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At least 271 records · Page 15

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NASA NTRS1979

On single-degree-of-freedom flutter induced by activated controls

It is shown that activation of the trailing-edge control of an airfoil leads to single-degree-of-freedom type instabilities which span over a very wide region of reduced frequencies k, including high values of k (unlike the nonactivated system). These instabilities are shown to be sensitive to changes in pitching axis location, control deflection phase angle, and values of the reduced frequency. These sensitivities of the single-degree-of-freedom system cause the activated airfoil to be potentially sensitive to changes in flight conditions, and may be the source of the many difficulties encountered in suppressing classical multi-degree-of-freedom flutter by means of active controls. The results presented herein relate to zero Mach number and to a 20% trailing-edge control surface.

Nissim, E.↗

NASA NTRS1983

An orthogonal coordinate grid following the three-dimensional viscous flow over a concave surface

Swept wings designed for laminar flow control exhibit both centrifugal and crossflow instabilities which produce streamwise vortices that can lead to early transition from laminar to turbulent flow in the presence of Tollmien-Schlichting waves. This paper outlines an iterative algorithm for generation of an orthogonal, curvilinear, coordinate grid following the streamlines of the three-dimensional viscous flow over a swept, concave surface. The governing equations for the metric tensor are derived from the Riemann-Christoffel tensor for an Euclidian geometry. Unit vectors along streamline, normal and binormal directions are determined. The governing equations are not solved directly, but are employed only as compatibility equations. The scale factor for the streamline coordinate is obtained by an iterative integration scheme on a 200 x 100 x 5 grid, while the other two scale factors are determined from definitions. Sample results are obtained which indicate that the compatibility equation error decreases linearly with grid step size. Grids smaller than 200 x 100 x 5 are found to be inadequate to resolve the grid curvature.

Dagenhart, J. R↗

NASA NTRS1986

Variable friction secondary seal for face seals

Vibration and stability of a primary seal ring is controlled by a secondary seal system. An inflatable bladder which forms a portion of the secondary seal varies the damping applied to this seal ring. The amplitude of vibration of the primary seal ring is sensed with a proximity probe that is connected to a microprocessor in a control system. The bladder pressure is changed by the control system to mitigate any sensed instability or vibration.

Dirusso, E.↗

NASA NTRS1988

Unsteady features of jets in lift and cruise modes for VTOL aircraft

Experiments were performed to simulate jet plume effects associated with VTOL aircraft in takeoff and cruise modes. A water facility was used to investigate the influence of inclination angle and separation distance on the three-dimensional fountain flowfield generated by two impinging jets operating at a jet Reynolds number of 250,000. Substantial differences in the flow features were observed for different spacings between the jets. Plume effects in cruise mode were simulated by a supersonic unheated jet parallel to a wall. Variation of the distance between the wall and the edge of the plume is shown to have a major controlling effect on the supersonic screech instability.

Kibens, V.↗

NASA NTRS1990

System Detects Vibrational Instabilities

Sustained vibrations at two critical frequencies trigger diagnostic response or shutdown. Vibration-analyzing electronic system detects instabilities of combustion in rocket engine. Controls pulse-mode firing of engine and identifies vibrations above threshold amplitude at 5.9 and/or 12kHz. Adapted to other detection and/or control schemes involving simultaneous real-time detection of signals above or below preset amplitudes at two or more specified frequencies. Potential applications include rotating machinery and encoders and decoders in security systems.

