Some Tunnel-Wall Effects on Transonic Flutter

Significant effects of wind-tunnel walls were observed on the transonic flutter boundaries of wall-mounted models during two flutter model research studies, a joint ONERA-NASA-Boeing program and a follow-on NASA exploratory study. In these studies, flutter experiments with cantilevered SST-type wing models were conducted in three different wind tunnels: the ONERA S2 tunnel at Modane, France; the NASA Langley transonic dynamics tunnel; and the NASA Ames 6-ft by 6-ft supersonic tunnel. The experimental results are compared to flutter boundaries calculated for the models in free air. The results indicate that transonic flutter boundaries can be affected by tunnel-wall interference, tunnel resonances, and shock-wave reflections, and that flutter model data accuracy is a function of model/tunnel size and tunnel wall porosity. However, models within the recommended size limits should give accurate results in transonic tunnels with normal ventilation. A flutter trend analysis for a two-dimensional wing demonstrating tunnel wall and resonance effects on flutter is also presented.