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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

A new look at numerical analyses of free-edge stresses in composite laminates

The edge stress problem for a + or - 45 deg graphite/epoxy laminate was examined. The reliability of the displacement formulated finite element method in analyzing the edge stress problem was investigated. Analyses of two well known elasticity problems, one involving a stress discontinuity and one a singularity, showed that the finite element analysis yields accurate stress distributions everywhere except in two elements closest to the stress discontinuity of singularity. Stress distributions for a + or - 45 deg laminate showed the same behavior near the singularity found in the well known problems with exact solutions. The displacement formulated finite element method appears to be a highly accurate technique for calculating interlaminar stress in composite laminates. The disagreement among the numerical methods was attributed to the unsymmetric stress tensor at the singularity.

Raju, I. S.↗
NASA NTRS1980

Space Shuttle Orbiter nose cap and wing leading edge certification test program

A reinforced carbon-carbon thermal protection system is used on the space shuttle orbiter vehicle's nose cap and wing leading edge regions where temperatures reach 1538 C (2800 F). To verify the analyses used to certify reinforced carbon-carbon for the first flight and operational missions, a multi-environment incremental test program was developed and implemented through the combined efforts of Rockwell International and the National Aeronautics and Space Administration. Three separate facilities at the Johnson Space Center were used to subject full-scale nose cap and wing leading edge test articles to simulated critical launch, on-orbit, and atmospheric entry environments.

Suppanz, M. J.↗
NASA NTRS1980

Leading-edge 'Vortex Flaps' for enhanced subsonic aerodynamics of slender wings

The 'Vortex Flap' is a novel concept aimed at reducing the lift-dependent drag due to leading-edge flow separation on highly swept, slender wings at high angles of attack. The suction effect of coiled vortices generated via controlled separation over leading-edge flap surfaces is utilized to produce an aerodynamic thrust component. This principle was verified through wind-tunnel experiments on 74-deg and 60-deg delta wings and a supersonic-cruise slender wing aircraft configuration. Adaptation of the vortex-flap for augmentation of roll-control on the 74-deg delta at high angles of attack also was demonstrated. Selected results are presented in this paper to indicate the potential of the vortex flap concept to enhance the low-speed performance, stability and control of slender wing aircraft.

Rao, D. M.↗
NASA NTRS1981

The coupling between flow instabilities and incident disturbances at a leading edge

It is now generally agreed that an external disturbance field, such as an incident acoustic wave, can effectively couple to instabilities of a flow past a trailing edge. One purpose of the present paper is to show that there are situations where a similar coupling can occur at a leading edge. The process is analyzed and the effects of experimentally controllable parameters are assessed. It is important to account for such phenomena when evaluating the effect of external disturbances on transition.

Goldstein, M. E.↗
NASA NTRS1981

Edge delamination in angle-ply composite laminates

Edge delamination has caused severe concern in the design and analysis of advanced composite materials and structures. Due to its complex nature, very limited knowledge for the problem is currently available. It involves not only geometric and material discontinuities but also inherently coupled mode I, II and III fracture in the layered anisotropic system. Based on complex-variable stress potentials in the anisotropic elasticity theory and eigenfunction expansion, exact orders of the crack-tip stress singularity and complete field solutions are obtained. Results are given for edge delaminated, angle-ply composites subjected to uniform axial extension for illustrative purposes. Effects of geometric, lamination, and crack variables are determined.

Wang, S. S.↗
NASA NTRS1981

Energetic ion beams at the edge of the plasma sheet - ISEE 1 observations plus a simple explanatory model

It has been found that energetic ions with energies in the range from 10 keV to 2 MeV are an important part of the geomagnetic tail particle population. A description is presented of energetic ion, three-dimensional distribution observations from ISEE 1 during 0400-2000 UT, May 3, 1978, a time when several encounters with the geomagnetic tail plasma sheet occurred. Pronounced energetic ion streaming is observed at each entry to and exit from the plasma sheet. Azimuthal gradients observed during these times indicate that this streaming occurs within two gyroradii of the plasma sheet edge. Streaming characteristics show both narrow and broad angular flows at arbitrary angles to B. These flows are observed simultaneously and individually. The time, energy, and pitch angle evolution of the ion fluxes show that ISEE 1 is encountering a dynamic and not a static equilibrium situation at the edge of the plasma sheet.