Bozeman, Richard J., Jr.↗

NASA NTRS1989

The SKY SHARK: an RPV Designed to Investigate the Pressure Distribution on a Lifting Surface

The objective was to design a remotely piloted vehicle which is capable of gathering in-flight pressure distribution data on a lifting test specimen, and then test the design by constructing a subscale demonstrator, to prove the flight worthiness of the concept. The technology demonstrator was scheduled for takeoff at approximately, 7:20 AM on Thursday April 27th. There was a light wind from the southeast. The plane was hand-launched and made an initial dip, most likely due to the poor trim conditions at launch. It then began to climb and bank into a left turn. The aircraft climbed to an altitude of approximately 150 ft and circled. The plane flew for several minutes and at times appeared to bump around, which was due to thermal activity disrupting the flight of the aircraft. The aircraft was brought slowly down in a power-off condition and glided in for a belly landing and landed without incident. Results of the flight test proved the general capability of the design to maintain flight stability throughout the take off, cruise, turning, and landing flight regimes. We were not able to demonstrate stability with the test specimen in place as the control surfaces designed to counteract the instabilities induced in the static system, winglets and ailerons, were not included.

Ziemba, Rob↗

NASA NTRS1992

The beta dependence of the collisionless tearing instability at the dayside magnetopause

The beta dependence (where beta is the ratio between plasma pressure to magnetic pressure) of the collisionless tearing instability at the dayside magnetopause current sheet was investigated analytically for several current sheet models. The results show that, when the asymmetry of the dayside magnetopause current sheet is taken into account, the beta dependence of the collisionless tearing instability at the dayside magnetopause is controlled by the magnetic field profile across the magnetopause current sheet.

Ding, D. Q.↗

NASA NTRS1993

Rotary-wing aeroelasticity with application to VTOL vehicles

A concise assessment is presented of the state of the art in the field of rotary-wing aeroelasticity (RWE). The basic ingredients of RWE are reviewed, including structural modeling, unsteady aerodynamic modeling, formulation of the equations of motion, and solution methods. Results illustrating these methods are presented for isolated blades and coupled rotor-fuselage problems. The application of active controls to suppress aeromechanical and aeroelastic instabilities and to reduce vibration in rotorcraft is discussed. Structural optimization with aeroelastic constraints, gust response analysis of helicopters, and aeroelastic problems in special VTOL vehicles are briefly examined.

Friedmann, Peretz P.↗

NASA NTRS1994

UV filters for lighting of plants

The wavelength dependent interaction of biological systems with radiation is commonly described by appropriate action spectra. Particularly effective plant responses are obtained for ultraviolet (UV) radiation. Excess shortwave UV-B radiation will induce genetic defects and plant damage. Besides the ecological discussion of the deleterious effects of the excess UV radiation there is increasing interest in horticultural applications of this spectral region. Several metabolic pathways leading to valuable secondary plant products like colors, odors, taste, or resulting in mechanical strength and vitality are triggered by UV radiation. Thus, in ecologically as well as in economically oriented experiments the exact generation and knowledge of the spectral irradiance, particularly near the UV absorption edge, is essential. The ideal filter 'material' to control the UV absorption edge would be ozone itself. However, due to problems in controlling the toxic and chemically aggressive, instable gas, only rather 'small ozone filters' have been realized so far. In artificial plant lighting conventional solid filter materials such as glass sheets and plastic foils (celluloseacetate or cellulosetriacetate) which can be easily handled have been used to absorb the UV-C and the excess shortwave UV-B radiation of the lamp emissions. Different filter glasses are available which provide absorption properties suitable for gradual changes of the spectral UV-B illumination of artificial lighting. Using a distinct set of lamps and filter glasses an acceptable simulation of the UV-B part of natural global radiation can be achieved. The aging of these and other filter materials under the extreme UV radiation in the lamphouse of a solar simulator is presently unavoidable. This instability can be dealt with only by a precise spectral monitoring and by replacing the filters accordingly. For this reason attempts would be useful to develop real ozone filters which can replace glass filters. In any case chamber experiments require a careful selection of the filter material used and must be accompanied by a continuous UV-B monitoring.

