No child's play

It's a birthday bash for science, and Leif Ristroph is struck by the motions of confetti in the air. The colourful rectangular bits of paper are showing complicated descent – flipping, fluttering, tumbling and gliding. So, Ristroph, in his lab at the Courant Institute of Mathematical Sciences, New York University, takes a thin, rectangular sheet of paper, puts a paperclip on its front end and launches it in the air. To his surprise, he finds that the paper plane's flight is quite stable. The Associate Professor of Mathematics now feels that he has a lot to learn from these simple airplanes about his research speciality: fluid dynamics.

Ristroph had always been curious about how birds glide and soar effortlessly. Analogous to that, paper airplanes hooked his interest. "The study started with simple curiosity about what makes a good paper airplane and specifically what is needed for smooth gliding," he says. "We discovered that the aerodynamics of how paper airplanes keep level flight is really very different from the stability of conventional aeroplanes."

But paper was not a reliable scientific specimen: it got flexed, bent and damaged. Humidity and other environmental factors also modified it. So, he took thin, rectangular sheets of plastic and 'flew' them under water.

Water and air are both fluids. They follow the same equations and laws of fluid dynamics, enabling simple paper-like plastic airplanes to fly in water. Plastic sheets are non-flexible; they can hit the bottom of a water tank and not get damaged; and they can be picked up and flown again. "This is very reproducible and makes them a good scientific subject to study," Ristroph says.