Robert M. Hall

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.

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.

AWS Figure of Merit (FOM) Developed Parameters from Static, Transonic Model Tests

This paper provides an approach to answer the question of whether one can rely solely on static data taken during a transonic model test to provide the certainty needed that a new aircraft will or will not have Abrupt Wing Stall (AWS) events during its flight operations. By comparing traditional- and alternate-static-Figures of Merit (FOMs) with the Free-To-Roll (FTR) response data, a rational basis for assessing the merits of using standard testing techniques for the prediction of AWS events has been established. Using the FTR response data as a standard, since these results compare well with flight, the conclusion from this study is that neither traditional nor alternate FOMs can be trusted to provide an indication as to whether a configuration will or will not have AWS tendencies. Even though these FOMs may flag features which have a high degree of correlation with the FTR response data, there are as many or more of these FOM flagged features which do not correlate. Thus, one needs to use the FTR rig to assess AWS tendencies on new configurations.

Transonic Free-to-Roll Analysis of Abrupt Wing Stall on Military Aircraft

Transonic free-to-roll and static wind-tunnel tests for four military aircraft-the AV-8B, the F/A-18C, the preproduction F/A-18E, and the F-16C-have been analyzed. These tests were conducted in the NASA Langley 16-Foot Transonic Tunnel as a part of the NASA/Navy/Air Force Abrupt Wing Stall Program. The objectives were to evaluate the utility of the free-to-roll test technique as a tool for predicting areas of significant uncommanded lateral motions and for gaining insight into the wing-drop and wing-rock behavior of military aircraft at transonic conditions. The analysis indicated that the free-to-roll results agreed well with flight data on all four models. A wide range of motions-limit-cycle wing rock, occasional and frequent damped wing drop/rock and wing-rock divergence-were observed

Development of a Free-To-Roll Transonic Test Capability

NASA Langley Research Center, Hampton, VirginiaAIAA-2003-0749ABSTRACTAs part of the NASA/Navy Abrupt Wing Stall Program, a relatively low-cost, rapid-access wind-tunnelfree-to-rollrig was developed. This rig combines the use of conventional models and test apparatuses toevaluate both transonic performance and wing-drop/rock tendencies in a single tunnel entry. A descriptionof the test hardware as well as a description of the experimental procedures is given. The free-to-roll testrig has been used successfully to assess the static and dynamic characteristics of three differentconfigurations--two configurations that exhibit uncommanded lateral motions, (pre-production F/A-18E andAV-8B),and one that did not (F/A-18C).SYMBOLS AND ABBREVIATIONS INTRODUCTIONAWS

RECOMMENDED EXPERIMENTAL PROCEDURES FOR EVALUATION OF ABRUPT WING STALL CHARACTERISTICS

This paper presents a review of the experimental program under the Abrupt Wing Stall (AWS) Program. Candidate figures of merit from conventional static tunnel tests are summarized and correlated with data obtained in unique free-to-roll tests. Where possible, free-to-roll results are also correlated with flight data. Based on extensive studies of static experimental figures of merit in the Abrupt Wing Stall Program for four different aircraft configurations, no one specific figure of merit consistently flagged a warning of potential lateral activity when actual activity was seen to occur in the free-to-roll experiments. However, these studies pointed out the importance of measuring and recording the root mean square signals of the force balance.

Development of a transonic free-to-roll test capability

As part of the NASA/Navy Abrupt Wing Stall Program, a relatively low-cost, rapid-access, free-to-roll test rig was developed on which conventional high-strength wind-tunnel models can be used to evaluate both transonic performance and wing-drop/rock behavior in a single tunnel entry. The overall objective was to demonstrate the utility of the free-to-roll test technique as a tool for identifying areas of significant uncommanded lateral activity during ground testing and for gaining insight into the wing-drop/rock behavior of military aircraft at transonic conditions. A description of the test hardware as well as a description of the experimental procedures is given. The free-to-roll test rig has been used successfully to assess the static and dynamic characteristics of four different configurations—two configurations that exhibited uncommanded lateral motions inflight (preproduction F/A-18E and AV-8B) and two that did not (F/A-18C, F-16C). Excellent agreement between free-to-roll results and flight was obtained for those configurations where flight data were available.

Review and Recommended Experimental Procedures For Evaluation Of Abrupt Wing Stall Characteristics

Ar eview of the experimental program for four different aircraft configurations conducted as part of the Abrupt Wing Stall Program has been made. Several candidate figures of merit from conventional static-tunnel tests are summarized and correlated with data obtained in unique free-to-roll tests. The conclusion from this study is that these figures of merit can by themselves give some indication as to whether an aircraft would experience uncommanded lateral activity caused by abrupt wing stall. However, no one specific figure of merit consistently flagged a warning of potential lateral activity when actual activity was seen to occur in the free-to-roll experiments. In fact, they yielded as many or more false indications of lateral activity then were seen in the free-to-roll response data. Excellent agreement between free-to-roll results and flight was obtained for those configurations where flight data were available.

Transonic Experimental Observations of Abrupt Wing Stall on an F/A-l8E Model

A transonic wind tunnel test of an 8% F/A-18E model was conducted in the NASA Langley Research Center (LaRC) 16 ft Transonic Tunnel (16-ft TT) to investigate on-surface flow physics during stall. The technical approach employed focused on correlating static (or time-averaged) and unsteady wind-tunnel test data to the unsteady wing-stall events using force, moment, pressure, and pressure-sensitive-paint measurements. This paper focuses on data obtained on the pre-production configuration of the F/A-18E aircraft at Mach number of 0.90. The flow unsteadiness occurring on the wing as the wing went through the stall process was captured using the time history of balance and pressure measurements and by calculating the root mean square (RMS) for a number of instrument signals. The second step was to gather global perspectives on the pressures influencing the wing stall process. The abrupt wing stall experienced by the 8% F/A-18E Model was observed to be an unsteady event triggered by the rapid advancement of separation, which had migrated forward from the trailing edge, to the leading-edge flap hingeline over a very small increment in angle of attack. The angle of attack at which this stall occurred varied, from run to run, over an 1 degree increment. The abrupt wing stall was observed, using pressure-sensitive-paint, to occur simultaneously on both wing panels or asymmetrically. The pressure-sensitive paint data and wing-root bending moment data were essential in providing insight to the flow structures occurring over the wing and the possible asymmetry of those flow structures. A repeatability analysis conducted on eight runs of static data provided a quick and inexpensive examination of the unsteady aerodynamic characteristics of abrupt wing stall. The results of the repeatability analysis agreed extremely well with data obtained using unsteady measurement techniques. This approach could be used to identify test conditions for more complex unsteady data measurements using special instrumentation.