Computational Study of F/A-18 E/F Abrupt Wing Stall in the Approach Configuration

Abstract

This computational study evaluated lateral instabilities observed during developmental testing of the preproduction F/A-18E/F in the power-approach configuration. These instabilities, described here as abrupt wing stall (AWS), occurred whenever the airplane exceeded 12-deg angle of attack. The AWS was quickly corrected in flight test by the closure of a vent at the wing root, without understanding its physical cause or cure. Computational solutions with both the vent open and closed provide insight into the likely cause of the instability, revealing two distinctive flow topologies. Specifically, all F/A-18 Hornet models depend upon strong vortex flows from the leading-edge extension (LEX) to provide lift at elevated angle of attack. Flow through the open vent displaces the LEX vortex core inboard and up off the wing's upper surface. This displacement dramatically weakens the flow stability over the wing's entire upper surface. Wind-tunnel studies performed in parallel by another researcher provide quantitative corroboration of the computational solutions. Finally, computational-fluid-dynamics metrics developed within the NASA/Navy/Industry AWS program are applied to determine their validity at this low-speed flight condition and modified to enhance their viability as predictive tools for AWS prone configurations.