Showing papers by "Langley Research Center published in 1973"

On the stabilization of decentralized control systems

Abstract: This paper considers the problem of stabilizing a linear time-invariant multivariable system by using several local feedback control laws. Each local feedback control law depends only on partial system outputs. A necessary and sufficient condition for the existence of local control laws with dynamic compensation to stabilize a given system is derived. This condition is stated in terms of a new notion, called "fixed modes," which is a natural generalization of the well-known concept of uncontrollable modes and unobservable modes that occur in centralized control system problems. A procedure that constructs a set of stabilizing feedback control laws is given.

Free vibrations of multilayered composite plates.

Abstract: Summary of some of the results of a recent study of the reliability and range of validity of two-dimensional plate theories in application to low-frequency free vibration analysis of simply supported, bidirectional, multilayered plates consisting of a large number of layers. These results show that for composite plates the error in the predictions of the classical plate theory is strongly dependent on the number and stacking of the layers, in addition to the degree of orthotropy of the individual layers and the thickness ratio of the plate.

A critical review of the experimental data for developed free turbulent shear layers

Abstract: Experimental shear layer data are reviewed and the results are compared to numerical predictions for three test cases. It was concluded from the study that many, if not most, of the apparent inconsistencies which exist in the interpretation of the experimental data for free shear layers result from confusing data taken in developed turbulent flows with those taken in transitional or developing flows. Other conclusions drawn from the study include the following: (1) The effects of Mach number are more uncertain primarily because of limited data and the absence of any turbulence measurements for supersonic shear layers. (2) The data available for heterogeneous shear layers are not sufficient to clearly establish the effect of density ratio on mixing rate.

Generalized radiation impedances and reflection coefficients of circular and annular ducts

Abstract: Morse's equation for the radiation impedance of a vibrating rigid piston is extended to give the generalized radiation impedances of all modes in circular and annular ducts which have arbitrary wall admittance and which terminate in an infinite baffle. The extended equation is reduced to a single infinite integral which is a function of the mode radiation directivity factors. An infinite matrix equation is derived which relates the generalized mode reflection coefficients to the radiation impedances. Results are presented which show that the duct-wall admittance is a significant parameter in the effect of duct-determination reflections and that mode-coupling reflection effects may be more important than direct-mode reflection effects.

Relaxation solutions for inviscid axisymmetric transonic flow over blunt or pointed bodies.

Abstract: A finite-difference relaxation method is presented for numerical solution of the full potential equation and exact boundary conditions for general axisymmetric bodies is inviscid, steady transonic flow. Body-normal coordinates are used in the nose region and sheared cylindrical coordinates are used on the afterbody to accommodate corners such as boattails and flares. An improved difference scheme is used which does not require that the flow be nearly alined with a coordinate direction in supersonic regions, and which treats either subsonic or supersonic free streams. Numerical results are illustrated for some simple classical shapes such as spheres and ellipsoids, and for more practical shapes like tangent-ogives with boattails. Special attention is given to bodies which have been studied for area-rule applications. Agreement with available experimental results is good in cases where viscous effects and wind-tunnel wall interference are not important.

Theoretical and Measured Electron-Density Distributions at High Altitudes

Abstract: A viscous shock-layer analysis has been developed and applied to the calculation of nonequilibrium-flow species distributions in the plasma layer of a blunt-nosed vehicle at high altitudes. The theoretical electron-density results obtained are in good agreement with those measured in flight for a hemisphere-9 cone entry vehicle. The flight measurements were obtained using electrostatic probes that protruded well into the shock layer. In addition, the theoretically obtained heavy-particle translational temperatures appear to agree fairly well with the electron temperatures that were measured in the flight experiments using voltage-swept thin-wire electrostatic probes. The influence of the reaction-rate coefficients on the calculated electron densities has been assessed and shown to be within the uncertainty in the flight data. The theoretical results demonstrate the importance of including in the chemical model the positive ions N2 + , O2 + , N + , and O + , in addition to NO + , for the high altitudes and velocities considered here.

Low speed aerodynamic characteristics of a 17 percent thick airfoil section designed for general aviation applications

Abstract: Wind-tunnel tests have been conducted to determine the low-speed two-dimensional aerodynamic characteristics of a 17-percent-thick airfoil designed for general aviation applications (GA(W)-1). The results were compared with predictions based on a theoretical method for calculating the viscous flow about the airfoil. The tests were conducted over a Mach number range from 0.10 to 0.28. Reynolds numbers based on airfoil chord varied from 2.0 million to 20.0 million. Maximum section lift coefficients greater than 2.0 were obtained and section lift-drag ratio at a lift coefficient of 1.0 (climb condition) varied from about 65 to 85 as the Reynolds number increased from about 2.0 million to 6.0 million.

