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At least 361 records · Page 20

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NASA NTRS1980

A new approach to active control of rotorcraft vibration

A state-variable feedback approach is utilized for active control of rotorcraft vibration. Fuselage accelerations are passed through undamped second-order filters with resonant frequencies at N/rev. The resulting outputs contain predominantly the N/rev vibration components, phase shifted by 180 deg, and are used to drive the blade pitch to cancel this component of fuselage vibration. The linear-quadratic-gaussian (LQG) method is used to design a feedback control system utilizing these filtered accelerations. The design is based on a nine-degree-of-freedom linear model of the Rotor System Research Aircraft (RSRA) in hover and is evaluated on a nonlinear blade-element simulation of the RSRA for this flight condition. The system is shown to essentially eliminate vibrations at N/rev in all axes. The required blade-pitch amplitude is within the capability of conventional actuators at the N/rev frequency.

Gupta, N. K.↗

NASA NTRS1980

A laboratory study on the dissociative recombination of vibrationally excited O2/+/ions

The dissociative recombination of vibrationally excited O2(+) ions is studied in light of the possible importance of this reaction in upper atmospheric chemistry. A plasma spectroscopy experiment was performed in a microwave cavity filled by an argon-oxygen mixture, with O(1S) production monitored by measurements of the 5577-A afterglow, the O2(+) density and the electron concentration. Plasma and optical data reveal the predominant afterglow ions to be Ar2(+) and O2(+), with an effective O(1S) dissociative recombination coefficient of 2.1 x 10 to the -8th cu cm/sec, corresponding to a quantum yield of 10%. Experiments with an argon-krypton-oxygen mixture reveal that vibrationally excited O2(+) ions are the chief source of the O(1S) atoms, with a specific recombination coefficient for the dissociation of O2(+)(2 pi g) into O(1S) and O(1D) of 4.2 x 10 to the -9th cu cm/sec. A comparison of the laboratory results with Atmospheric Explorer data on the 5577-A airglow implies that O2(+) ions in the sunlit ionosphere are vibrationally excited to the same degree as in the laboratory, with the vibrational relaxation of these ions much slower than dissociative recombination. Results also predict a dawn-twilight asymmetry in the effective O(1S) yield due to the normal variation of electron content.

Zipf, E. C.↗

NASA NTRS1982

Damping Vibration at an Impeller

Vibration of pump shaft is damped at impeller--where vibration-induced deflections are greatest--by shroud and seal. Damping reduces vibrational motion of shaft at bearings and load shaft places on them. Flow through clearance channel absorbs vibration energy.

Hager, J. A.↗

NASA NTRS1980

Vibration characteristics of a steadily rotating slender ring

Partial differential equations are derived to describe the structural vibrations of a uniform homogeneous ring which is very flexible because the radius is very large compared with the cross sectional dimensions. Elementary beam theory is used and small deflections are assumed in the derivation. Four sets of structural modes are examined: bending and compression modes in the plane of the ring; bending modes perpendicular to the plane of the ring; and twisting modes about the centroid of the ring cross section. Spatial and temporal characteristics of these modes, presented in terms of vibration frequencies and ratios between vibration amplitudes, are demonstrated in several figures. Given a sufficiently high rotational rate, the dynamics of the ring approach those of a vibrating string. In this case, the velocity of traveling wave in the material of the ring approaches in velocity of the material relative to inertial space, resulting in structural modes which are almost stationary in space.

Lallman, F. J.↗

NASA NTRS1981

Nonlinear in-plane and out-of-plane vibrations in solar arrays

This paper presents the results of analytical investigations of nonlinear effects in in-plane and out-of-plane vibrations of solar arrays caused by surface wrinkling of solar array blankets. It is shown that nonlinearities generated by wrinkles are important even for small deformations. Approximate analytical solutions describing the frequency and damping dependencies of the amplitudes of in-plane vibrations are derived. A new phenomenon - a parametrical resonance of out-of-plane vibrations induced by in-plane vibrations - is described.

Zak, M. A.↗

NASA NTRS1980

An active control system for helicopter vibration reduction by higher harmonic pitch

An analytical study defining the basic configuration of an active control system to reduce helicopter vibrations is presented. Theoretical results for a nonlinear four-bladed single rotor helicopter simulation are discussed, showing that vibration reductions on the order of 80-90% for airspeeds up to 150 kn can be expected when using a higher harmonic pitch in an active feedback control system. The rotor performance penalty associated with this level of vibration reduction is about 1-3% and the increase in rotor blade stresses is considered to be low. The location of sensor accelerometers proved to be significant for vibration reductions, and it is noted that the RTSA controller is tolerant of sensor signal noise.

