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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

A high accuracy sequential solver for simulation and active control of a longitudinal combustion instability

A high accuracy convection scheme using a sequential solution technique has been developed and applied to simulate the longitudinal combustion instability and its active control. The scheme has been devised in the spirit of the Total Variation Diminishing (TVD) concept with special source term treatment. Due to the substantial heat release effect, a clear delineation of the key elements employed by the scheme, i.e., the adjustable damping factor and the source term treatment has been made. By comparing with the first-order upwind scheme previously utilized, the present results exhibit less damping and are free from spurious oscillations, offering improved quantitative accuracy while confirming the spectral analysis reported earlier. A simple feedback type of active control has been found to be capable of enhancing or attenuating the magnitude of the combustion instability.

Shyy, W.↗
NASA NTRS1986

Individual-blade-control research in the MIT VTOL Technology Laboratory 1977-1985

A new, advanced system for active control of helicopters and its application to the solution of rotor aerodynamic and aeroelastic problems is described. Each blade is individually controlled in the rotating frame over a wide range of frequencies. Application of the system to gust alleviation, attitude stabilization, vibration alleviation, blade lag damping augmentation, stall flutter suppression, blade flapping stabilization, stall alleviation, and performance enhancement is outlined. The effectiveness of the system in achieving most of these applications is demonstrated by experimental results from wind tunnel tests of a model helicopter rotor with individual blade control. The feasibility of achieving many or all of the applications of individual blade control using the conventional helicopter swash plate is demonstrated, and the necessary control laws are presented.

Ham, N. D.↗
NASA NTRS1986

Helicopter individual-blade-control research at MIT 1977-1985

A new advanced system for active control of helicopters and its application to the solution of rotor aerodynamic and aeroelastic problems is described. Each blade is individually controlled in the rotating frame over a wide range of frequencies. Application of the system to gust alleviation, attitude stabilization, vibration alleviation, blade lag damping augmentation, stall flutter suppression, blade flapping stabilization, stall alleviation, and performance enhancement is outlined. The effectiveness of the system in achieving most of these applications is demonstrated by experimental results from wind tunnel tests of a model helicopter rotor with individual blade control. The feasibility of achieving many or all of the applications of individual blade control using the conventional helicopter swash plate is demonstrated, and the necessary control laws are presented.

Ham, Norman D.↗
NASA NTRS2010

Analytic Development of a Reference Profile for the First Entry in a Skip Atmospheric Entry

This note shows that a feasible reference drag profile for the first entry portion of a skip entry can be generated as a polynomial expression of the velocity. The coefficients of that polynomial are found through the resolution of a system composed of m + 1 equations, where m is the degree of the drag polynomial. It has been shown that a minimum of five equations (m = 4) are required to establish the range and the initial and final conditions on velocity and flight path angle. It has been shown that at least one constraint on the trajectory can be imposed through the addition of one extra equation in the system, which must be accompanied by the increase in the degree of the drag polynomial. In order to simplify the resolution of the system of equations, the drag was considered as being a probability density function of the velocity, with the velocity as a distribution function of the drag. Combining this notion with the introduction of empirically derived constants, it has been shown that the system of equations required to generate the drag profile can be successfully reduced to a system of linear algebraic equations. For completeness, the resulting drag profiles have been flown using the feedback linearization method of differential geometric control as a guidance law with the error dynamics of a second order homogeneous equation in the form of a damped oscillator. Satisfactory results were achieved when the gains in the error dynamics were changed at a certain point along the trajectory that is dependent on the velocity and the curvature of the drag as a function of the velocity. Future work should study the capacity to update the drag profile in flight when dispersions are introduced. Also, future studies should attempt to link the first entry, as presented and controlled in this note, with a more standard control concept for the second entry, such as the Apollo entry guidance, to try to assess the overall skip entry performance. A guidance law that includes an integral feedback term, as is the case in the actual Space Shuttle entry guidance and as is proposed in Ref 29, could be tried in future studies to assess whether its use results in an improvement of the tracking performance, and to evaluate the design needs when determining the control gains.

Garcia-Llama, Eduardo↗
NASA NTRS1988

A model for active control of helicopter air resonance in hover and forward flight

A coupled rotor/fuselage helicopter analysis is presented. The accuracy of the model is verified by comparing it with the experimental data. The sensitivity of the open loop damping of the unstable air resonance mode to such modeling effects as blade torsional flexibility, unsteady aerodynamics, forward flight, periodic terms, and trim solution is illustrated by numerous examples. Subsequently, the model is used in conjunction with linear optimal control theory to stabilize the air resonance mode. The influence of the modeling effects mentioned before on active air resonance control is then investigated.

