12,714 research outputs found
Review of antiskid and brake dynamics research
The behavior of various antiskid systems was investigated under controlled conditions. Results from utilizing a single main wheel of a DC-9 aircraft suggest that the systems investigated perform well under most circumstances but there may be room for improvement. For example, it was demonstrated that pressure bias modulation can adversely affect the response of antiskid systems to rapid changes in the runway friction level. Results also indicate that antiskid systems designed to operate at a slip ratio of approximately 0.1 can provide a maximum braking effort without undue loss in the cornering capability of the tire. Time histories of braking friction coefficient were shown to provide a means of determining antiskid system performance and for systems that employed pressure bias modulation it was shown that performance could also be estimated from time histories of brake pressure and torque
Status of recent aircraft braking and cornering research
The sources of degraded performance which sometimes occurs under adverse runway conditions, are investigated to obtain data necessary to the development of more advanced systems, in an effort to insure safe ground handling operations under all-weather conditions. Tire-to-ground friction characteristics are determined under braking conditions which closely resemble those of airplanes under heavy braking. Braking data from single-wheel landing loads track tests are related with those available from full-scale flight tests
Behavior of aircraft antiskid breaking systems on dry and wet runway surfaces: A slip-ratio-controlled system with ground speed reference from unbraked nose wheel
An experimental investigation was conducted at the Langley aircraft landing loads and traction facility to study the braking and cornering response of a slip ratio controlled aircraft antiskid braking system with ground speed reference derived from an unbraked nose wheel. The investigation, conducted on dry and wet runway surfaces, utilized one main gear wheel, brake, and tire assembly of a DC-9 series 10 airplane. During maximum braking, the average ratio of the drag force friction coefficient developed by the antiskid system to the maximum drag force friction coefficient available was higher on the dry surface than on damp and flooded surfaces, and was reduced with lighter vertical loads, higher yaw angles, and when new tire treads were replaced by worn treads. Similarly, the average ratio of side force friction coefficient developed by the tire under antiskid control to the maximum side force friction coefficient available to a freely rolling yawed tire decreased with increasing yaw angle, generally increased with ground speed, and decreased when tires with new treads were replaced by those with worn treads
Technique for measuring side forces on a banked aircraft with a free-swiveling nose gear
An experimental investigation was conducted at the Langley Research Center to determine a method for towing an aircraft to measure the side forces of a free-swiveling nose gear due to variations in bank angle. A F-106 aircraft and the Space Shuttle orbiter OV-101 were towed to measure side forces on full-size aircraft for bank angles up to 3 deg. These tests indicate that substantial side forces will occur if an aircraft is rolling on a runway in a banked attitude even when the nose gear is free to swivel. Corotation of a twin-tire nose gear appears to cause a substantial increase in side force due to bank angle compared with a nose gear with indepdendently rotating wheels
Improved Bounds on Restricted Isometry Constants for Gaussian Matrices
The Restricted Isometry Constants (RIC) of a matrix measures how close to
an isometry is the action of on vectors with few nonzero entries, measured
in the norm. Specifically, the upper and lower RIC of a matrix of
size is the maximum and the minimum deviation from unity (one) of
the largest and smallest, respectively, square of singular values of all
matrices formed by taking columns from . Calculation of
the RIC is intractable for most matrices due to its combinatorial nature;
however, many random matrices typically have bounded RIC in some range of
problem sizes . We provide the best known bound on the RIC for
Gaussian matrices, which is also the smallest known bound on the RIC for any
large rectangular matrix. Improvements over prior bounds are achieved by
exploiting similarity of singular values for matrices which share a substantial
number of columns.Comment: 16 pages, 8 figure
Quantum search algorithms on a regular lattice
Quantum algorithms for searching one or more marked items on a d-dimensional
lattice provide an extension of Grover's search algorithm including a spatial
component. We demonstrate that these lattice search algorithms can be viewed in
terms of the level dynamics near an avoided crossing of a one-parameter family
