Showing papers by "Langley Research Center published in 1981"

A crack-closure model for predicting fatigue crack growth under aircraft spectrum loading

Abstract: The development and application of an analytical model of cycle crack growth is presented that includes the effects of crack closure. The model was used to correlate crack growth rates under constant amplitude loading and to predict crack growth under aircraft spectrum loading on 2219-T851 aluminum alloy sheet material. The predicted crack growth lives agreed well with experimental data. The ratio of predicted to experimental lives ranged from 0.66 to 1.48. These predictions were made using data from an ASTM E24.06.01 Round Robin.

Characterization of delamination onset and growth in a composite laminate

Abstract: The onset and growth of delaminations in unnotched (+ or - 30/+ or - 30/90/90 bar) sub S graphite epoxy laminates is described quantitatively. These laminates, designed to delaminate at the edges under tensile loads, were tested and analyzed. Delamination growth and stiffness loss were monitored nondestructively. Laminate stiffness decreased linearly with delamination size. The strain energy release rate, G, associated with delamination growth, was calculated from two analyses. A critical G for delamination onset was determined, and then was used to predict the onset of delaminations in (+45 sub n/-45 sub n/o sub n/90 sub n) sub s (n=1,2,3) laminates. A delamination resistance curve (R curve) was developed to characterize the observed stable delamination growth under quasi static loading. A power law correlation between G and delamination growth rates in fatigue was established.

Linear Acoustic Formulas for Calculation of Rotating Blade Noise

Abstract: A unified approach is used to derive many of the current formulas for calculation of discrete frequency noise of helicopter rotors and propellers. Both compact and noncompact results are derived. The noncompact results are based on the solution of Ffowcs Williams-Hawkings (FW-H) equation. The compact formulations are obtained as the limit of noncompact source results. In particular, the linearized acoustic theories of Hawkings and Lowson, Farassat, Hanson, Woan and Gregorek, Succi, and Jou are discussed in this paper. An interesting thickness noise formula by Isom and its extension by Ffowcs Williams are also presented.

Stress-intensity factor equations for cracks in three-dimensional finite bodies

Abstract: Empirical stress intensity factor equations are presented for embedded elliptical cracks, semi-elliptical surface cracks, quarter-elliptical corner cracks, semi-elliptical surface cracks at a hole, and quarter-elliptical corner cracks at a hole in finite plates. The plates were subjected to remote tensile loading. Equations give stress intensity factors as a function of parametric angle, crack depth, crack length, plate thickness, and where applicable, hole radius. The stress intensity factors used to develop the equations were obtained from three dimensional finite element analyses of these crack configurations.

Two-dimensional aerodynamic characteristics of the NACA 0012 airfoil in the Langley 8 foot transonic pressure tunnel

Abstract: Data are presented for lift coefficients from near zero through maximum values at Mach numbers from 0.30 to 0.86 and Reynolds numbers of 3.0 x 10 to the sixth power with transition fixed. A limited amount of data is presented near zero and maximum lift for a Reynolds number of 6.0 x 10 to the sixth power with transition fixed. In addition, transition free data is presented through the Mach number range from 0.30 to 0.86 for near zero lift and a Reynolds number of 3.0 x 10 to the sixth power.

Finite element analysis of instability-related delamination growth

Abstract: A parametric study of postbuckled through-width delaminations in laminated coupons was performed. A finite element analysis was developed to analyze the coupons as a combination of linear and geometrically nonlinear components. Because most of the coupon configuration studied behaves linearly, the mixed linear and nonlinear analysis greatly reduced computational costs. The analysis was verified by comparing numerical with exact solutions for simple hypothetical problems. In addition, measured lateral deflections of postbuckled through-width delaminations in laminated coupons were compared with predicted deflections. In the parametric study, stress distributions and strain-energy release rates were calculated for various delamination lengths, delamination depths, applied loads, and lateral deflections. Also, a small number of coupons with through-width delaminations were fatigue tested to obtain delamination growth data. Calculated strain-energy release rates were compared with the observed growth rates to determine the relative importance of the Mode 1 and Mode 2 components of energy release. Growth process was dominated by G sub I.

