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At least 1,027 records · Page 57

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NASA NTRS1979

Convective instability when the temperature gradient and rotation vector are oblique to gravity. I - Fluids without diffusion

A linear stability analysis of fluid layers under uniform rotation (generally oblique to gravity) which possess both vertical and horizontal temperature gradients is made by considering ideal fluids without diffusion within a Boussinesq approximation. This simplified configuration is used to assess the preferred convective modes as a function of latitude on a planet like Jupiter. The tilted rotation vector introduces a preference for roll-like disturbances with north-south orientations, while the horizontal temperature gradient produces a thermal wind shear which favors convective rolls oriented parallel to the flow in an east-west direction. It is found that the horizontal temperature gradient needed to produce a preference for the axisymmetric or east-west rolls increases with an increasing rotation rate and a decreasing latitude. The parameter values for Jupiter are estimated with the use of a simple radiative convective model, indicating a preference for axisymmetric rolls at nearly all latitudes if the convection zone depth is greater than about 200 km below the one atmosphere pressure level and convective roll characteristics which contribute to an equatorial acceleration.

Hathaway, D. H.↗

NASA NTRS1983

Aircraft body-axis rotation measurement system

A two gyro four gimbal attitude sensing system having gimbal lock avoidance is provided with continuous azimuth information, rather than roll information, relative to the magnetic cardinal headings while in near vertical attitudes to allow recovery from vertical on a desired heading. The system is comprised of a means for stabilizing an outer roll gimbal that is common to a vertical gyro and a directional gyro with respect to the aircraft platform which is being angularly displaced about an axis substantially parallel to the outer roll gyro axis. A means is also provided for producing a signal indicative of the magnitude of such displacement as an indication of aircraft heading. Additional means are provided to cause stabilization of the outer roll gimbal whenever the pitch angle of the aircraft passes through a threshold prior to entering vertical flight and destabilization of the outer roll gimbal upon passing through the threshold when departing vertical flight.

Cowdin, K. T.↗

NASA NTRS1985

On cloud street development in three dimensional parallel and Rayleigh instabilities

Expected orientation angles and horizontal wavelengths of boundary layer rolls or cloud streets are determined from an analysis of a truncated spectral model of three dimensional shallow moist Boussinesq convection in a shearing environment. The nonlinear secondary circulations are organized into two dimensional forms by the height dependent wind field, and these rolls may develop from the combined effects of thermal stratification and mean wind shear. The associated thermal and parallel instability mechanisms are shown to be special cases of a single one. Only one mode is found when the stratification is unstable or neutral, but a second one is possible when the stratification is weakly stable. The first corresponds to relatively broadly spaced rolls having orientations for which the Fourier component of the roll perpendicular shear is nearly zero, but the second corresponds to relatively narrowly spaced rolls having orientations for which the Fourier coefficients of both the perpendicular and the parallel components of the shear are nearly equal.

Shirer, H. N.↗

NASA NTRS1985

Microstructure and fracture in SiC whisker reinforced 2124 aluminum composite

The microstructures of extruded and hot-rolled 2124 Al-15 percent (by weight) SiC whisker composites have been investigated, experimentally. Among the specific factors studied were: the strength of the whisker-matrix interfaces; (2) the presence of oxides; (3) the presence of defective whiskers; (4) and the presence of distribution of intermetallic compounds, impurities in the SiC(w) powder, and microstructural inhomogeneities. Modifications in the microstructure of the SiC/AL composites due to hot rolling and extrusion are illustrated in a series of microphotographs. It was found that hot rolling along the axis of extrusion was associated with some types of whisker damage, while the whiskers still retain their original orientation. Hot-rolling perpendicular to the axis of extrusion, however, tended to rotate the whiskers and produced a nearly isotropic material. Whisker free zones were virtually eliminated or reduced in size by hot rolling. In situ Auger fractography of the composite showed that the interfacial bonding between the SiC and the Al matrix was good and that Al2O2 had no significant influence on the fracture mechanics of the composite.

