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Hall, Robert M.

Publications and source records attributed to Hall, Robert M..

36 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

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.

McMillin, S. Naomi↗
NASA NTRS2003

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 199O's, 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.

Hall, Robert M.↗
NASA NTRS2000

Review of Skin Friction Measurements Including Recent High-Reynolds Number Results from NASA Langley NTF

This paper reviews flat plate skin friction data from early correlations of drag on plates in water to measurements in the cryogenic environment of The NASA Langley National Transonic Facility (NTF) in late 1996. The flat plate (zero pressure gradient with negligible surface curvature) incompressible skin friction at high Reynolds numbers is emphasized in this paper, due to its importance in assessing the accuracy of measurements, and as being important to the aerodynamics of large scale vehicles. A correlation of zero pressure gradient skin friction data minimizing extraneous effects between tests is often used as the first step in the calculation of skin friction in complex flows. Early data compiled by Schoenherr for a range of momentum thickness Reynolds numbers, R(sub Theta) from 860 to 370,000 contained large scatter, but has proved surprisingly accurate in its correlated form. Subsequent measurements in wind tunnels under more carefully controlled conditions have provided inputs to this database, usually to a maximum R(sub Theta) of about 40,000. Data on a large axisymmetric model in the NASA Langley National Transonic Facility extends the upper limit in incompressible R(sub Theta) to 619,800 using the van Driest transformation. Previous data, test techniques, and error sources ar discussed, and the NTF data will be discussed in detail. The NTF Preston tube and Clauser inferred data accuracy is estimated to be within -2 percent of a power-law curve fit, and falls above the Spalding theory by 1 percent at R(sub Theta) of about 600,000.

Watson, Ralph D.↗
NASA NTRS1998

Reynolds Number Effects at High Angles of Attack

Lessons learned from comparisons between ground-based tests and flight measurements for the high-angle-of-attack programs on the F-18 High Alpha Research Vehicle (HARV), the X-29 forward-swept wing aircraft, and the X-31 enhanced fighter maneuverability aircraft are presented. On all three vehicles, Reynolds number effects were evident on the forebodies at high angles of attack. The correlation between flight and wind tunnel forebody pressure distributions for the F-18 HARV were improved by using twin longitudinal grit strips on the forebody of the wind-tunnel model. Pressure distributions obtained on the X-29 wind-tunnel model at flight Reynolds numbers showed excellent correlation with the flight data up to alpha = 50 deg. Above (alpha = 50 deg. the pressure distributions for both flight and wind tunnel became asymmetric and showed poorer agreement, possibly because of the different surface finish of the model and aircraft. The detrimental effect of a very sharp nose apex was demonstrated on the X-31 aircraft. Grit strips on the forebody of the X-31 reduced the randomness but increased the magnitude of the asymmetry. Nose strakes were required to reduce the forebody yawing moment asymmetries and the grit strips on the flight test noseboom improved the aircraft handling qualities.

Fisher, David F.↗
NASA NTRS1998

Evaluation of Gritting Strategies for High Angle of Attack Using Wind Tunnel and Flight Test Data for the F/A-18

A subsonic study of high-angle-of-attack gritting strategies was undertaken with a 0.06-scale model of the F/A-18, which was assumed to be typical of airplanes with smooth-sided forebodies. This study was conducted in the Langley 7- by 10-Foot High-Speed Tunnel and was intended to more accurately simulate flight boundary layer characteristics on the model in the wind tunnel than would be possible by using classical, low-angle-of-attack gritting on the fuselage. Six-component force and moment data were taken with an internally mounted strain-gauge balance, while pressure data were acquired by using electronically scanned pressure transducers. Data were taken at zero sideslip over an angle-of-attack range from 0 deg to 40 deg and, at selected angles of attack, over sideslip angles from -10 deg to 10 deg. Free-stream Mach number was fixed at 0.30, which resulted in a Reynolds number, based on mean aerodynamic chord, of 1.4 x 10(exp 6). Pressure data measured over the forebody and leading-edge extensions are compared to similar pressure data taken by a related NASA flight research program by using a specially instrumented F/A-18, the High-Alpha Research Vehicle (HARV). Preliminary guidelines for high-angle-of-attack gritting strategies are given.

