IEEE Spectrum September, 2016 - 32

taKe FLight: pilots of the Wright brothers' first practical airplane

turned it by twisting its wood-and-fabric wings. after World War ii,
Northrop grumman released the XB-35 bomber, nicknamed "the flying
wing," to minimize drag over long ranges, but it proved difficult to steer.
the Bell X-5 was the first aircraft capable of "sweeping" its wings, or
angling them backward, during flight. decades later, NaSa and the
U.S. air Force vetted mission adaptive Wings, which could morph their
curvature for supersonic speeds. Next, they tested active aeroelastic
Wings, which leveraged forces generated by high-speed maneuvers
for better roll control. more recently, NaSa conducted 80 test flights of
the futuristic blended-wing X-48B before ending the program in 2013.

Bell X-5 1951

Wright Brothers
Flyer III 1905

Northrop XB-35 1946

hen i began to sketch the original diagrams for my flexible
wing in my home office, in 1994, I had no background
in aerospace engineering. I was also oblivious to the
many failed attempts to design a shape-changing wing that
preceded my own. I later learned that those earlier designs
were complex affairs incorporating hundreds of parts and
dozens of motors.
From the start, I resolved to make my design strong but
flexible, with no joints or hinges, and to forge it from a single
piece of material. Powered by one or two electric motors, it
would perform all the necessary functions for flight while
enduring the massive aerodynamic pressures created by
heavy aircraft flying at high speeds.
At this point, I was still working from home in my free time
and without a cent of external funding. Not long after I had
completed the first batch of sketches, though, I read a magazine article about the U.S. military's latest attempts to develop
a shape-changing airplane wing at Wright-Patterson Air Force
Base in Dayton, Ohio. I visited the base and showed the research
team my designs. Then in 1998, Wright-Patterson's Air Force
Research Laboratory awarded me a $100,000 contract to
develop a feasibility analysis for flexible wings. I didn't know it at
the time, but that funding was the first of more than $50 million
the U.S. government would spend over the next 18 years to test
my concept in various wind tunnels, and in flight.
To start, I spent the first check from Wright-Patterson
to design and rapidly prototype (using what is now called
3D printing) a plastic model of my wing and rent a wind tunnel at the University of Michigan, where I am a professor of
mechanical engineering, to conduct a few basic tests. My
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Boeing X-48B 2007

General
Dynamics F-111A
Mission Adaptive
Wing 1985

Boeing F/A-18 Active
Aeroelastic Wing 2005

design performed well in the wind tunnel and appeared scalable, so in 2001 the Air Force awarded me another contract
and more money to build a larger prototype that could be
tested at higher wind speeds.
It was about this time that I founded FlexSys in Ann Arbor,
Mich., and dubbed my wing FlexFoil. After several more successful tests with the larger prototype, my Air Force collaborators said they were ready to test our design on an actual
flight of a White Knight aircraft, which is the same plane that
carried the pioneering private space vehicle SpaceShipOne.
For these tests, as a safety precaution, we hung a
127-centimeter aluminum version of the FlexFoil control
surface vertically from the underside of the aircraft rather
than attaching it directly to the plane's wings. During flight,
we manipulated the camber and shape of FlexFoil remotely
from the cabin and monitored performance through pressure ports and thermal sensors that we had installed on the
suspended prototype to measure lift and drag.
The White Knight flew multiple tests for us in the Mojave
Desert in the summer of 2006. In all of them, FlexFoil demonstrated a significant reduction in drag while enduring
the stresses and temperature fluctuations of flight. We also
showed that at every stage of flight from takeoff to landing,
FlexFoil could reduce drag for a given amount of lift. Another
analysis by NASA showed that the technology could improve
lift over drag by up to 10 percent.
The best news, though, was that measurements collected
during those flights suggested that FlexFoil would boost fuel
efficiency by 8 to 12 percent if installed in place of flaps on new
airplanes, and by about 3 percent for retrofits, in which the
technology would be integrated into the existing flaps along
the wings' trailing edges. For comparison, the new winglets
that have recently been installed on many planes cost at least
$1 million per pair and deliver fuel savings of 4 to 5 percent.

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Table of Contents for the Digital Edition of IEEE Spectrum September, 2016