Time- and frequency-domain whirl-flutter analysis using a vortex particle method

The method of using the viscous vortex particle method (VVPM) to conduct whirl-flutter stability analysis is evaluated on a 4-bladed, soft in-plane tiltrotor model using the Rotorcraft Comprehensive Analysis System (RCAS). For the first time, coupled VVPM/RCAS transient simulations are used to calculate whirl-flutter stability. In addition, a novel concept of linearizing VPM solutions by perturbing trimmed VPM inflow velocities with dynamic inflow (DI) states is studied to make linearized stability predictions via eigenanalysis. Comparisons are made between several different levels of fidelity in prediction capabilities and against experimental data. Time-domain stability predictions are shown to capture the physical features of rotor airloads and rotor-to-wing interactional aerodynamics. Linearization of VPM solutions using DI is shown to accurately capture rotor airloads but currently does not properly capture the rotor-towing interaction. Baseline results are presented using VPM and uniform inflow (UI) wake models. DI is demonstrated to be significantly more accurate than UI and makes linearized stability predictions similar to those made using VPM. Linearized VPM frequency and damping predictions are shown to be insensitive to vortex particle resolution; however, a reduction in damping is shown for particle resolutions less than 2% of the rotor span using time-domain transient analysis and needs further investigation.