IEEE Spectrum September, 2016 - 31

Smooth moveS: For takeoff, FlexFoil

bends down by as much as 40 degrees
to maximize an aircraft's lift-to-drag
ratio [top]. While cruising, FlexFoil can
gradually flatten the wing's camber (or
curvature) to save fuel [middle]. When
encountering turbulence, FlexFoil
can rapidly twist into new shapes to
redistribute heavy loads. For example, a
flight control system can simultaneously
raise one end of the control surface and
lower the other end [bottom]. to minimize
drag, FlexFoil fits seamlessly against
a wing's trailing edge with weblike
connectors at each end that stretch as
the surface changes shape [below].

A plane could switch gears, so to speak, and achieve a
more optimal lift-to-drag ratio by changing the shape of its
wings. To understand how this would help, first consider
how wings generate lift. A standard airplane wing is more
curved on top than on the bottom, so that it diverts masses
of air downward, providing lift. The curved top surface of
the wing is mainly responsible for pushing the air downward, so, to some extent, enhancing the camber (or curvature) of this surface can improve a plane's lift-to-drag ratio.
For any airplane's combination of weight, altitude, and
speed, there is an ideal wing camber that delivers the
required lift and also achieves the lowest drag. Much of the
aerodynamic research to figure out which camber adjustments would deliver the best performance under specific
conditions has already been done. But the experts who carried out that theoretical work have had no means of implementing it on an actual plane until now.
Unlike wings with flaps, a flexible wing can smoothly adjust
its camber to minimize drag during flight. In fact, a flexible
wing could assume many more positions than a wing with
a traditional flap, allowing for much finer control over the
lift-to-drag ratio to match the evolving demands of a flight.
And if the flexible control surface fits smoothly against the
rest of the wing, these adjustments would create no additional drag from protruding joints and hinges.
The upshot is that altering the wings' camber during
flight allows aircraft designers to minimize drag depend-

ing on how much lift a plane needs for specific conditions.
For example, the burning of fuel during flight diminishes
the overall fuel weight, so the plane gradually requires
less lift. Currently, pilots contend with this weight loss by
reducing the aircraft's angle of attack, which is its angle
relative to the oncoming flow of air. A flexible wing could
simply morph the surface of its trailing edge continually
throughout an entire flight to achieve the optimal camber
for its current fuel weight.
The ability to fly well in a variety of flight conditions is
important because aviation rules limit the speeds and altitudes at which airplanes can travel. In the United States, the
Federal Aviation Administration, for example, reserves oddnumbered altitudes (such as 31,000 feet) for flights traveling north and east while south- and westbound flights stick
to even-numbered levels. Invariably, planes wind up cruising at a combination of speed and altitude that is not aerodynamically optimal. But a flexible wing could be tailored
to fly more efficiently at any assigned altitude or speed.
Aside from reducing drag, there is another potential benefit of shape-changing wings that passengers especially will
appreciate. These wings can also dampen the shaking of the
fuselage due to turbulence. This can be achieved by twisting
the wings' trailing edges in just the right way so as to reduce
the loads caused by turbulence and thus minimize the movement transmitted to the fuselage. This twisting would be done
automatically by a sophisticated flight control system.
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Table of Contents for the Digital Edition of IEEE Spectrum September, 2016