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At least 973 records · Page 54

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NASA NTRS1993

Stability of high speed compressible rotating Couette flow

In this study, we consider the linear stability of high speed, rotating Couette flow to two and three-dimensional disturbances in finite-gap spacings, including the full effects of compressibility and viscosity. Particularly, the combined effects of Mach number, Reynolds number, radial heating, and gap spacing are investigated. For a stationary outer cylinder, the primary instability is an axisymmetric mode independent of the Mach number. Increasing Mach numbers have a destabilizing effect for wide gaps, and a stabilizing effect for narrow gaps. For a sufficiently fast, counter-rotating outer cylinder, the primary instability becomes a three-dimensional traveling wave. Compressibility has a stabilizing effect on these modes regardless of the gap width; also, heating at the outer cylinder stabilizes the flow. Bicritical points for the primary instability corresponding to the crossover of the azimuthal wave numbers are determined for cylinders counter-rotating with equal angular speed.

Hatay, Ferhat F.↗

NASA NTRS1993

Weakly nonlinear stability of vicsous vortices in three-dimensional boundary layers

Attention is given to the weakly nonlinear stability of essentially viscous vortices in 3D boundary layers. These modes are unstable in the absence of crossflow, but the imposition of small crossflow has a stabilizing effect. Bassom and Hall (1991) demonstrated the existence of neutrally stable vortices for certain crossflow/wave number combinations, and the weakly nonlinear stability properties of these disturbances are described. It is shown that the effect of crossflow is to stabilize the nonlinear modes, and the present calculations allow stable finite-amplitude vortices to be found. Predictions are made concerning the likelihood of observing some of these viscous modes within a practical setting.

Bassom, Andrew P.↗

NASA NTRS1992

Measurements of a deep space station fractional frequency stability to the 10 exp -15 level

This article presents techniques for measuring frequency stability of a deep space station that was configured with a new X-band transmitter subsystem to enable high-quality radio science data to be obtained for a Gravity Wave Experiment. The instrument and test procedures for performing frequency stability measurements in the field are described. Test results from measurements made on various transmit receive subsystems, as well as the overall transmit-receive end-to-end systems below the antenna feed horn are presented. The subsystem test data proved to be useful for assessing the frequency stabilities of various subsystems. This assessment design improvements needed to meet stringent frequency stability requirements for forthcoming gravity wave experiments.

Otoshi, Tom Y.↗

NASA NTRS1992

Robust stabilization, robust performance, and disturbance attenuation for uncertain linear systems

This paper presents a linear quadratic regulator approach to the robust stabilization, robust performance, and disturbance attenuation of uncertain linear systems. The state-feedback designed systems provide both the robust stability with optimal performance and the disturbance attenuation with H-infinity-norm bounds. The proposed approach can be applied to matched and/or mismatched uncertain linear systems. For a matched uncertain linear system, it is shown that the disturbance attenuation robust-stabilizing controllers with or without optimal performance always exist and can be easily determined without searching; whereas, for a mismatched uncertain linear system, the introduced tuning parameters greatly enhance the flexibility of finding the disturbance-attenuation robust-stabilizing controllers.

Wang, Yeih J.↗

NASA NTRS1941

Notes on the Stability and Control of Tailless Airplanes

Problems involved in the stability and control of tailless airplanes are discussed. Such factors as the location of the aerodynamic center and its effect on the longitudinal stability, longitudinal trim with high-lift devices, the effects of various changes in the shape of the wing on lateral stability, and the effects of nacelles are covered. It appears that sufficient stability and controllability can be secured without sweepback. With sweepback, a flap over the center section of the wing may be used to serve the dual purpose of elevator control and high-lift device. Sweepback introduces undesirable stalling characteristics, however, and may require auxiliary devices to prevent stalling of the tips.