Hall, Robert M.↗
NASA NTRS1998

Impact of Fuselage Cross Section on the Stability of a Generic Fighter

Many traditional data bases, which involved smooth-sided forebodies, are no longer relevant for designing advanced aircraft. The current work provides data on the impact of chined-shaped fuselage cross section on the stability of a generic fighter configuration. Two different chined-shaped fuselages were tested upright and inverted. It was found that a fuselage with a 30" included chine angle resulted in significantly higher values of fuselage with a 100" included chine angle. This difference was attributed to the more beneficial vortical interaction between the stronger forebody vortices coming off of the sharper chine edges and the wing vortices. The longitudinal stability of the configuration with the sharper chine angle was also better because, based on pressures and flow visualization, the vortex burst over the wing was delayed until significantly higher values of a. Unstable rolling moment derivatives were also delayed to higher values of a for the sharper chine angle cross section. Furthermore, it was found that directional stability of both of the upright configurations, which had larger lofts in cross section above the chine lines than below the chine lines. was better than for the inverted configurations.

Hall, Robert M.↗
NASA NTRS1996

Overview of HATP Experimental Aerodynamics Data for the Baseline F/A-18 Configuration

Determining the baseline aerodynamics of the F/A-18 was one of the major objectives of the High-Angle-of-Attack Technology Program (HATP). This paper will review the key data bases that have contributed to our knowledge of the baseline aerodynamics and the improvements in test techniques that have resulted from the experimental program. Photographs are given highlighting the forebody and leading-edge-extension (LEX) vortices. Other data representing the impact of Mach and Reynolds numbers on the forebody and LEX vortices will also be detailed. The level of agreement between different tunnels and between tunnels and flight will be illustrated using pressures, forces, and moments measured on a 0.06-scale model tested in the Langley 7- by 10-Foot High Speed Tunnel, a 0.16-scale model in the Langley 30- by 60-Foot Tunnel, a full-scale vehicle in the Ames 80- by 120-Foot Wind Tunnel, and the flight F/A-18 High Alpha Research Vehicle (HARV). Next, creative use of wind tunnel resources that accelerated the validation of the computational fluid dynamics (CFD) codes will be described. Lastly, lessons learned, deliverables, and program conclusions are presented.

Hall, Robert M.↗
NASA NTRS1990

Experimental investigation of a new device to control the asymmetric flowfield on forebodies at large angles of attack

An exploratory experimental investigation of a new device to control the asymmetric flowfield on forebodies at large angles of attack has been conducted. The device is a rotatable forebody tip, which varies in cross section from circular at its base to elliptic at its tip. The device itself extends over a small portion of the aircraft or missile forebody. The device provides two important improvements. First, it replaced the normally random behavior of the nose side force as a function of nose tip orientation with a predictable and generally sinusoidal distribution and, second, the device showed promise for use as part of a vehicle control system, to be deflected in a prescribed manner to provide additional directional control for the vehicle. The device was tested on a cone/cylinder model having a 10 deg semiapex angle and on a 3.0 caliber tangent ogive model, each with a base diameter of 3.5 in, for angles of attack from 30 to 60 deg. Data were taken from 3 circumferential rows of pressure taps on each model at a Reynolds number of 84,000 based on cylinder diameter and by a helium-bubble flow visualization technique at a Reynolds number of 24,000.

Moskovitz, Cary A.↗
NASA NTRS1990

Navier-Stokes computations of vortex asymmetries controlled by small surface imperfections

Navier-Stokes analyses are employed to explore the driving mechanisms controlling asymmetric vortical flows with Re(D) = 0.8 million (Reynolds number based on maximum diameter) over a 3.5 caliber tangent-ogive cylinder at large angles of attack (alpha = 20, 30, and 40 degrees). All flowfield results are steady-state solutions to the three-dimensional, incompressible Navier-Stokes equations in the thin-layer approximation. The numerical results are temporally and spatially fully converged, and are in good agreement with experimental data. The major findings are: (1) for alpha not less than 30 degrees, the vortex flows are genuinely asymmetric yet recurrent; (2) asymmetric vortex patterns are highly sensitive toward such parameters as machine accuracy, grid topology, etc., unless triggered by a slight deformation (similar to an out-of-round nose tip) in the neighborhood of the apex; and (3) for alpha = 20 degrees, the flow is symmetric for both circular and elliptic cross-sectional shapes of the nose tip.