Design considerations for the airframe-integrated scramjet

Abstract: Research programs at the NASA Langley Research Center on the development of airframe-integrated scramjet concepts (supersonic combustion ramjet) are reviewed briefly. The design and performance of a specific scramjet configuration are examined analytically by use of recently developed and substantiated techniques on boundary-layer development, heat transfer, fuel-air mixing, heat-release rates, and engine-cycle analysis. These studies indicate that the fixed-geometry scramjet module will provide practical levels of thrust performance with low cooling requirements. Areas which need particular emphasis in further development work are the combustor design for low speeds and the integrated nozzle design.

Theoretical Post-Yielding Behavior of Composite Laminates Part II-Inelastic Macromechanics

Abstract: This paper and a subsequent one will present the results of an investiga tion which analytically relates the fiber/matrix nonlinearities to the post- yielding behavior of symmetric laminates made from unidirectional plies. This first paper is primarily concerned with the behavior of the single ply and the means for anticipating its response within a laminate. Boron and carbon/epoxies are considered along with a metal matrix composite. The qualitative features of the response of these unidirectional materials to normal, shear, and combined loads are discussed. The analysis (based on triangular finite element idealizations of regularly-spaced inclusion arrays) anticipates the level of nonlinearity that has been observed in composite shear and transverse tensile tests. Also, it reveals a nonlinear coupling that may exist between the combined shear and normal stress response.

Analytical and experimental studies of shock interference heating in hypersonic flows

Abstract: An analytical and experimental study is presented of the aerodynamic heating resulting from six types of shock interference patterns encountered in high speed flow. Centerline measurements of pressure and heat transfer distributions on basic bodies were obtained in four wind tunnels for Mach numbers from 6 to 20, specific heat ratios from 1.27 to 1.67, and free stream Reynolds numbers from 3 million to 25.6 million per meter. Peak heating and peak pressures up to 17 and 7.5 times stagnation values, respectively, were measured. In general, results obtained from semiempirical methods developed for each of the six types of interference agreed with the experimental peaks.

Inviscid Surface Streamlines and Heat Transfer on Shuttle-Type Configurations

Abstract: A method is developed which calculates laminar, transitional, and turbulent heating rates on arbitrary blunt-nosed three-dimensional bodies at angle of attack in hypersonic flow. The geometry of the body may be specified analytically, or generated from a doubly cubic spline fit to coordinate points. Inviscid surface streamlines are calculated from Euler's equation using a prescribed pressure distribution. Laminar and turbulent heating rates are determined along a streamline by applying the axisymmetric analog to solutions of the axisymmetric boundary-layer equations. The location of the transition region may be specified optionally by geometric location, momentum thickness Reynolds number, or integrated unit Reynolds number along a streamline. Transitional heating rates are then calculated as a weighted average of the local laminar and turbulent values. Either ideal gas or equilibrium air properties may be used. Results are presented for blunted circular cones, and a typical delta-wing space shuttle orbiter at angle of attack. In comparison with experimental data, the present method was found to yield accurate laminar heating rates and reasonably accurate transitional and turbulent heating rates. The computer program developed to calculate the results presented herein requires only a few seconds of computing time per streamline on the CDC 6600 computer.

Automated procedure for design of wing structures to satisfy strength and flutter requirements

Abstract: A pilot computer program was developed for the design of minimum mass wing structures under flutter, strength, and minimum gage constraints. The wing structure is idealized by finite elements, and second-order piston theory aerodynamics is used in the flutter calculation. Mathematical programing methods are used for the optimization. Computation times during the design process are reduced by three techniques. First, iterative analysis methods used to reduce significantly reanalysis times. Second, the number of design variables is kept small by not using a one-to-one correspondence between finite elements and design variables. Third, a technique for using approximate second derivatives with Newton's method for the optimization is incorporated. The program output is compared witH previous published results. It is found that some flutter characteristics, such as the flutter speed, can display discontinous dependence on the design variables (which are the thicknesses of the structural elements). It is concluded that it is undesirable to use such quantities in the formulation of the flutter constraint.

