touchdown!

To an aerodynamics expert watching the play, the real surprise is that, unlike a bullet, artillery shell, or unguided missile, a football doesn't naturally drift to one side when it flies a long distance. This puzzle so intrigued engineer William J. Rae that he has spent the last few years investigating the motion of a football, particularly during a long pass.

"It turns out that the flight of a football is almost as complicated as the flight of an airplane," says Rae of the mechanical and aerospace engineering department at the State University of New York at Buffalo. He uses his football studies as a way to involve students in his flight dynamics class.

Anyone who has thrown a football successfully knows from experience that one must give it a substantial spin about its long axis. Otherwise, it tumbles end over end instead of following the tight spiral of a well-thrown pass.

Ballistics engineers apply the same principle to stabilize the flight of artillery shells and missiles. However, when a shell and most other elongated, rapidly spinning objects travel in an arc through the air, they experience an aerodynamic twisting force, or torque, that causes them to drift to one side of their initial path.

That doesn't happen to a thrown football.

One clue to its apparently aberrant behavior may lie in the observation that a football thrown long distances not only spins but also usually wobbles. The football's nose doesn't point directly along the ball's path. Instead, as it moves forward, the ball's angled nose also precesses, tracing out a circle centered on the flight path. Studies show that a football typically spins about five times for every three times its nose circles.

Rae's computer simulations suggest that this motion introduces an additional aerodynamic force, known as the Magnus effect, that essentially cancels out a football's tendency to drift to one side during its flight. It isn't completely clear why this effect would be important for a football but not for a missile or bullet.

"This is work very much in progress," Rae says. "My period of doing computer simulations and twiddling knobs is over now. It's time to get into the wind tunnel and make some measurements."

These studies also make it easier to appreciate the tremendous skill of professional quarterbacks. "It's a credit to these fellows that they are able to throw tight passes over such long distances," Rae comments.