Nieh, T. G.↗

NASA NTRS1986

On cloud street development in three dimensions - Parallel and Rayleigh instabilities

Shirer, H. N.↗

NASA NTRS1990

Aeroservoelastic tailoring for lateral control enhancement

The application of an automated optimization approach to the integrated structural and control design of an aircraft wing and its control surface is illustrated. The performance index or cost function for optimization is the aileron hinge moment required to sustain a specified terminal roll rate at a chosen design speed at which aeroelastic effects are significant. The design variables used for aileron hinge-moment minimization are the aileron flap-to-chord ratio; the location of the aileron with respect to the roll axis; and the orientations of three advanced composite plies comprising 60 percent of the wing structure. The effects of aileron location and advanced composite ply orientation on the terminal roll rate and aileron hinge moment are examined. Optimization is seen to involve a compromise between a laminated structure/aileron combination that is effective in creating large rolling moments and one that minimizes the damping-in-roll. These two requirements are in conflict; the optimization driver must locate a compromise design point by moving the control surface and changing its size while also creating a suitable structural design.

Weisshaar, Terrence A.↗

NASA NTRS1992

The multiple-function multi-input/multi-output digital controller system for the AFW wind-tunnel model

A real time multiple-function digital controller system was developed for the Active Flexible Wing (AFW) Program. The digital controller system (DCS) allowed simultaneous execution of two control laws: flutter suppression and either roll trim or a rolling maneuver load control. The DCS operated within, but independently of, a slower host operating system environment, at regulated speeds up to 200 Hz. It also coordinated the acquisition, storage, and transfer of data for near real time controller performance evaluation and both open- and closed-loop plant estimation. It synchronized the operation of four different processing units, allowing flexibility in the number, form, functionality, and order of control laws, and variability in the selection of the sensors and actuators employed. Most importantly, the DCS allowed for the successful demonstration of active flutter suppression to conditions approximately 26 percent (in dynamic pressure) above the open-loop boundary in cases when the model was fixed in roll and up to 23 percent when it was free to roll. Aggressive roll maneuvers with load control were achieved above the flutter boundary. The purpose here is to present the development, validation, and wind tunnel testing of this multiple-function digital controller system.

Hoadley, Sherwood T.↗

NASA NTRS1992

The multiple-function multi-input/multi-output digital controller system for the AFW wind tunnel model

A real-time multiple-function digital controller system was developed for the Active Flexible Wing (AFW) Program. The digital controller system (DCS) allowed simultaneous execution of two control laws: flutter suppression and either roll trim or a rolling maneuver load control. The DCS operated within, but independently of, a slower host operating system environment, at regulated speeds up to 200 Hz. It also coordinated the acquisition, storage, and transfer of data for near real-time controller performance evaluation and both open- and closed-loop plant estimation. It synchronized the operation of four different processing units, allowing flexibility in the number, form, functionality, and order of control laws, and variability in selection of sensors and actuators employed. Most importantly, the DCS allowed for the successful demonstration of active flutter suppression to conditions approximately 26 percent (in dynamic pressure) above the open-loop boundary in cases when the model was fixed in roll and up to 23 percent when it was free to roll. Aggressive roll maneuvers with load control were achieved above the flutter boundary.

Hoadley, Sherwood T.↗

NASA NTRS1993

Experimental and numerical analysis of the wing rock characteristics of a 'wing-body-tail' configuration

Free-to-roll wind tunnel tests were conducted and a computer simulation exercise was performed in an effort to investigate in detail the mechanism of wing rock on a configuration that consisted of a highly-slender forebody and a 78 deg swept delta wing. In the wind tunnel test, the roll angle and wing surface pressures were measured during the wing rock motion. A limit cycle oscillation was observed for angles of attack between 22 deg and 30 deg. In general, the wind tunnel test confirmed that the main flow phenomena responsible for the wing-body-tail wing rock are the interactions between the forebody and the wing vortices. The variation of roll acceleration (determined from the second derivative of the roll angle time history) with roll angle clearly showed the energy balance necessary to sustain the limit cycle oscillation. Pressure measurements on the wing revealed the hysteresis of the wing rock process. First, second and nth order models for the aerodynamic damping were developed and examined with a one degree of freedom computer simulation. Very good agreement with the observed behavior from the wind tunnel was obtained.

