Shawn H. Woodson

Unsteady Computations of Abrupt Wing Stall Using Detached-Eddy Simulation

Unsteady computational fluid dynamics calculations are presented of the abrupt wing stall phenomenon on the preproduction F/A-18E using detached-eddy simulation. Detached-eddy simulation combines the efficiency of a Reynolds-averaged turbulence model near the wall with the fidelity of large-eddy simulation in separated regions. Because it uses large-eddy simulation in the separated regions, it is capable of predicting the unsteady motions associated with separated flows. Detached-Eddy Simulation has been applied to predict the unsteady shock motion present on the F/A-18E at transonic speeds over several angles of attack. Solution-based grid adaption is used on unstructured grids to improve the resolution in the separated region

Introduction to the Abrupt Wing Stall (AWS) Program

The Abrupt Wing Stall (AWS) Program has addressed the problem of uncommanded, transonic lateral motions, such as wing drop, with experimental, computational, and simulation tools. Background to the establishment of the AWS program is given as well as program objectives. In order to understand the fundamental flow mechanisms that caused the undesirable motions for a pre-production version of the F/A-18E, steady and unsteady flow field details were gathered from dedicated transonic wind-tunnel testing and computational studies. The AWS program has also adapted a free-toroll (FTR) wind-tunnel testing technique traditionally used for low-speed studies of lateral dynamic stability to the transonic flow regime. This FTR capability was demonstrated first in a proof-of-concept study and then applied to an assessment of four different aircraft configurations. Figures of merit for static testing and for FTR testing have been evaluated for two configurations that demonstrated wing drop susceptibility during full-scale flight conditions (the pre-production F/A-18E and the AV-8B at the extremes of its flight envelope) and two configurations that do not exhibit wing drop (the F/A-18C and the F-16C). Design insights have been obtained from aerodynamic computational studies of the four aircraft configurations and from computations quantifying the impact of the various geometric wing differences between the F/A-18C and the F/A-18E wings. Finally, the AWS program provides guidance for assessing, in the simulator, the impact of experimentally determined lateral activity on flight characteristics before going to flight. SYMBOLS AND ABBREVIATIONS

Accomplishments of the Abrupt-Wing-Stall Program

The Abrupt-Wing-Stall (AWS) Program has addressed the problem of uncommanded lateral motions, such as wing drop and wing rock, at transonic speeds. The genesis of this program was the experience of the F/A-18E/F program in the late 1990s, when wing drop was discovered in the heart of the maneuver envelope for the preproduction aircraft. Although the F/A-18E/F problem was subsequently corrected by a leading-edge flap scheduling change and the addition of a porous door to the wing fold fairing, the AWS program was initiated as a national response to the lack of technology readiness at the time of the F/A-18E/F development program. The AWS program objectives were to define causal factors for the F/A-18E/F experience, to gain insights into the flow physics associated with wing drop, and to develop methods and analytical tools so that future programs could identify this type of problem before going to flight test. The major goals of the AWS Program, the status of the technology before the program began, the program objectives, the accomplishments, and the impacts are reviewed. Lessons learned are presented for the benefit of programs that must assess whether a future vehicle will have uncommanded lateral motions before going to flight test.

Understanding Abrupt Wing Stall with Computational Fluid Dynamics.

We describe the computational-fluid-dynamics efforts and lessons learned during the four-year Abrupt Wing Stall national research program. The paper details the complex nature of the transonic flows encountered by modern U.S. fighter and attack aircraft during transonic maneuvering conditions. Topics include grid resolution, computational memory and processor requirements, turbulence modeling, steady and unsteady calculations, and Reynolds-averaged Navier-Stokes solutions compared with detached-eddy simulations for this highly complex, viscously dominated, shock-induced, massively separated class of flow. Examples include results obtained for F/A-18C, AV-8B, preproduction F/A-18E, and F-16C aircraft undergoing transonic maneuvering conditions. Various flap settings have been modeled and the computational results compared with extensive wind-tunnel data

