100,385 research outputs found
Towards Long-endurance Flight: Design and Implementation of a Variable-pitch Gasoline-engine Quadrotor
Majority of today's fixed-pitch, electric-power quadrotors have short flight
endurance ( 1 hour) which greatly limits their applications. This paper
presents a design methodology for the construction of a long-endurance
quadrotor using variable-pitch rotors and a gasoline-engine. The methodology
consists of three aspects. Firstly, the rotor blades and gasoline engine are
selected as a pair, so that sufficient lift can be comfortably provided by the
engine. Secondly, drivetrain and airframe are designed. Major challenges
include airframe vibration minimization and power transmission from one engine
to four rotors while keeping alternate rotors contra-rotating. Lastly, a PD
controller is tuned to facilitate preliminary flight tests. The methodology has
been verified by the construction and successful flight of our gasoline
quadrotor prototype, which is designed to have a flight time of 2 to 3 hours
and a maximum take-off weight of 10 kg.Comment: 6 page
Catastrophe versus instability for the eruption of a toroidal solar magnetic flux rope
The onset of a solar eruption is formulated here as either a magnetic
catastrophe or as an instability. Both start with the same equation of force
balance governing the underlying equilibria. Using a toroidal flux rope in an
external bipolar or quadrupolar field as a model for the current-carrying flux,
we demonstrate the occurrence of a fold catastrophe by loss of equilibrium for
several representative evolutionary sequences in the stable domain of parameter
space. We verify that this catastrophe and the torus instability occur at the
same point; they are thus equivalent descriptions for the onset condition of
solar eruptions.Comment: V2: update to conform to the published article; new choice for
internal inductance of torus; updated Fig. 2; new Figs. 3, 5, and
Mössbauer diffractometry on polycrystalline (57)Fe3Al
A Mossbauer powder diffractometer was used to measure diffraction patterns from polycrystalline foils of (Fe3Al)-Fe-57. The intensities of Bragg diffractions were measured as a function of the energy of the incident photon. The bee fundamental diffractions showed large changes in intensity as the incident energy was tuned through the nuclear resonances. These variations of diffraction intensity with incident energy were calculated with reasonable success using a kinematical theory of diffraction that included effects of coherent interference between x-ray Rayleigh scattering and, more importantly for these samples, Mossbauer scattering from nuclei having different hyperfine magnetic fields
Internal Gravity Waves Modulate the Apparent Misalignment of Exoplanets around Hot Stars
We propose that the observed misalignment between extra-solar planets and
their hot host stars can be explained by angular momentum transport within the
host star. Observations have shown that this misalignment is preferentially
around hot stars, which have convective cores and extended radiative envelopes.
This situation is amenable to substantial angular momentum transport by
internal gravity waves (IGW) generated at the convective-radiative interface.
Here we present numerical simulations of this process and show that IGW can
modulate the surface rotation of the star. With these two- dimensional
simulations we show that IGW could explain the retrograde orbits observed in
systems such as HAT-P-6 and HAT-P-7, however, extension to high obliquity
objects will await future three- dimensional simulations. We note that these
results also imply that individual massive stars should show temporal
variations in their v sini measurements.Comment: 6 pages, 2 figures, Accepted for publication in ApJ
Observation of strong electron dephasing in disordered CuGeAu thin films
We report the observation of strong electron dephasing in a series of
disordered CuGeAu thin films. A very short electron dephasing
time possessing very weak temperature dependence around 6 K, followed by an
upturn with further decrease in temperature below 4 K, is found. The upturn is
progressively more pronounced in more disordered samples. Moreover, a ln
dependent, but high-magnetic-field-insensitive, resistance rise persisting from
above 10 K down to 30 mK is observed in the films. These results suggest a
nonmagnetic dephasing process which is stronger than any known mechanism and
may originate from the coupling of conduction electrons to dynamic defects.Comment: to appear in Phys. Rev. Let
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