Damping Identification Sensitivity in Flutter Speed Estimation

TL;DR

The paper addresses flutter speed prediction by extracting modal parameters from Ground Vibration Testing using two frequency-domain SI methods, FRVF and LF, and validating them against N4SID on the XB-2 wing. A simplified 2-DoF aeroelastic model feeds these parameters into a classical $p$-$k$ flutter analysis to estimate onset speeds. The study demonstrates that FRVF and LF can yield flutter speeds within 3% and 5% of the N4SID benchmark, respectively, with damping identification—especially $\zeta_n$—being the key driver of accuracy. The findings support the use of FRVF and LF as computationally efficient alternatives for preliminary aeroelastic assessments in small-scale, UAV-class wings, while recognizing limitations and the need for broader scale validation.

Abstract

Predicting flutter remains a key challenge in aeroelastic research, with certain models relying on modal parameters, such as natural frequencies and damping ratios. These models are particularly useful in early design stages or for the development of small Unmanned Aerial Vehicles (maximum take-off mass below 7 kg). This study evaluates two frequency-domain system identification methods, Fast Relaxed Vector Fitting (FRVF) and the Loewner Framework (LF), for predicting the flutter onset speed of a flexible wing model. Both methods are applied to extract modal parameters from Ground Vibration Testing data, which are subsequently used to develop a reduced-order model with two degrees of freedom. Results indicate that FRVF and LF-informed models provide reliable flutter speed, with predictions deviating by no more than 3% (FRVF) and 5% (LF) from the N4SID-informed benchmark. The findings highlight the sensitivity of flutter speed predictions to damping ratio identification accuracy and demonstrate the potential of these methods as computationally efficient alternatives for preliminary aeroelastic assessments.

Figures (5)

• Figure 1: Flutter speed estimation workflow.
• Figure 2: XB-2 wing top view. Y-axis coming out of the page. Any apparent distortions are due to lens-induced optical effects. (Adapted from Dessena2023).
• Figure 3: Flexible wing 2 DoF model schematic.
• Figure 4: XB-2: Frequency (\ref{['fig:flut1a']}), damping ratio (\ref{['fig:flut1c']}), real (\ref{['fig:flut1b']}) and imaginary part (\ref{['fig:flut1d']}) of the eigenvalues vs $U_\infty$ plots for the aeroelastic model computed from data obtained via LF, FRVF, and N4SID for the baseline scenario.
• Figure 5: XB-2: Polar plot of the eigenvalues obtained from the LF, FRVF, and N4SID data for the baseline scenario.