ByTerry Ward
Photographs ByChristopher Payne
Published February 9, 2024
What’s more American than cheering as your football team sends a long bomb tightly spiraling toward its end zone target? It’s a tradition that stretches back to the late 19th century.
While today’s “pigskin” is no longer made with the pig’s bladder of football’s late-1800s origins (all pro and collegiate footballs are now constructed from cowhide leather with synthetic rubber interiors), the shape and dimensions of the NFL football have remained the same for roughly 100 years.
That the design from a century ago still perfectly suits the needs of today’s sport is a kind of “historical accident,” says Dr. Rabindra Mehta, chief of the experimental aero-physics branch at NASA Ames Research Center. “Compared to a baseball, a football is a more aerodynamic shape by design.”
Footballs are made from four individual panels of leather, leaf shaped and tapered at both ends, which are sewn together inside out. Next, the leather is steamed so that it becomes soft enough to turn right side out. An air bladder is then inserted and the football is laced together by hand.
A football’s shape is not actually a ball in the spherical sense of the word, but a prolate spheroid: oblong, with pointed ends that make it easier to grip. This shape and the way air flows around it helps the football to travel great distances.
Characteristics of the football’s surface–including the pebbling of the leather, the stitching of the panels, and the laces themselves–also allow airflow to stay attached longer and minimize drag, compared to something spherical like a baseball.
Picture a baseball traveling right, with air traveling past, moving left—this would be what’s called laminar flow, where air travels in a regular, smooth path. In a controlled environment (like a wind tunnel) the airflow would go straight left until it meets the ball, where it bends around until it reaches the “far side” of the ball (this point is called the “apex”). Then the air returns to traveling straight left, past the ball, without touching that far side of the ball.
When this happens, there’s a pressure difference between the front and back of the sphere which results in drag, the force that slows the ball down as it’s flying through the air. Golf ball manufacturers tackle this on spherical golf balls by dimpling the ball’s surface to help airflow stay attached longer, reducing drag, therefore letting the ball fly farther.
The thin layer of air on the ball’s surface is called a boundary layer, and a turbulent boundary layer creates turbulent flow—where a football’s design shines. Air meeting a football, with its textured surface and curved shape, would flow around the ball, staying attached longer to its surface than it can on a baseball’s. If the air is flowing left (and the ball is moving right) the air will move up, left, and down along the football’s bowed surface.
While the air may not hug a football’s curves all the way across, airflow does remain attached past its apex, resulting in a minimal wake and less drag. On a baseball, the air only makes a connection on half the ball, creating more of a wake and drag.
Drag can be “challenging to predict, particularly in odd-shaped objects like a football,” says Anette (Peko) Hosoi, Pappalardo professor of mechanical engineering at MIT. Drag depends on the shape of the wake, which, in a football, can vary depending on such factors as its orientation through the air, the velocity at which its thrown, and surface roughness.
Density of the surrounding air–a function of air temperature–also affects the boundary layer of a football and in turn its aerodynamics, Hosoi says.
“Warm air is less dense than cold air. If the air is less dense, there is less drag, so footballs may fly further on warmer days,” she says, adding that the phenomenon has been well documented in baseball, which clocks more home runs during hot and humid weather than the contrary.
A tight, spiraled throw wobbling not at all is indeed a thing of beauty, no matter the weather around it.
”The axis of the spin is aligned with the direction the ball will go,” says Mehta, likening it to the way a bullet flies. ”That’s what the quarterbacks are really good at doing.”
>>> Read full article>>>
Copyright for syndicated content belongs to the linked Source : National Geographic – https://www.nationalgeographic.com/photography/article/nfl-football-physics-design-super-bowl
