Vibration Testing
Engineers are developing less heavy,
slender carbon-composite wings for
the next generation of commercial
aircraft to improve the fuelefficiency
of conventional tube-and-wing airliners. But
low vibrational frequencies make these lighter wings
more flutter-prone than stiffer wings. Flutter thus
imposes optimization constraints on the aerospace
industry. The removal of these constraints is what the
US$7.2 million (€6.6 million) European FLEXOP (flutter
free FLight Envelope eXpansion for ecOnomical
Performance Improvement) set out to resolve.
“In 2014 we saw advances in the USA and realized
Europe had to start mastering flutter-suppression,” says
FLEXOP’s project leader, Dr Bálint Vanek, of the
Hungarian Computer Science and Control Institute,
SZTAKI. “We wanted to build a demonstrator, make and
test flexible materials, then link our technologies to nextgeneration
aircraft in a scale-up study.”
FLEXOP was initially focused on active flutter-control
but was soon extended to encompass passive load
alleviation by aeroelastic tailoring. Gusts make wings
bend and exert loads at the wing-root, but aeroelastic
tailoring can mitigate gust-loads by optimizing the
anisotropic qualities of carbon-fiber materials.
“Carbon-fiber is unidirectional, like the grain in
wood,’ says German Research Centre DLR’s head of load
analysis and aeroelastic design, Dr Wolf-Reiner Krüger.
“Its behavior depends on whether it is loaded along the
fibers or perpendicular to them. We simulated the
optimal thickness and direction of carbon-fiber layers for
passive load alleviation.” In a gust, the tailored wing not
only bends but twists, decreasing the local angle of attack
on the wing and reducing loads.
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Three wing designs were manufactured by Austrian
project partner, FACC: a stiff, reference wing, a tailored
wing and a third wing, with active flutter-control. “In the
region of flutter, unsteady aerodynamics occur naturally
around the wing,” explains flight-test manager Julius
Bartasevicius from the Technical University of Munich
(TUM), a FLEXOP project partner. “But moving ailerons
up and down can create additional unsteady
aerodynamics to counteract that. It’s about control
systems and modelling. You must either know at what
speed to actuate the ailerons, or measure axle vibrations
and channel that into a flight-control computer which
actuates flutter suppression.”
One of FLEXOP’s aims was to demonstrate to
certification agencies that active control surfaces could
ensure safe flight above flutter-speed.
MODEL & VIBRATION TESTS
DLR, TUM and industrial partner Airbus used
simulation to model flutter’s affect on wings. “We drew
on existing literature and military flutter-damping flighttests
to build a mathematical aircraft model,” says Vanek.
“We could predict flutter and show how our system could
implement control laws to achieve higher velocity.
“In simulations, we could push flutter-speed upwards
by 20%. We designed our first control system iteration
and simulated it with a full-aircraft model. We ran a
hardware-in-the-loop simulation with the actual flight
control computer to test our algorithms and see if we
could execute all commands in real-time.”
Considerable work went into optimizing the
configuration of special sensors built into the wings to
measure their behavior in flight. “We were initially
unsure whether we needed 0.1° or 10° actuator
“In simulations, we could push
flutter-speed upwards by 20%”
2
2 // Participants in the final
review of the FLEXOP
project at the Technical
University of Munich pose
with the demonstrator
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