Taylor, R. B.↗

NASA NTRS1982

Application of a computerized vibroacoustic data bank for random vibration criteria development

A computerized data bank system was developed for utilization of large amounts of vibration and acoustic data to formulate component random vibration design and test criteria. This system consists of a computer, graphics tablets, and a dry silver hard copier which are all desk top type hardware and occupy minimal space. Currently, the data bank contains data from the Saturn 5 and Titan 3 flight and static test programs. The vibration and acoustic data are stored in the form of power spectral density and one third octave band plots over the frequency range from 20 to 2000 Hz. The data were stored by digitizing each spectral plot by tracing with the graphics tablet. The digitized data were statistically analyzed, and the resulting 97.5 percent confidence levels were stored on tape along with the appropriate structural parameters. Standard extrapolation procedures were programmed for prediction of component random vibration test criteria for new launch vehicle and payload configurations. A user's manual is included to guide potential users through the programs.

Ferebee, R. C.↗

NASA NTRS1981

Design and evaluation of a state-feedback vibration controller

The effectiveness and practicality of the state-feedback vibration controller is demonstrated by computer simulations. It is shown that: (1) the required measurements can be reduced to fuselage accelerations; (2) the system is stable over a wide range of flight conditions; (3) a discrete design for the controller provides effective digital implementation at 80-Hz sampling; (4) attenuation produced by the actuators does not create a problem, and a simple lead compensator provides vibration reduction; and (5) a design based on a hover model worked well at five knots. At 40 and 120 knots, vibrations in all but the vertical acceleration were effectively suppressed. The state-feedback vibration controller has a rapid convergence time and implementation is easier than with multicyclic control since no on-line harmonic analysis or identification algorithms are required.

Du Val, R. W.↗

NASA NTRS1982

Rotor vibration caused by external excitation and rub

For turbomachinery with low natural frequencies, considerations have been recently required for rotor vibrations caused by external forces except unbalance one, such as foundation motion, seismic wave, rub and so forth. Such a forced vibration is investigated analytically and experimentally in the present paper. Vibrations in a rotor-bearing system under a harmonic excitation are analyzed by the modal technique in the case of a linear system including gyroscopic effect. For a nonlinear system a new and powerful quasi-modal technique is developed and applied to the vibration caused by rub.

Matsushita, O.↗

NASA NTRS1982

Vibration and destabilizing effects of floating ring seals in compressors

Operating experience on a compressor commissioned 12 years ago has presented an interesting history of sporadic increases in shaft vibration. Initial operation was satisfactory with low levels of vibration. However, after some time the shaft vibration level increased to several mils. Initially this was believed to be due to rotor unbalance from deposits formed in the passages due to process upsets. After cleaning up the rotor, operation was again increased. It was then found that the rotor vibration was primarily subsynchronous. Further investigation revealed that the original seal design was subject to wear and was no longer properly pressure balanced. A modified seal design was installed and it has operated successfully for the past six years.

Emerick, M. F.↗

NASA NTRS1983

Vibration characteristics of hexagonal radial rib and hoop platforms

Experiment and analysis have been used to characterize the modes of vibration of planar radial rib and hoop hexagonal platforms. Finite element analysis correlated very well with experimental results. The sensitivity of mode shapes and frequencies to cable stiffness and initial tension is presented. Threshold values have been identified, above which changes in cable stiffness do not affect the first few platform vibration modes. Primary vibration modes of the radial rib platform involve beam bending. Vibration modes of the hoop platform exhibit both beam bending and frame bending and torsion. Results indicate for low order polygonal structures, the radial rib concept produced a higher fundamental frequency. For high order polygonal structures, the hoop concept has the potential to achieve a higher fundamental frequency than the radial rib concept.

Belvin, W. K.↗

NASA NTRS1984

Evaluation of ride quality prediction methods for helicopter interior noise and vibration environments

The results of a simulator study conducted to compare and validate various ride quality prediction methods for use in assessing passenger/crew ride comfort within helicopters are presented. Included are results quantifying 35 helicopter pilots discomfort responses to helicopter interior noise and vibration typical of routine flights, assessment of various ride quality metrics including the NASA ride comfort model, and examination of possible criteria approaches. Results of the study indicated that crew discomfort results from a complex interaction between vibration and interior noise. Overall measures such as weighted or unweighted root-mean-square acceleration level and A-weighted noise level were not good predictors of discomfort. Accurate prediction required a metric incorporating the interactive effects of both noise and vibration. The best metric for predicting crew comfort to the combined noise and vibration environment was the NASA discomfort index.

