43 research outputs found
Redundant hydraulic secondary flight control systems behavior in failure conditions
The flight control systems, designed in order to assure the necessary safety level even in failure conditions, are generally
characterized by a proper redundant layout. The redundancies must be designed in order to assure an adequate system behavior when
some failures are present; in fact an incorrect layout may cause serious shortcomings concerning the response when some component is
not operational. Therefore the usual correct design activities request the complete analysis of the system behavior in failure condition.
The work analyses the response of a redundant secondary flight control hydraulic servo-mechanism equipped with some proper
equalization devices, when some of the most probable and representative failures are present. It must be noted that the redundancy
layout, designed in order to assure the necessary safety level even in failure conditions, may behave improperly during normal
operations, if the system architecture is unsuitable, when manufacturing defects are present. The improper behavior, generally
consisting of force fighting or speed fighting caused by different offsets or asymmetries between the two sections of the system, may be
usually overcome by means of a suitable equalization device. Therefore, the system behavior during and following the failure transient
greatly depends on both its redundancy architecture and related equalization device. The above mentioned problems have been studied
by means of an appropriate physical-mathematical model of a typical electro-hydraulic servo-mechanism prepared to the purpose,
performing a certain number of simulations of representative actuations in which different types of failures are accurately modeled. In
the opinion of the authors, this paper concerns a topic quite neglected but important in the technical literature. At the best of the authors’
knowledge, no specific scientific work in this field is available, excepting some industrial technical reports
Direct Imaging of Fine Structure in the Chromosphere of a Sunspot Umbra
High-resolution imaging observations from the Hinode spacecraft in the CaII H
line are employed to study the dynamics of the chromosphere above a sunspot. We
find that umbral flashes and other brightenings produced by the oscillation are
extremely rich in fine structure, even beyond the resolving limit of our
observations (0.22"). The umbra is tremendously dynamic, to the point that our
time cadence of 20 s does not suffice to resolve the fast lateral (probably
apparent) motion of the emission source. Some bright elements in our dataset
move with horizontal propagation speeds of 30 km/s. We have detected
filamentary structures inside the umbra (some of which have a horizontal
extension of ~1500 km) which, to our best knowledge, had not been reported
before. The power spectra of the intensity fluctuations reveals a few distinct
areas with different properties within the umbra that seem to correspond with
the umbral cores that form it. Inside each one of these areas the dominant
frequencies of the oscillation are coherent, but they vary considerably from
one core to another.Comment: Accepted for publication in Ap
Exotic fish in exotic plantations: a multi-scale approach to understand amphibian occurrence in the mediterranean region
Globally, amphibian populations are threatened by a diverse range of factors including habitat
destruction and alteration. Forestry practices have been linked with low diversity and
abundance of amphibians. The effect of exotic Eucalyptus spp. plantations on amphibian
communities has been studied in a number of biodiversity hotspots, but little is known of its
impact in the Mediterranean region. Here, we identify the environmental factors influencing
the presence of six species of amphibians (the Caudata Pleurodeles waltl, Salamandra salamandra,
Lissotriton boscai, Triturus marmoratus and the anurans Pelobates cultripes and
Hyla arborea/meridionalis) occupying 88 ponds. The study was conducted in a Mediterranean
landscape dominated by eucalypt plantations alternated with traditional use (agricultural,
montados and native forest) at three different scales: local (pond), intermediate (400
metres radius buffer) and broad (1000 metres radius buffer). Using the Akaike Information
Criterion for small samples (AICc), we selected the top-ranked models for estimating the
probability of occurrence of each species at each spatial scale separately and across all
three spatial scales, using a combination of covariates from the different magnitudes. Models
with a combination of covariates at the different spatial scales had a stronger support
than those at individual scales. The presence of predatory fish in a pond had a strong effect
on Caudata presence. Permanent ponds were selected by Hyla arborea/meridionalis over
temporary ponds. Species occurrence was not increased by a higher density of streams,
but the density of ponds impacted negatively on Lissotriton boscai. The proximity of ponds
occupied by their conspecifics had a positive effect on the occurrence of Lissotriton boscai
and Pleurodeles waltl. Eucalypt plantations had a negative effect on the occurrence of the
newt Lissotriton boscai and anurans Hyla arborea/meridionalis, but had a positive effect on
the presence of Salamandra salamandra, while no effect on any of the other species was detected.
In conclusion, eucalypts had limited effects on the amphibian community at the intermediate
and broad scales, but predatory fish had a major impact when considering all the
scales combined. The over-riding importance of introduced fish as a negative impact suggests that forest managers should prevent new fish introductions and eradicate fish
from already-occupied ponds whenever possible