Flight Experiment Demonstrates Improved Test Method

A flight experiment called the Aerostructures Test Wing (ATW)
conducted at NASA’s Dryden Flight Research Center, Edwards, Calif.,
successfully demonstrated a new software data analysis tool, the
flutterometer, which is designed to increase the efficiency of flight
flutter testing.

The experiment consisted of an 18-inch carbon fiber test wing with
surface-mounted piezoelectric strain actuators. The test wing was
mounted on a special ventral flight test fixture and flown on
Dryden’s F-15B Research Testbed aircraft.

Five flights consisted of increasing speeds and altitudes leading to
the final test point of Mach .85 at an altitude of 10,000 feet. At
each Mach and altitude, stability estimations of the wing were made
using accelerometer measurements in response to the piezoelectric
actuator excitation. The test wing was intentionally flown to the
point of structural failure, resulting in about a third of the
18-inch wing breaking off. This allowed engineers to record the
effectiveness of the flutterometer over the entire regime of flutter
testing, up to and including structural failure.

The actuators were moved at different magnitudes and frequency levels
to induce wing vibrations and excite the dynamics during flight.
Placement of the piezoelectric actuators was determined by NASA
Langley Research Center, Hampton, Va. engineer Mercedes Reaves to
maximize their effectiveness. The ATW experiment represents the first
time that piezoelectric actuators were used during a flight flutter
test.

“The data acquired during the Aerostructures Test Wing experiment
will help us improve the way we model structures, and helps to
validate the flutterometer concept” said Dryden project engineer
David Voracek. “The data that was created from these flights will be
invaluable to future flight flutter test engineers for research and
training.”

Potential benefits of this research include reduced time and cost
associated with aircraft certification by lowering the number of
flights required to clear a new or modified aircraft for flight, and
provision of a structural dynamics database for industry and
university flutter research.

The flutterometer is an on-line software tool that was loaded on
computers in Dryden’s control room for the experiment in order to
access the flight data. With this new technology, flight data can be
analyzed immediately using the newly developed software to determine
the stability properties of aircraft in flight. It is designed to
predict the flight conditions at which the onset of flutter may
occur. In this way, the flight envelope of an aircraft can be
determined more quickly and safely than using traditional approaches.
The ATW experiment was the first time the flutterometer was used on a
flight system that actually experienced flutter.

The flutterometer is an on-line software tool that was loaded on
computers in Dryden’s control room for the experiment in order to
access the flight data. With this new technology, flight data can be
analyzed immediately using the newly developed software to determine
the stability properties of aircraft in flight. It is designed to
predict the flight conditions at which the onset of flutter may
occur. In this way, the flight envelope of an aircraft can be
determined more quickly and safely than using traditional approaches.
The ATW experiment was the first time the flutterometer was used on a
flight system that actually experienced flutter.

“The flutterometer represents a significant advance for flight
flutter testing” said Dryden project engineer Rick Lind. “This tool
can result in dramatic decreases in time and cost for military and
commercial aircraft testing. The ATW experiment was a perfect
demonstration of how the unique facilities at NASA Dryden can be used
to develop tools that are beneficial to the entire aviation industry.”

The concept of a flutterometer was initially conceived by Dryden
structural dynamics engineer Len Voelker. Fellow engineers Rick Lind
and Marty Brenner developed the flight data algorithms that made the
flutterometer concept a reality. NASA was recently awarded a patent
for the flutterometer. Its software program combines the strengths
of analytical predictions and on-line estimation methods in the
development of a flutterometer concept. The flutterometer software
has previously been evaluated using simulations and wind tunnels
along with flight data from several aircraft types including NASA
Dryden’s F-18 Systems Research Aircraft.

Flutter is the rapid and self-excited vibration of wings, tail
surfaces, and other aircraft parts that can damage or destroy an
aircraft component. Flutter is caused by the flow of air across the
surface of the structure. Effectively, the aerodynamic forces couple
with the structural bending and twisting to result in the destructive
vibration. Flight flutter testing is the process of determining a
flight envelope within which an aircraft is safe to operate.
Traditional approaches for flight flutter testing do not accurately
predict the onset of instability so this testing is a very
time-consuming and expensive process.

The ATW was designed by NASA engineer Cliff Sticht and was
manufactured by Fiberset, Inc., located in Mojave, Calif.

– NASA –

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