Characterizing Control Effectiveness with a Flexible Control Surface

Lifting surface flexibility has been recognized as an important parameter in aircraft maneuvering performance since the early days of flight, causing typical aircraft configurations to experience degraded control effectiveness or even control reversal at high speeds. Traditional aeroelastic analysis methods focus on assessment of wing torque box stiffness assuming rigid control surface attachments, with a classical result for 2D airfoils being that the reversal speed is independent of elastic axis position. Over the last several decades, various observations have been made regarding the significant impact that control surface flexibility can have on control effectiveness, but there is very little published literature on expected trends associated with a flexible control surface. The contributions of this work are to characterize control effectiveness trends associated with a flexible trailing edge control surface on a 2D airfoil. It is shown that while reversal speeds are independent of elastic axis location, the control effectiveness of the airfoil below the reversal speed is highly dependent on the elastic axis location, and can be sensitive to control surface attachment stiffness. Additionally, a parameterization is defined which allows for comparison of wing torque box stiffness versus control surface stiffness contributions to overall airfoil control effectiveness.