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At least 271 records · Page 15

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NASA NTRS1974

The subsequence weight distribution of summed maximum length digital sequences

An attempt is made to develop mathematical formulas to provide the basis for the design of pseudorandom signals intended for applications requiring accurate knowledge of the statistics of the signals. The analysis approach involves calculating the first five central moments of the weight distribution of subsequences of hybrid-sum sequences. The hybrid-sum sequence is formed from the modulo-two sum of k maximum length sequences and is an extension of the sum sequences formed from two maximum length sequences that Gilson (1966) evaluated. The weight distribution of the subsequences serves as an approximation to the filtering process. The basic reason for the analysis of hybrid-sum sequences is to establish a large group of sequences with good statistical properties. It is shown that this can be accomplished much more efficiently using the hybrid-sum approach rather than forming the group strictly from maximum length sequences.

Weathers, G. D.↗

NASA NTRS1974

Minimum-weight designs for hat-stiffened composite panels under uniaxial compression

Optimum hat-stiffened compression panel designs are determined by a structural synthesis technique. The effects of simplifying assumptions made in the buckling analysis for the optimization program are investigated by a linked plate element program. Optimization results for hat-stiffened graphite-epoxy panels show a 50-percent weight savings over optimized aluminum panels. Composite panels are shown to possess a variety of proportions at nearly constant weight.

Agarwal, B.↗

NASA NTRS1975

Computer program to assess impact of fatigue and fracture criteria on weight and cost of transport aircraft

A preliminary design analysis tool for rapidly performing trade-off studies involving fatigue, fracture, static strength, weight, and cost is presented. Analysis subprograms were developed for fatigue life, crack growth life, and residual strength; and linked to a structural synthesis module which in turn was integrated into a computer program. The part definition module of a cost and weight analysis program was expanded to be compatible with the upgraded structural synthesis capability. The resultant vehicle design and evaluation program is named VDEP-2. It is an accurate and useful tool for estimating purposes at the preliminary design stage of airframe development. A sample case along with an explanation of program applications and input preparation is presented.

Tanner, C. J.↗

NASA NTRS1975

Effects of nacelle shape on drag and weight of a supersonic cruising aircraft

The quantitive relationship of cruise drag and nacelle shape was investigated for a representative advanced supersonic transport configuration. Nacelle shape parameters were systematically varied, and the effects of these variations on wave and friction drag were determined. The effects of changes in vehicle drag, propulsion weight, and specific fuel consumption on vehicle takeoff gross weight were computed. Generally, it was found that nacelle shapes such that the maximum cross-sectional area occurred at or near the nozzle exit resulted in the lowest wave drag. In fact, nacelle shapes were found that produce favorable interference effects (drag reduction) of such magnitude as to nearly offset the friction drag of the nacelle.

Bonner, E.↗

NASA NTRS1975

Automobile ride quality experiments correlated to iso-weighted criteria

As part of an overall study to evaluate the usefulness of ride quality criteria for the design of improved ground transportation systems an experiment was conducted involving subjective and objective measurement of ride vibrations found in an automobile riding over roadways of various roughness. Correlation of the results led to some very significant relationships between passenger rating and ride accelerations. The latter were collapsed using a frequency-weighted root mean square measure of the random vibration. The results suggest the form of a design criterion giving the relationship between ride vibration and acceptable automobile ride quality. Further the ride criterion is expressed in terms that relate to rides with which most people are familiar. The design of the experiment, the ride vibration data acquisition, the concept of frequency weighting and the correlations found between subjective and objective measurements are presented.

Healey, A. J.↗

NASA NTRS1976

Weight comparisons of optimized stiffened, unstiffened, and sandwich cylindrical shells made from composite or aluminum materials

This work presents optimum designs for unstiffened, hat stringer-stiffened and honeycomb sandwich cylinders under axial compression. Optimization results for graphite-epoxy cylinders show about a 50 percent weight savings over corresponding optimized aluminum cylinders for a wide loading range. The inclusion of minimum gage considerations results in a significant weight penalty, especially for a lightly loaded cylinder. Effects of employing a smeared stiffener buckling theory in the optimization program are investigated through comparison of results obtained from a more accurate branched shell buckling computer code. It was found that the stiffener cross-sectional deformations, which are usually ignored in smeared stiffener theory, result in about a 30 percent lower buckling load for the graphite-epoxy hat stiffened cylinder.

