With the steadiness of a surgeon, John McArthur, a graduate student in aerospace and mechanical engineering, positions a slightly curved plate about the length of a 12-inch ruler on a rod that protrudes from the force balance in USC’s Dryden wind tunnel.

The flow properties and the force balance calibrations in this tunnel, housed in the basement of the Rapp Research Building, have been carefully tested over three or four months.

McArthur’s faculty adviser, Geoff Spedding, a professor of aerospace and mechanical engineering in the USC Viterbi School of Engineering, and his colleagues have developed custom software over the last 10 years to perform an analysis of this flow field with extreme accuracy.

After making the measurements, they’ll be able to describe not only the lift and drag forces of this small curved wing, but also the spatial gradients of the airflow in which it was flying, which are difficult quantities to estimate.

No one has paid much attention to small-scale airfoil geometry – how the shapes of small flying things impact their flight capabilities – or to understanding the aerodynamics of winged flight on small scales, until quite recently, Spedding said, as he adjusts the cambered plate and aims the laser.

Spedding collaborates with biologists at Lund University, Sweden, who study wing flapping as live birds fly through a wind tunnel.

“The geometry and kinematics of bird flapping are complicated,” he said. “The question then arises, must this complexity be mimicked or are there more simple fundamental designs of small-scale aerodynamics that can be applied to build the next generation of small, remotely piloted flying machines?”

Spedding and a research group in USC’s Aerospace and Mechanical Engineering (AME) Department are busy pursuing those questions with data from their wind tunnel experiments.

AME graduate students machine the simplest possible wings – flat plates, curved plates and classical airfoils – and then plot the wind tunnel measurements, which sometimes turn out to be quite counterintuitive, on graphs that are scotch-taped to the laboratory door.

Winged flight has always fascinated Spedding, who is a zoologist by training after earning a Ph.D. in zoology from the University of Bristol, England. With the recent upsurge of interest in small-scale aerodynamics, he has begun to investigate what affects the aerodynamic performance of these very simple objects, which fly at very low speeds.

“Imagine a flying ruler, which is a simple flat plate,” he said, holding up a ruler he has retrieved from the top drawer of a desk. “How well could this fly when attached to a suitable airframe? How would we improve this design?”

It is remarkable that such questions qualify as topics of research, but Spedding said there are two reasons. “First, all of our textbooks on aerodynamics and on aircraft and helicopter flight have been developed for devices that are much larger and fly much faster. These aerodynamic models and analytical methods are, arguably, among some of the crowning intellectual and practical achievements of the past century.

“Modern aircraft are efficient and powerful, and routinely carry people and armaments over long distances,” he continued. “But few have paused to reflect on how a very small plane might work. In fact, most of the serious work has stopped at the scale of competition sailplanes.”

He wended his way out of the wind tunnel and back through the maze of instrumentation filling the basement of the Rapp laboratory, then climbed a flight of stairs leading to his office one floor above the wind tunnel.

Stepping inside, he reaches for a small plastic bat with a wingspan of about 20 centimeters dangling on a string from the ceiling.

When he isn’t collaborating with a group of biologists at Lund University, using live birds (thrushes) and bats, Spedding relies on wind-up or battery-powered toys, such as this red-eyed Halloween bat, to inspire him. He winds it up and gives it a gentle shove. The bat begins to flap its wings and flash its red eyes as it circles high above his desk. Gaining momentum, it lifts into a higher orbit.

“Newton’s laws of motion in action,” Spedding said with a grin.

How do Newton’s laws of motion work on much smaller scales? Just the same, Spedding noted. But because the viscosity of the air can no longer be ignored, the flows are very complex. And that is the second reason this research must be done. Even simple textbook problems become complicated. Standard aerodynamic models do not give researchers the answers.

“Paradoxically, it is far easier to predict, analyze and model the flow around a Boeing 747 than it is to predict the flow around our simple ruler flying at some reasonable speed,” Spedding said. “And much of the existing data in the literature is controversial and inconsistent, with little apparent incentive to force the issues to resolution.”

Then what sort of small-scale flying machine should he build?

“We could build just about anything,” Spedding said. “Suppose we built a small flying machine that can flit through crowded spaces, hover silently in a precise position, making observations of a moving target, abruptly reverse direction in times of danger and do all of this for more than an hour. Then it would report back to base as directed with images and other sensory information, such as chemical and pressure readings, visibility, heat and radioactivity measurements.”

It’s not the technology that is holding engineers back from building that plane, he said. It’s possible to build all kinds of small-scale electromechanical devices these days. It’s the design – what it would look like.

“It could look like a dragonfly. A moth. A bat. A bird. Or none of the above,” he said.