Suarez, Carlos J.↗

NASA NTRS1937

Theoretical span loading and moments of tapered wings produced by aileron deflection

The effect of tapered ailerons on linearly tapered wings is theoretically determined. Four different aileron spans are considered for each of three wing aspect ratios and each of four wing taper ratios. The change in lift on one half of the wing, the rolling moment, the additional induced drag, and the yawing moment, due to aileron deflection, are represented by non dimensional coefficients. Similar coefficients are given for the damping and yawing moments, the additional drag, and the change in lift, due to rolling. It was found possible to effect a fairly close agreement between the theoretical and experimental rolling moments by introducing into the theoretical expression for the rolling moment an effective change in angle of attack obtained from an analysis of flap data. The theoretical curves show that the highly tapered wing with long ailerons has a lower ratio of yawing to rolling moment and a lower additional induced drag than wings with less taper.

Pearson, H A↗

NASA NTRS1939

Flight Tests of Retractable Ailerons on a Highly Tapered Wing

A flight investigation was conducted to determine the lateral-control characteristics of retractable ailerons installed on a highly tapered wing. The effectiveness of the ailerons in producing roll was measured at various air speeds with full-span plain flaps both neutral and deflected 45 degrees. The direction of the yawing moment created by the ailerons was also noted. The lateral control provided by the retractable ailerons used in this investigation was approximately the same as that obtained with the plain ailerons of equal span with which the airplane was previously equipped. The amount of control available was found to be somewhat inadequate, apparently because of the rather short span of the ailerons (0.327 of the wing span). It is likely that, with an aileron span of from 0.50 to 0.60 of the wing span, a satisfactory degree of control would be obtained. With the full-span flaps deflected 45 degrees, the rolling action of the ailerons was increased about 30 percent over that obtained with the flaps neutral at the same speed. The yawing moment produced by the ailerons was in the same sense as the rolling moment, i.e., right roll was accompanied by yaw. Lag in the response of the rolling action to control application was not large enough to be noticed by the pilots. No appreciable control force was apparent to the pilots, which was considered somewhat undesirable. Minor modifications in the design of the ailerons, however, would probably correct this fault.

Wetmore, J W↗

NASA NTRS1994

Status of MSBS study at NAL

Two Magnetic Suspension and Balance Systems (MSBS) at the National Aerospace Laboratory (NAL) in Japan are introduced. They are the 10 cm MSBS and the 60 cm MSBS. They have 10 cm x 10 cm and 60 cm x 60 cm test sections. The control of suspending a model at the 10 cm MSBS is six degrees of freedom including the rolling moment control. The model for the rolling moment control has two pairs of small extra permanent magnets at both its ends plus a main cylindrical magnet. The rolling moment is generated by the magnetic forces acting on the extra magnets by controlled current passing through the four side coils independently. Test results show the roll angle of the model is controlled in this way. The dynamic calibration test was carried out at the MSBS in five degrees of freedom without the rolling moment control. The model is a simple cylindrical magnet magnetized along its axis. The obtained results show that the dynamic calibration with measured magnetic field intensity is much superior to that with the coil currents. The 60 cm MSBS was designed with some data obtained at the 10 cm one. It is fundamentally proportional to the 10 cm one in size and coil positions. The measured magnetic field intensity is not so strong as expected at design. It was operated first in 1993. The control is three degrees of freedom in the longitudinal direction.