Takahashi, M. D.↗
NASA NTRS1996

Cross Feedback Control of a Magnetic Bearing System: Controller Design Considering Gyroscopic Effects

For flywheel rotors or other rotors with significant ratios of moments of inertia, the influence of gyroscopic effects has to be considered. While conservative or damped systems remain stable even under gyroscopic effects, magnetically suspended rotors can be destabilized with increasing rotational speed. The influence of gyroscopic effects on the stability and behavior of a magnetic bearing system is analyzed. The analysis is carried out with a rigid body model for the rotor and a nonlinear model for the magnetic bearing and its amplifier. Cross feedback control can compensate gyroscopic effects. This compensation leads to better system performance and can avoid instability. Furthermore, the implementation of this compensation is simple. The main structure of a decentralized controller can still be used. It has only to be expanded by the cross feedback path.

Ahrens, Markus↗
NASA NTRS1991

Active vibration absorber for CSI evolutionary model: Design and experimental results

The development of control of large flexible structures technology must include practical demonstration to aid in the understanding and characterization of controlled structures in space. To support this effort, a testbed facility was developed to study practical implementation of new control technologies under realistic conditions. The design is discussed of a second order, acceleration feedback controller which acts as an active vibration absorber. This controller provides guaranteed stability margins for collocated sensor/actuator pairs in the absence of sensor/actuator dynamics and computational time delay. The primary performance objective considered is damping augmentation of the first nine structural modes. Comparison of experimental and predicted closed loop damping is presented, including test and simulation time histories for open and closed loop cases. Although the simulation and test results are not in full agreement, robustness of this design under model uncertainty is demonstrated. The basic advantage of this second order controller design is that the stability of the controller is model independent.

Bruner, Anne M.↗
NASA NTRS2016

Development of Semi-Empirical Damping Equation for Baffled Tank with Oblate Spheroidal Dome

Propellant slosh is a potential source of disturbance that can significantly impact the stability of space vehicles. The slosh dynamics are typically represented by a mechanical model of a spring-mass-damper. This mechanical model is then included in the equation of motion of the entire vehicle for Guidance, Navigation and Control analysis. The typical parameters required by the mechanical model include natural frequency of the slosh, slosh mass, slosh mass center location, and the critical damping ratio. A fundamental study has been undertaken at NASA MSFC to understand the fluid damping physics from a ring baffle in the barrel section of a propellant tank. An asymptotic damping equation and CFD blended equation have been derived by NASA MSFC team to complement the popularly used Miles equation at different flow regimes. The new development has found success in providing a nonlinear damping model for the Space Launch System. The purpose of this study is to further extend the semi-empirical damping equations into the oblate spheroidal dome section of the propellant tanks. First, previous experimental data from the spherical baffled tank are collected and analyzed. Several methods of taking the dome curvature effect, including a generalized Miles equation, area projection method, and equalized fill height method, are assessed. CFD simulation is used to shed light on the interaction of vorticity around the baffle with the locally curved wall and liquid-gas interface. The final damping equation will be validated by a recent subscale test with an oblate spheroidal dome conducted at NASA MSFC.

Yang, H. Q.↗
NASA NTRS1976

F-8C digital CCV flight control laws

A set of digital flight control laws were designed for the NASA F-8C digital fly-by-wire aircraft. The control laws emphasize Control Configured Vehicle (CCV) benefits. Specific pitch axis objectives were improved handling qualities, angle-of-attack limiting, gust alleviation, drag reduction in steady and maneuvering flight, and a capability to fly with reduced static stability. The lateral-directional design objectives were improved Dutch roll damping and turn coordination over a wide range in angle-of-attack. An overall program objective was to explore the use of modern control design methodilogy to achieve these specific CCV benefits. Tests for verifying system integrity, an experimental design for handling qualities evaluation, and recommended flight test investigations were specified.

