9,255 research outputs found
Neuromuscular control of wingbeat kinematics in Anna's hummingbirds (Calypte anna)
Hummingbirds can maintain the highest wingbeat frequencies of any flying vertebrate – a feat accomplished by the large pectoral muscles that power the wing strokes. An unusual feature of these muscles is that they are activated by one or a few spikes per cycle as revealed by electromyogram recordings (EMGs). The relatively simple nature of this activation pattern provides an opportunity to understand how motor units are recruited to modulate limb kinematics. Hummingbirds made to fly in low-density air responded by moderately increasing wingbeat frequency and substantially increasing the wing stroke amplitude as compared with flight in normal air. There was little change in the number of spikes per EMG burst in the pectoralis major muscle between flight in normal and low-density heliox (mean=1.4 spikes cycle^(–1)). However the spike amplitude, which we take to be an indication of the number of active motor units, increased in concert with the wing stroke amplitude, 1.7 times the value in air. We also challenged the hummingbirds using transient load lifting to elicit maximum burst performance. During maximum load lifting, both wing stroke amplitude and wingbeat frequency increased substantially above those values during hovering flight. The number of spikes per EMG burst increased to a mean of 3.3 per cycle, and the maximum spike amplitude increased to approximately 1.6 times those values during flight in heliox. These results suggest that hummingbirds recruit additional motor units (spatial recruitment) to regulate wing stroke amplitude but that temporal recruitment is also required to maintain maximum stroke amplitude at the highest wingbeat frequencies
Modeling and Simulation of the Second-Generation Orion Crew Module Air Bag Landing System
Air bags were evaluated as the landing attenuation system for earth landing of the Orion Crew Module (CM). An important element of the air bag system design process is proper modeling of the proposed configuration to determine if the resulting performance meets requirements. Analysis conducted to date shows that airbags are capable of providing a graceful landing of the CM in nominal and off-nominal conditions such as parachute failure, high horizontal winds, and unfavorable vehicle/ground angle combinations. The efforts presented here surround a second generation of the airbag design developed by ILC Dover, and is based on previous design, analysis, and testing efforts. In order to fully evaluate the second generation air bag design and correlate the dynamic simulations, a series of drop tests were carried out at NASA Langley's Landing and Impact Research (LandIR) facility. The tests consisted of a full-scale set of air bags attached to a full-scale test article representing the Orion Crew Module. The techniques used to collect experimental data, construct the simulations, and make comparisons to experimental data are discussed
The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441
We present time-series VI photometry of the metal-rich ([Fe/H] = -0.53)
globular cluster NGC 6441. Our color-magnitude diagram shows that the extended
blue horizontal branch seen in Hubble Space Telescope data exists in the
outermost reaches of the cluster. The red clump slopes nearly parallel to the
reddening vector. A component of this slope is due to differential reddening,
but part is intrinsic. The blue horizontal branch stars are more centrally
concentrated than the red clump stars. We have discovered about 50 new variable
stars near NGC 6441, among them eight or more RR Lyrae stars which are very
probably cluster members. Comprehensive period searches over the range 0.2-1.0
days yielded unusually long periods (0.5-0.9 days) for the fundamental
pulsators compared with field RR Lyrae of the same metallicity. Three similar
long-period RR Lyrae are known in other metal-rich globulars. With over ten
examples in hand, it seems that a distinct sub-class of RR Lyrae is emerging.
The observed properties of the horizontal branch stars are in reasonable
agreement with recent models which invoke deep mixing to enhance the
atmospheric helium abundance, while they conflict with models which assume high
initial helium abundance. The light curves of the c-type RR Lyrae seem to have
unusually long rise times and sharp minima. Reproducing these light curves in
stellar pulsation models may provide another means of constraining the physical
variables responsible for the anomalous blue horizontal branch extension and
sloped red clump observed in NGC 6441.Comment: 30 pages plus 6 EPS and 6 JPEG figures; uses AAS TeX. Accepted by the
Astronomical Journal. Minor changes include computing He abundance,
modifications to Figs 1 and 8, and expansion on idea that blue HB stars may
be produced in binarie
BIMA and Keck Imaging of the Radio Ring PKS 1830-211
We discuss BIMA (Berkeley Illinois Maryland Association) data and present new
high quality optical and near-IR Keck images of the bright radio ring PKS
1830-211. Applying a powerful new deconvolution algorithm we have been able to
identify both images of the radio source. In addition we recover an extended
source in the optical, consistent with the expected location of the lensing
galaxy. The source counterparts are very red, I-K=7, suggesting strong Galactic
absorption with additional absorption by the lensing galaxy at z=0.885, and
consistent with the detection of high redshift molecules in the lens.Comment: To be published in the ASP Conference Proceedings, 'Highly Redshifted
Radio Lines', Greenbank, W
The redshift of the gravitationally lensed radio source PKS1830-211
We report on the spectroscopic identification and the long awaited redshift
measurement of the heavily obscured, gravitationally lensed radio source PKS
1830-211, which was first observed as a radio Einstein ring. The NE component
of the doubly imaged core is identified, in our infrared spectrum covering the
wavelength range 1.5-2.5 microns, as an impressively reddened quasar at
z=2.507. Our redshift measurement, together with the recently measured time
delay (Lovell et al.), means that we are a step closer to determining the
Hubble constant from this lens. Converting the time delay into the Hubble
constant by using existing models leads to high values for the Hubble constant.
Since the lensing galaxy lies very close to the center of the lensed ring,
improving the error bars on the Hubble constant will require not only a more
precise time delay measurement, but also very precise astrometry of the whole
system.Comment: 11 pages, 2 figures, Accepted ApJ
Modeling and Simulation of the Second-Generation Orion Crew Module Air Bag Landing System
Air bags were evaluated as the landing attenuation system for earth landing of the Orion Crew Module (CM). Analysis conducted to date shows that airbags are capable of providing a graceful landing of the CM in nominal and off-nominal conditions such as parachute failure, high horizontal winds, and unfavorable vehicle/ground angle combinations, while meeting crew and vehicle safety requirements. The analyses and associated testing presented here surround a second generation of the airbag design developed by ILC Dover, building off of relevant first-generation design, analysis, and testing efforts. In order to fully evaluate the second generation air bag design and correlate the dynamic simulations, a series of drop tests were carried out at NASA Langley s Landing and Impact Research (LandIR) facility in Hampton, Virginia. The tests consisted of a full-scale set of air bags attached to a full-scale test article representing the Orion Crew Module. The techniques used to collect experimental data, develop the simulations, and make comparisons to experimental data are discussed
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