A study of gurney flaps and their influence on an airfoil in ground effect
As an aircraft wing approaches the ground, within a chord-length distance, it experiences an increase in lift and a decrease in drag. This is known as ―ground effect.‖ Furthermore, a Gurney flap positioned perpendicular to an airfoil’s trailing edge (with a 1% to 5% chord length) will also increase the lift. This thesis addresses the following question: Are the combined effects of these phenomena additive to any significant degree? Specifically, can the addition of a Gurney flap to an airfoil during ground effect increase the lift substantially, and are there any tradeoffs to be considered? Notable performance improvements could impact wing-in-ground-effect (WIG) aircraft. Such vehicles fly close to the ground (in ground effect [IGE]) to produce more lift than the wing would normally (out of ground effect [OGE]). WIG aircraft can be a small one-person aircraft or large transport aircraft like the Russian Ekranoplans. Currently, because these aircraft fly only IGE, they are literally inches off the surface of the ground. Further Gurney flap research could be beneficial for improving similar future aircraft. If the effects are additive, then WIG aircraft could be constructed to fly more safely at greater heights. This thesis was completed in three phases. First, a literature review was conducted to better understand ground effect and Gurney flaps. Interestingly, only one article was discovered that discussed combining both phenomena (for race car down-force enhancement applications). Second, basic and available two-dimensional computer codes were evaluated as potential tools for analysis and design. Only one was capable of analyzing an airfoil with a Gurney flap in ground effect. Third, v experiments were conducted in the low-speed wind tunnel at Wichita State University to directly evaluate the effect of a Gurney flap on airfoil performance in ground effect. Based on the results of this investigation, the following can be stated: JavaFoil, a computer program available online, reasonably models airfoil performance trends with Gurney flaps while in ground effect. Experiments indicate that the addition of a Gurney flap to an S7055 airfoil improves its lift, both in and out of ground effect (by up to 68%). Interestingly, the effects vary with angle of attack and are most pronounced at small values. But there are also tradeoffs to consider. Specifically, an increase in drag (approximately 48%) and a more nose-down pitching moment (about 85%) accompanies the lift improvement for the S7055 airfoil used for testing.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.