Measuring Aircraft Nonlinearity Across Aerodynamic Attitude Flight Envelope-Revisited With Symmetrized Aerodynamics

In previous research, the authors introduced a new asymmetric level aerodynamic attitude flight envelope. Further, nonlinearity of a six degree of freedom aircraft model was analyzed with nonlinear index theory across this nontraditional envelope. This paper revisits details concerning the experimental aerodynamic model used in the previous study after removing asymmetry and offset in the data. Asymmetry and offset in the force and moment coefficient data could originate from experimental error, model fabrication imperfections, vortex-dominated flow, data reduction flaws, or other sources. The purpose behind removing the asymmetry and offset is to facilitate analysis of the new aerodynamic attitude flight envelope with an ideal aircraft model so that fundamental relationships can be more easily observed, and to compare with the non-ideal case previously investigated. Literature shows that vortex-dominated flow causes side force, rolling moment, and yawing moment coefficient asymmetries. Based on the adapted and symmetrized aerodynamic data, a new aerodynamic attitude asymmetric level flight envelope is constructed and introduced. This angle of attack vs. sideslip angle envelope is an extension of the speed-altitude symmetric level flight envelope where in the former an aircraft can maintain a slipping horizontal flight condition. The new envelope provides enhanced insight to trimability-controllability regions for a model with ideal aerodynamic data characteristics. Aircraft dynamic characteristics frequently change in a nonlinear fashion across operating conditions. Nonlinear index theory is applied to the new symmetric aerodynamic attitude envelope. The index analysis exposes certain flight condition regions where nonlinearity strength is high.