Sawada, Hideo↗

NASA NTRS1996

Differential Canard deflection for generation of yawing moment on the X-31 with and without the vertical tail

The feasibility of augmenting the available yaw control power on the X-31 through differential deflection of the canard surfaces was studied as well as the possibility of using differential canard control to stabilize the X-31 with its vertical tail removed. Wind-tunnel tests and the results of departure criteria and linear analysis showed the destabilizing effect of the reduction of the vertical tail on the X-31. Wind-tunnel testing also showed that differential canard deflection was capable of generating yawing moments of roughly the same magnitude as the thrust vectoring vanes currently in place on the X-31 in the post-stall regime. Analysis showed that the X-31 has sufficient aileron roll control power that with the addition of differential canard as a yaw controller, the wind-axis roll accelerations will remain limited by yaw control authority. It was demonstrated, however, that pitch authority may actually limit the maximum roll rate which can be sustained. A drop model flight test demonstrated that coordinated, wind axis rolls could be performed with roll rates as high as 50 deg/sec (full scale equivalent) at 50 deg angle of attack. Another drop model test was conducted to assess the effect of vertical tail reduction, and an analysis of using differential canard deflection to stabilize the tailless X-31 was performed. The results of six-degree-of-freedom, non-linear simulation tests were correlated with the drop model flights. Simulation studies then showed that the tailless X-31 could be controlled at angles of attack at or above 20 deg using differential canard as the only yaw controller.

Whiting, Matthew Robert↗

NASA NTRS1996

Demonstration of the Feasibility of High Temperature Bearing Lubrication From Carbonaceous Gases

Research has been conducted on silicon nitride pin-on-disk sliding contacts at temperatures of up to 520 C, and four-ball rolling contacts with silicon nitride balls and 52100 steel or silicon nitride races at 590 C. These tests were conducted in a variety of gaseous environments in order to determine the effects of simulated engine exhaust gas on the carbonaceous gas decomposition lubrication scheme. In rolling tests with steel races and exhaust gas the wear track depth was roughly half that of tests run in nitrogen gas alone. The deposition of lubricous microcrystalline graphitic carbon on the rolling surfaces, generated from the carbon monoxide within the exhaust gas mixture, was verified by microfocused Raman spectroscopy. Ten-fold reductions in rolling wear could be achieved by the exhaust gas atmosphere in cases where water vapor was removed or not present. The exhaust gas mixture alone was not found to provide any lubricating effect on silicon nitride sliding contacts, where the rate of wear greatly exceeds the rate of carbon deposition. Directed admixture of acetylene (as low as 5% of the exhaust gas flow rates), has provided reductions in both wear volume and coefficient of friction by factors of 60X and 20X respectively for sliding contacts during the initial 80 m of sliding distance. Exhaust gas atmosphere with the acetylene admixture provided 65OX reductions in steady state wear rate compared to that measured for sliding contacts in dry N2. Such acetylene admixture also augments the ability of the exhaust gas atmosphere to lubricate high-temperature rolling contacts, with up to 25-fold reductions in wear track depth compared to those measured in the presence of N2 alone. In addition to providing some lubricating benefit itself, an important potential role of the exhaust gas from rich mixtures would be to shield bearings from 02. Such shielding enables surface deposition of lubricous pyrolytic carbon from the acetylene admixture, instead of combustion, rendering feasible the continuously replenished solid lubrication of high-temperature bearing surfaces.

Blanchet, Thierry A.↗

NASA NTRS1996

Lateral-Directional Eigenvector Flying Qualities Guidelines for High Performance Aircraft

This report presents the development of lateral-directional flying qualities guidelines with application to eigenspace (eigenstructure) assignment methods. These guidelines will assist designers in choosing eigenvectors to achieve desired closed-loop flying qualities or performing trade-offs between flying qualities and other important design requirements, such as achieving realizable gain magnitudes or desired system robustness. This has been accomplished by developing relationships between the system's eigenvectors and the roll rate and sideslip transfer functions. Using these relationships, along with constraints imposed by system dynamics, key eigenvector elements are identified and guidelines for choosing values of these elements to yield desirable flying qualities have been developed. Two guidelines are developed - one for low roll-to-sideslip ratio and one for moderate-to-high roll-to-sideslip ratio. These flying qualities guidelines are based upon the Military Standard lateral-directional coupling criteria for high performance aircraft - the roll rate oscillation criteria and the sideslip excursion criteria. Example guidelines are generated for a moderate-to-large, an intermediate, and low value of roll-to-sideslip ratio.