Agarwal, B. L.↗

NASA NTRS1976

An algorithm for the weighting matrices in the sampled-data optimal linear regulator problem

The sampled-data optimal linear regulator problem provides a means whereby a control designer can use an understanding of continuous optimal regulator design to produce a digital state variable feedback control law which satisfies continuous system performance specifications. A basic difficulty in applying the sampled-data regulator theory is the requirement that certain digital performance index weighting matrices, expressed as complicated functions of system matrices, be computed. Infinite series representations are presented for the weighting matrices of the time-invariant version of the optimal linear sampled-data regulator problem. Error bounds are given for estimating the effect of truncating the series expressions after a finite number of terms, and a method is described for their computer implementation. A numerical example is given to illustrate the results.

Armstrong, E. S.↗

NASA NTRS1976

Research requirements to reduce empty weight of helicopters by use of advanced materials

Utilization of the new, lightweight, high-strength, aerospace structural-composite (filament/matrix) materials, when specifically designed into a new aircraft, promises reductions in structural empty weight of 12 percent at recurring costs competive with metals. A program of basic and applied research and demonstration is identified with the objective of advancing the state of the art to the point where civil helicopters are confidently designed, produced, certified, and marketed by 1985. A structural empty-weight reduction of 12 percent was shown to significantly reduce energy consumption in modern high-performance helicopters.

Hoffstedt, D. J.↗

NASA NTRS1977

Analysis of turbofan propulsion system weight and dimensions

Weight and dimensional relationships that are used in aircraft preliminary design studies are analyzed. These relationships are relatively simple to prove useful to the preliminary designer, but they are sufficiently detailed to provide meaningful design tradeoffs. All weight and dimensional relationships are developed from data bases of existing and conceptual turbofan engines. The total propulsion system is considered including both engine and nacelle, and all estimating relations stem from physical principles, not statistical correlations.

Waters, M. H.↗

NASA NTRS1977

A method to estimate weight and dimensions of aircraft gas turbine engines. Volume 1: Method of analysis

Weight and envelope dimensions of aircraft gas turbine engines are estimated within plus or minus 5% to 10% using a computer method based on correlations of component weight and design features of 29 data base engines. Rotating components are estimated by a preliminary design procedure where blade geometry, operating conditions, material properties, shaft speed, hub-tip ratio, etc., are the primary independent variables used. The development and justification of the method selected, the various methods of analysis, the use of the program, and a description of the input/output data are discussed.

Pera, R. J.↗

NASA NTRS1976

Selecting class weights to minimize classification bias in acreage estimation

Preliminary results of experiments being performed to select optimal class weights for use with the maximum likelihood classifier in acreage estimation using remote sensor imagery are presented. These weights will be optimal in the sense that the bias will be minimized in the proportion estimate obtained from the classification results by sample counting. The procedure was tested using Landsat MSS data from an 8 by 9.6 km area of ground truth in Finney County, Kansas.

Belcher, W. M.↗

NASA NTRS1977

A multilevel approach for minimum weight structural design including local and system buckling constraints

A rational multilevel approach for minimum weight structural design of truss and wing structures including local and system buckling constraints is presented. Overall proportioning of the structure is achieved at the system level subject to strength, displacement and system buckling constraints, while the detailed component designs are carried out separately at the component level satisfying local buckling constraints. Total structural weight is taken to be the objective function at the system level while employing the change in the equivalent system stiffness of the component as the component level objective function. Finite element analysis is used to predict static response while system buckling behavior is handled by incorporating a geometric stiffness matrix capability. Buckling load factors and the corresponding mode shapes are obtained by solving the eigenvalue problem associated with the assembled elastic stiffness and geometric stiffness matrices for the structural system. At the component level various local buckling failure modes are guarded against using semi-empirical formulas. Mathematical programming techniques are employed at both the system and component level.

