High lift configuration of a slotted natural laminar flow airfoil

Abstract

Experiments were performed on a slotted natural laminar flow airfoil, the S414. The slotted natural laminar flow airfoil concept was developed to satisfy design constraints of high maximum lift and low profile drag. A two-element natural laminar flow configuration allows the typical rapid trailing-edge pressure recovery associated with NLF airfoils to be greatly reduced on the fore element, allowing for laminar flow across the entire airfoil upper surface. The slot provides a favorable injection of momentum to the flow over the aft-element upper surface, improving lift generation. However, an unfortunate side effect of the S414 is an abrupt, leading-edge stall type. This investigation focused on the development of a high-lift configuration of the S414 by altering the position of the aft element in order to characterize the feasibility of utilizing the aft element as a high-lift device.

Computational analysis was performed on the S414 to determine suitable aft-element positions for high lift. Two basic repositioning approaches were used; one that deflected the aft element to increase the airfoil camber while maintaining the slot width of the original airfoil, and one that targeted the utilization of the fore-element dumping velocity. In addition, a plain flap was incorporated into the aft element. Performance predictions for the new aft-element configurations were generated using the computational flow solver MSES, and four alternative aft-element riggings were selected for experimental testing. Tests were performed in the University of Illinois 2.8 ft × 4 ft wind tunnel at Re = 1.8 × 10^6, M = 0.18. Using knowledge gained from the experimental tests, a fifth, empirically-derived configuration was developed. Both computational and experimental results indicated that effective utilization of the fore-element dumping velocity results in the largest increase in Cl. Orienting the aft element such that the flow off the fore element was discharged into a region of low pressure over the aft-element upper surface reduced the pressure recovery requirements at the fore-element trailing edge, allowing for enhanced lift production. In addition, a momentum injection was provided to the flow over the aft-element upper surface, promoting increased lift generation by the aft element as well. These techniques coupled with the deflection of the aft-element plain flap resulted in a 34% increase in Cl,max.