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At least 145 records · Page 8

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NASA NTRS1989

Cornering and wear characteristics of the Space Shuttle Orbiter nose-gear tire

Tests of the Space Shuttle Orbiter nose-gear tire have been completed at NASA Langley's Aircraft Landing Dynamics Facility. The purpose of these tests was to determine the cornering and wear characteristics of the Space Shuttle Orbiter nose-gear tire under realistic operating conditions. The tire was tested on a simulated Kennedy Space Center runway surface at speeds from 100 to 180 kts. The results of these tests defined the cornering characteristics which included side forces and associated side force friction coefficient over a range of yaw angles from 0 deg to 12 deg. Wear characteristics were defined by tire tread and cord wear over a yaw angle range of 0 deg to 4 deg under dry and wet runway conditions. Wear characteristics were also defined for a 15 kt crosswind landing with two blown right main-gear tires and nose-gear steering engaged.

Davis, Pamela A.↗

NASA NTRS1991

Azimuthal Anisotropy In Radar Backscatter From The Ocean

Minima offset from nominal crosswind directions. Report describes experimental and theoretical study of azimuthal anisotropy in radar backscatter from surface of ocean. Important objective of this and related studies to enhance ability to sense speed and direction of surface wind remotely by use of airborne or spaceborne radar.

Li, F. K.↗

NASA NTRS1990

Aerodynamic Effect of Strakes on Two-Dimensional Tail Boom Models of the OH-58A and the OH-58D Helicopters

During hover and low speed flight, helicopters experience significant aerodynamic forces on the tail boom caused by the wake from the main and tail rotors and by crosswinds. These effects were simulated during a study conducted in the Langley 14 by 22 Foot Subsonic Tunnel on a 136 percent scaled 2-D tail boom model with cross sections representative of those on the U.S. Army OH-58A and the OH-58D helicopters. The effects of longitudinal strakes attached to the cross sections were studied. To obtain the aerodynamic forces acting on the cross sections, the flow incidence range on the scaled models was varied from -45 to 90 degrees and the models were tested through a dynamic pressure range of 5 to 15 psf. The results of the OH-58A and the OH-58D configurations show a significant improvement at conditions which represent right sideward flight by reducing the adverse aerodynamic side force when the strakes are installed. These data were used to calculate a change in tail rotor power for the full scale flight vehicle and indicated approx. a 5 to 6 percent average savings in right sideward flight for the critical velocity range of 0 to 30 knots. Increases in the tail boom normal force were noted due to the strakes. The results indicate a potential for reducing the directional control and tail rotor thrust required in the critical hover and right sideward flight speed range with a calculated minimum increase to main rotor power required and an overall net improvement in power of 0.5 percent for both the OH-58A and OH-58D.

Crowell, Cynthia A.↗

NASA NTRS1990

Distorted-wave born approximation calculations for turbulence scattering in an upward-refracting atmosphere

Weiner and Keast observed that in an upward-refracting atmosphere, the relative sound pressure level versus range follows a characteristic 'step' function. The observed step function has recently been predicted qualitatively and quantitatively by including the effects of small-scale turbulence in a parabolic equation (PE) calculation. (Gilbert et al., J. Acoust. Soc. Am. 87, 2428-2437 (1990)). The PE results to single-scattering calculations based on the distorted-wave Born approximation (DWBA) are compared. The purpose is to obtain a better understanding of the physical mechanisms that produce the step-function. The PE calculations and DWBA calculations are compared to each other and to the data of Weiner and Keast for upwind propagation (strong upward refraction) and crosswind propagation (weak upward refraction) at frequencies of 424 Hz and 848 Hz. The DWBA calculations, which include only single scattering from turbulence, agree with the PE calculations and with the data in all cases except for upwind propagation at 848 Hz. Consequently, it appears that in all cases except one, the observed step function can be understood in terms of single scattering from an upward-refracted 'skywave' into the refractive shadow zone. For upwind propagation at 848 Hz, the DWBA calculation gives levels in the shadow zone that are much below both the PE and the data.

