Investigation of Poststall Pitch Oscillations of an Aircraft Wind-Tunnel Model

A five-degree-of-freedom dynamic wind-tunnel rig is used in the observation of large-amplitude, stall-related and self-sustaining, pitch oscillations of a model aircraft. These oscillations are investigated on the dynamic wind-tunnel rig during a quasi-steady ramp input to the aircraft model's elevator surfaces in one-degree-of-freedom pitch mode and in the longitudinal two-degree-of-freedom pitch and heave modes. A mathematical model of the aerodynamics, incorporating the effects of dynamic stall, is proposed. The aerodynamic model is coupled to the test rig equations of motion, which include terms for joint friction, and its parameters are fitted to the experimental data. This latter process is achieved using continuation and bifurcation analysis, which also helped to reveal the influence of friction forces on the oscillatory behavior. The quality of the fit and the use of a phenomenological model allow a possible cause for these oscillations to be proposed.