Jones, Robert↗

NASA NTRS1958

Investigation of the Static Lateral Stability and Aileron Characteristics of a 0.067-Scale Model of the Bell X-2 Airplane at Mach Numbers of 2.29, 2.78, 3.22, and 3.71

Tests were performed in the high. Mach number test section of the Langley Unitary Plan wind tunnel to determine the static lateral stability. and aileron characteristics of a 0.067-scale model of the Bell X-2 airplane at Mach numbers of 2.29, 2. 78, 3.22, and. 3.71. The results of this investigation indicated that the directional stability of the model was low with directional instability occurring at Mach numbers higher than 3.1 and. angles of attack higher than about 5.0 deg (equivalent lift coefficient of about 0.18). The yaw due to aileron deflection was adverse and, with 10 deg of differential aileron deflection, large enough to overbalance the available directional restoring moment at all angles of attack higher than about 5.0 deg (equivalent lift coefficient of about 0.21) and Mach numbers higher than 2. 5. The model also had positive effective dihedral for all test attitudes and. Mach numbers. A combination of the lateral-stability parameters with the aileron characteristics to form a lateral-stability criterion for a maneuver using ailerons alone indicated that the model has characteristics which would. give unstable aperiodic behavior (divergence) over a large part of the test Mach number and angle-of-attack range.

Fournier, Roger H.↗

NASA NTRS1954

Methods of predicting helicopter stability

Some of the methods of predicting rotor stability derivatives have been reviewed. The methods by which these rotor derivatives are employed to estimate helicopter stability characteristics have been summarized. It is concluded that, although these methods are not always feasible for predicting absolute values of the stability of the helicopter, the effects on stability of changes in individual derivatives can generally be estimated satisfactorily.

Robert J Tapscott↗

NASA NTRS1956

A Theory for Stability and Buzz Pulsation Amplitude in Ram Jets and an Experimental Investigation Including Scale Effects

From a theory developed on a quasi-one-dimensional-flow basis, it is found that the stability of the ram jet is dependent upon the instantaneous values of mass flow and total pressure recovery of the supersonic diffuser and immediate neighboring subsonic diffuser. Conditions for stable and unstable flow are presented. The theory developed in the report is in agreement with the experimental data of NACA-TN-3506 and NACA-RM-L50K30. A simple theory for predicting the approximate amplitude of small pressure pulsation in terms of mass-flow decrement from minimum-stable mass flow is developed and found to agree with experiments. Cold-flow tests at a Mach number of 1.94 of ram-jet models having scale factors of 3.15:1 and Reynolds number ratios of 4.75:1 with several supersonic diffuser configurations showed only small variations in performance between geometrically similar models. The predominant variation in steady-flow performance resulted from the larger boundary layer in the combustion chamber of the low Reynolds number models. The conditions at which buzz originated were nearly the same for the same supersonic diffuser (cowling-position angle) configurations in both large and small diameter models. There was no appreciable variation in stability limits of any of the models when the combustion-chamber length was increased by a factor of three. The unsteady-flow performance and wave patterns were also similar when considered on a reduced-frequency basis determined from the relative lengths of the model. The negligible effect of Reynolds number on stability of the off-design configurations was not anticipated in view of the importance of boundary layer to stability, and this result should not be construed to be generally applicable. (author)

Trimpi, Robert L↗

NASA NTRS1937

Flight measurements of the dynamic longitudinal stability of several airplanes and a correlation of the measurements with pilots' observations of handling characteristics

The dynamic longitudinal stability characteristics of eight airplanes as defined by the period and damping of the longitudinal oscillations were measured in flight to determine the degree of stability that may be expected in conventional airplanes. An attempt was made to correlate the measured stability with pilots' opinions of the general handling characteristics of the airplanes in order to obtain an indication of the most desirable degree of dynamic stability. The results of the measurements show that the period of oscillation increases with speed. At low speeds a range of periods from 11 to 23 seconds was recorded for the different airplanes. At high speeds the periods ranged from 23 to 64 seconds. The damping showed no definite trend with speed.

Soule, Hartley A↗

NASA NTRS1941

Analysis and prediction of longitudinal stability of airplanes

An analysis has been made of the longitudinal stability characteristics of 15 airplanes as determined in flight. In the correlation of satisfactory and unsatisfactory characteristics with determined values, the derivative that expresses the ratio of static-restoring moments to elevator-control moments was found to represent most nearly the stability characteristics appreciated by the pilots. The analysis was extended to study the effects of various design features on the observed stability characteristics. Design charts and data are included that show the effects on longitudinal stability of relative positions of wing and tail, fuselage size and location, engine nacelles, and horizontal-tail arrangements.

