Experimental investigation of multielement airfoil lift hysteresis due to flap rigging

A study is reported on a particular type of lift hysteresis discovered while developing experimental geometry optimization techniques for high-lift airfoils. A modern three-element airfoil model with a remotely actuated flap was designed, tested, and used in low-speed wind tunnel experiments to investigate optimum flap positioning based on lift. Hysteresis in lift as a function of flap position was discovered when tests were conducted using continuous flow conditions. It was shown that optimum or near-optimum lift coefficients determined using continuous flow conditions exist over an extended range of flap positions when compared to those determined using traditional intermittent conditions.