44 research outputs found
A method of optimizing aircraft autostabilizer systems
A novel procedure for the optimization of aircraft
autostabilizer systems is presented. The procedure is
straightforward, and its application does not result in
demands for autostabilizer systems of prohibitive complexity.
Many important non-linear effects may be included with only
slight extra complication in the required calculations. The
procedure is applicable, in the first place, to piloted aircraft,
- the essence of the procedure being the assumption that the
purpose of thy: autostabilizer is to reduce the effort demanded
of the pilot in executing a given manoeuvre or attaining a
given response. Although the presence of the pilot is
explicitly taken into account in the calculations no form
of pilot's transfer function need be specified.
It is shown how the procedure may be modified to form
an approximate procedure for the optimization of autostabilizers
for pilotless aircraft having linear autostabilizer characteristics
and linear aircraft dynamics. The results of some
calculations presented herein support a suggestion that this
approximate optimization procedure may also be frequently
applied with success to pilotless aircraft having certain
non-linearities, either in the autostabilizer system or in
the aircraft dynamics
Application of the joined wing to tiltrotor aircraft
A study was made to determine the potential speed improvements and other benefits resulting from the application of the joined wing concept to tiltrotor aircraft. Using the XV-15 as a baseline, the effect of replacing the cantilever wing by a joined-wing pair was studied. The baseline XV-15 cantilever wing has a thickness/chord ratio of 23 percent. It was found that this wing could be replaced by a joined-wing pair of the same span and total area employing airfoils of 12 percent thickness/chord ratio. The joined wing meets the same static strength requirements as the cantilever wing, but increases the limiting Mach Number of the aircraft from M=0.575 to M=0.75, equivalent to an increase of over 100 knots in maximum speed. The joined wing configuration studied is lighter than the cantilever and has approximately 11 percent less wing drag in cruise. Its flutter speed of 245 knots EAS is not high enough to allow the potential Mach number improvement to be attained at low altitude. The flutter speed can be raised either by employing rotors which can be stopped and folded in flight at speeds below 245 knots EAS, or by modifying the airframe to reduce adverse coupling with the rotor dynamics. Several modifications of wing geometry and nacelle mass distribution were investigated, but none produced a flutter speed above 260 knots EAS. It was concluded that additional research is required to achieve a more complete understanding of the mechanism of rotor/wing coupling
Fluctuating Elastic Rings: Statics and Dynamics
We study the effects of thermal fluctuations on elastic rings. Analytical
expressions are derived for correlation functions of Euler angles, mean square
distance between points on the ring contour, radius of gyration, and
probability distribution of writhe fluctuations. Since fluctuation amplitudes
diverge in the limit of vanishing twist rigidity, twist elasticity is essential
for the description of fluctuating rings. We find a crossover from a small
scale regime in which the filament behaves as a straight rod, to a large scale
regime in which spontaneous curvature is important and twist rigidity affects
the spatial configurations of the ring. The fluctuation-dissipation relation
between correlation functions of Euler angles and response functions, is used
to study the deformation of the ring by external forces. The effects of inertia
and dissipation on the relaxation of temporal correlations of writhe
fluctuations, are analyzed using Langevin dynamics.Comment: 43 pages, 9 Figure
Imaging of the CCS 22.3 GHz emission in the Taurus Molecular Cloud complex
Thioxoethenylidene (CCS) is an abundant interstellar molecule, and a good
tracer of high density and evolutionary stage of dense molecular clouds. It is
also a suitable candidate for Zeeman splitting observations for its high
splitting factor and narrow thermal linewidths. We report here EVLA 22.3 GHz
observations of three dense molecular cores TMC-1, TMC-1C and L1521B in the
Taurus Molecular Cloud complex to image the CCS 2_1-1_0 transition. For all
three sources, the clumpy CCS emission is most likely tracing the starless
cores. However, these compact structures account for only ~ 1-13% of the
integrated emission detected in single-dish observations, indicating the
presence of significant large scale diffuse emission in favorable conditions
for producing CCS.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letters EVLA
special issue. The definitive version will be available at
http://iopscience.iop.org
VLA Observations of the Infrared Dark Cloud G19.30+0.07
We present Very Large Array observations of ammonia (NH3) (1,1), (2,2), and
CCS (2_1-1_0) emission toward the Infrared Dark Cloud (IRDC) G19.30+0.07 at
~22GHz. The NH3 emission closely follows the 8 micron extinction. The NH3 (1,1)
and (2,2) lines provide diagnostics of the temperature and density structure
within the IRDC, with typical rotation temperatures of ~10 to 20K and NH3
column densities of ~10^15 cm^-2. The estimated total mass of G19.30+0.07 is
~1130 Msun. The cloud comprises four compact NH3 clumps of mass ~30 to 160
Msun. Two coincide with 24 micron emission, indicating heating by protostars,
and show evidence of outflow in the NH3 emission. We report a water maser
associated with a third clump; the fourth clump is apparently starless. A
non-detection of 8.4GHz emission suggests that the IRDC contains no bright HII
regions, and places a limit on the spectral type of an embedded ZAMS star to
