SEARCH · Search NASA

Results for “lag”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3

All sources NASA NTRS DOE OSTI

NASA NTRS1994

Sensitivity of lag-damping correlations to structural and aerodynamic approximations of isolated experimental rotors in forward flight

The predictions of regressive lag-mode damping levels are correlated with the database of an isolated, soft-inplane, three-blade rotor operated untrimmed. The database was generated at the Army Aeroflightdynamics Directorate at Ames. The correlation covers a broad range of data, from near-zero thrust conditions in hover to high-thrust and highly stalled conditions in forward flight with advance ratio as high as 0.55 and shaft angle as high as 20 degrees. In the experimental rotor, the airfoil or blade portion has essentially uniform mass and stiffness distributions, but the root flexure has highly nonuniform mass and stiffness distributions. Accordingly, the structural approximations refer to four models of root-flexure-blade assembly. They range from a rigid flap-lag model to three elastic flap-lag-torsion models, which differ in modeling the root flexure. The three models of root-flexure are: three root springs in which the bending-torsion couplings are fully accounted for; a finite-length beam element with some average mass and stiffness distributions such that the fundamental frequencies match those of the experimental model; and accurate modal representation in which the actual mass and stiffness distributions of the experimental root-flexure-blade assembly are used in calculating the nonrotating mode shapes. The four models of root-flexure-blade assembly are referred to as the rigid flap-lag model, spring model, modified model and modal model. For each of these four models of the root-flexure-blade assembly, the predictions are based on the following five aerodynamic theories: ear theory, which accounts for large angle-of-attack and reverse-flow effects on lift, and has constant drag and pitching moment; quasisteady stall theory, which includes quasisteady stall lift, drag and pitching moment characteristics of the airfoil section, dynamics stall theory, which uses the ONERA dynamic stall models of lift, drag and pitching moment; dynamic wake theory, which is based on a finite-state three-dimensional wake model and includes all wake effects including both shed and trailing vorticity; and dynamics and wake theory, which combines both dynamic stall theory and dynamic wake theory and is a relatively complete aerodynamic representation.

Gaonkar, G. H.↗

NASA NTRS1943

Least-work analysis of the problem of shear lag in box beams

The distribution of stress in the cover sheets of thin-wall box beams is analyzed, with regard to the effect of shear deformation in the cover sheets, by the method of least work. Explicit results are obtained for a number of representative cases that show the influence of the following factors on the stress patterns. (1) Variation of stress in spanwise direction as given by elementary beam theory. (2) Value of a parameter called shear-lag aspect ratio which designates the product of span-width ratio of the beam and of the square root of the ratio of effective shear modulus and tensing modulus of the cover sheets. (3) Value of ratio of cover-sheet stiffness to side-web stiffness. (4) Variation of beam height in span direction. (5) Variation of beam width in span direction. (6) Variation of cover-sheet thickness in span direction. General conclusions are drawn from the results obtained. Among them the most important one appears to be the fact that the shear-lag effect depends primarily on the following tow quantities: (1) the value of the shear-lag aspect ratio. (2) the shape of the curve representing the product of the stress of elementary beam theory and of the cover-sheet thickness.

STRESS CALCULATIONS, LEAST-WORK↗

NASA NTRS1975

Lagged low order control systems with powered controls

With powered controls, a high order control system can be changed to a lower order control system with a lag, which is better suited to our position control skills and strategies. Separate groups of untrained men were trained on an acceleration control system, or on a rate control system with an exponential lag of either .5, 2, or 4 sec. Acquisition time was reliably shorter with all the lagged rate control systems than with the unlagged acceleration control system.

Poulton, E. C.↗

NASA NTRS1975

Time lag effects in the nucleation of particles in stellar atmospheres

When a system goes from a saturated state to a supersaturated one, the classical nucleation-rate formula is not instantly valid; rather, the nucleation rate relaxes exponentially (with a characteristic time - the time lag) to the value given by the steady-state formula. Under some circumstances, particularly those found in some cool stellar atmospheres, the time lag can be quite long and a decisve factor in determining the possibility of particle formation. Carbon condensation in the atmospheres of Mira variables is considered and found to be unlikely on the basis of classical nucleation theory because of the long time lags involved, unless the parameters describing the physical conditions in these atmospheres are significantly different from current estimates.

