Experimental characterization and control of airfoil lift under the influence of vortical gusts

This thesis demonstrates mitigation of the effect of artificial wind gusts on the lift coefficient of a DLR-F15 airfoil through dynamic trailing-edge flap scheduling in a wind tunnel environment. Atmospheric turbulence, where discrete encounters are often referred to as wind gusts, has been of interest to the aircraft industry for more than one century. Several incidents in aviation due to severe gust encounters, including the loss of entire aircraft, have been reported in the literature and media. The active mitigation of aircraft loads due to wind gust encounters promises to increase ride safety, passenger comfort and reduce the overall cost due to weight savings. Compared to similar investigations in the literature, the experiments in this contribution are conducted at comparably high Reynolds number, flow speed and flap motion rates. An electric servomotor system satisfies the need for highly dynamic actuation of the trailing-edge flap. The airfoil lift coefficient is computed from measurements of the airfoil surface pressure along the mid-span. Therefore, highly accurate steady, as well as time-resolved high frequency measurements of the surface pressure are taken. Artificial gusts are generated in the wind tunnel upstream of the research airfoil through fast pitching motions of a NACA-0021 airfoil. Real-time control of the airfoil lift coefficient is achieved through a combination of feedback, and feedforward control based on disturbance measurements with a hot wire anemometer. In order to obtain a full description of the system, the dynamic characteristics of all system components are analyzed and modeled before the control experiments. The airfoil lift response to unsteady actuation of the trailing-edge flap is investigated under sinusoidal motion of the flap at different rates and amplitudes. The findings from these experiments reveal some of the underlying flow physics in the aerodynamics of rapidly deflected control surfaces. Two models are calibrated and investigated in order to reflect the response in the lift coefficient to unsteady flap motions: a nonlinear model, as well as a simpler linear model. Another set of linear models is calibrated in order to reflect the response in the lift coefficient to the measured disturbance velocities. The characteristics of the inert flap actuation system are modeled by a simple lag element. Synthesis of these models yields the control loop, which is implemented and run on a standalone hardware controller in real-time. For the best control strategies, a great reduction in the intensity of the gust-induced peaks in the airfoil lift coefficient is reported. Further quantification and analysis of the measurement results reveals the pros and cons of different control strategies, and the impact of variations in the experimental parameters, such as the airfoil angle of attack.