Doehring, T.↗

NASA NTRS2004

Development and Characterization of a Low-Pressure Calibration System for Hypersonic Wind Tunnels

Minimization of uncertainty is essential for accurate ESP measurements at very low free-stream static pressures found in hypersonic wind tunnels. Statistical characterization of environmental error sources requires a well defined and controlled calibration method. A calibration system has been constructed and environmental control software developed to control experimentation to eliminate human induced error sources. The initial stability study of the calibration system shows a high degree of measurement accuracy and precision in temperature and pressure control. Control manometer drift and reference pressure instabilities induce uncertainty into the repeatability of voltage responses measured from the PSI System 8400 between calibrations. Methods of improving repeatability are possible through software programming and further experimentation.

Green, Del L.↗

NASA NTRS2018

In-Line Electromagnetic Actuator for Fuel Modulation

An in-line electromagnetic actuator with normally-open configuration has been developed for the purpose of exploring its application as a fuel modulator for the active control of combustor thermo-acoustic instabilities. The actuator is based on the spring-coil-plunger mechanism with the plunger designed to be suspended by crossed cantilever beam springs. Operational specification was set for a 1000 psi maximum inlet fuel pressure and modulation for a broad frequency bandwidth that encompasses frequencies that are typically associated with combustor thermo-acoustic instabilities. Various test results demonstrated fuel modulation up to 1200 Hz for coil excitation voltage between 2.3 and 4.6 V and current between 16 and 48 mA, respectively. The initial goal of this work was to demonstrate fuel modulation based on the concept of a normally-open fuel modulator with tethered plunger displacement.

Actuator↗

NASA NTRS2018

In-Line Electromagnetic Actuator for Fuel Modulation

Okojie, Robert S.↗

NASA NTRS1992

Recovery of postural equilibrium control following spaceflight

Decreased postural stability is observed in most astronauts immediately following spaceflight. Because ataxia may present postflight operational hazards, it is important to determine the incidence of postural instability immediately following landing and the dynamics of recovery of normal postural equilibrium control. It is postulated that postflight postural instability results from in-flight adaptive changes in central nervous system (CNS) processing of sensory information from the visual, vestibular, and proprioceptive systems. The purpose of the present investigation was to determine the magnitude and time course of postflight recovery of postural equilibrium control and, hence, readaptation of CNS processing of sensory information. Thirteen crew members from six spaceflight missions were studied pre- and postflight using a modified commercial posturography system. Postural equilibrium control was found to be seriously disrupted immediately following spaceflight in all subjects. Readaptation to the terrestrial environment began immediately upon landing, proceeded rapidly for the first 10-12 hours, and then proceeded much more slowly for the subsequent 2-4 days until preflight stability levels were reachieved. It is concluded that the overall postflight recovery of postural stability follows a predictable time course.

NASA Discipline Neuroscience↗

NASA NTRS1960

Investigation of the Subsonic Stability and Control Characteristics of a 1/7-Scale Model of the North American X-15 Airplane with and without Fuselage Forebody Strakes

An investigation of the low-subsonic stability and control characteristics of a l/7-scale free-flying model modified to represent closely the North American X-15 airplane (configuration 3) has been made in the Langley full-scale tunnel. Flight conditions at a relatively low altitude were simulated with the center of gravity at 16.0 percent of the mean aerodynamic chord. The longitudinal stability and control were considered to be satisfactory for all flight conditions tested. The lateral flight behavior was generally satisfactory for angles of attack below about 20 deg. At higher angles, however, the model developed a tendency to fly in a side-slipped attitude because of static directional instability at small sideslip angles. Good roll control was maintained to the highest angles tested, but rudder effectiveness diminished with increasing angle of attack and became adverse for angles above 40 deg. Removal of the lower rudder had little effect on the lateral flight characteristics for angles of attack less than about 20 deg but caused the lateral flight behavior to become worse in the high angle-of-attack range. The addition of small fuselage forebody strakes improved the static directional stability and lateral flight behavior of both configurations.