The distribution of carbon monoxide and ozone in the free troposphere

Abstract: The two-dimensional distributions of CO and O3 in the free troposphere during July and August, 1974, are discussed. The data confirm the previous findings that both of these gases are considerably more abundant in the northern hemisphere, but the degree of the asymmetry is somewhat different from what had been reported previously, especially for CO. When examined with respect to other available data sets, the conclusion is drawn that a pronounced seasonal cycle exists for CO in both hemispheres which may be driven by the likely seasonal cycle of the OH radical. The data also indicate that CO concentrations exhibit significant variability with height in the northern hemisphere, whereas southern hemispheric concentrations are quite constant with altitude except in cases where interhemispheric exchange of air may be occurring. A discussion on the vertical and horizontal transport processes inferred from the CO and O3 measurements is presented. The possible interdependence of the photochemical cycles of these two trace gases is also discussed.

Historical Development of Aircraft Flutter

Abstract: Introduction A EROELASTICITY, and in particular flutter, has influenced the evolution of aircraft since the earliest days of flight. This paper presents a glimpse of problems arising in these areas and how they were attacked by aviation's pioneers and their successors up to about the mid-1950s. The emphasis is on tracing some conceptual developments relating to the understanding and prevention of flutter including some lessons learned along the way. Because it must be light, an airplane necessarily deforms appreciably under load. Such deformations change the distribution of the aerodynamic load, which in turn changes the deformations; the interacting feedback process may lead to flutter, a self-excited oscillation, often destructive, wherein energy is absorbed from the airstream. Flutter is a complex phenomenon that must in general be completely eliminated by design or prevented from occurring within the flight envelope. The initiation of flutter depends directly on the stiffness, and only indirectly on the strength of an airplane, analogous to depending on the slope of the lift curve rather than on the maximum lift. This implies that the airplane must be treated not as a rigid body but as an elastic structure. Despite the fact that the subject is an old one, this requires for a modern airplane a large effort in many areas, including ground vibration testing, use of dynamically scaled wind-tunnel models, theoretical analysis, and flight flutter testing. The aim of this paper is to give a short history of aircraft flutter, with emphasis on the conceptual developments, from the early days of flight to about the mid-1950s. Work in flutter has been (and is being) pursued in many countries. As in nearly all fields, new ideas and developments in flutter have occurred similarly and almost simultaneously in diverse places in the world, so that exact assignment of priorities is often in doubt. Moreover, a definitive historical account would require several volumes; yet we hope to survey some of the main developments in a proper historical light, and in a way that the lessons learned may be currently useful. It is recognized that detailed documentation of flutter troubles has nearly always been hampered by proprietary conditions and by a reluctance of manufacturers to expose such problems. From our present perspective, flutter is included in the broader term aeroelasticity, the study of the static and dynamic response of an elastic airplane. Since flutter involves the problems of interaction of aerodynamics and structural deformation, including inertial effects, at subcritical as well as at critical speeds, it really involves all aspects of aeroelasticity. In a broad sense, aeroelasticity is at work in natural phenomena such as in the motion of insects, fish, and birds (biofluid-dynamics). In man's handiwork, aeroelastic problems of windmills were solved empirically four centuries ago in Holland with the moving of the front spars of the blades from about the midchord to the quarter-chord position (see the article by Jan Drees in list of Survey Papers). We now recognize that some 19th century bridges were torsionally weak and collapsed from aeroelastic effects, as did the Tacoma Narrows Bridge in spectacular fashion in 1940. Other aeroelastic wind-structure interaction pervades civil

Self-induced wing rock of slender delta wings

Abstract: As part of a research program aimed at exploring basic mechanisms that cause wing rock in combat aircraft, an investigation was conducted to study the aerodynamic factors which cause the low-speed wing rock exhibited by slender delta wings. A flat-plate delta wing with 80 deg leading-edge sweep was subjected to conventional static-force tests and dynamic wind-tunnel experiments which included forced-oscillation, rotary, and free-to-roll tests. In addition, visualization of the flow phenomena involved was obtained by observing tuft patterns and using a helium-bubble technique. This paper summarizes the results of this study. Fundamental information is presented on the aerodynamic mechanisms that cause the wing rock and the problem of mathematically modeling the aerodynamics and motions is discussed.