Hartmann, G. L.↗
NASA NTRS1998

The Role of Marangoni Convection for the FZ-Growth of Silicon

In growing crystals by the floating zone (FZ) technique under microgravity, the size restriction we have under earth conditions because of the hydrostatic pressure are avoided. Further, buoyancy related convection is eliminated to a high degree. But in the case of silicon, the gravity independent thermocapillary (Marangoni) convection is time-dependent even for small zone geometries. This has been demonstrated in several Technische Experimente unter Schwerelosigkeit (TEXUS) - technical experiments under reduced gravity) flights. Thus, to really take advantage of microgravity with respect to improve crystal quality, tools are required to control Marangoni convection in space facilities. Applying magnetic fields, convection can be influenced; fluid flow can either be damped (static magnetic fields) or overlaid by a regular flow regime (rotating magnetic fields). In floating zones of 8-10mm diameter and height (Ma approximately equals 6 x 10(exp 3)), a static magnetic field of about 2OOmT is sufficient to suppress time-dependent Marangoni convection to a high degree, but in dependence on the kind and the concentration of the added dopant, a new type of strongly pronounced dopant inhomogeneities have been detected. They are originated by thermoelectromagnetic convection. This can be avoided as well as detrimental effects on the radial dopant distribution by using rotating magnetic fields instead of static ones. Applying 75mT/50Hz to the FZ, the intensity of the dopant fluctuations is reduced to a high degree. Considering the rather low power consumption of rotating magnetic fields, this will be a useful tool for control or elimination of time-dependent Marangoni convection under microgravity. The strong time dependent character of thermocapillary flow and its influence on the temperature field has been measured in silicon half-zones for Marangoni numbers of Ma is approximately equal to 1-1.5 x 10(exp 4): temperature fluctuations up to 4C have been determined. Their frequency range was 0.1 and 0.4 Hz. Between certain thermocouple or sensor pairs, strong correlation has been detected.

Dold, P.↗
NASA NTRS2023

Flying Beyond Flutter with the X-56A Aircraft

The National Aeronautics and Space Administration X-56A Multi-Utility Technology Testbed was successfully flown beyond flutter using active feedback control methods. This report discusses the flight-test methods used during envelope expansion and presents the data collected. The frequency and damping of rigid-body flight dynamic modes and the first two structural modes were estimated from the flight data. The flutter mechanism (body freedom flutter) was found to match predictions with coupling occurring between the short period and the first symmetric wing bending mode. Flights up to the instability threshold and beyond were conducted using active feedback control. Several tests were conducted in flight where the control system was disabled momentarily to observe the unstable flutter mode and then reengaged; thus, demonstrating active suppression.

Jacob Schaefer↗
NASA NTRS2000

Flying Qualities Evaluation of a Commuter Aircraft With an Ice Contaminated Tailplane

During the NASA/FAA (Federal Aviation Administration) Tailplane Icing Program, pilot evaluations of aircraft flying qualities were conducted with various ice shapes attached to the horizontal tailplane of the NASA Twin Otter Icing Research Aircraft. Initially, only NASA pilots conducted these evaluations, assessing the differences in longitudinal flight characteristics between the baseline or clean aircraft, and the aircraft configured with an Ice Contaminated Tailplane (ICT). Longitudinal tests included Constant Airspeed Flap Transitions, Constant Airspeed Thrust Transitions, zero-G Pushovers, Repeat Elevator Doublets, and Simulated Approach and Go-Around tasks. Later in the program, guest pilots from government and industry were invited to fly the NASA Twin Otter configured with a single full-span artificial ice shape attached to the leading edge of the horizontal tailplane. This shape represented ice formed due to a 'Failed Boot' condition, and was generated from tests in the Glenn Icing Research Tunnel on a full-scale tailplane model. Guest pilots performed longitudinal handling tests, similar to those conducted by the NASA pilots, to evaluate the ICT condition. In general, all pilots agreed that longitudinal flying qualities were degraded as flaps were lowered, and further degraded at high thrust settings. Repeat elevator doublets demonstrated reduced pitch damping effects due to ICT, which is a characteristic that results in degraded flying qualities. Pilots identified elevator control force reversals (CFR) in zero-G pushovers at a 20 deg flap setting, a characteristic that fails the FAR 25 no CFR certification requirement. However, when the same pilots used the Cooper-Harper rating scale to perform a simulated approach and go-around task at the 20 deg flap setting, they rated the airplane as having Level I and Level II flying qualities respectively. By comparison, the same task conducted at the 30 deg flap setting, resulted in Level II flying qualities for the approach portion, and Level III for the go-around portion.The results of this program indicate that safe and acceptable flying qualities with an ICT condition, can be effectively assessed by task-oriented pilot maneuvers. In addition, other maneuvers such as repeat elevator doublets provide good qualitative and quantitative assessments of pitch damping and elevator effectiveness, which are characteristics that correlate well with pilot task ratings. The results of this testing indicate that the FAR 25 zero-G pushover maneuver, which requires no CFR during its execution, may be an overly conservative pass/fail criteria for aircraft certification.

