Online Monitoring Of Whirl Flutter During The Attila Wind Tunnel Testing

Abstract

This paper presents the methodology, implementation, and results of an online monitoring system (OLM) for whirl flutter during wind tunnel testing of the ATTILA tiltrotor aeroelastic testbed. The objective was to enable real-time tracking of modal parameters - i.e. frequencies, damping ratios, and mode shapes - under varying operating conditions to safely approach the whirl flutter stability boundary. A robust system combining measurement hardware with automated Operational Modal Analysis (OMA) using the Stochastic Subspace Identification (SSI), supplemented by matrix pencil method, was developed and deployed during the test campaign. Results from multiple configurations revealed amplitude dependent modal behavior and significant structural nonlinearities, particularly in the chord and torsion modes. These nonlinearities were also observed during Ground Vibration Tests (GVT) prior to wind tunnel entry, and required a proper trim of the rotor to limit their impact. The study demonstrates the critical role of real-time system identification and monitoring in identifying aeroelastic stability. It highlights the limitations of forecasts with linear simulation models when the actual hardware shows nonlinear dynamic behavior.