Approximate Convective-Heating Equations for Hypersonic Flows

Abstract: Laminar and turbulent heating-rate equations appropriate for engineering predictions of the convective heating rates about blunt reentry spacecraft at hypersonic conditions are developed. The approximate methods are applicable to both nonreacting and reacting gas mixtures for either constant or variable-entropy edge conditions. A procedure which accounts for variable-entropy effects and is not based on mass balancing is presented. Results of the approximate heating methods are in good agreement with existing experimental results as well as boundary-layer and viscous-shock-layer solutions.

Methods and models for predicting fatigue crack growth under random loading

Abstract: Papers are presented in the volume summarizing the baseline data, methodology, procedures, and results of a round-robin analysis which was conducted to predict the fatigue crack growth in 2219-T851 aluminum center-cracked specimens subjected to flight loading in random cycle-by-cycle format. The objective of the analysis was to assess whether data from constant-amplitude fatigue crack growth tests on center-cracked specimens can be used to predict fatigue crack growth lives under random loading. The following approaches are discussed in detail: a root-mean-square approach, a crack-closure model, a multi-parameter yield zone model, and a load-interaction model.

Design and experimental results for a flapped natural-laminar-flow airfoil for general aviation applications

Abstract: A natural-laminar-flow airfoil for general aviation applications, the NLF(1)-0416, was designed and analyzed theoretically and verified experimentally in the Langley Low-Turbulence Pressure Tunnel. The basic objective of combining the high maximum lift of the NASA low-speed airfoils with the low cruise drag of the NACA 6-series airfoils was achieved. The safety requirement that the maximum lift coefficient not be significantly affected with transition fixed near the leading edge was also met. Comparisons of the theoretical and experimental results show excellent agreement. Comparisons with other airfoils, both laminar flow and turbulent flow, confirm the achievement of the basic objective.

Mixed models and reduced/selective integration displacement models for nonlinear analysis of curved beams

Abstract: Simple mixed models are developed for use in the geometrically nonlinear analysis of deep arches. A total Lagrangian description of the arch deformation is used, the analytical formulation being based on a form of the nonlinear deep arch theory with the effects of transverse shear deformation included. The fundamental unknowns comprise the six internal forces and generalized displacements of the arch, and the element characteristic arrays are obtained by using Hellinger-Reissner mixed variational principle. The polynomial interpolation functions employed in approximating the forces are one degree lower than those used in approximating the displacements, and the forces are discontinuous at the interelement boundaries. Attention is given to the equivalence between the mixed models developed herein and displacement models based on reduced integration of both the transverse shear and extensional energy terms. The advantages of mixed models over equivalent displacement models are summarized. Numerical results are presented to demonstrate the high accuracy and effectiveness of the mixed models developed and to permit a comparison of their performance with that of other mixed models reported in the literature.

Comparison of reflectance with backscatter and absorption parameters for turbid waters.

Abstract: The relation of reflectance to backscatter and absorption parameters is investigated for waters more turbid than those of previous investigations. Experimental data are examined for river waters in which beam attenuation values range from 8.9 to 18.9 m(_1) at 550 nm. Attenuation, absorption, backscatter, and irradiance reflectance spectral properties are presented for wavelengths between 450 and 800 nm. Comparisons of reflectance with backscatter to absorption ratio and backscatter with absorption plus backscatter ratio indicate that data for turbid waters do not fit linear or polynomial models which are presently available in the literature.

