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Flight Flutter Testing of a Transport Aircraft with Wing Mounted
Store Suspension – Lessons Learned
Department of Aerospace Engineering
Middle East Technical University, 06531, ANKARA, TURKEY
Tel: 90-312-2104274, Fax:90-312-2101272
[email protected]
The methodology backed by relevant analysis results, ground vibration testing, flight flutter test results,
and their interpretation was presented for the flight flutter testing of a transport aircraft with wing
mounted external store suspension. Before the flight tests, extensive flutter analyses were performed by
MSC/Nastran for different weight and CG configurations to reduce the number of costly flutter flight tests
based on the results of flutter analysis, and decide on the aircraft test configurations. Flutter analysis
phase was supported by the relevant ground vibration testing to update the beam-like half aircraft
dynamic model of the modified aircraft. Finally, six sortie of flight test was performed with an
instrumented aircraft for flutter testing of the modified aircraft for two different weight and CG
configurations, and envelope expansion was achieved up to the dive speed of the basic aircraft. Analysis
results and test results indicate that although the flutter speeds of the modified aircraft were lowered
compared to the basic aircraft, the overall damping of the wing vibration modes stayed above the values
required by the flutter regulation MIL-A-8870C for the wing structure modes up to about 10 Hz which is a
typical frequency range that can be excited by control surface kicks. Lessons learned from the flight test
program, specifically related to the practical difficulty of executing flutter test with the particular
transport aircraft type, reliable data gathering and data analysis methods were elaborated.
Aeroelastic flutter is the complex interaction of aerodynamic, elastic, and inertia forces producing an
unstable, usually divergent oscillation of the aircraft structure or the external component added to the
aircraft by modification. Airworthiness regulations require that stability within the flight envelope of the
aircraft must be demonstrated by flight flutter tests. In practice, the most common flight flutter test data
analysis procedure is to track the variation of the damping of the aircraft structure with the airspeed. Some
examples of other flutter prediction methods include Flutter Margin, Envelope Function, Nissim and
Gilyard [Dimitriadis 2001] or the Eigenvector Orientations [Pidaparti 2001]. Common excitation means
used in practice include control surface pulses, oscillating control surfaces, thrusters, inertia exciters,
aerodynamic vanes, and random atmospheric turbulence [Kehoe 1995], [Brenner 1997]. The excitation
system must provide adequate excitation over the desired frequency range with adequate force levels.
Among these excitation systems, manual control surface pulses are the most common and affordable type
of excitation on most aircraft. The advantages and drawbacks of the various excitation techniques are
discussed very nicely in [Kehoe 1995].
Several flutter incidents occurred in the past involve the carriage of external stores as well as pylon
mounted engines [Garrick 1981]. The store carriage is an area of much concern even today, particularly
with the many store configurations that an airplane can carry. Wing mounted external store installations
Kayran, A. (2005) Flight Flutter Testing of a Transport Aircraft with Wing Mounted Store Suspension – Lessons Learned. In Flight Test –
Sharing Knowledge and Experience (pp. 18-1 – 18-22). Meeting Proceedings RTO-MP-SCI-162, Paper 18. Neuilly-sur-Seine, France: RTO.
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