Characterization of structural and aerodynamic stiffness and damping associated with wing limit cycle oscillations

The mechanism of transonic limit cycle oscillations (LCO) was studied for the NASA ARW-2 which unexpectedly encountered LCO during a wind tunnel test. The theoretical free vibration modes were used to transform the system to modal coordinates and were fitted to the measured accelerometer responses using a least squares mode fitting process. It was found that the first wing bending free vibration mode adequately represented the fundamental behavior of the measured LCO response. A single degree-of-freedom (DOF) oscillator model of the structure was developed but it had significant deficiencies when compared to the measured response characteristics.; Ground vibration test results indicated a possible interaction between the wing and the wind tunnel turntable mounting fixture. This motivated the development of a structural model that represented the wing and its mounting fixture as a two degree-of-freedom oscillator. The 2-DOF model was a significant improvement over the 1-DOF model. Very good agreement with experiment was seen when the 2-DOF model was excited with the forces resulting from the measured pressures. The overall behavior of this model indicated that the wind tunnel base flexibility characteristics have a significant influence on the structural model response and likely have an influence on the measured LCO characteristics.; A basic aerodynamic model was developed which represented the aerodynamics as an oscillating fluid driven by the wing displacement. The form of the aerodynamic equation was modified so that strictly aerodynamic frequency and damping coefficients were not present in the wing forcing terms. This approach did not require a prior assumption of the explicit composition of the aerodynamic spring and damper. The basic aerodynamic model showed similar behavior when compared to the experimental data. The wing forces tracked the aerodynamic forces reasonably well when the aerodynamic forces were oscillating in an organized harmonic fashion. However, the wing forces did not adequately describe the aerodynamic forces when the aerodynamic oscillations were less organized. In addition, the aerodynamic forces showed some large amplitude excursions that were not tracked well by the wing force, which indicated the presence of additional coupling mechanisms that are not included in the simple aerodynamic model.