Flights of Fancy : Science File / An exploration of issues and trends affecting science and the environment : Aerodynamics --as much art as science-- has applications both practical and playful, like reducing drag on trucks and improving flying disks.
A recent USC engineering seminar became more interesting than its title--Flight Dynamics of Spin-Stabilized Systems--when the speaker started throwing at the listeners examples of his subject: hollow plastic-foam cylinders.
“Go ahead, throw them around,” said aeronautics professor Peter Lissaman. “I throw them at the chairman of the department.”
The undergraduates were soon whizzing the cylinders--which look a lot like soda can coolers--to and fro across the classroom, whapping the unwary in the back of the head.
Actually, the cylinders are can coolers. When not soaring, the Can-o-SOAR-us, as Irvine inventor Doug Malewicki has dubbed his creation, can snugly hug a Coca-Cola can and keep the brown bubbly nicely chilled.
Things that fly have long captivated college students and other gravity-bound humans. Tinkerers and inventors such as Malewicki hope to brainstorm the next great flying toy, while academics such as Lissaman ponder such nettlesome questions as how much spin quarterbacks should put on passes.
“Humans always seem to want to fly,” said David Silverglate, a Santa Cruz inventor who came up with the Woosh flying ring, a round plastic rim with Spandex stretched over it. “You can project a bit of yourself on the toy and fly on it.”
“I love everything that relates to wind and water,” said Lissaman, who splits his time between USC and the Art Center College of Design in Pasadena.
Part of what attracts curiosity seekers to the field is that most of the problems have no easy, exact answers.
Even the fastest, biggest computers can offer only a partial glimpse.
“You take something as simple as a Frisbee,” Lissaman said. “There’s no computer that will predict its dynamics.”
Thus the study of aerodynamics is as much art as science. Nevertheless, the field does have practical applications. Lissaman’s work has included the design of one of the bubble-like protrusions that sit atop the cabs of tractor-trailer trucks to reduce air resistance and fuel consumption. He helped design the human-powered Gossamer Condor aircraft. He consults with auto manufacturers on making their cars slide more easily through air.
He has also worked out equations that describe the flight path of footballs, which is not as simple as it might at first appear. Because a spinning object tries to keep the direction of its spin, the football maintains its tight spiral toward a receiver despite buffeting winds. Without enough spin, it tumbles awkwardly to the ground.
Spin also gives rise to a second phenomenon known as gyroscopic precession. The air hitting the front of the football tries to push it up. Counterintuitively, that upward force causes the motion to drift sideways. It’s the same phenomenon that causes tops to wobble and washing machines to hop around.
But Lissaman doubts that Dan Marino or Steve Young would learn much from his results. Through practice and intuition, NFL quarterbacks already “know exactly how to launch spin-stabilized spheroids to go where they want them to go,” he said.
The equations do explain why toy foam footballs are so hard to throw, though. Because a foam football is solid instead of an inflated shell, the mass is, on average, closer to the central axis, diluting the stabilizing benefits of spinning.
Just as lighter objects are easier to stop, spinning objects tip over more easily when their mass is closer to the axis. To throw a foam football gracefully, a quarterback has to add extra spin.
Fins can counteract this tendency toward tumbling. Alan Adler, a Stanford University mechanical engineering professor who also owns a toy company, created the Aerobie Football, which is basically a foam football with what looks like a boat propeller sticking out the back. The OddzOn toy company in Campbell, Calif., took a similar approach in designing the Vortex football, which looks like a miniature football mounted on top of a rocket.
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But more interesting than footballs--to aerodynamicists, at least--are flying disks. The granddaddy of them all, the Frisbee, was invented in 1948 by California carpenter Fred Morrison. Wham-O started selling the toy nine years later and has since sold more than 100 million of them.
Despite their success, some have thought Frisbees could be easier to throw. “Whenever I threw a Frisbee, I could see the problem with them sinking very fast,” Adler said.
Thus began his 20-year quest to build a better flying disk, which is essentially a circular wing. The disk’s curved upper surface produces an upward lifting force as an airplane wing does. But because the lift acts near the front, the resulting precession causes the disk to veer sideways in flight and roll over. If the lift force could be shifted toward the center, the precession would be canceled out, and the disk would fly straighter and farther.
A decade into his journey, Adler invented the Aerobie Flying Ring, which holds the record for farthest-thrown object at 1,257 feet, more than four football fields. (Imprinted on the ring is this warning: “Throw only in a clear space to an alert catcher.”) The ring’s secret, Adler said, is a small ridge along the outside. “It kills some of the lift up front, just enough to bring it into perfect balance,” he said.
According to Silverglate, the porous fabric of his Woosh ring similarly reduces the lift in the front.
However, when Adler applied the idea to a solid disk, it didn’t work. Not the first time, not the first few hundred times. Only last year did he finalize work on the Aerobie Superdisc. “This is it, the design I’ve been searching for 20 years,” he wrote in his notes.
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In the end, the design was almost identical to one from a decade earlier, a testament to the finickiness of aerodynamics and the importance of serendipity. The slight tweaks “made the difference between a good flier and an unstable one,” Adler said.
Chance is also how the Can-o-SOAR-us came to be. While working on a different project, Malewicki had assembled a foot-long foam tube with four bolts taped to one end. “It was just a blob with bolts,” he said. On a whim, he flung it in the air and watched in amazement as it glided about 30 feet.
The current incarnation has shrunk in both size and weight. A plastic ring has replaced the bolts. Unlike other flying toys, the Can-o-SOAR-us is stable in flight even without spin.
“Six-year-olds can throw it half a football field,” Malewicki said. “This thing keeps going and going and going and going . . . “
Silverglate noted that most flying-toy designers live in the Golden State. “We’re all loony California types,” he said. “We’re not well-grounded people.”
