Investigation of aerodynamic hysteresis on 5-DOF manoeuvre rig in wind tunnel tests

This paper describes an innovative control-based dynamic test method that has been developed to capture nonlinear and unsteady aerodynamic effects, such as static hysteresis, in wind tunnel testing. An aircraft model is mounted on a five-degree-of-freedom manoeuvre rig in a low-speed wind tunnel. A high-speed wireless message collection-distribution network, specially developed for the rig, allows distributed motion parameters to be recorded with synchronized time-stamps in microseconds for phase-sensitive analysis; a low command-response loop latency of under ten milliseconds makes real-time feedback control in closed loop practicable.

Self-excited oscillations in the aircraft model pitch degree of freedom (DOF) is the original focus of the study, aimed at revealing nonlinear aerodynamics at stall conditions. Thereafter, simple elevator feedback was introduced to stabilise the aircraft model through a range of angles of attack. By switching on/off of feedback controls, the instabilities and control effectiveness on equilibrium branches are varied, and responses that switch between equilibria and limit cycle attractors are analysed.

Slow ramp elevator inputs with feedback are tested first in a pitch-only configuration to continuously track bifurcations of equilibrium points, on which two hysteresis loops in different incidence ranges are found. Then, a 2DOF (model pitch plus arm pitch) configuration is used to provide both heave and pitch degrees of freedom of the aircraft model. Tests in this set-up reveal existence of static hysteresis loops in the lift dependence on angle of attack, with indication of onset of asymmetric flow separation.

Furthermore, by also releasing aircraft roll and yaw degrees of freedom and gradually adding corresponding control augmentation, nonlinear discontinuous and asymmetric aerodynamic effects are revealed via by a slow elevator sweep in this 4DOF conguration.

Overall, by introducing feedback control to multi-DOF wind tunnel tests, the stability of the released degrees of freedom could be augmented and aircraft model instabilities suppressed, which provides benefits in tracking and observing equilibrium branches. A static hysteresis phenomenon has been observed in ramp input tests with feedback controls, associated with discontinuous switching between different steady state solution branches.