In-flight Output Only Modal Analysis of Aircraft Structural Dynamics

This work describes the application of the output-only modal analysis to the study of the structural dynamic parameters of the Brazilian Air Force F-5E aircraft, in several flight conditions. The applied methodology is based on a technique to estimate the frequency response functions and extract the modal parameters using only the structural dynamic response data, without assuming the knowledge of the excitation forces. The aircraft was multiply excited in flight with impulsive forces from pyrotechnical devices known as bonkers. These devices are attached to different position along the airframe. The in-flight structural response has been acquired by accelerometers distributed along the wings, fuselage and empenages of the aircraft. The Enhanced Frequency Domain Decomposition (EFDD) technique was chosen to identify the dynamic parameters of the airframe. This technique is based on the hypotheses that the system is randomly excited with a broad band spectrum with almost constant power spectral density. The system identification procedure is based on the single value decomposition (SVD) of the power spectral densities of system output signals, estimated by the usual Fast Fourier Transform (FFT) method. This procedure has been applied to different flight conditions to evaluate the modal parameters and the aeroelastic stability trends of this airframe. The experimental results obtained by this methodology were compared with the predicted results supplied by aeroelastic numerical models in order to check the consistency of the proposed output-only methodology.