Nonlinear analysis of random gust response

In the present research, the nonlinear unsteady aerodynamics is incorporated in the application of Matched Filter Theory to determine the aerodynamic response to random gust. The analysis is focused on the gust response at low Mach numbers. Two airplane configurations, the F-18 HARV and F-16XL, have been tested in forced oscillation to provide the necessary data. The atmospheric turbulence is assumed to be characterized by the von Karman gust power spectral density function. The nonlinear aerodynamic model is set up through Fourier functional analysis; while the linear aerodynamic model is calculated with the unsteady Quasi-Vortex-Lattice method. Both nonlinear and linear aerodynamic models are used to provide the airplane aerodynamics for comparison. The static results show that nonlinear unsteady aerodynamics produce at least 50% ∼ 60% higher maximum lift responses than the linear unsteady aerodynamics. The pitching moment coefficients are more negative for the F-18 HARV and more positive for the F-16XL with the nonlinear aerodynamic models than the linear ones under the prescribed random gust In plunging motion, the dynamic results show that the use of nonlinear unsteady aerodynamics will result in 48.7% ∼ 90.1% higher possible maximum gust load factors than with linear unsteady aerodynamics.