Description and calibration of the Langley unitary plan wind tunnel

Abstract: The two test sections of the Langley Unitary Plan Wind Tunnel were calibrated over the operating Mach number range from 1.47 to 4.63. The results of the calibration are presented along with a a description of the facility and its operational capability. The calibrations include Mach number and flow angularity distributions in both test sections at selected Mach numbers and tunnel stagnation pressures. Calibration data are also presented on turbulence, test-section boundary layer characteristics, moisture effects, blockage, and stagnation-temperature distributions. The facility is described in detail including dimensions and capacities where appropriate, and example of special test capabilities are presented. The operating parameters are fully defined and the power consumption characteristics are discussed.

Determination of Airplane Model Structure From Flight Data by Using Modified Stepwise Regression

Abstract: The linear and stepwise regressions are briefly introduced, then the problem of determining airplane model structure is addressed. The MSR was constructed to force a linear model for the aerodynamic coefficient first, then add significant nonlinear terms and delete nonsignificant terms from the model. In addition to the statistical criteria in the stepwise regression, the prediction sum of squares (PRESS) criterion and the analysis of residuals were examined for the selection of an adequate model. The procedure is used in examples with simulated and real flight data. It is shown that the MSR performs better than the ordinary stepwise regression and that the technique can also be applied to the large amplitude maneuvers.

Rapid inversion of limb radiance data using an emissivity growth approximation

Abstract: The time-consuming nature of limb relaxation-type inversion algorithms is due primarily to the numerous integrations over an absorption band to obtain forward radiance values with which to compare measured values. A new method has been devised for the quick and accurate (0.5% error) calculation of single gas broadband (approximately 100 per cm) limb radiance. The method uses a precalculated data base consisting of homogeneous path emissivity vs mass path data for a wide range of temperature and pressure. A 50-km altitude range, 1-km resolution, constituent inversion employing this method requires under 1 sec of computational time when run on modern computer hardware. The method does not rely upon a priori statistical knowledge.

Measurement of aircraft speed and altitude

Abstract: Problems involved in measuring speed and altitude with pressure-actuated instruments (altimeter, airspeed indicator, true-airspeed indicator, Machmeter, and vertical-speed indicator) are examined. Equations relating total pressure and static pressure to the five flight quantities are presented, and criteria for the design of total and static pressure tubes are given. Calibrations of typical static pressure installations (fuselage nose, wing tip, vertical fin, and fuselage vent) are presented, various methods for flight calibration of these installations are described, and the calibration of a particular installation by two of the methods is described in detail. Equations are given for estimating the effects of pressure lag and leaks. Test procedures for the laboratory calibration of the five instruments are described, and accuracies of mechanical and electrical instruments are presented. Operational use of the altimeter for terrain clearance and vertical separation of aircraft is discussed, along with flight technical errors and overall altitude errors of aircraft in cruise operations. Altitude-measuring techniques based on a variety of properties of the Earth and the atmosphere are included. Two appendixes present airspeed and altitude tables and sample calculations for determining the various flight parameters from measured total and static pressures.

The role of atmospheric deposition in an eastern U.S. deciduous forest

Abstract: Atmospheric sources contributed significantly to the annual flux of trace metals and sulfate to the forest floor of Walker Branch Watershed, a forested catchment in the southeastern United States. Atmospheric deposition supplied from 14% (Mn) to≈40% (Zn, Cd, So 4 = ) to 99% (Pb) of the annual flux to the forest floor; the remainder was attributable to internal element cycling. The measured water solubility of these metals in suspended and deposited particles indicates that they may be readily mobilized following deposition. Dry deposition constituted a major fraction of the total annual atmospheric input of Cd and Zn (≈20%), SO=(≈35%), Pb(≈55%), and Mn (≈90%); however, wet deposition rates for single events exceeded dry deposition rates by one to four orders of magnitude. Interception of rain by the canopy resulted in loss of Cd, Mn, Pb, Zn, and SO= from the canopy, but uptake of H+ which increased with increasing free acidity of the incoming rain, and with increasing residence time of the rain on the leaf surface.

