4,294 research outputs found
A flight test method for pilot/aircraft analysis
In high precision flight maneuvres a pilot is a part of a closed loop pilot/aircraft system. The assessment of the flying qualities is highly dependent on the closed loop characteristics related to precision maneuvres like approach, landing, air-to-air tracking, air-to-ground tracking, close formation flying and air-to air refueling of the receiver. The object of a research program at DFVLR is the final flight phase of an air to ground mission. In this flight phase the pilot has to align the aircraft with the target, correct small deviations from the target direction and keep the target in his sights for a specific time period. To investigate the dynamic behavior of the pilot-aircraft system a special ground attack flight test technique with a prolonged tracking maneuvres was developed. By changing the targets during the attack the pilot is forced to react continously on aiming errors in his sights. Thus the closed loop pilot/aircraft system is excited over a wide frequency range of interest, the pilot gets more information about mission oriented aircraft dynamics and suitable flight test data for a pilot/aircraft analysis can be generated
True airspeed measured by airborne laser Doppler velocimeter
Velocimeter utilizing carbon dioxide laser measures true airspeed of aircraft. Results of flight tests indicate that clear-weather airspeeds can be measured with accuracy better than 0.1% at altitudes up to 3000 meters; measurements can be made at much greater altitudes in cloudy or turbid air
Modeling pN2 through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures
Nitrogen is a major nutrient for all life on Earth and could plausibly play a
similar role in extraterrestrial biospheres. The major reservoir of nitrogen at
Earth's surface is atmospheric N2, but recent studies have proposed that the
size of this reservoir may have fluctuated significantly over the course of
Earth's history with particularly low levels in the Neoarchean - presumably as
a result of biological activity. We used a biogeochemical box model to test
which conditions are necessary to cause large swings in atmospheric N2
pressure. Parameters for our model are constrained by observations of modern
Earth and reconstructions of biomass burial and oxidative weathering in deep
time. A 1-D climate model was used to model potential effects on atmospheric
climate. In a second set of tests, we perturbed our box model to investigate
which parameters have the greatest impact on the evolution of atmospheric pN2
and consider possible implications for nitrogen cycling on other planets. Our
results suggest that (a) a high rate of biomass burial would have been needed
in the Archean to draw down atmospheric pN2 to less than half modern levels,
(b) the resulting effect on temperature could probably have been compensated by
increasing solar luminosity and a mild increase in pCO2, and (c) atmospheric
oxygenation could have initiated a stepwise pN2 rebound through oxidative
weathering. In general, life appears to be necessary for significant
atmospheric pN2 swings on Earth-like planets. Our results further support the
idea that an exoplanetary atmosphere rich in both N2 and O2 is a signature of
an oxygen-producing biosphere.Comment: 33 pages, 11 figures, 2 tables (includes appendix), published in
Astrobiolog
The equilibrium intrinsic crystal-liquid interface of colloids
We use confocal microscopy to study an equilibrated crystal-liquid interface
in a colloidal suspension. Capillary waves roughen the surface, but locally the
intrinsic interface is sharply defined. We use local measurements of the
structure and dynamics to characterize the intrinsic interface, and different
measurements find slightly different widths of this interface. In terms of the
particle diameter , this width is either (based on structural
information) or (based on dynamics), both not much larger than the
particle size. This work is the first direct experimental visualization of an
equilibrated crystal-liquid interface.Comment: 6 pages; revised version, submitted to PNA
Transverse excitations of ultracold matter waves upon propagation past abrupt waveguide changes
The propagation of ultracold atomic gases through abruptly changing waveguide
potentials is examined in the limit of non-interacting atoms. Time-independent
scattering calculations of microstructured waveguides with discontinuous
changes in the transverse harmonic binding potentials are used to mimic
waveguide perturbations and imperfections. Three basic configurations are
examined: step-like, barrier-like and well-like with waves incident in the
ground mode. At low energies, the spectra rapidly depart from single-moded,
with significant transmission and reflection of excited modes. The high-energy
limit sees 100 percent transmission, with the distribution of the transmitted
modes determined simply by the overlap of the mode wave functions and
interference.Comment: 20 pages, 7 figures, under review PR
New excitations in bcc He - an inelastic neutron scattering study
We report neutron scattering measurements on bcc solid % He. We studied
the phonon branches and the recently discovered ''optic-like'' branch along the
main crystalline directions. In addition, we discovered another, dispersionless
"optic-like'' branch at an energy around 1 meV (~11K). The properties of
the two "optic-like" branches seem different. Since one expects only 3 acoustic
phonon branches in a monoatomic cubic crystal, these new branches must
represent different type of excitations. One possible interpretation involves
localized excitations unique to a quantum solid.Comment: 4 pages, 3 figures, accepted by PRB, Rapid Communication
Testing T Invariance in the Interaction of Slow Neutrons with Aligned Nuclei
The study of five-fold (P even, T odd) correlation in the interaction of slow
polarized neutrons with aligned nuclei is a possible way of testing the time
reversal invariance due to the expected enhancement of T violating effects in
compound resonances. Possible nuclear targets are discussed which can be
aligned both dynamically as well as by the "brute force" method at low
temperature. A statistical estimation is performed of the five-fold correlation
for low lying p wave compound resonances of the Sb, Sb and
I nuclei. It is shown that a significant improvement can be achieved
for the bound on the intensity of the fundamental parity conserving time
violating (PCTV) interaction.Comment: 22 pages, 5 figures, published versio
Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets
We investigated the effect of intermolecular dipolar interactions on a cubic
3D ensemble of 5X5X4=100 nanomagnets, each with spin . We employed the
Landau-Lifshitz-Gilbert equation to solve for the magnetization curves
for several values of the damping constant , the induction sweep rate,
the lattice constant , the temperature , and the magnetic anisotropy
field . We find that the smaller the , the stronger the maximum
induction required to produce hysteresis. The shape of the hysteresis loops
also depends on the damping constant. We find further that the system
magnetizes and demagnetizes at decreasing magnetic field strengths with
decreasing sweep rates, resulting in smaller hysteresis loops. Variations of
within realistic values (1.5 nm - 2.5 nm) show that the dipolar interaction
plays an important role in the magnetic hysteresis by controlling the
relaxation process. The dependencies of and of are presented
and discussed with regard to recent experimental data on nanomagnets.
enhances the size of the hysteresis loops for external fields parallel to the
anisotropy axis, but decreases it for perpendicular external fields. Finally,
we reproduce and test an curve for a 2D-system [M. Kayali and W. Saslow,
Phys. Rev. B {\bf 70}, 174404 (2004)]. We show that its hysteretic behavior is
only weakly dependent on the shape anisotropy field and the sweep rate, but
depends sensitively upon the dipolar interactions. Although in 3D systems,
dipole-dipole interactions generally diminish the hysteresis, in 2D systems,
they strongly enhance it. For both square 2D and rectangular 3D lattices with
, dipole-dipole interactions can cause
large jumps in the magnetization.Comment: 15 pages 14 figures, submitted to Phys. Rev.
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