Gilbert, Kenneth E.↗

NASA NTRS1989

Experimental investigation of the dependence of radar backscattering on wind speed, wind stress and wave height

During summer 1988, radar measurements were performed in conjunction with detailed environmental observations on Lake Washington at the University of Washington Sand Point field station. Radar data were collected at 5.3 and 10 GHz for incidence angles between 30 and 60 deg with VV-polarization. The environmental measurements included wind speed and direction, large-wave heights, the high-frequency portion of the wave spectrum, humidity, and air and water temperatures. The small-scale wave spectrum was measured using a resistance wire gauge. The results show that backscatter increased with wind speed as expected. However, little difference was observed in the scattering coefficient for upwind and crosswind directions. The results also indicated an increase in the amplitude of small waves with friction velocity.

Gogineni, S. P.↗

NASA NTRS1991

Approach and landing simulator for Space Shuttle Orbiter touchdown conditions

This study describes a computer simulation that provides an accurate calculation of the touchdown conditions. The program is small enough to operate on a personal computer. The program simulates the Orbiter trajectory from an altitude of 10,000 ft to the earth's surface. It assumes the Orbiter files close to its reference altitude profile, with negligible crosswind effects. Flight data have proven both assumption to be valid. By incorporating these assumptions with the Shuttle guidance logic, a quick approximation can be made of the touchdown speed and downrange distance to an accuracy within 400 ft of the mainframe prediction.

Walyus, Keith D.↗

NASA NTRS1991

Mission safety evaluation report for STS-39, postflight edition

After a delay of approximately 2 months due to a rollback from the pad to replace the External Tank door lug housing, Space Shuttle Discovery was launched from NASA-Kennedy at 7:33 a.m. Eastern Daylight Time on 28 April 1991. STS-39 was the first unclassified DoD Shuttle mission. On 28 April, countdown proceeded normally through the T-20 minute hold. No significant problems were encountered except for the Operations Sequence-2 recorder starting unexpectedly; it was stopped by an uplink command. Discovery landed on KSC runway 15 at 2:55 p.m. EDT on 6 May 1991. This was the second time in 6 months that the Space Shuttle was diverted to KSC for landing because of high winds at Edwards AFB, Calif. This was also the 7th of 40 Shuttle missions to land at KSC in the history of the Space Shuttle Program. The Main Landing Gear outer right tire shredded 3 of the 16 cords due to either an uneven landing or a maximum force breaking test during rollout. Contributing factors to the tire cord shredding were the development of last minute crosswinds and reluctance of the ground controllers to distract the Shuttle pilots with warnings of the low flight path. As a corrective action, communication procedures will be modified for future flights.

Hardie, Kenneth O.↗

NASA NTRS1990

Mission safety evaluation report for STS-38, postflight edition

After several months delay due to Liquid Hydrogen (LH2) leaks in the External Tank (ET) 17 in LH2 umbilical disconnect, Atlantis was launched at 6:48 p.m. Eastern Standard Time on 15 Nov. 1990. Atlantis was cleared for launch after the replacement of the ET 17 in umbilical disconnect and a successful tanking test on 24 Oct. 1990. No technical concerns were encountered during the countdown. The end of the STS-38 mission was extended by one day because of high crosswinds at the primary landing site, Edwards Air Force Base (EAFB), Calif. On the next day, 20 Nov. 1990, the winds were too high for a safe landing at EAFB, and a decision was made to select the Shuttle Landing Facility (SLF) at the Kennedy Space Center (KSC) at the primary landing site. The deorbit burn was performed on orbit 79 of the STS-38 mission. Reentry and descent were effected with no problems. Postlanding inspection of Atlantis' exterior found little evidence of damage. There has been a concern about returning an Orbiter to KSC, because the last mission to do so experienced tire failure. Atlantis' tires and brakes were in excellent condition with no indication of excessive wear.