Walker, G. H.↗

NASA NTRS1976

An experimental investigation of the flap-lag stability of a hingeless rotor with comparable levels of hub and blade stiffness in hovering flight

An experimental investigation of the flap-lag stability of a hingeless rotor in hovering flight is presented and discussed. The rotor blade and hub configuration were selected such that the hub and blade had comparable levels of bending stiffness. Experimental measurements of the lag damping were made for various values of rotor rotational speed and blade pitch angle. Specifically at a blade pitch angle of 8 deg at three-quarters radius, the lag damping was determined over a range of rotational speeds from 200 RPM to 320 RPM and also over a range of blade pitch angles from 0 deg to 8 deg.

Curtiss, H. C., Jr.↗

NASA NTRS1981

A time lag study of the vertical motion simulator computer system

A study was performed to evaluate an experimental method to determine time lags in real-time computer systems as the one associated with the Vertical Motion Simulator at Ames Research Center. The approach was to use an ordinary frequency analyzer to measure the phase difference between inputs and outputs of the computer system. The various elements of the program and computational architecture were modeled. Various factors, such as computer frame time and input frequency, were varied so that they were representative of the operational use of the simulator facilities. Experimentally determined results were compared with predictions derived from the simulation models. The results indicate that the frequency analyzer can be readily used to evaluate time lags in systems of this type. Differences between predicted and measured phase values indicate that the hardware and software imparts a time lag of about 5 msec to this facility.

Cleveland, W. B.↗

NASA NTRS1982

Time lags in solar modulation

Von Rosenvinge and Paizis (1981) have discussed the amplitude variations of low energy cosmic rays, taking into account intensity-time variations which occurred starting in late 1973 and continued into mid-1975. The present investigation is concerned with the occurrence of so-called hysteresis effects during the considered period. A graph is presented which shows a comparison of the time-intensity plots for 35.3-82 MeV protons (340 MV) and the Deep River neutron monitor. A time lag can be observed between the neutron monitor intensity and the 35.3-82 MeV proton intensity. The same data is replotted to show a hysteresis loop. A plot for 35.1-82 MeV/n helium is also provided. It is found that all measured values of the time lag for protons and helium are consistent with those reported by Klecker et al. (1980). It is pointed out that the time lag for the anomalous helium is not consistent with the model of O'Gallagher (1975).

Paizis, C.↗

NASA NTRS1983

Lagged average forecasting, an alternative to Monte Carlo forecasting

A 'lagged average forecast' (LAF) model is developed for stochastic dynamic weather forecasting and used for predictions in comparison with the results of a Monte Carlo forecast (MCF). The technique involves the calculation of sample statistics from an ensemble of forecasts, with each ensemble member being an ordinary dynamical forecast (ODF). Initial conditions at a time lagging the start of the forecast period are used, with varying amounts of time for the lags. Forcing by asymmetric Newtonian heating of the lower layer is used in a two-layer, f-plane, highly truncated spectral model in a test forecasting run. Both the LAF and MCF are found to be more accurate than the ODF due to ensemble averaging with the MCF and the LAF. When a regression filter is introduced, all models become more accurate, with the LAF model giving the best results. The possibility of generating monthly or seasonal forecasts with the LAF is discussed.

Hoffman, R. N.↗

NASA NTRS1984

Flap-lag damping of an elastic rotor blade with torsion and dynamic inflow in hover from symbolically generated equations

A program written in FORTRAN IV and coded into subroutines for subsequent numerical study is used to derive symbolically the governing equations of motion of the elastic rotor blade and the dynamic inflow equations. By rearranging the coefficients of the equations, the blade and dynamic inflow equations are converted into equations in a multiblade coordinated system. The final multiblade equations can accommodate any number of elastic blade modes. It is noted that with a given data set, the entire process, from the derivation of the equations to the numerical calculations, is computerized and requires only limited user interface. To demonstrate the usefulness of the approach, damping data in hover with and without dynamic inflow effects are presented for various rotor blade models, including flap-lag, flap-lag-torsion, quasi-static torsion, and rigid blade lag-flap.

