[ Nondestructive testing: thermal imaging of airframes ]
Quartzline heat
source
Infrared camera
Microcomputer and
image processing
hardware
Scanning table
Illustration by Phil Saunders
Aircraft fuselage
Camera controller
the application of the coating. If the researchers purchased
communication-grade optical fiber from a vendor, they would
have to remove the coating from the fiber, use a laser to write
the fiber Bragg gratings and then reapply the coating. “When
you do it that way, however, that sometimes degrades the per-
formance of the grating or even the optical fiber as a whole,”
Cramer said. “By etching the lines during the fabrication
process, it allows us to get higher-sensitivity gratings, and we
can much better control the entire process.”
By altering the fiber coatings, researchers can make sensors
that are sensitive to strain, temperature, moisture ingression
or the presence of certain chemical species, Cramer said. He
hopes that the fiber-sensor technology will be ready to fly in
10 years. “As with anything that flies, a lot of these technolo-
gies have to buy their way on, showing they can help improve
performance or improve safety or reduce weight,” Cramer said.
Farther in the future, quantum dot technologies could help
pilots and crews monitor rapidly changing external temperatures. Mool Gupta, professor of electrical engineering at
the University of Virginia (U.S.A.), and one of his graduate
students, Devin Pugh-Thomas, recently demonstrated lumi-nescence-based temperature sensing by embedding CdSe(ZnS)
quantum dots into a quartz matrix. According to Gupta, these
materials respond to temperature fluctuations on a very fast
timescale—microseconds to nanoseconds.
Manufacturing technologies
To provide cooling during powered flight, gas turbine engines
have tiny holes (less than 1 mm in diameter) drilled into their
blades, nozzle guide vanes, combustion chambers and afterburners. Some engine components need hundreds or thousands of these holes drilled, so that the air flow through them
produces a film that protects their surfaces from combustion
gases. Typically a high-power Nd:YAG laser is used to punch
these narrow holes.
For future aircraft, Gupta said, scientists are developing
micro- and nano-textured surfaces that would be laser-etched
onto the external surfaces of aircraft. Not only could these textures alter the airflow properties over these surfaces, but certain
textures—which look almost like tiny lotus flowers—make the
surfaces more hydrophobic and thus less prone to potentially
deadly ice formation.
Because they don’t carry a human crew and passengers,
unmanned air vehicles (UAVs) can make handy test platforms
for far-future manufacturing processes and power sources.
Just this summer, researchers at the University of Southampton (U.K.) built a UAV with a 2-m wingspan by “printing”
all of the component parts using a laser-sintering machine. The
process, also known as laser additive manufacturing, fabricates
plastic objects by building them up, thin layer by thin layer,
and fusing the material with laser heat.
According to the Southampton team, laser sintering enables
engineers to build highly tailored shapes that would be difficult,
if not cost-prohibitive, to make using traditional manufacturing
techniques. The pieces of the group’s UAV wings and fuselage
“snap-fit” together without fasteners, and the electric-powered
plane reached a top speed of nearly 100 mph (160 kph).
Solar and laser power in the air
Recently, solar power for both UAVs and piloted aircraft has
become an intense area of study, as part of a larger focus on
electricity as an alternative energy source for aviation. The first
human flight in a solar-powered craft took place in 1979, but
the “plane” was actually a hang glider fitted with a small electric motor that was photovoltaically charged prior to its flight.
Currently, a Swiss team called the Solar Impulse Project is
trying to build a solar-powered fixed-wing plane that would
circumnavigate the globe with a pilot, possibly as early as 2013.
Experiments with solar-powered UAVs emphasize longevity
in the air; the lightweight Zephyr, built by the British defense
firm Qinetiq, flew continuously for more than 336 hours in
July 2010.
The sun isn’t the only potential source of photovoltaic
energy for aircraft. Researchers at a Seattle-based company,
LaserMotive, say that beams from a near-infrared laser could
keep photovoltaic UAVs flying longer. LaserMotive is building on technology it developed for a NASA “space elevator”
competition in 2009.
Ring laser gyroscopes
Pilots who fly aircraft equipped with ring laser gyroscopes to
determine their location and orientation can thank a French
physicist who lived a century ago—George Sagnac, who was
trying to prove the now-discredited theory that there is an
“ether” through which light propagates and relative to which it
travels at a fixed velocity. In 1913, Sagnac spun an interferometer around on a turntable and observed interference between
