Comparison of in-Flight Measured and Computed Aeroelastic Damping: Modal Identification Procedures and Modeling Approaches

Abstract

AbstrAct:The Operational Modal Analysis technique is a methodology very often applied for the identification of dynamic systems when the input signal is unknown. 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. Such approach is an adequate way for measuring the aircraft aeroelastic response due to random input, like atmospheric turbulence. The in-flight structural response has been measured by accelerometers distributed along the aircraft wings, fuselage and empennages. The Enhanced Frequency Domain Decomposition technique was chosen to identify the airframe dynamic parameters. This technique is based on the hypothesis that the system is randomly excited with a broadband spectrum with almost constant power spectral density. The system identification procedure is based on the Single Value Decomposition of the power spectral densities of system output signals, estimated by the usual Fast Fourier Transform method. This procedure has been applied to different flight conditions to evaluate the modal parameters and the aeroelastic stability trends of the airframe under investigation. 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. The objective of this paper is to compare in-flight measured aeroelastic damping against the corresponding parameters computed from numerical aeroelastic models. Different aerodynamic modeling approaches should be investigated such as the use of source panel body models, cruciform and flat plate projection. As a result of this investigation it is expected the choice of the better aeroelastic modeling and Operational Modal Analysis techniques to be included in a standard aeroelastic certification process.

Experimental Validation of a Flight Simulation Model for Slightly Flexible Aircraft

“…There is some evidence that the use of the potential theory generally leads to conservative damping ratios, and thus to conservative flutter velocities, which are attributed to conservative aerodynamic damping calculations: for example, Smith et al [40] indicated that "divergence and possibly flutter speeds predicted by lower-order aerodynamics may be overly conservative" and that "the utilization of linear aerodynamic models may cause overly conservative predictions in divergence and flutter speed." This tendency was observed also for higher velocities (Mach 0.85) by Follador et al [41] using ZAERO; the authors found aerodynamic damping ratios for the first wing bending mode up to 40% lower when compared to the damping ratios calculated from flight-test data. All thise evidence points to limitations of the potential theory to model the aerodynamic damping.…”

Real Time Measurement of Airplane Flutter via Distributed Acoustic Sensing

“…The damping of the two modes of flutter (bending and torsion) was theoretically analyzed with a frequency domain technique that is described in [21]. Figure 11 shows a plot of the damping factor for the two modes as a function of airspeed.…”

Normal Modes Study on Half Wing Structure of N219 Aircraft Using Computational Method

“…Normal modes analysis is one of part of the dynamic parameter for understanding the sensitivity of aeroelastic stability behavior [1]. Aeroelasticity phenomenon in-volve the study of the interaction between aerodynamic, elastic forces and inertial [2].…”