The effect of systematic error in forced oscillation wind tunnel test apparatuses on determining nonlinear unsteady aerodynamic stability derivatives

One of the basic problems of flight dynamics is the formulation of aerodynamic forces and moments acting on an aircraft in arbitrary motion. Classically conventional stability derivatives are used for the representation of aerodynamic loads in the aircraft equations of motion. However, for modern aircraft with highly nonlinear and unsteady aerodynamic characteristics undergoing maneuvers at high angle of attack and/or angular rates the conventional stability derivative model is no longer valid. Attempts to formulate aerodynamic model equations with unsteady terms are based on several different wind tunnel techniques: for example, captive, wind tunnel single degree-of-freedom, and wind tunnel free-flying techniques. One of the most common techniques is forced oscillation testing. However, the forced oscillation testing method does not address the systematic and systematic correlation errors from the test apparatus that cause inconsistencies in the measured oscillatory stability derivatives. The primary objective of this study is to identify the possible sources and magnitude of systematic error in representative dynamic test apparatuses. Using a high fidelity simulation of a forced oscillation test rig modeled after the NASA LaRC 12-ft tunnel machine, Design of Experiments and Monte Carlo methods, the sensitivities of the longitudinal stability derivatives to systematic errors are computed. Finally, recommendations are made for improving the fidelity of wind tunnel test techniques for nonlinear unsteady aerodynamic modeling for longitudinal motion.