Gilruth, R R↗

NASA NTRS1950

Theoretical stability derivatives of thin sweptback wings tapered to a point with sweptback or sweptforward trailing edges for a limited range of supersonic speeds

The stability derivatives valid for a limited range of supersonic speeds are presented for a series of sweptback wings tapered to a point with sweptback or sweptforward trailing edges. These wings were derived by modifying the trailing edge of a basic triangular wing so that it coincided with lines drawn from the wing tips to the wing axis of symmetry. The stability derivatives were formulated by using the pressure distributions previously obtained for the basic triangular wing for angle of attack, constant vertical acceleration, sideslip, pitching, rolling, and yawing. Explicit expressions are given for the stability derivatives with respect to principal body axes, and conversion formulas are provided for the transformation to stability axes. The results are limited to Mach numbers for which the wing is contained within the Mach cones springing from the vertex and from the trailing edge of the center section of the wing.

Malvestuto, Frank S , Jr↗

NASA NTRS1947

Stability and control characteristics of an airplane model having a 45.1 degree swept-back wing with aspect ratio 2.50 and taper ratio 0.42 and a 42.8 degree swept-back horizontal tail with aspect ratio 3.87 and taper ratio 0.49

Tests were made of an airplane model having a 45.1 degree swept-back wing with aspect ratio 2.50 and taper ratio 0.42 and a 42.8 degree swept-back horizontal tail with aspect ratio 3.87 and taper ratio 0.49 to determine its low-speed stability and control characteristics. The test Reynolds number was 2.87 x 10(6) based on a mean aerodynamic chord of 2.47 feet except for some of the aileron tests which were made at a Reynolds number of 2.05 x 10(6). With the horizontal tail located near the fuselage juncture on the vertical tail, model results indicated static longitudinal instability above a lift coefficient that was 0.15 below the lift coefficient at which stall occurred. Static longitudinal stability, however, was manifested throughout the life range with the horizontal tail located near the top of the vertical tail. The use of 10 degrees negative dihedral on the wing had little effect on the static longitudinal stability characteristics. Preliminary tests of the complete model revealed an undesirable flat spot in the yawing-moment curves at low angles of attack, the directional stability being neutral for yaw angles of plus-or-minus 2 degrees. This undesirable characteristic was improved by replacing the thick original vertical tail with a thin vertical tail and by flattening the top of the dorsal fairing.

Schuldenfrei, Marvin↗

NASA NTRS1993

Large time-step stability of explicit one-dimensional advection schemes

There is a wide-spread belief that most explicit one-dimensional advection schemes need to satisfy the so-called 'CFL condition' - that the Courant number, c = udelta(t)/delta(x), must be less than or equal to one, for stability in the von Neumann sense. This puts severe limitations on the time-step in high-speed, fine-grid calculations and is an impetus for the development of implicit schemes, which often require less restrictive time-step conditions for stability, but are more expensive per time-step. However, it turns out that, at least in one dimension, if explicit schemes are formulated in a consistent flux-based conservative finite-volume form, von Neumann stability analysis does not place any restriction on the allowable Courant number. Any explicit scheme that is stable for c is less than 1, with a complex amplitude ratio, G(c), can be easily extended to arbitrarily large c. The complex amplitude ratio is then given by exp(- (Iota)(Nu)(Theta)) G(delta(c)), where N is the integer part of c, and delta(c) = c - N (less than 1); this is clearly stable. The CFL condition is, in fact, not a stability condition at all, but, rather, a 'range restriction' on the 'pieces' in a piece-wise polynomial interpolation. When a global view is taken of the interpolation, the need for a CFL condition evaporates. A number of well-known explicit advection schemes are considered and thus extended to large delta(t). The analysis also includes a simple interpretation of (large delta(t)) total-variation-diminishing (TVD) constraints.

Leonard, B. P.↗

NASA NTRS1993

Extraction of stability and control derivatives from orbiter flight data

The Space Shuttle Orbiter has provided unique and important information on aircraft flight dynamics. This information has provided the opportunity to assess the flight-derived stability and control derivatives for maneuvering flight in the hypersonic regime. In the case of the Space Shuttle Orbiter, these derivatives are required to determine if certain configuration placards (limitations on the flight envelope) can be modified. These placards were determined on the basis of preflight predictions and the associated uncertainties. As flight-determined derivatives are obtained, the placards are reassessed, and some of them are removed or modified. Extraction of the stability and control derivatives was justified by operational considerations and not by research considerations. Using flight results to update the predicted database of the orbiter is one of the most completely documented processes for a flight vehicle. This process followed from the requirement for analysis of flight data for control system updates and for expansion of the operational flight envelope. These results show significant changes in many important stability and control derivatives from the preflight database. This paper presents some of the stability and control derivative results obtained from Space Shuttle flights. Some of the limitations of this information are also examined.

