11,092 research outputs found
Trends in aircraft design
The improved performance of aircraft during the past decade has resulted
in the need for new design and production techniques. Particular examples are
integral construction and the use of sandwich panels. Although these processes
are costly, especially when applied to titanium and steel construction, their use
is likely to be necessary, at least to some extent. on many supersonic aircraft.
The supersonic airliner is no exception to this and the paper discusses the design
aspects of this type of aircraft which have a bearing on production problems. It
is concluded that more research aimed at reducing the cost of sophisticated forms
of construction is required
The teaching of aircraft design
Aircraft Design has been taught at the College of Aeronautics since
1946. The course is at postgraduate level and is of two years duration.
In the first year the students are given three exercises in component
design which aim to teach a logical approach and the fundamentals of the
subject. During the second year each student works as a member of a
team engaged in the design of a complete aircraft, which is chosen to be
of a type currently being investigated by industry. The project aircraft
invariably incorporates experimental features and the design work is
therefore of the nature of research
Embedded Solitons in Lagrangian and Semi-Lagrangian Systems
We develop the technique of the variational approximation for solitons in two
directions. First, one may have a physical model which does not admit the usual
Lagrangian representation, as some terms can be discarded for various reasons.
For instance, the second-harmonic-generation (SHG) model considered here, which
includes the Kerr nonlinearity, lacks the usual Lagrangian representation if
one ignores the Kerr nonlinearity of the second harmonic, as compared to that
of the fundamental. However, we show that, with a natural modification, one may
still apply the variational approximation (VA) to those seemingly flawed
systems as efficiently as it applies to their fully Lagrangian counterparts. We
call such models, that do not admit the usual Lagrangian representation,
\textit{semi-Lagrangian} systems. Second, we show that, upon adding an
infinitesimal tail that does not vanish at infinity, to a usual soliton ansatz,
one can obtain an analytical criterion which (within the framework of VA) gives
a condition for finding \textit{embedded solitons}, i.e., isolated truly
localized solutions existing inside the continuous spectrum of the radiation
modes. The criterion takes a form of orthogonality of the radiation mode in the
infinite tail to the soliton core. To test the criterion, we have applied it to
both the semi-Lagrangian truncated version of the SHG model and to the same
model in its full form. In the former case, the criterion (combined with VA for
the soliton proper) yields an \emph{exact} solution for the embedded soliton.
In the latter case, the criterion selects the embedded soliton with a relative
error .Comment: 10 pages, 1 figur
Simple computer method provides contours for radiological images
Computer is provided with information concerning boundaries in total image. Gradient of each point in digitized image is calculated with aid of threshold technique; then there is invoked set of algorithms designed to reduce number of gradient elements and to retain only major ones for definition of contour
Indexed Regulation
Seminal work by Weitzman (1974) revealed that prices are preferred to quantities when marginal benefits are relatively flat compared to marginal costs. We extend this comparison to indexed policies, where quantities are proportional to an index, such as output. We find that policy preferences hinge on additional parameters describing the first and second moments of the index and the ex post optimal quantity level. When the ratio of these variables’ coefficients of variation divided by their correlation is less than two, indexed quantities are preferred to fixed quantities. A slightly more complex condition determines when indexed quantities are preferred to prices. Applied to the case of climate change, we find that quantities indexed to GDP are preferred to fixed quantities for about half of the 19 largest emitters, including the United States and China, while (consistent with previous work) prices dominate for all countries.price, quantity, regulation, uncertainty, policy, environment, climate change
SAR antenna calibration techniques
Calibration of SAR antennas requires a measurement of gain, elevation and azimuth pattern shape, boresight error, cross-polarization levels, and phase vs. angle and frequency. For spaceborne SAR antennas of SEASAT size operating at C-band or higher, some of these measurements can become extremely difficult using conventional far-field antenna test ranges. Near-field scanning techniques offer an alternative approach and for C-band or X-band SARs, give much improved accuracy and precision as compared to that obtainable with a far-field approach
The Tropos Software Development Methodology: Processes, Models and Diagrams
Tropos is a novel agent-oriented software development methodology founded on two key features: (i) the notions of agent, goal, plan and various other knowledge level concepts are fundamental primitives used uniformly throughout the software development process; and (ii) a crucial role is assigned to requirements analysis and specification when the system-to-be is analyzed with respect to its intended environment. This paper provides a (first) detailed account of the Tropos methodology. In particular, we describe the basic concepts on which Tropos is founded and the types of models one builds out of them. We also specify the analysis process through which design flows from external to system actors through a goal analysis and delegation. In addition, we provide an abstract syntax for Tropos diagrams and other linguistic constructs
A weak turbulence theory for incompressible magnetohydrodynamics
We derive a weak turbulence formalism for incompressible magnetohydrodynamics. Three-wave interactions lead to a system of kinetic equations for the spectral densities of energy and helicity. The kinetic equations conserve energy in all wavevector planes normal to the applied magnetic field B0ê[parallel R: parallel]. Numerically and analytically, we find energy spectra E± [similar] kn±[bot bottom], such that n+ + n− = −4, where E± are the spectra of the Elsässer variables z± = v ± b in the two-dimensional case (k[parallel R: parallel] = 0). The constants of the spectra are computed exactly and found to depend on the amount of correlation between the velocity and the magnetic field. Comparison with several numerical simulations and models is also made
- …