Limit-cycle oscillation flight-test results for asymmetric store configurations

A conventional approach for flutter certification of asymmetric external store configurations on fighter aircraft is to analyze each side of the aircraft using half-span models. The flutter speed for the asymmetric configuration is then assumed to be no less than the flutter speed of either half-span solution. Recent certification efforts on the F-16 have involved asymmetrically carried stores that were previously certified for symmetric carriage. For the present work, asymmetric F-16 external store configurations were studied to investigate their aeroelastic behavior and relationship to similar symmetrically loaded external store configurations. Linear flutter analyses utilizing fullspan finite element and aerodynamic models were used to predict the flutter boundaries of the asymmetric external store configurations. It was found that solutions using full-span models predicted lower flutter speeds in some cases for an asymmetric configuration than the comparable flutter speeds using the half-span model. The mode shapes of the asymmetric configuration indicate coupling mechanisms on at least one wing are similar to the symmetric configurations; however, they are unique in that the frequencies of these modes have shifted. Flutter flight tests on the F-16 were performed to validate the analytical results. The test results from the asymmetrically loaded configurations were compared to the test results for similar symmetrically loaded configurations. The oscillatory response behavior of the asymmetric configurations is shown to be primarily antisymmetric. Also, the flight-test results show limit-cycle oscillation behavior that correlates to the full-span linear flutter analysis. Specifically, it is shown that two flutter sensitive half-span configurations can be combined into one asymmetric configuration that exhibits no aeroelastic instabilities in flight. Conversely, it is shown that two aeroelastically stable, half-span configurations can be combined asymmetrically, which results in a configuration with lower, aeroelastically critical flutter speeds. These cases demonstrate the need for flutter analyses of the full aircraft structure for certification of asymmetric external store configurations.