Schmit, L. A., Jr.↗

NASA NTRS1977

Transmission in nonuniform ducts - A comparative evaluation of finite element and weighted residuals computational schemes

The Method of Weighted Residuals (MWR) and the Finite Element Method (FEM) are considered as computational schemes in the problem of acoustic transmission in nonuniform ducts. MWR is presented in an improved form which includes the interaction of acoustic modes (irrotational) and hydrodynamic modes (rotational). FEM is based on a weighted residuals formulation using eight noded isoparametric elements. Both are applicable to two-dimensional and axially symmetric problems. Calculations are made for several sample problems to demonstrate accuracy and relative efficiency. One test case has implications in the phenomenon of subsonic acoustic choking and it is found that a large transmission loss is not an automatic consequence of propagation against a high subsonic mean flow.

Eversman, W.↗

NASA NTRS1978

Friction and wear of polyethylene oxide polymer having a range of molecular weights

Sliding friction and wear experiments were conducted at light loads (25 to 250 g) with various molecular weights of the polyethylene oxide polymer sliding on itself and iron. Results of the experimental investigation indicate that: (1) the coefficient of friction for the polymer decreases with increasing molecular weight; (2) friction coefficient is higher for the polymer sliding on itself than it is for the polymer sliding on iron; (3) at sufficiently high loads localized surface melting occurs and the friction coefficient is the same for the polymer sliding on itself and iron; (4) fracture cracks develop in the sliding wear track at higher but not lower sliding velocities, reflecting a strain rate sensitivity to crack initiation, and (5) the friction coefficient for the polymer sliding on iron increases with the formation of a polymer film on the iron surface.

Buckley, D. H.↗

NASA NTRS1978

A finite element method for transmission in non-uniform ducts without flow Comparison with the method of weighted residuals

A finite element method is derived for studying the transmission of sound in nonuniform ducts without flow. The method is based on a weighted residuals technique and eight noded isoparametric elements. The method employs two computational schemes: the Helmholtz equation found through conservation equations, and the conservation equations themselves. Solutions are found for transmission and reflection coefficients for both two-dimensional and circular geometries. Numerical solutions are compared with those for a method of weighted residuals in the form of a modified Galerkin method, showing good agreement.

Astley, R. J.↗

NASA NTRS1978

Effect of weighting on time sidelobe suppression

Weighting simulations in the time domain were considered to shape the time-compressed pulse waveform. The digital input radar data were 32 bit I,Q, and simulated data from a point target, imaged by a Seasat-A type system. Weighting functions tested include stepped-amplitude distributions, (with 1 through 5 steps), and the cosine-squared plus pedestal distribution. Effects treated include mainlobe ratio, signal to noise ratio, and nearest sidelobe suppression.

Dicenzo, A.↗

NASA NTRS1979

A method to estimate weight and dimensions of large and small gas turbine engines

A computerized method was developed to estimate weight and envelope dimensions of large and small gas turbine engines within + or - 5% to 10%. The method is based on correlations of component weight and design features of 29 data base engines. Rotating components were estimated by a preliminary design procedure which is sensitive to blade geometry, operating conditions, material properties, shaft speed, hub tip ratio, etc. The development and justification of the method selected, and the various methods of analysis are discussed.

Onat, E.↗

NASA NTRS1976

EDIN0613P weight estimating program

The weight estimating relationships and program developed for space power system simulation are described. The program was developed to size a two-stage launch vehicle for the space power system. The program is actually part of an overall simulation technique called EDIN (Engineering Design and Integration) system. The program sizes the overall vehicle, generates major component weights and derives a large amount of overall vehicle geometry. The program is written in FORTRAN V and is designed for use on the Univac Exec 8 (1110). By utilizing the flexibility of this program while remaining cognizant of the limits imposed upon output depth and accuracy by utilization of generalized input, this program concept can be a useful tool for estimating purposes at the conceptual design stage of a launch vehicle.

Hirsch, G. N.↗