Coordinated adaptive washout for motion simulators

Abstract: This paper introduces a new method of providing motion cues to a moving base six-degree-of-freedom flight simulator utilizing nonlinear filters. Coordinated adaptive filters, used to coordinate translational and rotational motion, are derived based on the method of continuous steepest descent, and the basic concept of the digital controllers used for the uncoordinated heave and yaw cues is also presented. The coordinated adaptive washout method is illustrated by an application in a six-degree-of-freedom fixed-base environment.

High-Reynolds-Number Cryogenic Wind Tunnel

Abstract: Theoretical considerations indicate that cooling the wind-tunnel test gas to cryogenic temperatures will provide a large increase in test Reynolds number with no increase in dynamic pressure while reducing the tunnel drive-power requirements. Studies have been made to determine the expected variations of Reynolds number and other parameters over wide ranges of Mach number, pressure, and temperature with due regard to avoiding liquefaction and adverse real-gas effects. Practical operational procedures have been developed in a low-speed prototype cryogenic wind tunnel. Aerodynamic experiments in the facility have demonstrated the theoretically predicted variations in Reynolds number and drive power. Force and moment measurements on a wing model mounted on a water-jacketed strain-gage sting balance have demonstrated the feasibility of operation of such balances in a cryogenic environment.

A Quick Accurate Method to Measure the Dielectric Constant of Microwave Integrated-Circuit Substrates (Short Papers)

Abstract: A technique is described that makes possible the accurate measurement of the dielectric constant of microwave integrated-circuit substrates. The substrate is metallized on all sides, hence forming a tiny resonant cavity, and the resonant frequencies are determined either from transmission or reflection. The dielectric constant is then calculated to an accuracy of better than 1 percent.

Bayesian Estimation of Life Parameters in the Weibull Distribution

Abstract: This paper develops a Bayesian analysis of the scale and shape parameters in the Weibull distribution and the corresponding reliability function with respect to the usual life-testing procedures. For the scale parameter I¸, Bayesian estimates of I¸ and reliability are obtained for the uniform, exponential, and inverted gamma prior probability densities. Bhattacharya's results [J. Am. Stat. Assn. 62, 48-62 1967] for the one-parameter exponential life-testing distribution are reduced to a special case of these results. The paper develops a fully Bayesian analysis of both the scale and shape parameters I¸ and I¾ by assuming independent prior distributions; since in the latter case, analytical tractability is not possible, Bayesian estimates are obtained through a conjunction of a Monte Carlo simulation and numerical-integration techniques. In both cases, the paper carries out a computer simulation and makes a comparison between the Bayesian and the corresponding minimum-variance unbiased, or maximum likelihood, estimates. As expected, the Bayesian estimates are superior.

Motion software for a synergistic six-degree-of-freedom motion base

Abstract: Computer software for the conversion of fixed-base simulations into moving-base simulations utilizing a synergistic six-degree-of-freedom motion simulator has been developed. This software includes an actuator extension transformation, inverse actuator extension transformation, a centroid transformation, and a washout circuit. Particular emphasis is placed upon the washout circuitry as adapted to fit the synergistic motion simulator. The description of the washout circuitry and illustration by means of a sample flight emphasize that translational cue representation may be of good fidelity, but care in the selection of parameters is very necessary, particularly in regard to anomalous rotational cues.

Heat Transfer to a Hemispherical Body in a Supersonic Argon Plasma

Abstract: Heat transfer was mcasured for conducting (copper) and nonconducting (teflon) models in a Mach 4.6 low-density Ar plasma in the absence and presence of magnetic fields up to 0.5 Telsa. Results show the importance of local wall currents both with and without magnetic field. For the conducting model, an electron flux entering the surface increased the stagnation-region heat flux compared to the nonconducting model. For the nonconducting model, theory predicts well the heat transfer for zero field but fails to predict the slight increase at the stagnation region with high magnetic field. (DLC)

Compensation based on linearized analysis for a six degree of freedom motion simulator

Abstract: The inertial response characteristics of a synergistic, six-degree-of-freedom motion base are presented in terms of amplitude ratio and phase lag as functions of frequency data for the frequency range of interest (0 to 2 Hz) in real time, digital, flight simulators. The notch filters which smooth the digital-drive signals to continuous drive signals are presented, and appropriate compensation, based on the inertial response data, is suggested. The existence of an inverse transformation that converts actuator extensions into inertial positions makes it possible to gather the response data in the inertial axis system.

