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.
Spedding, left, watches as John McArthur aims a laser at his newly
machined wing and adjusts the airflow for a flight experiment in USC's
Dryden wind tunnel.
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.
Today the airflow in the tunnel will be seeded by a fog of
one-micron-diameter smoke particles and illuminated by consecutive
pulses from a dual-head laser. Each light pulse will be five
nanoseconds in length and consecutive flashes will be separated by
about 150 microseconds. The laser will flash at a rate of 10 Hertz, so
each pulse pair will be regenerated every 1/10th of a second. Images of
the airflow will be captured on a charge-coupled device (CCD) array
camera and saved in real time.
McArthur’s faculty adviser, Geoff Spedding, a professor of aerospace
and mechanical 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 says, as he adjusts the cambered plate
and aims the laser.
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
says. “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 counter-intuitive, 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, he specialized in animal aero- and-
hydrodynamics. 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
“Imagine a flying ruler, which is a simple flat plate,” he says,
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 says there are two reasons for it. “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.
McArthur, left, examines the curved edge of a plate that he machined and will test in a later wind tunnel experiment.
“Modern aircraft are efficient and powerful, and routinely carry people
and armaments over long distances,” he continues. “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
Of Bats and Thrushes
He wends his way out of the wind tunnel and back through the maze of
instrumentation filling the basement of the Rapp laboratory, then
climbs 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
When he isn’t collaborating with a group of biologists at Lund
University, Sweden, 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 grins.
How do Newton’s laws of motion work on much smaller scales? Just
the same, Spedding notes. 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 says. “And much
of the existing data in the literature is controversial and
inconsistent, with little apparent incentive to force the issues to
Measuring the wingspan of a thrush.
Then what sort of small-scale flying machine should he build?
“We could build just about anything,” Spedding says. “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
It’s not the technology that is holding engineers back from building
that plane, he says. 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 says.
Assistant Professor Eva Kanso and Professor Tony Maxworthy, who holds
the Smith International Professorial Chair in Mechanical Engineering,
are members of Spedding’s AME research team addressing those and other
problems in biological locomotion.
In 1979, Maxworthy, a member of the National Academy of Engineering,
was the first scientist to realize and demonstrate that many simple
wings in oscillatory flapping motion will generate strong swirling
currents, or vortices, of fluid at the front edge of the wing, and that
the forces associated with this strong rotational motion will be both
beneficial and controllable. In many cases, the presence of these
complex, time-varying fluid motions can make the difference between
flying or being grounded.
Since then, the leading edge vortex (or LEV) has become a staple of
those seeking to understand the aerodynamics of both insect and bird
flight. Maxworthy predicts that it is “very likely that any
successfully engineered device will have to have some similar means of
generating and then controlling such fluid motions.”
Similar principles apply to swimming fish, he says. Researchers
are currently working on experimental modeling of the forces and flow
fields generated by fin and tail motions.
Kanso, a mechanical engineer who earned her Ph.D. from the University
of California, Berkeley, is the third member of the team.
Interested in the interaction of shape changes with wake dynamics, she
is attempting to unlock the mathematical keys to underwater
“There’s still a lot to learn from bird flight before a reasonable
airfoil design can be proposed,” Spedding says, “or before we start
pasting bird feathers on our small flying machines.”
But the day will come when this planet is populated by both animals and
small autonomous vehicles whose forms and behaviors may — or may not —
mimic the natural world. That day isn’t too far away.