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At least 649 records · Page 36

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NASA NTRS1975

Computer programs for calculating the static longitudinal aerodynamic characteristics of wing-body-tail configurations

Four computer programs developed to calculate the longitudinal aerodynamic characteristics of wing-body and wing-body-tail combinations are presented. The R1307 program is based on a linear method and is limited to the small range of angles of attack for which the lift and moment characteristics of wings and bodies are linear with angle of attack. The CRSFLW program is based on a crossflow method of predicting the forces and moments on bodies alone or wing-body combinations over a large angle of attack range. The SUBSON program predicts the longitudinal aerodynamic characteristics of wing-body-tail combinations at subsonic speeds and at angles of attack for which symmetrical pairs of vortices are shed from the body nose and the leading and side edges of the lifting surfaces. Program SUPSON predicts the longitudinal aerodynamic characteristics of wing-body-tail combinations at supersonic speeds in the same angle-of-attack range. A description of the use of each program, instructions for preparation of input, a description of the output, program listings, and sample cases for each program are included.

Mendenhall, M. R.↗

NASA NTRS1977

Rolling moments in a trailing vortex flow field

Pressure distributions are presented which were measured on a wing in close proximity to a tip vortex of known structure generated by a larger, upstream semispan wing. Overall loads calculated by integration of these pressures are checked by independent measurements made with an identical model mounted on a force balance. Several conventional methods of wing analysis are used to predict the loads on the following wing. Strip theory is shown to give uniformly poor results for loading distribution, although predictions of overall lift and rolling moment are sometimes acceptable. Good results are obtained for overall coefficients and loading distribution by using linearized pressures in vortex-lattice theory in conjunction with a rectilinear vortex. The equivalent relation from reverse-flow theory that can be used to give economic predictions for overall loads is presented.

Mcmillan, O. J.↗

NASA NTRS1977

Rolling moments in a trailing vortex flow field

Mcmillan, O. J.↗

NASA NTRS1978

A method for predicting full scale buffet response with rigid wind tunnel model fluctuating pressure data. Volume 1: Prediction method development and assessment

The method requires unsteady aerodynamic forces, natural airplane modes, and the measured pressure data as input. A gust response computer program is used to calculate buffet response due to the forcing function posed by the measured pressure data. By calculating both symmetric and antisymmetric solutions, upper and lower bounds on full-scale buffet response are formed. Comparisons of predictions with flight test results are made and the effects of horizontal tail loads and static aeroelasticity are shown. Discussions are also presented on the effects of primary wing torsion modes, chordwise and spanwise phase angles, and altitude.

Cunningham, A. M., Jr.↗

NASA NTRS1979

The application of finite element techniques to acoustic transmission in lined ducts with flow

The finite element method (FEM) is used to analyze the propagation of sound in two-dimensional nonuniform ducts carrying a compressible subsonic mean flow. Galerkin and residual least squares (RLS) methods with natural and forced boundary conditions are considered. The accuracy of FEM results for the eigenvalue and transmission problems is assessed by comparison with alternative numerical schemes for nonuniform ducts. The results presented and those from associated investigations indicate that modal coupling is a significant feature of the acoustic field, especially at high Mach numbers. A multimodal model therefore appears to be essential if any reliable conclusions are to be drawn in the context of turbofan inlet regions. Improvements to the eigenvalue scheme following the implementation of higher-order Hermitian elements indicate a similar modification for the transmission problem.

Astley, R. J.↗

NASA NTRS1980

Influence of pressure driven secondary flows on the behavior of turbofan forced mixers

A finite difference procedure was developed to analyze the three dimensional subsonic turbulent flows in turbofan forced mixer nozzles. The method is based on a decomposition of the velocity field into primary and secondary flow components which are determined by solution of the equations governing primary momentum, secondary vorticity, thermal energy, and continuity. Experimentally, a strong secondary flow pattern was identified which is associated with the radial inflow and outflow characteristics of the core and fan streams and forms a very strong vortex system aligned with the radial interface between the core and fan regions. A procedure was developed to generate a similar generic secondary flow pattern in terms of two constants representing the average radial outflow or inflow in the core and fan streams as a percentage of the local streamwise velocity. This description of the initial secondary flow gave excellent agreement with experimental data. By identifying the nature of large scale secondary flow structure and associating it with characteristic mixer nozzle behavior, it is felt that the cause and effect relationship between lobe design and nozzle performance can be understood.

Anderson, B.↗

NASA NTRS1985

Stiffness control of large space structures

A method of using internal force producing dual element/actuators for vibration suppression of large space structures is proposed. The technique is applied to a vibrating string and to a low order system. Response feedback control for the vibrating string and selected modal control are used for achieving the modal damping. The actuators may be electrically powered and are suitable for structures with extremely low rigidity.

