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Large aircraft, such as the Lockheed C-5A, can be forced to resonate on the ground in a large number of closely coupled vibration modes which involve the combined motion of lifting and control surfaces, fuselage and engines. During flight, atmospheric disturbances can also excite these vibrational resonances, though, under normal conditions, they are damped to a safe level because the airstream is able to extract energy from the vibrating structure. However, there exists the aeroelastic phenomenon called flutter---under certain conditions the structure is able to extract energy from the airstream and the amplitude of a resonance can very rapidly increase to a destructive level. Clearly, the damping of all resonances must remain positive throughout a wide range of flight conditions. This is verified by flight flutter test programs during which aircraft are proven safe at an airspeed and altitude before proceeding to a higher airspeed. In one method of flutter testing of large aircraft, the resonant modes are excited during flight by oscillatory forces from aerodynamic vanes. A frequency sweep technique is used; the frequency of the oscillatory forces is varied continuously from about 1 to 30 Hz. Accelerometers or other transducers indicate the response at various locations on the aircraft. After an excitation sweep, the frequencies and measures of damping of the resonances are determined, and a decision is made about the safety of a higher airspeed.

A hybrid frequency response technique and its application to aircraft flight flutter testing