Reddy, T. S. R.↗

NASA NTRS1987

High-angle-of-attack pneumatic lag and upwash corrections for a hemispherical flow direction sensor

As part of the NASA F-14 high angle of attack flight test program, a nose mounted hemispherical flow direction sensor was calibrated against a fuselage mounted movable vane flow angle sensor. Significant discrepancies were found to exist in the angle of attack measurements. A two fold approach taken to resolve these discrepancies during subsonic flight is described. First, the sensing integrity of the isolated hemispherical sensor is established by wind tunnel data extending to an angle of attack of 60 deg. Second, two probable causes for the discrepancies, pneumatic lag and upwash, are examined. Methods of identifying and compensating for lag and upwash are presented. The wind tunnel data verify that the isolated hemispherical sensor is sufficiently accurate for static conditions with angles of attack up to 60 deg and angles of sideslip up to 30 deg. Analysis of flight data for two high angle of attack maneuvers establishes that pneumatic lag and upwash are highly correlated with the discrepancies between the hemispherical and vane type sensor measurements.

Whitmore, Stephen A.↗

NASA NTRS1989

Flap-lag stability data for a small-scale isolated hingeless rotor in forward flight

An isolated, hingeless rotor with discrete flap and lead-lag flexures and relatively rigid blades was tested in the Aeroflightdynamics Directorate's 7- by 10-Foot Wind Tunnel. The lead-lag stability of a structurally simple rotor configuration in forward flight was determined. The model tested had no cyclic pitch control, and was therefore operated untrimmed at several collective pitch angles, at shaft angles from 0 deg to -20 deg, and at advance ratios as high as 0.55. Two inplane natural frequencies, 0.61/rev and 0.72/rev, were tested for configuration both with and without structural flap lag coupling. Concomitant hover testing of the model was also conducted. Representative plots of the frequency and damping data are presented to show general trends, and complete tabular data and model properties information are included for use in detailed correlation exercises. The most prominent feature of the forward flight data is an abrupt increase in damping with advance ratio at certain high-speed, high shaft-angle conditions, with high flapping loads. The hover data are consistent with previous experimental and theoretical results for hingeless rotors without kinematic couplings. Overall, the data quality is very good and the data are expected to be useful in the development and validation of rotor aeroelastic stability analyses.

Mcnulty, Michael J.↗

NASA NTRS1988

A signal filter with zero phase lag

Rotorcraft and rotorcraft models are tested at the NASA Ames National Full-Scale Aerodynamics Complex (NFAC). The models tested in the NFAC wind tunnels are controlled by a remote control console located in a control room. Certain critical information must be displayed on the control console to monitor the rotor to insure that the rotor is within operational limits. The signal for these parameters is complex (with ac and dc components) and is derived from pitch and flapping transducers on the rotor head. The pitch and flapping cyclics are derived from resolver circuits that indicate the magnitude and phase of the cyclics. Conventional filter circuits used to separate the ac and dc components have a phase lag on the ac component, which will introduce an error in the cyclic vector. To overcome this error, a filter with zero phase lag needed to be developed. This paper discusses a rotorcraft circuit that was designed to have an ac/dc filter with zero phase lag over the frequency range.

Forsyth, T. J.↗

NASA NTRS1992

Lag Angle Compensation in a Space Borne Scanning Lidar

The so-called 'lag angle' is inherent in any spaceborne scanning lidar system. It is also important in systems where the scanning causes a significant pointing error during the laser interrogating pulse echo time. In a heterodyne lidar system, the lag angle, if uncompensated, will significantly reduce the signal-to-noise ratio. An investigation of lag angle is presented.