Chen, J. C.↗

NASA NTRS1986

Hybrid position/force control of multi-arm cooperating robots

This paper extends the theory of hybrid position/force control to the case of multi-arm cooperating robots. Cooperation between n robot arms is achieved by controlling each arm such that the burden of actuation is shared between the arms in a nonconflicting way as they control the position of and force on a designated point on an object. The object, which may or may not be in contact with a rigid environment, is assumed to be held rigidly by n robot end-effectors. Natural and artificial position and force constraints are defined for a point on the object and two selection matrices are obtained to control the arms. The position control loops are designed based on each manipulator's Cartesian space dynamic equations. In the position control subspace, a feature is provided which allows the robot arms to exert additional forces/torques to achieve compression, tension, or torsion in the object without affecting the execution of the motion trajectories. In the force control subspace, a method is introduced to minimize the total force/torque magnitude square while realizing the net desired force/torque on the environment.

Hayati, Samad↗

NASA NTRS1987

Predictions of F-111 TACT aircraft buffet response and correlations of fluctuating pressures measured on aluminum and steel models and the aircraft

Results of buffet research that was conducted as part of the joint USAF/NASA F-111 TACT Research Program are presented. The correlation of wind tunnel and flight measurements of buffet excitation showed that there generally was good agreement between measurements of pressure fluctuations on the models and aircraft in regions of separated flow. At shock-wave boundaries of the separated flow, correlations of pressure fluctuations were not so good, due to Reynolds number and static elastic effects. The buffet prediction method, which applies a forcing function that is obtained by real-time integration of pressure time histories with the natural modes, is described. The generalized forces, including the effects of wing and tail, correlations of predicted and measured damping, and correlations of predicted and measured buffet response are presented. All presented data are for a Mach number of 0.8 with wing-sweep angles of 26 and 35 deg for a range of angles-of-attack that include buffet onset to high intensity buffeting. Generally, the buffet predictions were considered to be quite good particularly in light of past buffet-prediction experience.

Coe, Charles F.↗

NASA NTRS1986

Riblets for aircraft skin-friction reduction

Energy conservation and aerodynamic efficiency are the driving forces behind research into methods to reduce turbulent skin friction drag on aircraft fuselages. Fuselage skin friction reductions as small as 10 percent provide the potential for a 250 million dollar per year fuel savings for the commercial airline fleet. One passive drag reduction concept which is relatively simple to implement and retrofit is that of longitudinally grooved surfaces aligned with the stream velocity. These grooves (riblets) have heights and spacings on the order of the turbulent wall streak and burst dimensions. The riblet performance (8 percent net drag reduction thus far), sensitivity to operational/application considerations such as yaw and Reynolds number variation, an alternative fabrication technique, results of extensive parametric experiments for geometrical optimization, and flight test applications are summarized.

Walsh, Michael J.↗

NASA NTRS1987

The structure and stability of coronal magnetic fields

The high degree of symmetry often assumed in studies of the structure and stability of coronal magnetic field configurations is restrictive and can yield misleading results. Fully three-dimensional numerical methods for constructing force-free equilibria and for examining their stability properties are developed which make no assumptions about symmetry. A test of the stability analysis has been performed by applying it to the Gold-Hoyle twisted flux tube, which is known to be kink-unstable if the helical field makes more than about one turn between the line-tying end-plates. The preliminary result is that the critical number of turns is about 1.1, in good agreement with the previous best estimate. However, it is found that the growth rate, which has not been discussed previously, is orders of magnitude smaller than expected, even when the flux tube is twisted far beyond the stability limit.

Mcclymont, A. N.↗

NASA NTRS1988

Coronal magnetic fields produced by photospheric shear

The magnetofrictional method for computing force-free fields is used to examine the evolution of the magnetic field of a line dipole, when there is relative shearing motion between the two polarities. It is found that the energy of the sheared field can be arbitrarily large compared with the potential field. It is also found that it is possible to fit the magnetic energy, as a function of shear amplitude, by a simple functional form. The fit parameters depend only on the distribution of normal field in the photosphere and the form of the shearing displacement. They show that the energy is relatively more enhanced if the shear occurs: (1) where the normal field is strongest; and/or (2) in the inner region of the dipole, near the axis; and/or (3) over a large fraction of the dipole area.

Klimchuk, J. A.↗

NASA NTRS1990

Time domain flutter analysis of cascades using a full-potential solver

A time domain approach is used to determine the dynamic aeroelastic stability of a cascade of blades. The structural model for each blade is a typical section with two degrees of freedom. The aerodynamic model is the unsteady, two-dimensional, full-potential flow through the cascade of airfoils. The unsteady equations of motion for the structure and the fluid are integrated simultaneously in time starting with the steady flowfield and a small initial disturbance applied to the airfoils. The motion of each blade is analyzed to determine the aeroelastic stability of the cascade. The effect of interblade phase angle is included in the analysis by allowing each blade to have an independent motion and considering a number of blade passages. Calculations are made using an airfoil section and structural parameters that are representative of a propfan. The results are compared with those from a separate frequency domain analysis. Good agreement between the results is observed. With the time domain approach, it is possible to consider nonlinear structural models and nonlinear force-displacement relations. The method allows a realistic simulation of the motion of the fluid and the cascade blades for a better physical understanding and it also has the potential for saving computational time when compared to the frequency domain approach for the flutter analysis of cascades.