Hill, William C.↗

NASA NTRS1991

A video-aided study of sea spikes in radar backscatter at moderate incidence

Backscatter from a lake was measured at a 37-deg incidence angle using an FM-CW radar operating on X band and C band simultaneously. A limited amount of Ka-band data was also collected at 50 deg incidence. A video camera recorded the activity within the footprint looking upwind and crosswind. The video showed that breaking waves at all look directions and wedge-shaped waves when looking upwind consistently produced spikes in the radar time series. However, 62 percent to 90 percent of the spikes were not attributable to any surface structure visible on the video picture except wave crests. These spikes may be due to specular reflections or micro-scale breaking too small to be seen. Individual contributions to the backscattering coefficient were calculated for unattributed spikes, breaker spikes, and wedge spikes.

Bush, Daniel A.↗

NASA NTRS1992

A gravity current model for the May 18, 1980 Mount St. Helens plume

Observations of the stratospheric plume from the May 18, 1980 Mount St. Helens eruption suggest that it spread in the crosswind direction as an intrusive gravity current, as it was transported downwind. Grain size analyses of the plinian tephra are consistent with this model, suggesting that to distances of many hundreds of kilometers, turbulent atmospheric diffusion played a secondary role in plume spreading and tephra dispersal.

Bursik, M. I.↗

NASA NTRS1992

Sampling errors in the vertical fluxes of potential temperature and moisture measured by aircraft during FIFE

This paper contributes to understanding the particular problems encountered during FIFE 87 as a result of utilizing relatively short (15 km) averaging lengths for the flux estimates. The implications of employing data that had been high-pass filtered with a five-km cutoff wavelength for flux estimates are also investigated. It is found that a 15-km run length is just adequate for low-level flux estimates as long as the runs are crosswind.

Grossman, Robert L.↗

NASA NTRS1991

Rotational wind indicator enhances control of rotated displays

Rotation by 108 deg of the spatial mapping between a visual display and a manual input device produces large spatial errors in a discrete aiming task. These errors are not easily corrected by voluntary mental effort, but the central nervous system does adapt gradually to the new mapping. Bernotat (1970) showed that adding true hand position to a 90 deg rotated display improved performance of a compensatory tracking task, but tracking error rose again upon removal of the explicit cue. This suggests that the explicit error signal did not induce changes in the neural mapping, but rather allowed the operator to reduce tracking error using a higher mental strategy. In this report, we describe an explicit visual display enhancement applied to a 108 deg rotated discrete aiming task. A 'wind indicator' corresponding to the effect of the mapping rotation is displayed on the operator-controlled cursor. The human operator is instructed to oppose the virtual force represented by the indicator, as one would do if flying an airplane in a crosswind. This enhancement reduces spatial aiming error in the first 10 minutes of practice by an average of 70 percent when compared to a no enhancement control condition. Moreover, it produces adaptation aftereffect, which is evidence of learning by neural adaptation rather than by mental strategy. Finally, aiming error does not rise upon removal of the explicit cue.

Cunningham, H. A.↗

NASA NTRS1995

Aircraft Sensor Platform Has Increased Angular Range

Mechanism rotates and translates instrument platform within pressure housing in aircraft to aim remote-sensing instrument toward target on ground below. Enables instrument to look under aircraft structure at larger fore and aft angles without having to deploy instrument into air stream outside. Also provides 10 degrees of yaw compensation, reducing further need for adjustment of attitude of aircraft to keep target in sight. With yaw compensation, pilot can fly with wings level and nose pointed into crosswind while on desired flight path over target.

