James Chung

Computational Methods for Stability and Control (COMSAC): The Time Has Come

Powerful computational fluid dynamics (CFD) tools have emerged that appear to offer significant benefits as an adjunct to the experimental methods used by the stability and control community to predict aerodynamic parameters. The decreasing costs for and increasing availability of computing hours are making these applications increasingly viable as time goes on and the cost of computing continues to drop. This paper summarizes the efforts of four organizations to utilize high-end computational fluid dynamics (CFD) tools to address the challenges of the stability and control arena. General motivation and the backdrop for these efforts will be summarized as well as examples of current applications.

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

Study of Icing Effects on Performance and Controllability of an Accident Aircraft

The activities at NASA John H. Glenn Research Center at Lewis Field in support of a National Transportation Safety Board accident investigation are described. An icing research tunnel (IRT) test was conducted in January 1998. Most conditions for the test were based on raw and derived data from the flight data recorder recovered from the accident and on the current understanding of the meteorological conditions near the accident. Using a two-dimensional Navier-Stokes code, the flowfield and resultant lift and drag were calculated for the wing section with various ice shapes accreted in the IRT test. Before the final calculations could be performed extensive examinations of geometry smoothing and turbulence modeling were conducted

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.

Transonic computational fluid dynamics calculations on preproduction F/A-18E for stability and control

Computational fluid dynamics was used to predict the longitudinal and lateral/directional stability and control characteristics of an 8%-scale wind tunnel model of the preproduction F/A-18E Super Hornet at two transonic Mach numbers without any prior knowledge of existing wind tunnel or flight test data. The tetrahedral unstructured software system was used to generate and analyze grids during this computational study. The longitudinal stability and control characteristics of the aircraft were evaluated using three different horizontal tail deflections. Before evaluating nonzero horizontal tail deflections, coarse, medium and fine grids of the preproduction F/A-18E with a horizontal tail deflection of 0 deg were used in a grid resolution study to determine the grid density that was required to accurately calculate the forces and moments of the aircraft. The grid resolution study indicated that the medium grid was adequate at Mach 0.8 whereas the fine grid was necessary at Mach 0.9. The medium and fine grids with tail deflections of -6 and 6 deg were then generated and analyzed at Mach 0.8 and Mach 0.9, respectively, to determine the longitudinal stability and control characteristics of the aircraft. The lateral/ directional stability and control characteristics of the preproduction F/A-18E were evaluated using a range of sideslip angles for several different angles of attack at Mach 0.8 and 0.9. The computational results compared very favorably to the existing wind tunnel data.

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.

Navier-Stokes analysis of flowfield characteristics of an ice-contaminated aircraft wing

An analytical study was performed as part of the NASA Lewis support of a National Transportation Safety Board (NTSB) aircraft accident investigation. The study was focused on the performance degradation associated with ice contamination on the wing of a commercial turbo-prop-powered aircraft. Based upon the results of an earlier numerical study conducted by the authors, a prominent ridged-ice formation on the subject aircraft wing was selected for detailed flow analysis using 2-dimensional (2-D), as well as, 3-dimensional (3-D) Navier-Stokes computations. This configuration was selected because it caused the largest lift decrease and drag increase among all the ice shapes investigated in the earlier study. A grid sensitivity test was performed to find out the influence of grid spacing on the lift, drag, and associated angle-of-attack for the maximum lift (C(sub lmax)). This study showed that grid resolution is important and a sensitivity analysis is an essential element of the process in order to assure that the final solution is independent of the grid. The 2-D results suggested that a severe stability and control difficulty could have occurred at a slightly higher angle-of-attack (AOA) than the one recorded by the Flight Data Recorder (FDR). This stability and control problem was thought to have resulted from a decreased differential lift on the wings with respect to the normal loading for the configuration. The analysis also indicated that this stability and control problem could have occurred whether or not natural ice shedding took place. Numerical results using an assumed 3-D ice shape showed an increase of the angle at which this phenomena occurred of about 4 degrees. As it occurred with the 2-D case, the trailing edge separation was observed but started only when the AOA was very close to the angle at which the maximum lift occurred.

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.

Computational Study of the Abrupt-Wing-Stall Characteristics of F/A-18E and F-16C

Steady-state computational-fluid-dynamic (CFD) simulations are used to gain an understanding of the physics behind the abrupt-wing-stall (AWS) phenomenon and to arrive at static figures of merit (FOMs). Navier‐Stokes solutions are obtained using the NASA Langley Research Center developed TetrUSS simulation suite, which is based on tetrahedral, unstructured grids. The physics of the AWS phenomenon is understood by comparing CFD simulation results on two aircraft; a preproduction F/A-18E configuration, which exhibits AWS phenomenon under certain geometric and flow conditions, and an F-16C aircraft configuration that does not. The computational results are used to understand the possible causes of AWS by comparing the detailed flowfields between the two configurations under a variety of flow conditions. Based on this approach, a number of static figures of merit are developed to predict AWS. The potential FOMs include the break in the lift and wing-root bending moment vs angle of attack α and the rate of change of sectional lift with respect to α .A companion paper describes a similar study for the AV-8B Harrier and F/A-18C.

Development and Assessment of CFD Methods for Integrated Simulation of Air Vehicle Stability and Control

The Naval Air Systems Command (NAVAIR), NASA Langley Research Center, AS&M, and GDIT inc. have joined together in a cooperative effort to develop a computational modeling and simulation suite capable of handling air vehicle stability. The funding and computer resources were provided by the DoD High Performance Computing Modernization Office (HPCMO) under the Common High Performance Computing Software Support Initiative (CHSSI). This project is under Collaborative Simulation and Testing (CST) portfolio to provide scalable software for military application to reduce risk in weapons system development and to provide information to senior decision makers throughout the life cycle of the system. This paper summarizes the three year effort to develop a methodology to utilize CFD analysis for an improved test and evaluation process on fixed-wing air vehicle stability and control problems and to demonstrate an integrated analysis capability of airframe-inlet-compressor interaction for a high-performance military jet.