Bakhle, Milind A.↗

NASA NTRS1993

Atmospheric sulfur hexafluoride - Sources, sinks and greenhouse warming

An estimate is obtained of worldwide production of SF6, from which a global emission rate is derived and extrapolated for the next 20 years. The atmospheric lifetime of SF6 is then estimated based on a known mechanism (e.g., photolysis and atmospheric oxidation) and/or on the mass balance method. Finally, the radiative forcing of SF6 is calculated based on recent laboratory IR absorption data, and the expected warming over the time period 1950-2010 is computed for several emission scenarios. Calculations showed that SF6 is 3 times more effective as a greenhouse gas compared to CFC 11 on a per-molecule basis. However, based on projected emission scenarios, the expected warming from SF6 through 2010 is small (0.004 C), compared to the warming from CO2 and other trace gases (0.8 C).

Ko, Malcolm K. W.↗

NASA NTRS1995

Progress in HTS trapped field magnets: J(sub c), area, and applications

Progress in trapped field magnets is reported. Single YBCO grains with diameters of 2 cm are made in production quantities, while 3 cm, 4 1/2 cm and 6 cm diameters are being explored. For single grain tiles: J(sub c) is approximately 10,000 A/cm(exp 2) for melt textured grains; J(sub c) is approximately 40,000 A/cm2 for light ion irradiation; and J(sub c) is approximately 85,000 A/cm(exp 2) for heavy ion irradiation. Using 2 cm diameter tiles bombarded by light ions, we have fabricated a mini-magnet which trapped 2.25 Tesla at 77K, and 5.3 Tesla at 65K. A previous generation of tiles, 1 cm x 1 cm, was used to trap 7.0 Tesla at 55K. Unirradiated 2.0 cm tiles were used to provide 8 magnets for an axial gap generator, in a collaborative experiment with Emerson Electric Co. This generator delivered 100 Watts to a resistive load, at 2265 rpm. In this experiment activation of the TFMs was accomplished by a current pulse of 15 ms duration. Tiles have also been studied for application as a bumper-tether system for the soft docking of spacecraft. A method for optimizing tether forces, and mechanisms of energy dissipation are discussed. A bus bar was constructed by welding three crystals while melt-texturing, such that their a,b planes were parallel and interleaved. The bus bar, an area of approximately 2 cm(exp 2), carried a transport current of 1000 amps, the limit of the testing equipment available.

Weinstein, Roy↗

NASA NTRS1996

Passive Magnetic Bearing With Ferrofluid Stabilization

A new class of magnetic bearings is shown to exist analytically and is demonstrated experimentally. The class of magnetic bearings utilize a ferrofluid/solid magnet interaction to stabilize the axial degree of freedom of a permanent magnet radial bearing. Twenty six permanent magnet bearing designs and twenty two ferrofluid stabilizer designs are evaluated. Two types of radial bearing designs are tested to determine their force and stiffness utilizing two methods. The first method is based on the use of frequency measurements to determine stiffness by utilizing an analytical model. The second method consisted of loading the system and measuring displacement in order to measure stiffness. Two ferrofluid stabilizers are tested and force displacement curves are measured. Two experimental test fixtures are designed and constructed in order to conduct the stiffness testing. Polynomial models of the data are generated and used to design the bearing prototype. The prototype was constructed and tested and shown to be stable. Further testing shows the possibility of using this technology for vibration isolation. The project successfully demonstrated the viability of the passive magnetic bearing with ferrofluid stabilization both experimentally and analytically.

Jansen, Ralph↗

NASA NTRS1996

Motion of Cesium Atoms in the One-Dimensional Magneto-Optical Trap

The force to which Cs atoms are subjected in the one-dimensional magneto-optical trap (lD-MOT) is calculated, and properties of this force are discussed. Several methods to increase the number of Cs atoms in the lD-MOT are presented on the basis of the analysis of the capture and escape of Cs atoms in the ID-MOT.

Li, Yimin↗

NASA NTRS1996

Study of the Mutual Interaction Between a Wing Wake and an Encountering Airplane

In an effort to increase airport productivity, several wind-tunnel and flight-test programs are currently underway to determine safe reductions in separation standards between aircraft. These programs are designed to study numerous concepts from the characteristics and detection of wake vortices to the wake-vortex encounter phenomenon. As part of this latter effort, computational tools are being developed and utilized as a means of modeling and verifying wake-vortex hazard encounters. The objective of this study is to assess the ability of PMARC, a low-order potential-flow panel method, to predict the forces and moments imposed on a following business-jet configuration by a vortex interaction. Other issues addressed include the investigation of several wake models and their ability to predict wake shape and trajectory, the validity of the velocity field imposed on the following configuration, modeling techniques and the effect of the high-lift system and the empennage. Comparisons with wind-tunnel data reveal that PMARC predicts the characteristics for the clean wing-body following configuration fairly well. Non-linear effects produced by the addition of the high-lift system and empennage, however, are not so well predicted.

Walden, A. B.↗