Dabney, Philip W.↗

NASA NTRS1997

Sensitivity of Runway Occupancy Time (ROT) to Various Rollout and Turnoff (ROTO) Factors: Complete Set of Plotted Data - Volume 2

The Terminal Area Productivity (TAP) research program was initiated by NASA to increase the airport capacity for transport aircraft operations. One element of the research program is called Low Visibility Landing and Surface Operations (LVLASO). A goal of the LVLASO research is to develop transport aircraft technologies which reduce Runway Occupancy Time (ROT) so that it does not become the limiting factor in the terminal area operations that determine the capacity of a runway. Under LVLASO, the objective of this study was to determine the sensitivity of ROT to various factors associated with the Rollout and Turnoff (ROTO) operation for transport aircraft. The following operational factors were studied and are listed in the order of decreasing ROT sensitivity: ice/flood runway surface condition, exit entrance ground speed, number of exits, high-speed exit locations and spacing, aircraft type, touchdown ground speed standard deviation, reverse thrust and braking method, accurate exit prediction capability, maximum reverse thrust availability, spiral-arc vs. circle-arc exit geometry, dry/slush/wet/snow runway surface condition, maximum allowed deceleration, auto asymmetric braking on exit, do not stow reverse thrust before the exit, touchdown longitudinal location standard deviation, flap setting, anti-skid efficiency, crosswind conditions, stopping on the exit and touchdown lateral offset.

Goldthorpe, S. H.↗

NASA NTRS1997

Sensitivity of Runway Occupancy Time (ROT) to Various Rollout and Turnoff (ROTO) Factors

Goldthorpe, S. H.↗

NASA NTRS1997

Spectral Density of Laser Beam Scintillation in Wind Turbulence: Theory - Part 1

The temporal spectral density of the log-amplitude scintillation of a laser beam wave due to a spatially dependent vector-valued crosswind (deterministic as well as random) is evaluated. The path weighting functions for normalized spectral moments are derived, and offer a potential new technique for estimating the wind velocity profile. The Tatarskii-Klyatskin stochastic propagation equation for the Markov turbulence model is used with the solution approximated by the Rytov method. The Taylor 'frozen-in' hypothesis is assumed for the dependence of the refractive index on the wind velocity, and the Kolmogorov spectral density is used for the refractive index field.

Balakrishnan, A. V.↗

NASA NTRS1998

Inviscid Interactions Between Wake Vortices and Shear Layers

Aircraft trailing vortices can be influenced significantly by atmospheric conditions such as crosswind, turbulence, and stratification. According to the NASA 1994 and 1995 field measurement program in Memphis, Tennessee, the descending aircraft wake vortices could stall or be deflected at the top of low-level temperature inversions that usually produce pronounced shear zones. Numerical simulations of vortex/shear interactions with ground effects have been performed by several groups. Burnham used a series of evenly spaced line vortices at a particular altitude to model the ground shear layer of the cross- wind. He found that the wind shear was swept up around the downwind vortex and caused the downwind vortex to move upward, and claimed that the effect was actually produced by the vertical gradient in the wind shear rather than by the wind shear directly, because uniformly distributed wind-shear vortices would have no effect on the trailing vortex vertical motion. Recently, Proctor et al. numerically tested the effects of narrow shear zones on the behavior of the vortex pair, motivated by the observation of the Memphis field data. The shear-layer sensitivity tests indicated that the downwind vortex was more sensitive and deflected to a higher altitude than its upwind counterpart. The downstream vortex contained vorticity of opposite sign to that of the shear. There was no detectable preference for the downwind vortex (or upwind vortex) to weaken (or strengthen) at a greater rate.

Zheng, Z. C.↗

NASA NTRS1998

Shear-Layer Effects on Trailing Vortices

Crosswind shear can influence the trailing vortex trajectories significantly, according to both field measurement and numerical simulations. Point vortex models are used in this paper to study the fluid dynamic mechanism in the interactions between trailing vortex pair and shear layers. It has been shown that the shear-layer deformation causes the vortex descent history difference in the two vortices of the vortex pair. When a shear layer is below the vortex pair with the same sign as the left vortex, the right vortex descends less than the left vortex. When the same shear layer is above the vortex pair, the right vortex descends more. The descent altitudes of the two vortices are the same when they go through a constant, non-deformed shear layer. Those trends are in agreement with Navier-Stokes simulations.

Zheng, Z. C.↗