15,203 research outputs found
Investigation of aeroelastic stability phenomena of a helicopter by in-flight shake test
The analytical capability of the helicopter stability program is discussed. The parameters which are found to be critical to the air resonance characteristics of the soft in-plane hingeless rotor systems are detailed. A summary of two model test programs, a 1/13.8 Froude-scaled BO-105 model and a 1.67 meter (5.5 foot) diameter Froude-scaled YUH-61A model, are presented with emphasis on the selection of the final parameters which were incorporated in the full scale YUH-61A helicopter. Model test data for this configuration are shown. The actual test results of the YUH-61A air resonance in-flight shake test stability are presented. Included are a concise description of the test setup, which employs the Grumman Automated Telemetry System (ATS), the test technique for recording in-flight stability, and the test procedure used to demonstrate favorable stability characteristics with no in-plane damping augmentation (lag damper removed). The data illustrating the stability trend of air resonance with forward speed and the stability trend of ground resonance for percent airborne are presented
Optical guiding in meter-scale plasma waveguides
We demonstrate a new highly tunable technique for generating meter-scale low
density plasma waveguides. Such guides can enable electron acceleration to tens
of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by
optical field ionization induced by two time-separated Bessel beam pulses: The
first pulse, a J_0 beam, generates the core of the waveguide, while the delayed
second pulse, here a J_8 or J_16 beam, generates the waveguide cladding. We
demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths
with on axis plasma densities as low as N_e0=5x10^16 cm^-3
On the volume functional of compact manifolds with boundary with constant scalar curvature
We study the volume functional on the space of constant scalar curvature
metrics with a prescribed boundary metric. We derive a sufficient and necessary
condition for a metric to be a critical point, and show that the only domains
in space forms, on which the standard metrics are critical points, are geodesic
balls. In the zero scalar curvature case, assuming the boundary can be
isometrically embedded in the Euclidean space as a compact strictly convex
hypersurface, we show that the volume of a critical point is always no less
than the Euclidean volume bounded by the isometric embedding of the boundary,
and the two volumes are equal if and only if the critical point is isometric to
a standard Euclidean ball. We also derive a second variation formula and apply
it to show that, on Euclidean balls and ''small'' hyperbolic and spherical
balls in dimensions 3 to 5, the standard space form metrics are indeed saddle
points for the volume functional
Analytical design and evaluation of an active control system for helicopter vibration reduction and gust response alleviation
An analytical study was conducted to define the basic configuration of an active control system for helicopter vibration and gust response alleviation. The study culminated in a control system design which has two separate systems: narrow band loop for vibration reduction and wider band loop for gust response alleviation. The narrow band vibration loop utilizes the standard swashplate control configuration to input controller for the vibration loop is based on adaptive optimal control theory and is designed to adapt to any flight condition including maneuvers and transients. The prime characteristics of the vibration control system is its real time capability. The gust alleviation control system studied consists of optimal sampled data feedback gains together with an optimal one-step-ahead prediction. The prediction permits the estimation of the gust disturbance which can then be used to minimize the gust effects on the helicopter
An alternative approach to regularity for the Navier-Stokes equations in critical spaces
In this paper we present an alternative viewpoint on recent studies of
regularity of solutions to the Navier-Stokes equations in critical spaces. In
particular, we prove that mild solutions which remain bounded in the space
do not become singular in finite time, a result which was proved
in a more general setting by L. Escauriaza, G. Seregin and V. Sverak using a
different approach. We use the method of "concentration-compactness" +
"rigidity theorem" which was recently developed by C. Kenig and F. Merle to
treat critical dispersive equations. To the authors' knowledge, this is the
first instance in which this method has been applied to a parabolic equation.
We remark that we have restricted our attention to a special case due only to
a technical restriction, and plan to return to the general case (the
setting) in a future publication.Comment: 41 page
On a Localized Riemannian Penrose Inequality
Consider a compact, orientable, three dimensional Riemannian manifold with
boundary with nonnegative scalar curvature. Suppose its boundary is the
disjoint union of two pieces: the horizon boundary and the outer boundary,
where the horizon boundary consists of the unique closed minimal surfaces in
the manifold and the outer boundary is metrically a round sphere. We obtain an
inequality relating the area of the horizon boundary to the area and the total
mean curvature of the outer boundary. Such a manifold may be thought as a
region, surrounding the outermost apparent horizons of black holes, in a
time-symmetric slice of a space-time in the context of general relativity. The
inequality we establish has close ties with the Riemannian Penrose Inequality,
proved by Huisken and Ilmanen, and by Bray.Comment: 16 page
Quantum Manifestation of Elastic Constants in Nanostructures
Generally, there are two distinct effects in modifying the properties of
low-dimensional nanostructures: surface effect (SS) due to increased
surface-volume ratio and quantum size effect (QSE) due to quantum confinement
in reduced dimension. The SS has been widely shown to affect the elastic
constants and mechanical properties of nanostructures. Here, using Pb nanofilm
and graphene nanoribbon as model systems, we demonstrate the QSE on the elastic
constants of nanostructures by first-principles calculations. We show that
generally QSE is dominant in affecting the elastic constants of metallic
nanostructures while SS is more pronounced in semiconductor and insulator
nanostructures. Our findings have broad implications in quantum aspects of
nanomechanics
Journal Staff
Discoba (Excavata) is an ancient group of eukaryotes with great morphological and ecological diversity. Unlike the other major divisions of Discoba (Jakobida and Euglenozoa), little is known about the mitochondrial DNAs(mtDNAs) of Heterolobosea. We have assembled a complete mtDNA genome from the aggregating heterolobosean amoeba, Acrasis kona, which consists of a single circular highly AT-rich (83.3%) molecule of 51.5 kb. Unexpectedly, A. kona mtDNA is missing roughly 40% of the protein-coding genes and nearly half of the transfer RNAs found in the only other sequenced heterolobosean mtDNAs, those of Naegleria spp. Instead, over a quarter of A. kona mtDNA consists of novel open reading frames. Eleven of the 16 protein-coding genes missing from A. kona mtDNA were identified in its nuclear DNA and polyA RNA, and phylogenetic analyses indicate that at least 10 of these 11 putative nuclear-encoded mitochondrial (NcMt) proteins arose by direct transfer from the mitochondrion. Acrasis kona mtDNA also employs C-to-U type RNA editing, and 12 homologs of DYW-type pentatricopeptide repeat (PPR) proteins implicated in plant organellar RNA editing are found in A. kona nuclear DNA. A mapping of mitochondrial gene content onto a consensus phylogeny reveals a sporadic pattern of relative stasis and rampant gene loss in Discoba. Rampant loss occurred independently in the unique common lineage leading to Heterolobosea + Tsukubamonadida and later in the unique lineage leading to Acrasis. Meanwhile, mtDNA gene content appears to be remarkably stable in the Acrasis sister lineage leading to Naegleria and in their distant relatives Jakobida
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