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At least 937 records · Page 52

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NASA NTRS1983

High angle-of-attack aerodynamics of a strake-canard-wing V/STOL fighter configuration

High angle-of-attack aerodynamic data are analyzed for a strake-canard-wing V/STOL fighter configuration. The configuration represents a twin-engine supersonic V/STOL fighter aircraft which uses four longitudinal thrust-augmenting ejectors to provide vertical lift. The data were obtained in tests of a 9.39 percent scale model of the configuration in the NASA Ames 12-Foot Pressure Wind Tunnel, at a Mach number of 0.2. Trimmed aerodynamic characteristics, longitudinal control power, longitudinal and lateral/directional stability, and effects of alternate strake and canard configurations are analyzed. The configuration could not be trimmed (power-off) above 12 deg angle of attack because of the limited pitch control power and the high degree of longitudinal instability (28 percent) at this Mach number. Aerodynamic center location was found to be controllable by varying strake size and canard location without significantly affecting lift and drag. These configuration variations had relatively little effect on the lateral/directional stability up to 10 deg angle of attack.

Durston, D. A.↗

NASA NTRS1983

Size estimates of Titan's aerosols based on Voyager high-phase-angle images

Limits on the physical properties of the scattering haze near the top of Titan's atmosphere are derived from data obtained from seven high-phase-angle images from Voyager 1 and 2. From the ratio of the intensities observed at two different high phase angles, an estimate can be made of the forward-scattering lobe of the single-scattering phase function. Comparing the forward-scattering estimate with diffraction lobes from particles of different radii, it is concluded that the average radius of the particles found in the upper few tenths of an optical depth exceeds 0.19 micron. Judging from data observed at four different phase angles, the haze particles probably have a refractive index near 1.6 and a mean size of about 0.5 micron, if the widths of their diffraction peaks are close to those for equal-volume spheres. However, the highly polarizing nature of the particles over a broad wavelength-bandpass (Tomasko and Smith, 1982; West et al, 1983) combined with their forward-scattering behavior makes it very unlikely that the particles are spherical. The nonsphericity contributes to the uncertainty about the radii of the particles, but it is thought that the average radius is several tenths of a micron.

Rages, K.↗

NASA NTRS1984

Stagger angle dependence of inertial and elastic coupling in bladed disks

Conditions which necessitate the inclusion of disk and shaft flexibility in the analysis of blade response in rotating blade-disk-shaft systems are derived in terms of nondimensional parameters. A simple semianalytical Rayleigh-Ritz model is derived in which the disk possesses all six rigid body degrees of freedom, which are elastically constrained by the shaft. Inertial coupling by the rigid body motion of the disk on a flexible shaft and out-of-plane elastic coupling due to disk flexure are included. Frequency ratios and mass ratios, which depend on the stagger angle, are determined for three typical rotors: a first stage high-pressure core compressor, a high bypass ratio fan, and an advanced turboprop. The stagger angle controls the degree of coupling in the blade-disk system. In the blade-disk-shaft system, the stagger angle determines whether blade-disk motion couples principally to the out-of-plane or in-plane motion of the disk on the shaft. The Ritz analysis shows excellent agreement with experimental results.

Crawley, E. F.↗

NASA NTRS1984

Method for correction of errors in observation angles for limb thermal emission measurements

Thermal emission measurements of the earth's stratospheric limb from space platforms require an accurate knowledge of the observation angles for retrieval of temperature and constituent distributions. Without the use of expensive stabilizing systems, however, most observational instruments do not meet the required pointing accuracies, thus leading to large errors in the retrieval of atmospheric data. This paper describes a self-constituent method of correcting errors in pointing angles by using information contained in the observed spectrum. Numerical results based on temperature inversions of synthetic thermal emission spectra with assumed random errors in pointing angles are presented.

Abbas, M. M.↗

NASA NTRS1984

Signal statistics of the radar echoes: Angle-of-arrival statistics, part 2.2A

The Doppler spectra, measured with a vertical antenna beam, are characterized by essentially two different kinds of distributions, one has a very narrow and the other a broad spectral width. The former is accepted as due to reflection whereas the latter is due to scattering. It can happen that both kinds of spectra are observed simultaneously. Superimposed on a fairly broad Gaussian signal spectrum, very narrow signal spikes are evident. Because some spikes occur at different Doppler frequencies, the subreflections have to be assumed to move with different radial velocities. To experimentally verify this reasonable assumption, it is proposed to measure the angle of arrival of the different signal returns. One can here make use of the essential advantage that the spikes can be filtered in frequency (called Doppler sortening or sharpening). By means of the cross spectrum analysis with the interferometer technique (measuring phase differences between spaced antennas), their angle of arrival can be measured. This will allow determination of the distribution of the angles of arrival.