early-B or later. From the NH3 emission we find G19.30+0.07 is composed of
three distinct velocity components, or "subclouds." One velocity component
contains the two 24 micron sources and the starless clump, another contains the
clump with the water maser, while the third velocity component is diffuse, with
no significant high-density peaks. The spatial distribution of NH3 and CCS
emission from G19.30+0.07 is highly anti-correlated, with the NH3 predominantly
in the high-density clumps, and the CCS tracing lower-density envelopes around
those clumps. This spatial distribution is consistent with theories of
evolution for chemically young low-mass cores, in which CCS has not yet been
processed to other species and/or depleted in high-density regions.Comment: 29 pages, 9 figures, accepted for publication by ApJ. Please contact
the authors for higher resolution versions of the figure
Evidence for dust evolution within the Taurus Complex from Spitzer images
We present Spitzer images of the Taurus Complex (TC) and take advantage of
the sensitivity and spatial resolution of the observations to characterize the
diffuse IR emission across the cloud. This work highlights evidence of dust
evolution within the translucent sections of the archetype reference for
studies of quiescent molecular clouds. We combine Spitzer 160 um and IRAS 100
um observations to produce a dust temperature map and a far-IR dust opacity map
at 5' resolution. The average dust temperature is about 14.5K with a dispersion
of +/-1K across the cloud. The far-IR dust opacity is a factor 2 larger than
the average value for the diffuse ISM. This opacity increase and the
attenuation of the radiation field (RF) both contribute to account for the
lower emission temperature of the large grains. The structure of the TC
significantly changes in the mid-IR images that trace emission from PAHs and
VSGs. We focus our analysis of the mid-IR emission to a range of ecliptic
latitudes where the zodiacal light residuals are small. Within this cloud area,
there are no 8 and 24 um counterparts to the brightest 160 um emission
features. Conversely, the 8 and 24 um images reveal filamentary structure that
is strikingly inconspicuous in the 160 um and extinction maps. The IR colors
vary over sub-parsec distances across this filamentary structure. We compare
the observed colors with model calculations quantifying the impact of the RF
intensity and the abundance of stochastically heated particles on the dust SED.
To match the range of observed colors, we have to invoke variations by a factor
of a few of both the interstellar RF and the abundance of PAHs and VSGs. We
conclude that within this filamentary structure a significant fraction of the
dust mass cycles in and out the small size end of the dust size distribution.Comment: 43 pages, 13 figures, accepted for publication in Ap
Tracing the evolutionary stage of Bok globules: CCS and NH3
We pursue the investigation of a previously proposed correlation between
chemical properties and physical evolutionary stage of isolated low-mass
star-forming regions. In the past, the NH3/CCS abundance ratio was suggested to
be a potentially useful indicator for the evolutionary stage of cloud cores. We
aim to study its applicability for isolated Bok globules. A sample of 42 Bok
globules with and without signs of current star formation was searched for
CCS(2-1) emission, the observations were complemented with NH3 measurements
available in the literature and own observations. The abundance ratio of both
molecules is discussed with respect to the evolutionary stage of the objects
and in the context of chemical models. The NH3/CCS ratio could be assessed for
18 Bok globules and is found to be moderately high and roughly similar across
all evolutionary stages from starless and prestellar cores towards internally
heated cores harbouring protostars of Class 0, Class I or later. Bok globules
with extremely high CCS abundance analogous to carbon-chain producing regions
in dark cloud cores are not found. The observed range of NH3/CCS hints towards
a relatively evolved chemical state of all observed Bok globules.Comment: 12 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
[Abridged] We present a molecular survey of the starless cores L1498 and
L1517B. These cores have been selected for their relative isolation and
close-to-round shape, and they have been observed in a number of lines of 13
molecular species (4 already presented in the first part of this series): CO,
CS, N2H+, NH3, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+. Using a
physical model of core structure and a Monte Carlo radiative transfer code, we
determine for each core a self-consistent set abundances that fits
simultaneously the observed radial profile of integrated intensity and the
emergent spectrum towards the core center (for abundant species, optically thin
isopologues are used). From this work, we find that L1498 and L1517B have
similar abundance patterns, with most species suffering a significant drop
toward the core center. This occurs for CO, CS, CH3OH, SO, C3H2, HC3N, C2S,
HCN, H2CO, HCO+, and DCO+, which we fit with profiles having a sharp central
hole. The size of this hole varies with molecule: DCO+, HCN, and HC3N have the
smallest holes while SO, C2S and CO have the largest holes. Only N2H+ and NH3
are present in the gas phase at the core centers. From the different behavior
of molecules, we select SO, C2S, and CH3OH as the most sensitive tracers of
molecular depletion. Comparing our abundance determinations with the
predictions from current chemical models we find order of magnitude
discrepancies. Finally, we show how the ``contribution function'' can be used
to study the formation of line profiles from the different regions of a core.Comment: 22 pages, 12 figures, A&A accepte