68,202 research outputs found
Digital simulation of V/STOL aircraft for autopilot research
Simulations of V/STOL aircraft for autopilot research were introduced as examples of large scale systems. A hierarchy of simulations was assembled, and a modular organization was given to the simulations. The dynamics of the system were subdivided into translational and rotational degrees of freedom, based on the different frequencies at which significant variations in motion variables, control forces and moments occur in two subsystems. This is the basis of the autopilot partitioning into two smaller control problems
Nuclear Reactions Rates Governing the Nucleosynthesis of Ti44
Large excesses of Ca44 in certain presolar graphite and silicon carbide
grains give strong evidence for Ti44 production in supernovae. Furthermore,
recent detection of the Ti44 gamma-line from the Cas A SNR by CGRO/COMPTEL
shows that radioactive Ti44 is produced in supernovae. These make the Ti44
abundance an observable diagnostic of supernovae. Through use of a nuclear
reaction network, we have systematically varied reaction rates and groups of
reaction rates to experimentally identify those that govern Ti44 abundance in
core-collapse supernova nucleosynthesis. We survey the nuclear-rate dependence
by repeated calculations of the identical adiabatic expansion, with peak
temperature and density chosen to be 5.5xE9 K and 1E7 g/cc, respectively, to
approximate the conditions in detailed supernova models. We find that, for
equal total numbers of neutrons and protons (eta=0), Ti44 production is most
sensitive to the following reaction rates: Ti44(alpha,p)V47,
alpha(2alpha,gamma)C12, Ti44(alpha,gamma)Cr48, V45(p,gamma)Cr46. We tabulate
the most sensitive reactions in order of their importance to the Ti44
production near the standard values of currently accepted cross-sections, at
both reduced reaction rate (0.01X) and at increased reaction rate (100X)
relative to their standard values. Although most reactions retain their
importance for eta > 0, that of V45(p,gamma)Cr46 drops rapidly for eta >=
0.0004. Other reactions assume greater significance at greater neutron excess:
C12(alpha,gamma)O16, Ca40(alpha,gamma)Ti44, Al27(alpha,n)P30, Si30(alpha,n)S33.
Because many of these rates are unknown experimentally, our results suggest the
most important targets for future cross section measurements governing the
value of this observable abundance.Comment: 37 pages, LaTex, 17 figures, 8 table
A formal structure for advanced automatic flight-control systems
Techniques were developed for the unified design of multimode, variable authority automatic flight-control systems for powered-lift STOL and VTOL aircraft. A structure for such systems is developed to deal with the strong nonlinearities inherent in this class of aircraft, to admit automatic coupling with advanced air traffic control, and to admit a variety of active control tasks. The aircraft being considered is the augmentor wing jet STOL research aircraft
Rotational properties of two-component Bose gases in the lowest Landau level
We study the rotational (yrast) spectra of dilute two-component atomic Bose
gases in the low angular momentum regime, assuming equal interspecies and
intraspecies interaction. Our analysis employs the composite fermion (CF)
approach including a pseudospin degree of freedom. While the CF approach is not
{\it a priori} expected to work well in this angular momentum regime, we show
that composite fermion diagonalization gives remarkably accurate approximations
to low energy states in the spectra. For angular momenta (where
and denote the numbers of particles of the two species, and ), we
find that the CF states span the full Hilbert space and provide a convenient
set of basis states which, by construction, are eigenstates of the symmetries
of the Hamiltonian. Within this CF basis, we identify a subset of the basis
states with the lowest -level kinetic energy. Diagonalization within
this significally smaller subspace constitutes a major computational
simplification and provides very close approximations to ground states and a
number of low-lying states within each pseudospin and angular momentum channel
Configuration management and automatic control of an augmentor wing aircraft with vectored thrust
An advanced structure for automatic flight control logic for powered-lift aircraft operating in terminal areas is under investigation at Ames Research Center. This structure is based on acceleration control; acceleration commands are constructed as the sum of acceleration on the reference trajectory and a corrective feedback acceleration to regulate path tracking errors. The central element of the structure, termed a Trimmap, uses a model of the aircraft aerodynamic and engine forces to calculate the control settings required to generate the acceleration commands. This report describes the design criteria for the Trimmap and derives a Trimmap for Ames experimental augmentor wing jet STOL research aircraft
Analytic invariant charge and the lattice static quark-antiquark potential
A recently developed model for the QCD analytic invariant charge is compared
with quenched lattice simulation data on the static quark-antiquark potential.
By employing this strong running coupling one is able to obtain the confining
quark-antiquark potential in the framework of the one-gluon exchange model. To
achieve this objective a technique for evaluating the integrals of a required
form is developed. Special attention is paid here to removing the divergences
encountered the calculations. All this enables one to examine the asymptotic
behavior of the potential at both small and large distances with high accuracy.
An explicit expression for the quark-antiquark potential, which interpolates
between these asymptotics, and satisfies the concavity condition, is proposed.
The derived potential coincides with the perturbative results at small
distances, and it is in a good agreement with the lattice data in the
nonperturbative physically-relevant region. An estimation of the parameter
is obtained for the case of pure gluodynamics. It is found to
be consistent with all the previous estimations of in the
framework of approach in hand.Comment: LaTeX2e, 10 pages with 3 EPS figure
Measurement of macroscopic plasma parameters with a radio experiment: Interpretation of the quasi-thermal noise spectrum observed in the solar wind
The ISEE-3 SBH radio receiver has provided the first systematic observations of the quasi-thermal (plasma waves) noise in the solar wind plasma. The theoretical interpretation of that noise involves the particle distribution function so that electric noise measurements with long antennas provide a fast and independent method of measuring plasma parameters: densities and temperatures of a two component (core and halo) electron distribution function have been obtained in that way. The polarization of that noise is frequency dependent and sensitive to the drift velocity of the electron population. Below the plasma frequency, there is evidence of a weak noise spectrum with spectral index -1 which is not yet accounted for by the theory. The theoretical treatment of the noise associated with the low energy (thermal) proton population shows that the moving electrical antenna radiates in the surrounding plasma by Carenkov emission which becomes predominant at the low frequencies, below about 0.1 F sub P
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