Semianalytic Monte Carlo radiative transfer model for oceanographic lidar systems

Abstract: A semianalytic Monte Carlo radiative transfer model (SALMON) has been developed which is particularly well-suited for addressing oceanographic lidar systems. SALMON is based on the method of expected values in which an analytical estimate is made of the probability of collection by a remote receiver of scattered or emitted photons at appropriate points in the stochastically constructed underwater photon trajectory. Sample results indicate that a substantial reduction in both variance and computer resources can be realized by using SALMON, as compared with more conventional Monte Carlo approaches, to study radiative transfer mechanisms associated with lidar systems.

Preliminary design of large reflectors with flat facets

Abstract: A concept for approximating curved antenna surfaces using flat facets is discussed. A preliminary design technique for determining the size of the reflector surface facets necessary to meet antenna surface accuracy requirements is presented. A proposed large microwave radiometer satellite (MRS) is selected as an application, and the far-field electromagnetic response of a faceted reflector surface is compared with that from a spherical reflector surface.

Chemical kinetic analysis of hydrogen-air ignition and reaction times

Abstract: An anaytical study of hydrogen air kinetics was performed. Calculations were made over a range of pressure from 0.2 to 4.0 atm, temperatures from 850 to 2000 K, and mixture equivalence ratios from 0.2 to 2.0. The finite rate chemistry model included 60 reactions in 20 species of the H2-O2-N2 system. The calculations also included an assessment of how small amounts of the chemicals H2O, NOx, H2O2, and O3 in the initial mixture affect ignition and reaction times, and how the variation of the third body efficiency of H2O relative of N2 in certain key reactions may affect reaction time. The results indicate that for mixture equivalence ratios between 0.5 and 1.7, ignition times are nearly constant; however, the presence of H2O and NO can have significant effects on ignition times, depending on the mixture temperature. Reaction time is dominantly influenced by pressure but is nearly independent of initial temperature, equivalence ratio, and the addition of chemicals. Effects of kinetics on reaction at supersonic combustor conditions are discussed.

Satellite and Correlative Measurements of the Stratospheric Aerosol. I: An Optical Model for Data Conversions

Abstract: A description is presented of an empirically based model of stratospheric aerosol optical properties (size distributions and refractive indices) and their variations. The need for such a model arose in the data validation and archival programs for two satellite sensors, SAM II and SAGE. These programs require the ability to convert measurements of a given aerosol macroproperty (e.g., volume extinction coefficient, volume backscatter coefficient, particle number or mass per unit volume) to best estimates of other aerosol macroproperties, and to assess quantitatively the uncertainties in the conversion process. The described model provides the information on size distributions, refractive indices and their variations necessary for these tasks, and also defines a procedure for combining the model information with empirical data in a way that facilitates automatic data processing. Although the model was developed for use in the satellite validation and archival programs, it also has proven useful in other studies of stratospheric aerosol.

Satellite and Correlative Measurements of the Stratospheric Aerosol. II: Comparison of Measurements Made by SAM II, Dustsondes and an Airborne Lidar

Abstract: Results are shown from the first set of measurements conducted to validate extinction data from the Stratospheric Aerosol Measurement II (SAM II). Dustsonde-measured number density profiles and lidar-measured backscattering profiles for two days are converted to extinction profiles, and are shown to agree within their respective uncertainties at all heights above the tropopause. Near the tropopause, agreement depends on use of model size distributions with larger particles, having radii greater than 0.6 microns. The presence of such large particles is supported by measurements made elsewhere, is suggested by the in situ size distribution measurements reported, and is likely to have an important bearing on the radiative impact of the total stratospheric aerosol. It is concluded that the SAM II extinction data and uncertainty estimates are supported.

Comparison of Experimental and Theoretical Turbulence Reduction from Screens, Honeycomb, and Honeycomb-Screen Combinations

Abstract: A 1/2-scale model of a portion of the NASA Langley 8-foot transonic pressure tunnel was used to conduct some turbulence reduction research. The experimental results are correlated with various theories. Screens alone reduce axial turbulence more than the lateral turbulence; whereas, honeycomb alone reduce lateral turbulence more than axial turbulence. Because of this difference, the physical mechanism for decreasing turbulence for screens and honeycomb must be completely different. Honeycomb with a downstream screen is an excellent combination for reducing turbulence.