Vibration Testing
“We believe we can implement
systems on passenger aircraft
that could fly in ten years”
“That defined the 2km airspace box we had available. Ten
seconds of flying at 60m/sec flutter-speed takes 600m
and 15 seconds is already half your airspace. Obviously,
you must first reach that speed, then turn around in a
confined area.” Rapid acceleration to flutter-speed
necessitated a jet engine and an airbrake enabled the
demonstrator to quickly slow back down within its 2km
visibility-envelope.
“The ground control-station was a Mercedes Sprinter
van with integrated antennas, laptops and work-places
for the crew. The pilot has a standard radio-control
transmitter. The back-up pilot is always ready to assume
control if something happens. The operator guides the
aircraft on a map, telling it to bank, turn or change
altitude, while the engineer monitors engine parameters
and aircraft health. It was a tough test-campaign. 70
people had worked on the aircraft for four years, so the
moment it actually took off, flies and lands – it was a
great feeling,” Bartasevicius says.
On August 1 2019, FLEXOP’s demonstrator finally
attained Bavarian skies.
“After two flights, we installed the fiber Bragg
system and started collecting data,” says Vanek. “We
could test changes in wing-shape during bank turns,
high g-loading and nose-down, nose-up maneuvers.”
This enabled Vanek’s team to validate their models,
which were closely matched by flight-test data. The
aeroelastically-tailored wing did indeed behave as
predicted, providing better load alleviation than the
reference wing. “It was nice to see the expected trends,’
says Vanek. “Now we can refine our tools and
methodology with some confidence that our models
match the aircraft’s behavior. “We showed that what we
claim in simulation actually happens in flight.’
NEXT STEPS
The delayed timetable left no time to test the third wing,
with active flutter-control. But that will now fly in
FLiPASED, a recentlyannounced
74 MARCH \\ AEROSPACETESTINGINTERNATIONAL.COM
follow-on project.
Meanwhile, FLEXOP’s industrial
relevance is being explored in an
Airbus scale-up study, led by TU Delft’s Dr
Roeland De Breuker, who spent a
sabbatical at Airbus during the project.
“We applied tools and methods
validated in our test-flight to a long-range
passenger aircraft,’ says De Breuker. “We
used a derivative aircraft based on the
Airbus XRF1 research model. Our
objective was 7% fuel reduction or a 20%
payload increase – we achieved 8% fuelreduction
and 25% payload increase.
“We believe we can implement
systems on passenger aircraft that could
fly in ten years.”
FLEXOP dealt separately with
SHAKING RESULTS
The German Aerospace Centre (DLR)
conducted ground vibration tests (GVT) on
the FLEXOP’s 7m-wingspan demonstrator
to determine its structural dynamic
characteristics. First the shakers were
connected to the structure and all modes
of interest were captured at four excitation
levels. A low-energy random signal was
sent to the structure, then a swept sine
signal to excite the modes on a higher
level. Eigenfrequencies, mode-shape
vectors and damping estimates were
determined. The data was used to update
the finite element model
design Dr Wolf-Reiner Krüger. “But an
assembled aircraft has rivets, cables and
systems which may vibrate. Free-play and
friction are typical sources of non-linearity.
Geometrically, the FLEXOP aircraft’s
V-tail may cause non-standard behavior.
We tested for non-linearities, looking at
amplitude-dependent vibration.”
“In FLEXOP, we captured one mode
which was highly non-linear,’ says Govers.
“That tended to occur at a junction or
where there’s friction. It’s important to
understand the degree of non-linearity and
we’re developing ways to measure it.”
aeroelastic tailoring and flutter-control,
but Airbus is interested in the benefits achievable from
combining both technologies in one wing. “There’s a
trade off, because you need flexibility for aeroelastic
tailoring but you need stiffness for active control
surfaces to work effectively,” says De Breuker. “The
question is: where’s the sweet spot between them?”
Finding the trade off will be FLiPASED’s focus.
“Now we can use existing know-how and
“There’s always a little discrepancy
between the design and how it was
“We usually think of aircraft as
liner – if you double the input,
the output will double,” says DLR’s
head of load analysis and aeroelastic
infrastructure to begin test flights as soon as possible,”
says Vanek. “For Airbus and Boeing, this flutter problem
comes up continually. New aircraft don’t happen every
year, but it’s on their next-generation horizon. We’ve
collaborated with EASA and FAA to ensure this isn’t just
an academic example but really tied to certification.
We’ve built sophisticated aircraft design tools from
scratch and got some nice results. But aero-servo-elastic
interactions between aerodynamics, structures and
control systems could create even greater efficiencies.’ \\
manufactured,” says DLR team
leader, Dr Yves Govers.
The DLR team has novel
methods for capturing nonlinear
phenomena during GVT.
4 // The carbon-fibre
reinforced composite
demonstrator for FLEXOP
was assembled at
Göttingen, Germany
4
/AEROSPACETESTINGINTERNATIONAL.COM