Rottger, J.↗

NASA NTRS1985

Pitch Angle Distributions of Solar Energetic Particles and the Local Scattering Properties of the Interplanetary Magnetic Field

An approximate solution of the Fokker-Planck equation containing pitch angle scattering and adiabatic focusing is discussed. For modest focusing effects the omnidirectional density obeys an ordinary diffusion equation with a modified diffusion coefficient. The anisotropic part of the distribution function is properly normalized and split into an even and an odd part. The even part is determined by the ratio between the scattering mean free path and the focusing length and by the degree of polarization of the magnetic field fluctuations. The odd part is determined by the deviation of the pitch angle scattering from isotropic scattering. The method supplies a powerful tool to obtain the local characteristics of pitch angle scattering. It is insensitive to long lasting solar injections and to moderate radial variations of the mean free path. The method is applied to solar particle events observed on Helio-1 and -2.

Beeck, J.↗

NASA NTRS1985

The cosmic ray differential diurnal variation dependences on the zenith angle and the geomagnetic disturbance

Simultaneous and continuous muon measurements in two opposite azimuthal directions under equal zenith angles demonstrated the importance of this method for cosmic ray diurnal variation investigations. Lately these measurements were extended by means of improved telescopes. The obtained cosmic ray diurnal variations were presented as intensity differential curves. Theoretical investigations connected the properties of these curves with some interplanetary spece parameters. The harmonics of these curves were interpreted physically. Some order difference curves were introduced. In earlier works some dependences between the parameters characterizing the first and the second harmonics of the differential intensity curves and the geomagnetic activity were found. Then all measurements were carried out under only one zenith angle. The results of investigations of similar dependences using data of simultaneous measurements under three different zenith angles are presented.

Kavlakov, S.↗

NASA NTRS1984

Using the solar elevation angle and radiance conversion to normalize forest spectral signatures

A number of spectral signals associated with forest cover were selected in order to test the usefulness of adjustments to Landsat MSS digital count values. The signatures were taken from 26 independent regional land cover inventories in the northeastern US. A regression analysis was conducted which revealed a significant relationship between digital count values and variation in the solar elevation angles in MSS bands four, six, and seven for deciduous forests and in all MSS bands for conifers. When signatures were adjusted for solar elevation angles and MSS sensor calibration differences, the dependence on solar elevation angle was reduced. The adjustments also significantly reduced the variance with Level II forest categories among scenes.

Middleton, E. M.↗

NASA NTRS1984

Dependence of NOAA-AVHRR recorded radiance on scan angle, atmospheric turbidity and unresolved cloud

Experimental evidence on the scan angle and sun angle dependence of radiance recorded by the Advanced Very High Resolution Radiometer (AVHRR) devices on the NOAA-6 and NOAA-7 satellites is presented. The effects of atmospheric turbidity at various scan angles is shown, and simulations of angular anisotropy and recorded radiance are compared with the recorded digital data from the AVHRR obtained over the Great Plains area of the US. Evidence is presented on the effects of unresolved cloud on the recorded radiance and vegetative indices from uniform, vegetative targets.

Piwinski, D. J.↗

NASA NTRS1984

Effect of angle of attack on rotor trailing-edge noise

Previous analyses of boundary layer trailing edge noise for large rotors have used zero blade angle of attack as input data. Attention is presently given to the important effects of blade angle of attack changes on rotor trailing edge noise in the case of a UH-1 helicopter. The primary effect is in the low to mid-frequency range, where noise level increases with angle of attack.

Chou, S.-T.↗

NASA NTRS1985

Estimates of leaf area index from spectral reflectance of wheat under different cultural practices and solar angle

The influence of management practices and solar illumination angle on the leaf area index (LAI) was estimated from measurements of wheat canopy reflectance evaluated by two methods, a regression formula and an indirect technique. The date of planting and the time of irrigation in relation to the stage of plant growth were found to have significant effects on the development of leaves in spring wheat. A reduction in soil moisture adversely affected both the duration and magnitude of the maximum LAI for late planting dates. In general, water stress during vegetative stages resulted in a reduction in maximum LAI, while water stress during the reproductive period shortened the duration of green LAI in spring wheat. Canopy geometry and solar angle also affected the spectral properties of the canopies, and hence the estimated LAI. Increase in solar zenith angles resulted in a general increase in estimated LAI obtained from both methods.

Asrar, G.↗

NASA NTRS1985

A model of the polarization position-angle swings in BL Lacertae objects

The polarization position-angle swings that have been measured in a number of BL Lacertae objects and highly variable quasars are interpreted in terms of shock waves which illuminate (by enhanced synchrotron radiation) successive transverse cross sections of a magnetized, relativistic jet. The jet is assumed to have a nonaxisymmetric magnetic field configuration of the type discussed in the companion paper on the equilibria of force-free jets. For a jet that is viewed at a small angle to the axis, the passage of a shock will give rise to an apparent rotation of the polarization position angle whose amplitude can be substantially larger than 180 deg. The effects of freely propagating shocks are compared with those of bow shocks which form in front of dense obstacles in the jet, and specific applications to 0727 - 115 and BL Lacertae are considered. In the case of 0727 - 115, it is pointed out that the nonuniformity of the swing rate and the apparent oscillations of the degree of polarization could be a consequence of relativistic aberration.

