Experimental Investigation of Aerodynamic Hysteresis Using a Five-Degree-of-Freedom Wind-Tunnel Maneuver Rig

The high incidence aerodynamics of a lightweight jet trainer aircraft model has been investigated using a novel five degree-of-freedom (DoF) dynamic manoeuvre rig, recently updated with improved actuation and data acquisition systems, in the 7' x 5' closed-section low-speed wind tunnel at the University of Bristol. The major focus was to identify the nonlinear and unsteady aerodynamic characteristics specific to the stall region and which affect free-to-move aircraft model behaviour. First, the unstable equilibrium states in the limit cycle regions were stabilized, and so observed, over a wide range of angles of attack using a simple elevator feedback control law based on pitch angle and pitch-rate sensor measurements.

Tests with two degrees-of-freedom, namely the aircraft model and rig arm pitch angles, revealed the existence of static hysteresis in the normal force acting on the aircraft model in the stall region. Unlocking the aircraft model in roll and yaw accompanied by feedback stabilization of the lateral-directional modes of motion demonstrated onset of asymmetric aerodynamic rolling and yawing moments in this four degree-of-freedom configuration. This observation implicitly indicates a link between the static hysteresis in the normal aerodynamic force with an onset of aerodynamic asymmetry. The experimental results show the efficiency of the updated multi-degree-of-freedom actively controlled manoeuvre rig in providing insight into complicated aerodynamic effects within the stall region.