Williams, D. J.↗
NASA NTRS1981

The influence of leading-edge thrust on twisted and cambered wing design for supersonic cruise

A study of leading-edge thrust phenomena at supersonic speeds has shown that although these forces are not large, they can be a significant factor in the design of wings for supersonic cruise. It is seen that the rather severe twisted and cambered wing surfaces resulting from the application of present design methods, which ignore leading-edge thrust, can be replaced by more moderate surfaces with little or no loss in aerodynamic efficiency if realistic possibilities for the attainment of some fraction of the theoretical thrust are taken into account.

Carlson, H. W.↗
NASA NTRS1981

Trailing edge noise prediction using Amiet's method

Amiet's (1976, 1978) solution to the problem of airfoil trailing edge noise prediction is discussed in light of the results of evanescent wave theory's application to the measured surface pressure behavior near the trailing edge of an airfoil with a turbulent boundary layer. The method employed by Amiet has the advantage of incorporating the effect of finite chord in its solution. The assumed form of the pressure distribution is examined as well as the constant turbulent boundary layer convection assumption, which is found to be unnecessarily restrictive.

Brooks, T. F.↗
NASA NTRS1981

Mass loss of TEOS-coated RCC subjected to the environment at the shuttle wing leading edge

Coated, reinforced carbon-carbon (RCC) is used for the leading edges of the Space Shuttle. The mass loss characteristics of RCC specimens coated with tetra-ethyl-ortho-silicate (TEOS) were determined for conditions which simulated the entry environment expected at the stagnation area of the wing leading edge. Maximum specimen temperature was 1632 K. Specimens were exposed for up to 100 missions. Stress levels up to 8.274 MPa caused an average increase in oxidation of 6 percent over unstressed specimens. Experimentally determined mass losses were compared with those predicted by an existing empirical analysis.

Stroud, C. W.↗
NASA NTRS1982

Experimental study of delta wing leading-edge devices for drag reduction at high lift

The drag reduction devices selected for evaluation were the fence, slot, pylon-type vortex generator, and sharp leading-edge extension. These devices were tested on a 60 degree flatplate delta (with blunt leading edges) in the Langley Research Center 7- by 10-foot high-speed tunnel at low speed and to angles of attack of 28 degrees. Balance and static pressure measurements were taken. The results indicate that all the devices had significant drag reduction capability and improved longitudinal stability while a slight loss of lift and increased cruise drag occurred.

Johnson, T. D., Jr.↗
NASA NTRS1982

Computation of two-dimensional turbulent flow at subsonic Mach numbers over thick trailing edges

An implicit time marching finite difference method is used to predict two dimensional turbulent flow at a Reynolds number of 440,000 and a Mach number of 0.574 over a shortened NACA 0012 airfoil with a trailing edge of 4.5% thickness and semicircular shape. The flow is found to be unsteady but periodic in the trailing edge region. Thus, lift and drag fluctuate at small amplitudes around mean values and at distinct frequencies.

Drescher, P.↗
NASA NTRS1982

Reliability of the finite element method for calculating free edge stresses in composite laminates

The interlaminar normal stress distributions along the interface between the +45 deg and -45 deg plies of a graphite/epoxy laminate, obtained by various investigators, were found to disagree in both magnitude and sign. The reliability of the displacement-formulated finite element method in analyzing the edge-stress problem of a composite laminate is investigated. The history of the edge-stress problem is reviewed, and two well-known elasticity problems, one involving a stress discontinuity and one a singularity, are analyzed. The finite element analysis in these problems yields accurate stress distributions everywhere except in two elements closest to the stress discontinuity or singularity. Stress distributions for a + or -45 deg ply laminate near the singularity were similar to those of the two elasticity problems, demonstrating the methods, accuracy for calculating interlaminar stresses in composite laminates. The disagreement between the numerical methods was attributed to the unsymmetric stress tensor at singularity.

