Nonlinear, Transonic Flutter Prediction for F-16 Stores Configuration Clearance

Wing flutter, or more accurately limit cycle oscillation (LCO), has been an issue for the F-16 since its operational deployment. Different store configurations and the permutations of those configurations after weapons are released will cause LCO to either disappear or appear. Unfortunately, the current method used by engineers for predicting LCO onset is based on linear, subsonic aerodynamic theory with no corrections for transonic effects. Predictions using this method are often good in frequency, but can be far off in predicting onset speed, forcing flutter engineers to rely more on experience and interpolation from similar configurations to design flight test parameters. During flight tests, very specific and stringent guidelines are adopted to ensure the aircraft does not encounter classical flutter or excessive LCO; consequently, these tests require a large investment in resources and time to validate any particular store configuration. A new approach, incorporating inherent nonlinearities that drive LCO is investigated in the following research. This approach (called ZTAIC - ZONA\u27s Transonic Aerodynamic Influence Coefficient method) uses steady Cp data in conjunction with the Transonic Equivalent Strip (TES) method to generate a transonic modal aerodynamic influence coefficient (AIC) matrix which accounts for wing thickness effects and shock structure. This AIC is then used in the g-method flutter solution methodology (incorporating a damping perturbation technique) to extend the classical linear p-k flutter solution methodology to include first-order aerodynamic damping effects. Two F-16 store configurations are examined using the g-method to correlate predicted flutter onset speeds, frequencies and character (classical flutter, typical LCO, or non-typical LCO) with those found in flight test. Additionally, an investigation of the aerodynamic effects of modeling underwing stores on the flutter solution is accomplished