Konigl, A.↗

NASA NTRS1985

Microwave inversion of leaf area and inclination angle distributions from backscattered data

The backscattering coefficient from a slab of thin randomly oriented dielectric disks over a flat lossy ground is used to reconstruct the inclination angle and area distributions of the disks. The disks are employed to model a leafy agricultural crop, such as soybeans, in the L-band microwave region of the spectrum. The distorted Born approximation, along with a thin disk approximation, is used to obtain a relationship between the horizontal-like polarized backscattering coefficient and the joint probability density of disk inclination angle and disk radius. Assuming large skin depth reduces the relationship to a linear Fredholm integral equation of the first kind. Due to the ill-posed nature of this equation, a Phillips-Twomey regularization method with a second difference smoothing condition is used to find the inversion. Results are obtained in the presence of 1 and 10 percent noise for both leaf inclination angle and leaf radius densities.

Lang, R. H.↗

NASA NTRS1986

Exploratory investigation of deflectable forebody strakes for high angle of attack yaw control

A deflectable strake concept was investigated on a conical forebody to evaluate its yaw control potential at high angles of attack. In exploratory low-speed tunnel tests using a generic delta wing fighter configuration, antisymmetrically deflected strakes provided useful levels of yaw power at angles of attack when the conventional rudder became totally degraded. Symmetrical strakes prevented side force development at high angles of attack, and provided pitch control through symmetrical deflection. The strake performance was sensitive to its circumferential position on the forebody due to varying interaction of strake vortices with the wing and vertical tail. The low Reynolds number results of this study provided a favorable initial validation of the concept, subject to verification in regard to scale effects.

Rao, D. M.↗

NASA NTRS1985

What are the best radar wavelengths, incidence angles and polarizations for geologic applications? A statistical approach

Linear discriminant analysis of multifrequency and multipolarization radar scatterometer data of lava flows and sedimentary rocks indicates that the lava flows can be separated by age and the sedimentary rocks can be discriminated from one another. The optimum wavelengths, polarizations and incidence angles among those available for these problems was determined by the discriminant analysis program. For separation of the lava flows, shorter wavelengths, smaller incidence angles and horizontal polarization are best. A SIR-C radar configuration could provide nearly complete discrimination of these lava flows. Conversely, the longer wavelengths, larger incidence angles and vertical polarization was preferred for sedimentary rocks, perhaps due to the slight vegetation cover. Satisfactory classification of sedimentary rocks requires more radar data than for the lavas. These results are potentially useful both for radar system configuration and for geological applications. The method developed here may provide a rationale for user specification of imaging system parameters.

Blom, R.↗

NASA NTRS1986

Probing the magnetic field of radio pulsars - A reexamination of polarization position angle swings

A vacuum magnetic field is assumed in the present calculation of the polarization position angle as a function of pulse longitude and polarization-limiting radius in radio pulsars. The polarization-limiting radius is estimated on the basis of results of Cheng and Ruderman (1980) and Stinebring (1982). The average change in position angle is also calculated, and this is found to be consistent with observations studied by Narayan and Vivekanand (1983); an alternative interpretation of the data is given, however, which is further supported by the frequency dependence of polarization angle swing in the Crab pulsar.

Barnard, J. J.↗

NASA NTRS1985

Pivotable strakes for high angle of attack control

A series of pivotable strakes have been tested in combination with a 44-deg swept wing to determine their ability to provide pitch control at high angles of attack. The tests were carried out in the Langley 7 X 10-Foot High-Speed tunnel at nominal Mach numbers of 0.3 to 0.4. A total of five strakes with various aspects ratios and shapes were tested at negative deflection angles of 5, 10, 15, and 20 deg through an angle of attack range up to approximately 50 deg. In addition, a cranked delta wing with 70-deg sweep was tested with two different pivotable apex flaps. This paper shows lift, drag and moment data for the complete configurations as well as for the separate contributions of the strakes and fuselage nose which are mounted on a second balance. Generally, strake deflection was found to have a significant effect on moment while lift/drag changed very little.

Bobbitt, P. J.↗

NASA NTRS1986

Angle-resolved Auger electron spectra induced by neon ion impact on aluminum

Auger electron emission from aluminum bombarded with 1 to 5 keV neon ions was studied by angle-resolved electron spectroscopy. The position and shape of the spectral features depended on the incident ion energy, angle of ion incidence, and electron take-off angle with respect to the aluminum surface. These spectral dependencies were interpreted in terms of the Doppler shift given to the Auger electron velocity by the excited atom ejected into the vacuum. For oblique ion incidence it is concluded that a flux of high energy atoms are ejected in a direction close to the projection of the ion beam on the target surface. In addition, a new spectral feature was found and identified as due to Auger emission from excited neon in the aluminum matrix.

Pepper, S. V.↗