A generalized dynamic aerodynamic coefficient model for flight dynamics applications

A generalized dynamic aerodynamic coefficient model to represent the effect of unsteady aerodynamics for aircraft is presented in this paper. The model is of the nonlinear algebraic form with the coefficients being functions of frequency and determined from a set of large amplitude oscillatory experimental data by using least-squares fitting. In applications, a general dynamic motion at any instant is modelled by an equivalent harmonic motion with an appropriate frequency. The resulting model is a generalization of the conventional dynamic derivatives with the values of the latter being now dependent on the motion. To verify this model, expressions for lift, drag and pitching moment coefficients of an F-18 HARV configuration are generated with one set of large-amplitu de harmonic oscillatory data. The computed results from these models are compared with good agreement to other sets of data in harmonic oscillation with a smaller amplitude and in constant pitch-rate motions. These models are also used in bifurcation analysis and control study for the same F-18 HARV configuration. The results show significant differences in the equilibrium surfaces and dynamic stability. It is also shown that control gains developed with the conventional quasi-steady aerodynamic data may not be adequate when the effect of unsteady aerodynamics is significant.