Wissinger, A. B.↗

NASA NTRS1991

Approximations for inclusion of rotor lag dynamics in helicopter flight dynamics models

Approximate forms are suggested for augmenting linear rotor/body response models to include rotor lag dynamics. Use of an analytically linearized rotor/body model has shown that the primary effect comes from the additional angular rate contributions of the lag inertial response. Addition of lag dynamics may be made assuming these dynamics are represented by an isolated rotor with no shaft motion. Implications of such an approximation are indicated through comparison with flight test data and sensitivity of stability levels with body rate feedback.

Mckillip, Robert, Jr.↗

NASA NTRS1945

Pressure Lag in Tubing Used in Flight Research

Tests described in this report were undertaken to obtain a quantitative measure of the pressure lag in typical pressure-tubing systems used by the Ames Aeronautical Laboratory in flight research investigations. Lag measurements were made with both single-direction and oscillating pressure changes. Single-direction pressure changes were investigated to determine if the lag in orifice-pressure lines and in the research airspeed and altitude measuring systems of pursuit-type airplane undergoing flight tests was sufficient to cause an appreciable error in the record of a sudden pressure change. Oscillating pressure changes were investigated with particular reference to the accuracy of pressure peaks in pressure-distribution measurements during the time of buffeting conditions as found in stalls. (author)

Turner, Howard L↗

NASA NTRS1961

An Investigation of the Effects of the Time Lag Due to Long Transmission Distances upon Remote Control: Tracking Experiments - Phase 1

A series of pursuit tracking tasks were performed incorporating a transport lag in the control loop. The target was a mixture of four sine waves, the fastest having a frequency of 16 cycles per minute at full speed. An attempt was made to design the experiments so that they would provide data applicable to remote control of a ground vehicle over long transmission distances. Three programs were run. In each the time lag was placed between the control and the display. In the first program a velocity control was used and the operator was told that his knob controlled a vehicle, the problem represented a road 9 and he was to drive his vehicle along the road 9 using the delayed vehicle position as feedback for whatever means he desired. The objective was not to match the display traces. In the second program a velocity control was used, and the operator was told that the problem trace represented a road and the delayed trace represented a vehicle and he was to keep them together. The objective was to match display traces. The third program was identical with the first, except that an acceleration control was used rather than a velocity control. Target speeds used were full speed, 1/2 speed, 1/4 speed, 1/8 speed, and 1/16 speed. Time lags were 1/4 second, l/2 second, 1 second, 1-1/2 second, 2 second, 3 second, and 6 seconds. The experimental results are presented in the last section of this report.

Adams, James L.↗

NASA NTRS1998

Hard X-Ray Lags in GRO J1719-24

We have used the Fourier cross spectra of GRO J1719-24. as obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20-,50 and 50-100 keV energy bands as a function of Fourier frequency in the interval 0.002-0.488 Hz. Our analysis covers the entire approximately 80 day X-ray outburst of this black-hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50-100 keV band, lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.072 +/- 0.010 rad in the frequency interval 0.02-0.20 Hz. The time lags of GRO J1719-24 decrease with frequency as a power law, with index 1.04 +/- 0.13 for frequencies >/= 0.01 Hz. These results are discussed together with those obtained in recent similar studies of the black-hole candidates Cyg X-1 and GRO J0422+32.

VanderHooft, Frank↗

NASA NTRS1999

Hard X-Ray Lags in GRO J1719--24

We have used the Fourier cross spectra of GRO J1719-24, obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20 50 and 50-100 keV energy bands as a function of Fourier frequency in the interval 0.002-0.488 Hz. Our analysis covers the entire about 80 day X-ray outburst of this black hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.072 + 0.010 rad in the frequency interval 0.02-0.20 Hz. The peak phase lags in the interval 0.02-0.20 Hz are about twice those of Cyg X-1 and GRO J0422+32. These results are consistent with models for Comptonization regions composed of extended nonuniform clouds around the central source.

VanDerHooft, F.↗