Hassell, James L., Jr.↗

NASA NTRS1989

Controller reduction for effective interdisciplinary design of active structures

Control problems of large aerospace structures are intrinsically interdisciplinary and require strategies which address the complete interaction between flexible structures, electromechanical actuators and sensors, and feedback control algorithms. Current research and future directions which will require an interdisciplinary team effort in dynamics, control and optimization of such structures are being surveyed. It is generally agreed that the dynamics of space structures require large scale discrete modeling, resulting in thousands of discrete unknowns. Proven control strategies, on the other hand, employ a low order controller that is based on a reduced order model of structures. Integration of such low order controllers and large scale dynamics models often leads to serious deterioration of the closed loop stability margin and even instability. To alleviate this stability deterioration while low order controllers remain effective, the following approach was investigated: (1) retain low order controllers based on reduced order models of structures as the basic control strategy; (2) introduce a compensator that will directly account for the deterioration of stability margin due to controller-structure integration; and (3) assess overall performance of the integrated control structure system by developing measures of suboptimality in the combination of (1) and (2). The benefits include: simplicity in the design of basic controllers, thus facilitating the optimization of structure control interactions; increased understanding of the roles of the compensator so as to modify the structure as well as the basic controller, if necessary, for improved performance; and adaptability to localize controllers by viewing the compensator as a systems integration filter.

Balas, Mark J.↗

NASA NTRS1999

Stability of the Tilt Modes of an Actively Controlled Flywheel Analyzed

Applications of strongly gyroscopic rotors are becoming important, including flywheels for terrestrial and space energy storage and various attitude control devices for spacecraft. Some of these applications, especially the higher speed ones for energy storage, will have actively controlled magnetic bearings. These bearings will be required where speeds are too high for conventional bearings, where adequate lubrication is undesirable or impossible, or where bearing losses must be minimized for efficient energy storage. Flywheel rotors are highly gyroscopic, and above some speed that depends on the bandwidth of the feedback system, they always become unstable in an actively controlled magnetic bearing system. To assess ways to prevent instability until speeds well above the desired operating range, researchers at the NASA Lewis Research Center used a commercial controls code to calculate the eigenvalues of the tilt modes of a rigid gyroscopic rotor supported by active magnetic bearings. The real part of the eigenvalue is the negative of the damping of the mode, and the imaginary part is approximately equal to the mode s frequency.

Brown, Gerald V.↗

NASA NTRS2020

Projection-Based Inter-Agent Collision Avoidance in Dual Agent Systems

Inter-agent collisions can occur in otherwise dynamically-stable (i.e., Lyapunov stable) dual-agent leaderfollower systems. These inter-agent collisions between the leader and the follower happen during the transient phase of the system’s evolution, although the steady-state behavior of the system is asymptotically/exponentially stable. Therefore, to avoid such inter-agent collisions, it is essential to control the relative error trajectory between the leader and the follower during the transient phase of the system’s evolution. In this paper, we introduce a novel projection operator based modelreference control architecture that can mitigate impending inter-agent collisions by modifying the transient dynamics of relative trajectories. This controller augments the follower’s baseline controller and consists of two essential components: a collision-free reference model based on the projection operator and a model reference tracking controller to guide the follower to follow the reference-model. This paper defines the concept of transient-instability in leader-follower systems, introduces collision mitigation controller architecture, and presents an illustrative example demonstrating its effectiveness.

Bayard, David S.↗

NASA NTRS1989

A design procedure for the handling qualities optimization of the X-29A aircraft

A design technique for handling qualities improvement was developed for the X-29A aircraft. As with any new aircraft, the X-29A control law designers were presented with a relatively high degree of uncertainty in their mathematical models. The presence of uncertainties, and the high level of static instability of the X-29A caused the control law designers to stress stability and robustness over handling qualities. During flight test, the mathematical models of the vehicle were validated or corrected to match the vehicle dynamic behavior. The updated models were then used to fine tune the control system to provide fighter-like handling characteristics. A design methodology was developed which works within the existing control system architecture to provide improved handling qualities and acceptable stability with a minimum of cost in both implementation as well as software verification and validation.

Bosworth, John T.↗