In case you haven’t heard, there’s a little football game being played over at Taylor Field next Sunday between a team from Calgary and a team from Baltimore. Canadian football is known as a pass-happy game, so I thought I’d delve into the aerodynamics of a flying football.
Football aerodynamics, however, isn’t something you just look up in an encyclopedia. Instead, I got on the World Wide Web, did a search for “football” and “aerodynamics,” and immediately came up with Dr. Peter Lissaman, Adjunct Professor of Aerodynamics at the University of Southern California and one of the world’s leading experts in the aerodynamics of spinning objects. (In fact, he just recently gave a seminar in Las Vegas on that very topic.)
To be stable in flight, a football has to spin; otherwise the air rushing up under its tip as it descends causes it to start to tumble, and it falls from the sky like (pardon the cliche) a wounded duck, because although properly thrown, a football is quite streamlined, when it tumbles, it exposes much more surface to wind resistance, which slows it dramatically.
Spinning stabilizes a football because of something called “angular momentum.” A spinning object rotates around an axis, and it resists having that axis tilted. The best example of this is bicyle wheels: everybody knows that it’s easy to balance on a fast-moving bicycle, but it’s hard to balance on a slow-moving one. That’s because the fast-spinning bicycle wheels resist being tilted over onto their side, and so help keep you upright.
Generally, the lighter the object, the faster it must be spun to make it stable. Dr. Lissaman used the (good Grey-Cup) example of tossing a can of beer to a friend while keeping it more or less upright. If it’s full, it will remain stable with little or no spin applied to the can; if it’s empty, you have to spin it pretty hard to make sure it doesn’t tumble through the air. Footballs aren’t terribly heavy, so they need a fair amount of spin to remain stable.
Furthermore, the faster an object is thrown, the harder it must spin to remain stable, because it must resist more air pressure. In football, that means that a long pass must be spun harder than a short pass.
Although no definitive studies on footballs have been done, it’s common knowledge in the military among artillery personnel that shells–which, like footballs, are spun to keep them stable–drift in the direction of the spin: that is, a shell spinning clockwise as seen from the rear will drift to the right, while a spell spinning counter-clockwise will drift to the left. The same, Dr. Lissaman is certain, holds true of footballs. Quarterbacks, he believes, unconsciously compensate for this drift when they’re aiming their throws, so it’s not readily apparent.
As to why a football drifts in the direction of spin: Dr. Lissaman says it’s because the spin slowly slows down as the football descends toward the receiver. When a spinning object begins to slow, its axis “precesses” — tilts — in the direction of the spin. Once the nose of the descending football has tilted in the direction of the spin, the flow of air around it forces it in that direction.
Exactly how much a football drifts in the direction of spin he’s not sure of, because although he and his students have assembled all the equations necessary to calculate it, the precise aerodynamic qualities of a football have never been measured, and so they don’t have accurate figures to plug into those equations.
However, they soon may, because down at Mississippi State University, according to Keith Konig, Professor of Aerospace Engineering at that school, “we’re in the process right now of looking at lift and drag on footballs,” both as an academic exercise for students and because manufacturers are interested in ways they can improve football performance.
To conduct their tests, the Mississippi State researchers mount footballs on a spinning shaft inside the wind-tunnel. The shaft’s angle and rate of spin can both be adjusted. They’re finding that spinning balls go further than non-spinning balls not only because they remain more stable, but also because they’re more efficient at cleaving the air. It appears, Dr. Konig said, that a non-spinning ball “splits” the air, forcing it away from the ball, which increases drag, while air tends to remain “attached” to the surface of a spinning ball, which results in less turbulence, less drag–and therefore farther flight.
Another reason footballs can be thrown so far, Dr. Konig said, is because their shape actually generates a little bit of lift. In cross-section, he pointed out, a football is shaped something like an airplane wing. If the football is thrown at the right upward angle, the air underneath it has a higher pressure than the air above it, which creates lift.
But, Dr. Konig hastened to add, there are “many variables,” such as the fact that the football isn’t perfectly symmetrical–it has laces on one side which also play a role in the football’s flight.
He and his team will continue conducting studies, the data from which will no doubt be fascinating to Dr. Lissaman as well…but next Sunday, Calgary and Baltimore will conduct their own tests right here at Taylor Field, and the only data that will matter to them will be the numbers displayed on the scoreboard at the end of the game.