Ranaudo, Richard J.↗
NASA NTRS1976

Frequency domain approach for evaluation of stochastic control of elastic spacecraft

An attempt is made to develop a reduced-order filter for an elastic spacecraft. The frequency-domain approach is resorted to both for arriving at an appropriate ROF and for evaluating the resulting stochastic flight controller. The frequency-domain approach employed is contrasted to the time-domain approach previously utilized by the authors. A linearized deterministic dynamical model of a solar electric propulsion spacecraft is analyzed. Flexibility modes were incorporated profitably into the ROF model, and the stochastic controllers exhibit notching of structural resonances reminiscent of classical notch filters. Accurate estimates of modal parameters (damping, natural frequency) are deemed crucial, and use of worst-case values in the ROF design is recommended.

Larson, V.↗
NASA NTRS1978

Transonic static and dynamic stability characteristics of a finned projectile configuration

Static and dynamic stability tests were made of a finned projectile configuration with the aft-mounted fins arranged in a cruciform pattern. The tests were made at free stream Mach numbers of 0.7, 0.9, 1.1, and 1.2 in the Langley 8-foot transonic pressure tunnel. Some of the parameters measured during the tests were lift, drag, pitching moment, pitch damping, and roll damping. Configurations tested included the body with undeflected fins, the body with various fin deflections for control, and the body with fins removed. Theoretical estimates of the stability derivatives were made for the fins on configuration.

Boyden, R. P.↗
NASA NTRS1982

On passive damping mechanisms in large space structures

The significance is explained of even tiny amounts of passive energy dissipation to ensure successful stabilization of large, flexible space structures. Study of scale effects on various mechanisms indicates that modal damping ratios are likely to decrease as size increases in a family of similar structures. Paper focuses on thermal dissipation induced by strain gradients during vibration of monolithic configurations. Past work and the expected magnitudes of this damping are reviewed, along with reasons why it is, to some degree, under the designer's control. In the search for the highest practical values, unidirectional metallic composites and other arrangements are examined

Ashley, H.↗
NASA NTRS1987

On incorporating damping and gravity effects in models of structural dynamics of the SCOLE configuration

The damping for structural dynamic models of flexible spacecraft is usually ignored and then added after modal frequencies and mode shapes are calculated. It is common practice to assume the same damping ratio for all modes, although it is known that damping due to bending and that due to torsion are sometimes ignored. Two methods of including damping in the modeling process from its onset are examined. First, the partial derivative equations of motion are analyzed for a pinned-pinned beam with damping. The end conditions are altered to handle bodies with mass and inertia for the Spacecraft Control Laboratory Experiment (SCOLE) configuration. Second, a massless beam approximation is used for the modes with low frequencies, and a clamped-clamped system is used to approximate the modes for arbitrarily high frequency. The model is then modified to include gravity effects and is compared with experimental results.

Taylor, Larry↗
NASA NTRS1985

Analysis of the free-tip rotor wind-tunnel test results

The results from a wind tunnel test of a small scale free-tip rotor are analyzed. The free-tip rotor has blade tips that are free to weathervane into the tip's relative wind, thus producing a more uniform lift around the azimuth. The free-tip assembly, which includes the controller, functioned flawlessly throughout the test. In a test of the free-tip's response after passing through a vertical air jet, the tip pitched freely and in a controlled manner. Analysis of the tip's response characteristics showed the free-tip system's damped natural frequency to be 5.2 per rev. Tip pitch angle responses to the local airstream are presented for an advance-ratio range of 0.1 to 0.397 and for a solidity weighted rotor lift coefficient range of 0.038 to 0.092. Harmonic analysis of the responses showed a dominance by the first harmonic. As a result of the tip being free, forward flight power requirements were reduced by 8% or more. More power reduction was recorded for high thrust conditions. In addition to the power reduction, flatwise blade bending moments were reduced by as much as 30% at the inboard blade stations.

Stroub, Robert H.↗