An Overview of Artificial Gravity

Abstract: The unique characteristics of artificial gravity that affect human performance and physiology in an artificial gravity environment are reviewed. The rate at which these unique characteristics change decreases very rapidly with increasing radius of a rotating vehicle used to produce artificial gravity. Reducing their influence on human performance or physiology by increasing radius becomes a situation of very rapidly diminishing returns. A review of several elements of human performance has developed criteria relative to the sundry characteristics of artificial gravity. A compilation of these criteria indicates that the maximum acceptable rate of rotation, leg heaviness while walking, and material handling are the factors that define the minimum acceptable radius. The ratio of Coriolis force to artificial weight may also be significant. Based on current knowledge and assumptions for the various criteria, a minimum radius between 15.2 and 16.8 m seems desirable.

A parallel variable metric optimization algorithm

Abstract: An algorithm, designed to exploit the parallel computing or vector streaming (pipeline) capabilities of computers is presented. When p is the degree of parallelism, then one cycle of the parallel variable metric algorithm is defined as follows: first, the function and its gradient are computed in parallel at p different values of the independent variable; then the metric is modified by p rank-one corrections; and finally, a single univariant minimization is carried out in the Newton-like direction. Several properties of this algorithm are established. The convergence of the iterates to the solution is proved for a quadratic functional on a real separable Hilbert space. For a finite-dimensional space the convergence is in one cycle when p equals the dimension of the space. Results of numerical experiments indicate that the new algorithm will exploit parallel or pipeline computing capabilities to effect faster convergence than serial techniques.

Application of sonic-boom minimization concepts in supersonic transport design

Abstract: The applicability of sonic boom minimization concepts in the design of large supersonic transport airplanes capable of a 2500-nautical-mile range at a cruise Mach number of 2.7 is considered. Aerodynamics, weight and balance, and mission performance as well as sonic boom factors, have been taken into account. The results indicate that shock-strength nominal values of somewhat less than 48 newtons/sq m during cruise are within the realm of possibility. Because many of the design features are in direct contradiction to presently accepted design practices, further study of qualified airplane design teams is required to ascertain sonic boom shock strength levels actually attainable for practical supersonic transports.

Photosensor aperture shaping to reduce aliasing in optical-mechanical line-scan imaging systems.

Abstract: Optical-mechanical scanning techniques are generally employed in instruments specifically designed to characterize variations in scene brightness spectrally or radiometrically. The effect of aliasing, which can be caused by line-scan sampling, on the spatial detail of the reconstructed image has therefore been of little concern. Emphasis of some recent applications of optical-mechanical scanning techniques is, however, on the spatial characterization of the scene. As is shown here, such images can be severely degraded by aliasing. Photosensor aperture shaping and line-scan spacing are investigated as a means for reducing this degradation.

Development of the Viking Parachute Configuration by Wind-Tunnel Investigation

Abstract: Several experimental investigations to obtain the drag performance of 10% scale-model disk-gap-band-type parachute assemblies trailing in the Viking forebody wake were conducted over the range of Mach 0.2 to 2.6. The wind-tunnel tests varied both the canopy trailing distance and ratio of suspension line length to canopy diameter. The data obtained permitted optimization of both parameters. Parachute drag performance in the forebody wake is markedly degraded transonically beginning at Mach 0.6, reaching a minimum value at Mach 1.0, then approaches the subsonic value for the selected configuration as Mach number is increased to 1.4. Further increase in Mach number causes the drag coefficient to again decline. A 20% increase in parachute-alone transonic drag coefficient was obtained by increasing suspension-line length ratio from 1.16 to 1.73.

Multipath Modeling for Aeronautical Communications

Abstract: One of the fundamental technical problems in aeronautical digital communications is that of multipath propagation between aircraft and ground terminal. This paper examines in detail a model of the received multipath signal that is useful for application of modern detection and estimation theories. The model treats arbitrary modulation and covers the selective and nonselective cases. The necessarily nonstationary statistics of the received signal are determined from the link geometry and the surface roughness parameters via a Kirchhoff solution. Use of the model in solving detection and estimation problems is outlined. Results of a flight experiment are presented and compared to a digital simulation for verification of the model.

Three-Dimensional Separation for Interaction of Shock Waves with Turbulent Boundary Layers

Abstract: For the interaction of shock waves with turbulent boundary layers, obtained experimental three-dimensional separation results and correlations with earlier two-dimensional and three-dimensional data are presented. It is shown that separation occurs much earlier for turbulent three-dimensional than for two-dimensional flow at hypersonic speeds.