Characterisation of wind tunnel observed, large-amplitude pitch limit-cycles

A five-degree-of-freedom dynamic wind tunnel rig is used in the observation of large amplitude self-sustaining pitch oscillations of a model aircraft. These oscillations arise 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 dynamic stall, is proposed. The model is coupled to the equations of motion of the rig, which include terms for friction, and fitted to the experimental data. This later process is achieved using continuation and bifurcation analysis and revealed the influence of friction forces on the oscillatory behavior. The quality of the fit and the use of a phenomenological model allows a possible cause for these oscillations to be proposed.