Estimation of Nonlinear Aerodynamic Roll Models for Identification Of Uncommanded Rolling Motions

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Detailed analysis of wind-tunnel data in free-to-roll testing of one aircraft model, the preproduction F/A-18E, in a transonic tunnel is presented. The main purpose is to identify possible uncommanded rolling motions of the full-scale aircraft by examining the roll dynamic characteristics of the model in the tunnel. To improve the tunnel balance data, the bearing-friction effect on the balance rolling-moment coefficient was removed. The corrected rolling-moment coefficients are then modeled through a fuzzy-logic algorithm. The resulting aerodynamic models are employed in calculating all roll derivatives by a central-difference scheme. The proposed wing-drop theory relies on the values of the relative aerodynamic stiffness in the rolling equation of motion, which is assumed to be composed of two terms: derivatives with respect to the roll angle alone and second-order derivatives of the rolling-moment coefficient with respect to the roll angle and angle of attack. Wing drop is predicted if the relative aerodynamic stiffness changes sign from that of the overall motion and if the contributions to the rolling-moment coefficient from both the first-order and second-order derivatives are of the same sign and are small in value. This is equivalent to the vanishing of a frequency with damping in the vibration theory. It is found that at a low angle of attack and a transonic Mach number, the dynamic motion is wing rock. As the angle of attack is increased, the wing-drop condition is initially exhibited with a single event, then with multiple occurrences of wing drop at a higher angle of attack, with the magnitude of roll-off angles changing with time. It is also found that wing-rock motion in the tunnel on a free-to-roll test rig is mostly caused by the unstable effect of time rate of sideslip angle, not by the traditional roll damping due to roll rate.