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

What is it about?

Aircraft that enter a region of the flight envelope in which the incidence to the freestream flow is large experience wing stall - where the flow starts to separate from the surface. Under these conditions the aerodynamic loads become nonlinear and often unsteady (time-dependent) too. The latter sometimes causes hysteresis (double-valued relationship between the load and the incidence). The response of the aircraft is affected by these complex flow phenomena but they are difficult to predict and to observe in a wind tunnel. Usually, in addition to being time-dependent, the stalled flow becomes asymmetric. This paper reports on recent advances with a multi-degree-of-freedom dynamic wind tunnel rig - the 'manoeuvre rig' - designed to exhibit such behaviour in experiments in a repeatable and measurable way. The paper reveals static hysteresis and asymmetry in the stalled flow over an aircraft model, providing deep insight into its nonlinear oscillatory behaviour previously reported. Such evidence of sensitive flow structures and their effects in multiple degrees of freedom are extremely difficult to observe in traditional wind tunnel testing. This rig therefore opens new opportunities to testing of aircraft models under such conditions such that the aerodynamic causes of their nonlinear responses can be better understood.

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