18,362 research outputs found

    Results of including geometric nonlinearities in an aeroelastic model of an F/A-18

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    An integrated, nonlinear simulation model suitable for aeroelastic modeling of fixed-wing aircraft has been developed. While the author realizes that the subject of modeling rotating, elastic structures is not closed, it is believed that the equations of motion developed and applied herein are correct to second order and are suitable for use with typical aircraft structures. The equations are not suitable for large elastic deformation. In addition, the modeling framework generalizes both the methods and terminology of non-linear rigid-body airplane simulation and traditional linear aeroelastic modeling. Concerning the importance of angular/elastic inertial coupling in the dynamic analysis of fixed-wing aircraft, the following may be said. The rigorous inclusion of said coupling is not without peril and must be approached with care. In keeping with the same engineering judgment that guided the development of the traditional aeroelastic equations, the effect of non-linear inertial effects for most airplane applications is expected to be small. A parameter does not tell the whole story, however, and modes flagged by the parameter as significant also need to be checked to see if the coupling is not a one-way path, i.e., the inertially affected modes can influence other modes

    Nominal Wage Stickiness and Aggregate Supply in the Great Depression

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    Building on earlier work by Eichengreen and Sachs, we use data for 22 countries to study the role of wage stickiness in propagating the Great Depression. Recent research suggests that monetary shocks, transmitted internationally by the gold standard, were a major cause of the Depression. Accordingly, we use money supplies and other aggregate demand shifters as instruments to identify aggregate supply relationships. We find that nominal wages adjusted quite slowly to falling prices, and that the resulting increases in real wages depressed output. These findings leave open the question of why wages were so inertial in the face of extreme labor market conditions.

    A Partition Theorem for a Randomly Selected Large Population

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    We state and prove a proposition on partitioning of a randomly selected large population into stationary and non-stationary populations by using a property of stationary population identity. Applicability of this theorem for practical purposes is summarized at the end.Comment: 7 pages, a new result in population dynamic

    Credit risk rating at large U.S. banks

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    Large banks use internally developed credit rating systems to differentiate the riskiness of their commercial loans. Internal ratings are an essential ingredient of effective credit risk management for such banks, whose commercial borrowers may number in the tens of thousands. This article describes these rating systems, how their design varies across institutions, and how they are used in risk management. The article also outlines conceptual and practical difficulties currently faced by banks in achieving accurate and consistent ratings and describes ways in which some institutions have attempted to deal with these difficulties. This article is based on a detailed review of policy documents and internal management reports from the fifty largest U.S. bank holding companies and interviews by the authors at a selection of these institutions.Credit ratings ; Risk

    Hot-bench simulation of the active flexible wing wind-tunnel model

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    Two simulations, one batch and one real-time, of an aeroelastically-scaled wind-tunnel model were developed. The wind-tunnel model was a full-span, free-to-roll model of an advanced fighter concept. The batch simulation was used to generate and verify the real-time simulation and to test candidate control laws prior to implementation. The real-time simulation supported hot-bench testing of a digital controller, which was developed to actively control the elastic deformation of the wind-tunnel model. Time scaling was required for hot-bench testing. The wind-tunnel model, the mathematical models for the simulations, the techniques employed to reduce the hot-bench time-scale factors, and the verification procedures are described

    A 100 pc Elliptical and Twisted Ring of Cold and Dense Molecular Clouds Revealed by Herschel Around the Galactic Center

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    Thermal images of cold dust in the Central Molecular Zone of the Milky Way, obtained with the far-infrared cameras on board the Herschel satellite, reveal a ~3 × 10^7 M_☉ ring of dense and cold clouds orbiting the Galactic center. Using a simple toy model, an elliptical shape having semi-major axes of 100 and 60 pc is deduced. The major axis of this 100 pc ring is inclined by about 40° with respect to the plane of the sky and is oriented perpendicular to the major axes of the Galactic Bar. The 100 pc ring appears to trace the system of stable x_2 orbits predicted for the barred Galactic potential. Sgr A⋆ is displaced with respect to the geometrical center of symmetry of the ring. The ring is twisted and its morphology suggests a flattening ratio of 2 for the Galactic potential, which is in good agreement with the bulge flattening ratio derived from the 2MASS data

    Primary transit of the planet HD189733b at 3.6 and 5.8 microns

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    The hot Jupiter HD 189733b was observed during its primary transit using the Infrared Array Camera on the Spitzer Space Telescope. The transit depths were measured simultaneously at 3.6 and 5.8 microns. Our analysis yields values of 2.356 +- 0.019 % and 2.436 +- 0.020$ % at 3.6 and 5.8 microns respectively, for a uniform source. We estimated the contribution of the limb-darkening and star-spot effects on the final results. We concluded that although the limb darkening increases by ~0.02-0.03 % the transit depths, and the differential effects between the two IRAC bands is even smaller, 0.01 %. Furthermore, the host star is known to be an active spotted K star with observed photometric modulation. If we adopt an extreme model of 20 % coverage with spots 1000K cooler of the star surface, it will make the observed transits shallower by 0.19 and 0.18 %. The difference between the two bands will be only of 0.01 %, in the opposite direction to the limb darkening correction. If the transit depth is affected by limb darkening and spots, the differential effects between the 3.6 and 5.8 microns bands are very small. The differential transit depths at 3.6 and 5.8 microns and the recent one published by Knutson et al.(2007) at 8 microns are in agreement with the presence of water vapour in the upper atmosphere of the planet. This is the companion paper to Tinetti et al. (2007b), where the detailed atmosphere models are presented.Comment: 6 pages, 4 figures, Astrophysical Journal 675. Accepted Nov 21, 20007, to appear on March 10, 200
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