RECOMMENDATIONS FOR CFD PROCEDURES FOR PREDICTING ABRUPT WING STALL

This paper summarizes the lessons learned from the computational fluid dynamics (CFD) effort of the joint NASA/Navy/Air Force Abrupt Wing Stall (AWS) Program, discusses the results, and makes recommendations for approaches to be used in future aircraft programs to identify uncommanded lateral characteristics early in the design phase of an aircraft development program. The discussion also suggests CFD procedures and figures of merit for use in predicting and quantifying AWS tendencies and vulnerabilities of the proposed designs. Topics addressed include critical parameters that can be used to identify uncommanded lateral activity in the transonic flow regime, and the geometric parameters that were the primary contributors to the adverse lateral activity observed on pre-production F/A-18E/F aircraft. In addition, differences in steady-state and averaged time-accurate CFD solutions for the F/A18E in the AWS region of interest are analyzed and compared with existing unsteady experimental data to determine the utility and accuracy of the unsteady approach. Lastly, proposed CFD figures of merit are critically evaluated as indicators of possible AWS tendencies, and screening procedures for the identification of AWS are suggested.

Recommendations for Computational-Fluid-Dynamics Procedures for Predicting Abrupt Wing Stall

We summarize the lessons learned from the computational-fluid-dynamics effort of the joint NASA/Navy/Air Force Abrupt Wing Stall Program, discusses the results, and makes recommendations for approaches to be used in future aircraft programs to identify uncommanded lateral characteristics early in the design phase of an aircraft development program. The discussion also suggests procedures and figures of merit for use in predicting and quantifying rapid and severe wing-stall tendencies and vulnerabilities of the proposed designs. Topics addressed include critical parameters that can be used to identify uncommanded lateral activity in the transonic flow regime and the geometric parameters that were the primary contributors to the adverse lateral activity observed on preproduction F/A-18E/F aircraft. In addition, differences in steady-state and averaged time-accurate computational solutions for the F/A-18E in the abrupt-wing-stall region of interest are analyzed and compared with existing unsteady experimental data to determine the utility and accuracy of the unsteady approach. Lastly, proposed computational figures of merit are critically evaluated as indicators of possible abrupt separation tendencies, and screening procedures for the identification of those tendencies are suggested.

Understanding Abrupt Wing Stall with CFD

This paper describes the Computational Fluid Dynamics (CFD) efforts and lessons learned during the four-year Abrupt Wing Stall national research program. The paper details the complex nature of the transonic flows encountered by modern U.S. fighter and attack aircraft during transonic maneuvering conditions. Topics include grid resolution, computational memory and CPU requirements, turbulence modeling, steady and unsteady calculations, and Reynolds-Averaged-Navier-Stokes solutions compared with Detached Eddy Simulations for this highly complex, viscously-dominated, shock-induced, massively-separated class of flow. Examples include results obtained for F/A-18C, AV-8B, pre-production F/A-18E, and F-16N aircraft undergoing transonic maneuvering conditions. Various flap settings have been modeled and the CFD results compared with extensive wind tunnel data. The comparisons illustrate the results obtained from both structured and unstructured CFD codes. The utility and accuracy of the various computational solvers is evaluated by qualitative comparisons of surface oil flow and pressure sensitive paint results obtained in wind tunnels for some of the models as well as by detailed quantitative pressure coefficient data where experimental results exist. Static lift coefficients are compared between CFD codes as well as the experimental data for each of the aircraft considered in this study.

Accomplishments of the Abrupt Wing Stall (AWS) Program and Future Research Requirements

The Abrupt Wing Stall (AWS) Program has addressed the problem of uncommanded lateral motions, such as wing drop and wing rock, at transonic speeds. The genesis of this Program was the experience of the F/A-18E/F Program in the late 199Os, when wing drop was discovered in the heart of the maneuver envelope for the pre-production aircraft. While the F/A-18E/F problem was subsequently corrected by a leading-edge flap scheduling change and the addition of a porous door to the wing fold fairing, the AWS Program was initiated as a national response to the lack of technology readiness available at the time of the F/A-18E/F Development Program. The AWS Program objectives were to define causal factors for the F/A-18E/F experience, to gain insights into the flow physics associated with wing drop, and to develop methods and analytical tools so that future programs could identify this type of problem before going to flight test. The paper reviews, for the major goals of the AWS Program, the status of the technology before the program began, the program objectives, accomplishments, and impacts. Lessons learned are presented for the benefit of future programs that must assess whether a vehicle will have uncommanded lateral motions before going to flight test. Finally, recommended future research needs are presented in light of the AWS Program experience.