4,088 research outputs found
Empirical relation between induced velocity, thrust, and rate of descent of a helicopter rotor as determined by wind-tunnel tests on four model rotors
The empirical relation between the induced velocity, thrust, and rate of vertical descent of a helicopter rotor was calculated from wind tunnel force tests on four model rotors by the application of blade-element theory to the measured values of the thrust, torque, blade angle, and equivalent free-stream rate of descent. The model tests covered the useful range of C(sub t)/sigma(sub e) (where C(sub t) is the thrust coefficient and sigma(sub e) is the effective solidity) and the range of vertical descent from hovering to descent velocities slightly greater than those for autorotation. The three bladed models, each of which had an effective solidity of 0.05 and NACA 0015 blade airfoil sections, were as follows: (1) constant-chord, untwisted blades of 3-ft radius; (2) untwisted blades of 3-ft radius having a 3/1 taper; (3) constant-chord blades of 3-ft radius having a linear twist of 12 degrees (washout) from axis of rotation to tip; and (4) constant-chord, untwisted blades of 2-ft radius. Because of the incorporation of a correction for blade dynamic twist and the use of a method of measuring the approximate equivalent free-stream velocity, it is believed that the data obtained from this program are more applicable to free-flight calculations than the data from previous model tests
Controllable binding of polar molecules and meta-stability of 1-D gases with attractive dipole forces
We explore one-dimensional (1-D) samples of ultracold polar molecules with
attractive dipole-dipole interactions and show the existence of a repulsive
barrier due to a strong quadrupole interaction between molecules. This barrier
can stabilize a gas of ultracold KRb molecules and even lead to long-range
wells supporting bound states between molecules. The properties of these wells
can be controlled by external electric fields, allowing the formation of long
polymer-like chains of KRb, and studies of quantum phase transitions by varying
the effective interaction between molecules. We discuss the generalization of
those results to other systems
A Cryogenic Infrared Calibration Target
A compact cryogenic calibration target is presented that has a peak diffuse
reflectance, , from m).
Upon expanding the spectral range under consideration to m) the observed performance gracefully degrades to at the band edges. In the implementation described, a
high-thermal-conductivity metallic substrate is textured with a pyramidal
tiling and subsequently coated with a thin lossy dielectric coating that
enables high absorption and thermal uniformity across the target. The resulting
target assembly is lightweight, has a low-geometric profile, and has survived
repeated thermal cycling from room temperature to K. Basic design
considerations, governing equations, and test data for realizing the structure
described are provided. The optical properties of selected absorptive materials
-- Acktar Fractal Black, Aeroglaze Z306, and Stycast 2850 FT epoxy loaded with
stainless steel powder -- are characterized and presented
Structure Relations and Darboux Contractions for 2D 2nd Order Superintegrable Systems
Two-dimensional quadratic algebras are generalizations of Lie algebras that
include the symmetry algebras of 2nd order superintegrable systems in 2
dimensions as special cases. The superintegrable systems are exactly solvable
physical systems in classical and quantum mechanics. Distinct superintegrable
systems and their quadratic algebras can be related by geometric contractions,
induced by In\"on\"u-Wigner type Lie algebra contractions. These geometric
contractions have important physical and geometric meanings, such as obtaining
classical phenomena as limits of quantum phenomena as and
nonrelativistic phenomena from special relativistic as , and the
derivation of the Askey scheme for obtaining all hypergeometric orthogonal
polynomials as limits of Racah/Wilson polynomials. In this paper we show how to
simplify the structure relations for abstract nondegenerate and degenerate
quadratic algebras and their contractions. In earlier papers we have classified
contractions of 2nd order superintegrable systems on constant curvature spaces
and have shown that all results are derivable from free quadratic algebras
contained in the enveloping algebras of the Lie algebras in
flat space and on nonzero constant curvature spaces. The
quadratic algebra contractions are induced by generalizations of
In\"on\"u-Wigner contractions of these Lie algebras. As a special case we
obtained the Askey scheme for hypergeometric orthogonal polynomials. Here we
complete this theoretical development for 2D superintegrable systems by showing
that the Darboux superintegrable systems are also characterized by free
quadratic algebras contained in the symmetry algebras of these spaces and that
their contractions are also induced by In\"on\"u-Wigner contractions. We
present tables of the contraction results
Current Taxonomic Status of the Plesiosaur Pantasaurus Striatus from the Upper Jurassic Sundance Formation, Wyoming
Plesiosaur material has been known from the Redwater Shale member of the Sundance Formation (Jurassic: Oxfordian) of Wyoming for over 100 years, but has received little research attention. Here we report on the taxonomic status of a long-necked cryptocleidoid plesiosaur from the Redwater Shale, the correct identity of which is Pantosaurus striatus Marsh 1893. The taxon Muraenosaurus reedii Mehl 1912 is shown to be a junior synonym of Pantosaurus striatus. Pantosaurus is described on the basis of the holotype and referred specimens, and found to be a cryptocleidoid plesiosaur possessing between 35 and 40 cervical vertebrae. These vertebrae are very similar in proportion and anatomical detail to those of Muraenosaurus leedsii from the Oxford Clay of England. However, the forelimb of Pantosaurus is diagnostic and differs from that of Muraenosaurus in several particulars, the most important being the relatively large size of the radius and its corresponding humeral articulation. Although no cranial material is available at this time, we believe that Pantosaurus striatus is a valid taxon
Preliminary Report on the Osteology and Relationships of a New Aberrant Cryptocleidoid Plesiosaur from the Sundance Formation, Wyoming
The cryptocleidoid plesiosaur Tatenectes laramiensis, new genus, is described from the Redwater Shale Member of the Sundance Formation, Narrona County, Wyoming. The holotype of this species was a partial skeleton that has since been lost. A neotype is designated that preserves the same elements present in the holotype. A second specimen is referred to the taxon, and this specimen includes cranial material. The preserved cranial elements are the left squamosal, a partial right frontal, several isolated teeth, the parasphenoid, and large portions of the left and right pterygoids. The skull shares many traits with that of Kimmerosaurus, a cryptocleidoid plesiosaur from the Kimmeridge Clay of England. However, the palate is derived, and resembles those of the poorly-understood cimoliasaurid plesiosaurs of the Creoceno of the southern hemisphere. This similarity is established via comparison with the skull of an undescribed taxoo from late Jurassic of Cuba. The cryptocleidoid plesiosaurs underwent an extensive radiation in the Late Jurassic, and more research attention is needed, beginning with additional preparation and collection of Tatenectes
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