AEROSPACETESTINGINTERNATIONAL.COM // MARCH 2020 73
commands,” Vanek says. “Sizing actuators differently
creates different mass penalties, so we had to understand
the limits and trade-offs.”
Conventional data acquisition systems were too
heavy, so a Raspberry Pi computer was used to process
the data from lightweight internal sensing. “We installed
fiber-Bragg sensors, which gave us 300 measurements
per wing-pair at 200Hz grade,” he says. “We used
accelerometers and gyroscopes to measure normal
acceleration, bending and torsional angular rate and
recreate wing behavior.”
Once manufactured, the wings underwent ground
vibration tests (GVT) at DLR’s Göttingen Aeroelasticity
Research Institute. “We validated our models and
identified the global dynamic properties of assembled
structures,” says Krüger. “We tested at four excitation
levels to ascertain eigenfrequencies and mode shapes.’
Around 100 triaxial acceleration sensors measured how
wings reacted to electrodynamic shaker inputs, enabling
reciprocal damping estimates to be calculated. “While the
aircraft was vibrated, data-acquisition techniques could
also be tested,” he adds. Data from integral wing sensors
differed less than 2% from DLR’s external, state-of-theart
sensor results, confirming an ability to measure
wings in flight with laboratory-like precision,” he adds.
FLEXOP’s 65kg, 7m-wingspan unmanned
demonstrator aircraft was shaped by both experimental
and industrial requirements. “US flutter-mitigation
flight-tests use blended wing-body demonstrators,” Vanek
3
TOWARDS INDUSTRIALIZATION
The three-year €3.8 million FLiPASED (FLight Phase adaptive Aero-
Servo-Elastic aircraft Design) continues where FLEXOP left off and is
targeting reductions of 10% in fuel-efficiency and 20% in peak gustresponse
amplitude by 2022 in wings.
“The focus is multidisciplinary design optimization for tailored wing
structures with control also in the loop,’ says Dr Roeland De Breuker.
TU Delft Aerospace Structures and Computational Mechanics’ section
leader. “FLEXOP had two distinct objectives: aeroelastic tailoring and
flutter-control. But during my five-month sabbatical with Airbus in
Ottobrunn, we focused on the combination of active control surfaces
and aeroelastic tailoring in one design.’
“FLiPASED will investigate the same questions of aero-servoelasticity,’
he continues. “Servo means the control part. We will consider
structures, aerodynamics and control in one optimization process.
By understanding their interactions, we can make better trade-offs
between conflicting requirements and build even more efficient wings.”
For De Breuker, FLiPASED represents an important step to
industrialization.
“FLEXOP showed aeroelastic tailoring and flutter-control to be
feasible, at a low level of technology-readiness,” he says. “But the
reliability of actuators needs to improve before we can achieve
certification and implement flutter-control in passenger aircraft. Tailored
aeroelastic materials must also be certified.
“That’s an expensive campaign, because each carbon-composite
stacking-sequence requires an individual certification process. But
an Airbus A350 already has highly advanced wings with many control
possibilities. Airbus could include some degree of flutter-control in its
next project, which would fly in ten to fifteen years.”
“A 65kg aircraft is
not a toy. We had to
design a flight
termination system
and demonstrate
redundancy”
explains. “But we opted for a conventional, wing-fuselage
aircraft which was more industrially relevant.”
Demonstrating control of flutter meant making
wings which were prone to it, going against established
design aerodynamic and structural design thinking.
Vanek says, “As wingspan decreases, oscillation
frequency increases and we can’t show anything
interesting. We went with the highest wingspan we could
handle to get a flutter-speed below 60m/sec.”
This demanded a frequency below 8Hz, achieved
using a 7m wingspan with additional masses attached to
tune flutter-speed downwards.
BEYOND VISUAL LINE OF SIGHT
Test-flights at Oberpfaffenhofen Airport near Munich
were first scheduled for 2018. But German Civil Aviation
Authority unmanned aerial vehicle (UAV) regulations are
not well-defined and stringent safety requirements made
for a lengthy development process. “There are offices
around the airfield and a public road at one end of the
runway,’ says Vanek. “A 65kg aircraft is not a toy. We had
to design a flight termination system and demonstrate
redundancy, so that if one component or system fails, the
aircraft still flies. We simulated the flight beforehand,
preparing the crew to conduct it sequentially and
formulate contingency plans.’
Critically, current German rules preclude flying UAVs
beyond visible line-of-sight. “Staying within visual range
means a maximum distance from the pilot of 1km in
either direction,” says test-flight manager Bartasevicius.
Vibration Testing
3 // The FLEXOP flight
demonstrator lands after its
25 minute maiden flight
last November
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