of quantum random walks. We give approximations for both the level-splitting at
the avoided crossing and the effectively two-dimensional subspace of the full
Hilbert space spanning the level crossing. This makes it possible to give the
leading order behaviour for the search time and the localisation probability in
the limit of large lattice size including the leading order coefficients. For
d=2 and d=3, these coefficients are calculated explicitly. Closed form
expressions are given for higher dimensions
Regular quantum graphs
We introduce the concept of regular quantum graphs and construct connected
quantum graphs with discrete symmetries. The method is based on a decomposition
of the quantum propagator in terms of permutation matrices which control the
way incoming and outgoing channels at vertex scattering processes are
connected. Symmetry properties of the quantum graph as well as its spectral
statistics depend on the particular choice of permutation matrices, also called
connectivity matrices, and can now be easily controlled. The method may find
applications in the study of quantum random walks networks and may also prove
to be useful in analysing universality in spectral statistics.Comment: 12 pages, 3 figure
Mixed Models and Reduction Techniques for Large-Rotation, Nonlinear Analysis of Shells of Revolution with Application to Tires
An effective computational strategy is presented for the large-rotation, nonlinear axisymmetric analysis of shells of revolution. The three key elements of the computational strategy are: (1) use of mixed finite-element models with discontinuous stress resultants at the element interfaces; (2) substantial reduction in the total number of degrees of freedom through the use of a multiple-parameter reduction technique; and (3) reduction in the size of the analysis model through the decomposition of asymmetric loads into symmetric and antisymmetric components coupled with the use of the multiple-parameter reduction technique. The potential of the proposed computational strategy is discussed. Numerical results are presented to demonstrate the high accuracy of the mixed models developed and to show the potential of using the proposed computational strategy for the analysis of tires
Coronal Seismology and the Propagation of Acoustic Waves Along Coronal Loops
We use a combination of analytical theory, numerical simulation, and data
analysis to study the propagation of acoustic waves along coronal loops. We
show that the intensity perturbation of a wave depends on a number of factors,
including dissipation of the wave energy, pressure and temperature gradients in
the loop atmosphere, work action between the wave and a flow, and the
sensitivity properties of the observing instrument. In particular, the scale
length of the intensity perturbation varies directly with the dissipation scale
length (i.e., damping length) and the scale lengths of pressure, temperature,
and velocity. We simulate wave propagation in three different equilibrium loop
models and find that dissipation and pressure and temperature stratification
are the most important effects in the low corona where the waves are most
easily detected. Velocity effects are small, and cross-sectional area
variations play no direct role for lines-of-sight that are normal to the loop
axis. The intensity perturbation scale lengths in our simulations agree very
well with the scale lengths we measure in a sample of loops observed by TRACE.
The median observed value is 4.35x10^9 cm. In some cases the intensity
perturbation increases with height, which is likely an indication of a
temperature inversion in the loop (i.e., temperature that decreases with
height). Our most important conclusion is that thermal conduction, the primary
damping mechanism, is accurately described by classical transport theory. There
is no need to invoke anomalous processes to explain the observations.Comment: To appear in the Dec. 1, 2004 issue of the Astrophysical Journa
Pulsar Timing Probes of Primordial Black Holes and Subhalos
Pulsars act as accurate clocks, sensitive to gravitational redshift and
acceleration induced by transiting clumps of matter. We study the sensitivity
of pulsar timing arrays (PTAs) to single transiting compact objects, focusing
on primordial black holes and compact subhalos in the mass range from to well above . We find that the Square Kilometer
Array can constrain such objects to be a subdominant component of the dark
matter over this entire mass range, with sensitivity to a dark matter
sub-component reaching the sub-percent level over significant parts of this
range. We also find that PTAs offer an opportunity to probe substantially less
dense objects than lensing because of the large effective radius over which
such objects can be observed, and we quantify the subhalo concentration
parameters which can be constrained.Comment: 18 pages, 6 figure
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