Davidson, John B.↗

NASA NTRS1995

Time-to-Passage Judgments in Nonconstant Optical Flow Fields

The time until an approaching object will pass an observer (time to passage, or TTP) is optically specified by a global flow field even in the absence of local expansion or size cues. Kaiser and Mowafy have demonstrated that observers are in fact sensitive to this global flow information. The present studies investigate two factors that are usually ignored in work related to TTP: (1) non-constant motion functions and (2) concomitant eye rotation. Non-constant velocities violate an assumption of some TTP derivations, and eye rotations may complicate heading extraction. Such factors have practical significance, for example, in the case of a pilot accelerating an aircraft or executing a roll. In our studies, a flow field of constant-sized stars was presented monocularly on a large screen. TIP judgments had to be made on the basis of one target star. The flow field varied in its acceleration pattern and its roll component. Observers did not appear to utilize acceleration information. In particular, TTP with decelerating motion were consistently underestimated. TTP judgments were fairly robust with respect to roll, even when roll axis and track vector were decoupled. However, substantial decoupling between heading and track vector led to a decrement in performance, in both the presence and the absence of roll.

Kaiser, Mary K.↗

NASA NTRS1998

Direction-Finding Measurements of Heliospheric 2-3 kHz Radio Emissions

Using data from the Voyager 1 plasma wave instrument, a series of direction-finding measurements is presented for the intense 1992-93 heliospheric 2- to 3-kHz radio emission event, and several weaker events extending into 1994. Direction-finding measurements can only be obtained during roll maneuvers, which are performed about once every three months. Two parameters can be determined from the roll-induced intensity modulation, the azimuthal direction of arrival (measured around the roll axis), and the modulation index (the peak-to-peak amplitude divided by the peak amplitude). Measurements were made at two frequencies, 1.78 and 3.11 kHz. No roll modulation was observed at 1.78 kHz, which is consistent with an isotropic source at this frequency. In most cases an easily measurable roll modulation was detectable at 3.11 kHz. Although the azimuth angles have considerable scatter, the directions of arrival at 3.11 kHz can be organized into three groups, each of which appears to be associated with a separate upward drifting feature in the radio emission spectrum. The first group, which is associated with the main 1992-93 event, is consistent with a source located near the nose of the heliosphere. The remaining two groups, which occur after the main 1992-93 event, have azimuth angles well away from the nose of the heliosphere. The modulation indexes vary over a large range, from 0.06 to 0.61, with no obvious trend. Although the variations in the directions of arrival and modulation indicies appear to reflect changes in the position and angular size of the source, it is also possible that they could be caused by refraction or scattering due to density structures in the solar wind.

Gurnett, Donald A.↗

NASA NTRS1999

Predicted Variations in Flow Patterns in a Horizontal CVD Reactor

Expressions in terms of common reactor operating parameters were derived for the ratio of the Grashof number to the Reynolds number, Gr/Re, the ratio of the Grashof to the square of 2 the Reynolds number, Gr/Re(exp 2), and the Rayleigh number, Ra. Values for these numbers were computed for an example horizontal CVD reactor and compared to numerical simulations to gauge their effectiveness as predictors of the presence or absence of transverse and longitudinal rolls in the reactor. Comparisons were made for both argon and hydrogen carrier gases over the pressure range 2- 101 kPa. Reasonable agreement was achieved in most cases when using Gr/Re to predict the presence of transverse rolls and Ra to predict the presence of longitudinal rolls. The ratio Gr/Re(exp 2) did not yield useful predictions regarding the presence of transverse rolls. This comparison showed that the ratio of the Grashof number to the Reynolds number, as well as the Rayleigh number, can be used to predict the presence or absence of transverse and longitudinal rolls in a horizontal CVD reactor for a given set of reactor conditions. These predictions are approximate, and care must be exercised when making predictions near transition regions.

Kuczmarski, Maria A.↗