Hartwich, PETER-M.↗
NASA NTRS1989

Experimental investigation of the F/A-18 vortex flows at subsonic through transonic speeds

A subsonic-to-transonic speed wind tunnel study has been conducted to deepen understanding of the nature and possibilities for control of vortical flows associated with wing leading edge extensions (LEXs) on the F/A-18 aircraft; these vortical phenomena encompass vortex breakdown and vortex interactions with vertical stabilizers. Wind tunnel results were correlated with in-flight flow visualizations as well as handling-qualities trend data from a USN F-18 with LEX fences to improve the vertical tail buffet environment. Attention is given to the sensitivity of the vortex flows to Re and Mach numbers, reduced vertical tail excitation due to the presence of a LEX fence, and the interpretation of off-body flow visualizations.

Erickson, Gary E.↗
NASA NTRS1989

Effects of nose bluntness, roughness, and surface perturbations on the asymmetric flow past slender bodies at large angles of attack

The effects of such geometric perturbations as variations of model-tip sharpness and roughness, as well as discrete surface perturbations, on the asymmetric flow past slender bodies is experimentally investigated for the cases of a cone/cylinder model having a 10-deg semiapex angle and a 3.0-caliber tangent ogive model. Both models have base diameters of 3.5 inches, and were tested in laminar flow conditions at angles-of-attack in the 30-60 deg range. Single, discrete roughness elements were represented by beads; bead effectiveness was judged on the basis of the extent to which they affected the flowfield in various conditions.

Moskovitz, Cary A.↗
NASA NTRS1988

Effects of surface perturbations on the asymmetric vortex flow over a slender body

An experimental investigation of the effects of surface perturbations on the asymmetric flow past a slender body has been conducted for laminar flow conditions. Beads with diameters ranging from 3/32 to 12/32 in. were attached near the apex of a cone/cylinder model having a base diameter of 3.5 in. and a cone semiapex angle of 9 deg at an angle of attack of 40 deg in an attempt to alter the sense of the asymmetric vortex flow pattern. Circumferential position as well as longitudinal location were varied to determine the most effective bead position. Whether or not the beads were effective in controlling the magnitude and direction of the vortex asymmetries was determined by 3 circumferential rows of pressure taps and by a helium-bubble flow visualization technique. The most effective circumferential position was found to be approximately 140 deg from the windward ray. While holding this circumferential position constant, the effect of bead size at three stations further along the body was also investigated. It was found that the size of the bead necessary to reverse the asymmetry increased more rapidly than the growth in cylinder radius. In general, these results indicate that discrete geometric imperfections on a body's surface can force asymmetry in a given direction if they are sufficiently large relative to the local radius.

Moskovitz, Cary A.↗
NASA NTRS1987

Remote detection of boundary-layer transition by an optical system

This instrument development program was funded because of the urgent need to measure boundary-layer transition in wind tunnels. In the course of this development program, a prototype was designed, built, and tested. Recent transonic experiments in the Boeing Model Transonic Wind Tunnel show that the interferometer results correlate very well with sublimating chemical tests.

Hall, Robert M.↗
NASA NTRS1987

Forebody and missile side forces and the time analogy

A review of the 2-D, impulsively started cylinder literature is given to identify a number of physical parameters that can impact the nondimensional time at which vortex asymmetries begin. These parameters are then related to 3-D testing and explain the lack of data repeatability. The implications of these parameters on 3-D computational modeling is also discussed. An ad hoc form of the time analogy is then used to analyze and organize a slender body data base. An important result of the analysis is that the sectional side force in the nose region is a key to understanding the side force behavior for the configuration as a whole. The nose side force at the first pressure station is shown, for the data analyzed, to uniquely determine the rest of the side force development for laminar flow and to correlate with roll orientation independent of angle of attack and Reynolds number.

Hall, Robert M.↗
NASA NTRS1987

Influence of Reynolds number on forebody side forces for 3.5-diameter tangent-ogive bodies

Vortex asymmetries can arise on aircraft and missile forebodies at high angles of attack and have proven difficult to simulate in the wind tunnel because of a large array of parameters that can influence the test results. One of the most important of these parameters is Reynolds number. The current report compares, for tangent-ogive forebodies with fineness ratios of 3.5, flow patterns from oil-flow photographs to features in pressure distributions. The pressure data base is then used to illustrate the development of the sectional side force coefficient along both the tangent-ogive nose and an attached cylindrical afterbody for Reynolds numbers of 0.2, 0.8 and 3.8 million, based on maximum diameter, and angles of attack of 40, 50 and 60 deg. General commments concerning the effects of Reynolds number are presented, as well as an analysis of the effects of Reynolds number on the sectional side force coefficient near and at the apex of the pointed body. All data presented are for freestream Mach numbers on the order of 0.25.

Hall, Robert M.↗