20,576 research outputs found
Flight control systems properties and problems, volume 1
This volume contains a delineation of fundamental and mechanization-specific flight control characteristics and problems gleaned from many sources and spanning a period of over two decades. It is organized to present and discuss first some fundamental, generic problems of closed-loop flight control systems involving numerator characteristics (quadratic dipoles, non-minimum phase roots, and intentionally introduced zeros). Next the principal elements of the largely mechanical primary flight control system are reviewed with particular emphasis on the influence of nonlinearities. The characteristics and problems of augmentation (damping, stability, and feel) system mechanizations are then dealt with. The particular idiosyncracies of automatic control actuation and command augmentation schemes are stressed, because they constitute the major interfaces with the primary flight control system and an often highly variable vehicle response
Feasibility of conventional control techniques for large highly coupled elastic boost vehicles Final report
Control techniques for large highly-coupled elastic boost vehicles with elastic and fuel slosh mode frequencies close to desired control frequencies - motion equation
Lattice effects in the LaSrCuO compounds
Systematic Raman studies on several cuprates (YBaCuO, YBaCuO or BiSrCaCuO) have shown that at optimal doping the compounds
are at the edge of lattice instability; once this level is exceeded, by means
of doping or applying external hydrostatic pressure, the changes in the
transition temperature are accompanied by spectral modifications. There are
strong indications that the reduction in T is correlated with a
separation into nanoscale phases, which involve the oxygen atoms of the
CuO planes. In this work, modifications with doping in the Raman
spectra of the LaSrCuO compound are presented,
which show that spin or charge ordering is coupled with lattice distortions in
the whole doping region.Comment: 6 pages, 6 figure
Magnetic, Transport, and Thermal Properties of Single Crystals of the Layered Arsenide BaMn2As2
Growth of BaMn2As2 crystals using both MnAs and Sn fluxes is reported. Room
temperature crystallography, anisotropic isothermal magnetization M versus
field H and magnetic susceptibility chi versus temperature T, electrical
resistivity in the ab plane rho(T), and heat capacity C(T) measurements on the
crystals were carried out. The tetragonal ThCr2Si2-type structure of BaMn2As2
is confirmed. After correction for traces of ferromagnetic MnAs impurity phase
using M(H) isotherms, the inferred intrinsic chi(T) data of the crystals are
anisotropic with chi_{ab}/chi_{c} \approx 7.5 at T = 2 K. The temperature
dependences of the anisotropic chi data suggest that BaMn2As2 is a collinear
antiferromagnet at room temperature with the easy axis along the c axis, and
with an extrapolated Neel temperature T_N \sim 500 K. The rho(T) decreases with
decreasing T below 310 K but then increases below \sim 50 K, suggesting that
BaMn2As2 is a small band-gap semiconductor with an activation energy of order
0.03 eV. The C(T) data from 2 to 5 K are consistent with this insulating ground
state, exhibiting a low temperature Sommerfeld coefficient gamma = 0.0(4)
mJ/mol K^2. The Debye temperature is determined from these data to be theta_D =
246(4) K. BaMn2As2 is a potential parent compound for ThCr2Si2-type
superconductors.Comment: 7 pages, 6 figures; v2: typos corrected, additional data and
discussion, accepted for publication in Phys. Rev.
Space Shuttle flying qualities and flight control system assessment study, phase 2
A program of flying qualities experiments as part of the Orbiter Experiments Program (OEX) is defined. Phase 1, published as CR-170391, reviewed flying qualities criteria and shuttle data. The review of applicable experimental and shuttle data to further define the OEX plan is continued. An unconventional feature of this approach is the use of pilot strategy model identification to relate flight and simulator results. Instrumentation, software, and data analysis techniques for pilot model measurements are examined. The relationship between shuttle characteristics and superaugmented aircraft is established. STS flights 1 through 4 are reviewed from the point of view of flying qualities. A preliminary plan for a coordinated program of inflight and simulator research is presented
The Urban Food Question in the Context of Inequality and Dietary Change: A Study of Schoolchildren in Accra
Diets are changing globally, as agricultural and food systems have become globalised. Understanding how patterns of globalisation affect welfare is a key development question, but we know little about the way that the globalisation of food systems impacts different groups. This study explores food security and consumption among schoolchildren in Accra. We use a novel approach based on triangulation of primary data on food consumption and a synthesis of secondary literature on food trade, policy and urban food environment. Thus, we bridge a divide between micro-level analyses of food consumption and macro-level studies of food systems. We find that socio-economic status is a critical dimension, with poorer children more vulnerable to food insecurity and narrow dietary diversity. However, the consumption of packaged and processed foods, often sugar-rich and nutrient-poor, cuts across wealth groups. We argue that the urban food question today is defined by two intersecting phenomena: inequality and dietary change. The urban poor continue to face the fundamental challenge of adequate food access amidst a food environment that provides consumers with unhealthy and cheap food options. Therefore, food policy needs to regulate imports of cheap, unhealthy and enticing food
- …