Iliff, Kenneth W.↗

NASA NTRS1993

Inherent stability of central element coaxial liquid-liquid injectors

Most TRW liquid bi-propellant rocket engines built over the past thirty-plus years have employed a central element coaxial pintle injector and have operated with liquid/liquid propellant injection. This injector is a patented design exclusive to TRW and has unique features that make the rocket engine combustion characteristics different from those of other types of injector engine designs. Its many benefits include excellent combustion performance, efficient deep throttling, adaptability to low cost manufacturing, and high reliability. Approximately 200 pintle injector engines of various sizes and operating on a variety of propellants have been flown without a single inflight failure. An especially important feature of the pintle injector engine is its apparent inherent combustion stability. In over thirty years of development, testing, and production, TRW has never experienced combustion instability in any of its pintle injector engine designs. This has been true of engines operating over a range of thrust from 5 to 250,000 lbs. on earth-storable hypergolic propellants and a large number of smaller engines operating on a variety of propellants (21 combinations) in long duration-firing, pulsing (down to 2 msec), and deep throttling (as much as 19:1) modes. Operating chamber pressures have ranged from 10 to 3,500 psia. This record is particularly impressive given that typical TRW design practice does not consider combustion instability as an issue and no pintle engine has ever employed stability-enhancing features, such as baffles or acoustically resonant chambers. In spite of this, TRW engines have operated stably in regimes not possible with other types of injectors. Various physical explanations and combustion process models for this favorable stability characteristic have been postulated. However, a definitive study that unequivocally establishes the important stabilizing mechanisms still remains to be conducted.

Stoddard, Frank J.↗

NASA NTRS1992

Linear stability theory and three-dimensional boundary layer transition

The viewgraphs and discussion of linear stability theory and three dimensional boundary layer transition are provided. The ability to predict, using analytical tools, the location of boundary layer transition over aircraft-type configurations is of great importance to designers interested in laminar flow control (LFC). The e(sup N) method has proven to be fairly effective in predicting, in a consistent manner, the location of the onset of transition for simple geometries in low disturbance environments. This method provides a correlation between the most amplified single normal mode and the experimental location of the onset of transition. Studies indicate that values of N between 8 and 10 correlate well with the onset of transition. For most previous calculations, the mean flows were restricted to two-dimensional or axisymmetric cases, or have employed simple three-dimensional mean flows (e.g., rotating disk, infinite swept wing, or tapered swept wing with straight isobars). Unfortunately, for flows over general wing configurations, and for nearly all flows over fuselage-type bodies at incidence, the analysis of fully three-dimensional flow fields is required. Results obtained for the linear stability of fully three-dimensional boundary layers formed over both wing and fuselage-type geometries, and for both high and low speed flows are discussed. When possible, transition estimates form the e(sup N) method are compared to experimentally determined locations. The stability calculations are made using a modified version of the linear stability code COSAL. Mean flows were computed using both Navier Stokes and boundary-layer codes.

Spall, Robert E.↗

NASA NTRS1994

Stabilization of thermocapillary convection by means of nonplanar flow oscillations

Nonplanar flow oscillations have been shown to be effective in stabilizing buoyancy-induced Rayleigh-Benard convection. The present study was initiated to see if thermocapillary convection of the Marangoni type might also be stabilized by the same means. When surface deflection can be ignored, significant stabilization occurs. However, when the operating parameters are such that surface deflection is nonnegligible, destabilization can occur, in contrast to Rayleigh-Benard convection. Mechanisms for both stabilization and destabilization are discussed.

Kelly, Robert E.↗

NASA NTRS1994

Convection and morphological stability during directional solidification

For growth of a vicinal face at constant velocity, the effect of anisotropic interface kinetics on morphological stability is calculated for a binary alloy. The dependence of the interface kinetic coefficient on crystallographic orientation is based on the motion and density of steps. Anisotropic kinetics give rise to traveling waves along the crystal-melt interface, and can lead to a significant enhancement of morphological stability. The stability enhancement increases as the orientation approaches a singular orientation and as the solidification velocity increases. Shear flows interact with the traveling waves and, depending on the direction of the flow, may either stabilize or destabilize the interface. Specific calculations are carried out for germanium-silicon alloys.

Coriell, Sam R.↗