Whitcomb, J. D.↗
NASA NTRS1982

Space Shuttle Orbiter leading edge structural subsystem thermal performance

An extensive qualification test program and the STS-1 flight of the Space Shuttle Orbiter have provided the data necessary to verify the performance of the Orbiter thermal protection system. The reinforced carbon-carbon leading edge structural subsystem is used on areas of the orbiter where temperatures exceed 2300 F. The subsystem consists of the ROC nose cap and wing leading edge panels, metallic attachments, internal insulation, and interface tiles. Thermal response data from the qualification tests and the STS-1 flight, postflight inspection, and analytical predictions support the conclusion that the thermal performance of the subsystem verified the design.

Curry, D. M.↗
NASA NTRS1982

Generation of instability waves at a leading edge

This paper describes the generation of instability waves downstream of a leading edge by an imposed upstream disturbance. Two cases are considered. The first is concerned with mean flows of the Blasius type wherein the instabilities are represented by Tollmien-Schlichting waves. It is shown that the latter are generated fairly far downstream of the edge and are the result of a wave length reduction process that tunes the free stream disturbances to the Tollmien-Schlichting wave length. The other case is concerned with inflectional, uni-directional, transversely sheared mean flows. Such idealized flows provide a fairly good local representation to the nearly parallel flows in jets. They can support inviscid instabilities of the Kelvin-Helmholtz type. The various mathematically permissible mechanisms that can couple these instabilities to the upstream disturbances are discussed. The results are compared to some acoustic measurements and conclusions are drawn about the generation of the instabilities in these flows.

Goldstein, M. E.↗
NASA NTRS1982

Reduction of solar cell efficiency by edge defects across the back-surface-field junction - A developed perimeter model

Material imperfections, impurity clusters and fabrication defects across the back-surface-field junction can degrade the performance of high-efficiency solar cells. The degradation from defects appearing on the circumference of a solar cell is analyzed using a two-region developed perimeter device model. The width of the defective perimeter region is characterized by the range or the distance-of-influence of the defective edge and this width is about two diffusion lengths. The defective edge is characterized by a surface recombination velocity. Family of theoretical curves and numerical examples are presented to show that significant reduction of open-circuit voltage can occur in high-efficiency cells which are thin compared with the diffusion length. In one example, the degradation is decreased from 135 mV to 75 mV when the cell size is increased from 10 to 100 times the diffusion length in a thin cell whose thickness is 1% of the diffusion length.

Sah, C. T.↗
NASA NTRS1974

Effects of wing trailing-edge truncation on aerodynamic characteristics of a NASA supercritical-wing research airplane model

The Langley 8-foot transonic pressure tunnel was used at Mach numbers from 0.80 to 1.00 to determine the effects of wing trailing-edge truncation on the aerodynamic characteristics of a 0.0625-scale model of a NASA TF-8A supercritical-wing research airplane. The effects of trailing-edge truncations of 1, 2, and 3 percent of the local streamwise chord on the longitudinal aerodynamic characteristics and the wing section characteristics are presented.

Bartlett, D. W.↗
NASA NTRS1982

Wind-tunnel investigation of effects of wing-leading-edge modifications on the high angle-of-attack characteristics of a T-tail low-wing general-aviation aircraft

Exploratory tests have been conducted in the NASA-Langley Research Center's 12-Foot Low-Speed wind Tunnel to evaluate the application of wing-leading-edge devices on the stall-departure and spin resistance characteristics of a 1/6-scale model of a T-tail general-aviation aircraft. The model was force tested with an internal strain-gauge balance to obtain aerodynamic data on the complete configuration and with a separate wing balance to obtain aerodynamic data on the outer portion of the wing. The addition of the outboard leading-edge droop eliminated the abrupt stall of the windtip and maintained or increased the resultant-force coefficient up to about alpha = 32 degrees. This change in slope of the resultant-force coefficient curve with angle of attack has been shown to be important for eliminating autorotation and for providing spin resistance.

White, E. R.↗
NASA NTRS1983

Internal and edge cracks in a plate of finite width under bending

Internal and edge cracks were studied by using Reissner's transverse shear theory. The effect of stress-free boundaries on the stress intensity factors in plates under bending were investigated. Among the results found, particularly interesting are those relating to the limiting cases of the crack geometries. The numerical results are given for a single internal crack, two collinear cracks, and two edge cracks. The effect of Poisson's ratio on the stress intensity factors was studied.

Boduroglu, H.↗