Longitudinal and Lateral-Directional Coupling Effects on Nonlinear Unsteady Aerodynamic Modeling from Flight Data

To investigate the coupling effects between the longitudinal and lateral-directional aerodynamics, a nonlinear normal force model, set up with a fuzzylogic algorithm by using one longitudinal maneuver flight of an F-16XL configuration, is used to predict the normal force coefficients for the same configuration in a rapid rolling maneuver with large amplitudes. It is found that significant discrepancies between the predicted values and those from flight data analysis occur when roll rates are high. Pitch damping is also shown to be dependent on pitch rates, and roll rates as well. Furthermore, the time rate of angle of attack may cause the roll and yaw derivatives to vary significantly. Airplane maneuvering time history effects are seen to significantly alter the longitudinal and lateraldirectional stability characteristics.