Leatherwood, J. D.↗

NASA NTRS1984

Vibrational and rotational excitation of CO in comets. Part 1: Non-equilibrium calculations. Part 2: Results of the calculation for standard bright comet, comet Iras-Araki-Alcock and comet Halley

The vibrational and rotational excitation of the CO molecule in cometary comae were investigated using a model which includes IR vibrational pumping by the solar flux, vibrational and rotational radiative decay, and collisional coupling among rotational states. Steady state was not assumed in solving the rate equations. The evolution of a shell of CO gas was monitored as it expanded from the nucleus into the outer coma. Collisional effects were treated using a kinetic temperature profile derived from theoretical work on the coma energy balance. The kinetic temperature was assumed to be extremely cold in the inner coma; this has significant consequences for the CO excitation. If optical depth effects are ignored, only low J transitions will be significantly excited in comets observed at high spatial resolution. Ground-based observations of CO co-vibrational and rotational transitions will be extremely difficult due to lack of sensitivity and/or terrestrial absorption. However, CO should be detectable from a large comet with favorable observing geometry if the CO is a parent molecule present at the 10% level (or greater) relative to H2O. Observations using cooled, spaceborne instruments should be capable of detecting CO emission from even moderately bright comets.

Chin, G.↗

NASA NTRS1984

Refinement and evaluation of helicopter real-time self-adaptive active vibration controller algorithms

A Real-Time Self-Adaptive (RTSA) active vibration controller was used as the framework in developing a computer program for a generic controller that can be used to alleviate helicopter vibration. Based upon on-line identification of system parameters, the generic controller minimizes vibration in the fuselage by closed-loop implementation of higher harmonic control in the main rotor system. The new generic controller incorporates a set of improved algorithms that gives the capability to readily define many different configurations by selecting one of three different controller types (deterministic, cautious, and dual), one of two linear system models (local and global), and one or more of several methods of applying limits on control inputs (external and/or internal limits on higher harmonic pitch amplitude and rate). A helicopter rotor simulation analysis was used to evaluate the algorithms associated with the alternative controller types as applied to the four-bladed H-34 rotor mounted on the NASA Ames Rotor Test Apparatus (RTA) which represents the fuselage. After proper tuning all three controllers provide more effective vibration reduction and converge more quickly and smoothly with smaller control inputs than the initial RTSA controller (deterministic with external pitch-rate limiting). It is demonstrated that internal limiting of the control inputs a significantly improves the overall performance of the deterministic controller.

Davis, M. W.↗

NASA NTRS1982

A formulation of rotor-airframe coupling for design analysis of vibrations of helicopter airframes

A linear formulation of rotor airframe coupling intended for vibration analysis in airframe structural design is presented. The airframe is represented by a finite element analysis model; the rotor is represented by a general set of linear differential equations with periodic coefficients; and the connections between the rotor and airframe are specified through general linear equations of constraint. Coupling equations are applied to the rotor and airframe equations to produce one set of linear differential equations governing vibrations of the combined rotor airframe system. These equations are solved by the harmonic balance method for the system steady state vibrations. A feature of the solution process is the representation of the airframe in terms of forced responses calculated at the rotor harmonics of interest. A method based on matrix partitioning is worked out for quick recalculations of vibrations in design studies when only relatively few airframe members are varied. All relations are presented in forms suitable for direct computer implementation.

Kvaternik, R. G.↗

NASA NTRS1984

Vibration and buckling of general periodic lattice structures

A method is presented for vibration and buckling analysis of arbitrary lattice structures having repetitive geometry in any combination of coordinate directions. The approach is based on exact member theory for representing the stiffness of an individual member subject to axial load, and in the case of vibration, undergoing harmonic oscillation. The method is an extension of previous work that was limited to specific geometries. The resulting eigenvalue problem is of the size associated with the repeating element of the structure. A computer program has been developed incorporating the theory and results are given for vibration of rectangular platforms and a large antenna structure having rotational symmetry. Buckling and vibration results for cable-stiffened rings are also given.

Anderson, M. S.↗

NASA NTRS1984

Laser measurements of the effects of vibrational energy on the reactions of CN

Pulsed laser photolysis of C2N2 at 193 nm has been used as a source of CN radicals in both the v-double-prime = 0 and v-double-prime = 1 levels. Individual rovibronic levels of these radicals were measured as a function of time with a tunable dye laser. From these measurements the rate constants for the reaction of each of these vibrational levels with H2, O2, CO, CO2, N2, HCN, C2N2, and CH4 have been determined. Some enhancement in the rate constant with vibrational energy which could not be ascribed to quenching was observed for O2, CH4, and H2. Only vibrational quenching was observed for HCN, N2, CO2, CO and C2N2. In the CO case the vibrational quenching rate appears to be significantly enhanced by complex formation during the quenching process.

Sayah, N.↗

NASA NTRS1984

The importance of nonlinearity on the higher harmonic control of helicopter vibration

The effect of nonlinearity on the higher harmonic control (HHC) of helicopter vibration is investigated using a nonlinear aeroelastic simulation. A nonlinear solution is proposed which relates the HHC inputs to vibration outputs on the basis of a Volterra functional series. The Volterra series solution is shown to reduce to a vector polynomial equation relating HHC inputs to vibration outputs at any harmonic frequency. The nonlinear transfer relationship is identified from a nonlinear vibration analysis computer program, and the identification model is examined in detail. Improvements to current HHC algorithms are presented, and several Kalman filter divergence problems are quantified.

Molusis, J. A.↗