Comparison of vane type external excitation and Free Air Turbulence Methods for flutter flight testing

Structure excitation plays a key role in evaluating the stability of the aircraft during flutter flight test. Various techniques are used for the excitation of aircraft during flutter flight test. Each technique has its own merits and demerits depending upon the type and application of exciters. This paper describes the comparison of rotating cylinder vane type excitation system (external) with free air turbulence (natural). Different performance parameters like excitation force, frequency range and phase difference of the flutter exciters were evaluated based on the flight test data. Free air turbulence was able to excite low frequency modes (<; 20 Hz) with small amplitudes. Whereas the rotating cylinder system excited the complete required range (up to 50 Hz) with significantly higher amplitudes. Major difference can be seen in the performance like high signal to noise ratio, suitable identification of elastic modes, appropriate in-phase and out-of-phase excitation to the aircraft as compared to free air turbulence. Rotating cylinder excitation systems is a more efficient way of doing flutter flight testing as it results in significant reduction in time of flight tests and better estimation of structural identification.