Testing airplane fabrics

The following considerations determine the strength of airplane fabrics: 1. maximum air forces acting on the surfaces (including local stresses); 2. tensions produced in the fabrics, in the directions of both warp and filling; 3. factor of safety required. The question of the permissible depression of the fabric as affecting the aerodynamic requirements in regard to the maintenance of shape of the section, the tenacity and extensibility of the layer of dope, its strength and its permeability to water is almost as important

Collisionless microinstabilities in stellarators I - analytical theory of trapped-particle modes

This is the first of two papers about collisionless, electrostatic micro-instabilities in stellarators, with an emphasis on trapped-particle modes. It is found that, in so-called maximum-$J$ configurations, trapped-particle instabilities are absent in large regions of parameter space. Quasi-isodynamic stellarators have this property (approximately), and the theory predicts that trapped electrons are stabilizing to all eigenmodes with frequencies below the electron bounce frequency. The physical reason is that the bounce-averaged curvature is favorable for all orbits, and that trapped electrons precess in the direction opposite to that in which drift waves propagate, thus precluding wave-particle resonance. These considerations only depend on the electrostatic energy balance, and are independent of all geometric properties of the magnetic field other than the maximum-$J$ condition. However, if the aspect ratio is large and the instability phase velocity differs greatly from the electron and ion thermal speeds, it is possible to derive a variational form for the frequency showing that stability prevails in a yet larger part of parameter space than what follows from the energy argument. Collisionless trapped-electron modes should therefore be more stable in quasi-isodynamic stellarators than in tokamaks.Comment: 9 pages, 1 figur

Collisionless microinstabilities in stellarators II - numerical simulations

Microinstabilities exhibit a rich variety of behavior in stellarators due to the many degrees of freedom in the magnetic geometry. It has recently been found that certain stellarators (quasi-isodynamic ones with maximum-$J$ geometry) are partly resilient to trapped-particle instabilities, because fast-bouncing particles tend to extract energy from these modes near marginal stability. In reality, stellarators are never perfectly quasi-isodynamic, and the question thus arises whether they still benefit from enhanced stability. Here the stability properties of Wendelstein 7-X and a more quasi-isodynamic configuration, QIPC, are investigated numerically and compared with the National Compact Stellarator Experiment (NCSX) and the DIII-D tokamak. In gyrokinetic simulations, performed with the gyrokinetic code GENE in the electrostatic and collisionless approximation, ion-temperature-gradient modes, trapped-electron modes and mixed-type instabilities are studied. Wendelstein 7-X and QIPC exhibit significantly reduced growth rates for all simulations that include kinetic electrons, and the latter are indeed found to be stabilizing in the energy budget. These results suggest that imperfectly optimized stellarators can retain most of the stabilizing properties predicted for perfect maximum-$J$ configurations.Comment: 15 pages, 40 figure

Variational Auto-Encoders for Satellite Images of Fields

This thesis is situated at the overlap of probabilistic machine learning and remote sensing as it analyses the application of variational auto-encoders to satellite images of fields with the final objective of image classification. The rising availability of high-resolution satellite images of fields increases the need for compressing the images in order to keep maintenance and inference of machine learning models on a feasible and cost-efficient scale. Machine learning, in general, has proven to offer auspicious methods for summarising high-dimensional data into a lower-dimensional representation. Variational auto-encoders are a modern and advanced representation learning algorithm and are the topic of research in this thesis. An extensive hyperparameter search for the implemented networks is performed. The best architecture is selected and compared against conventional computer vision methods. The research shows that summarising high-resolution satellite images with variational auto-encoders is possible. It will, however, still take a performance hit on the classification tasks in comparison to the conventional computer vision techniques. The findings show the potential that variational auto-encoders offer for image compression but also that the used method needs further refinement in order to beat conventional approaches

Spiral aftereffect and the time interval between successive stimulations

Thesis (Ph.D.)--Boston University.The purpose of this study was (1) to test whether or not a theoretical model is capable of handling the data generated by this experiment, and, more broadly, whether the present model is tenable; (2) to narrow down the several possible interpretations of a previous study of the effect of speed of rotation upon aftereffect. The viewing of a rotating spiral gives rise to an aftereffect of motion. The model used herein took as its starting point the fact that one retinal element is stimulated before some other nearby retinal element during the inspection period in movement aftereffect studies. By means of transforming this difference in time of stimulation into a difference in amount of excitation in two different firing elements, the model generated a number of predictions which are in accord with the experimental data in the literature. The model was then applied to the problem of the present study. The model predicted that holding the (time) interval between stimulations constant would produce constant aftereffect, and that varying the length of the interval would produce variations in aftereffect [TRUNCATED

Take-off of heavily loaded airplanes

In the present article, several suggestions will be made for shortening the otherwise long take-off distance. For the numerical verification of the process, I will use a graphic method for determining the take-off distance of seaplanes

Pressure Measurements During Flight

The general idea was to record the air pressure on the spot, that is, without intervening tubes, by some simple automatic registering device. This was therefore made in such small dimensions that it could be inserted into the wing between the ribs and stand in direct connection with the pressure openings. The results of different experiments are presented along with the best locations for locating the instruments for recording the pressure

Take-off distance for airplanes

There are two principal ways in which the total take-off distance (taxying plus hovering plus the first part of the climb) can be reduced to a minimum. These are: 1) taxying and hovering until the maximum speed has been attained close to the ground and then changing to a steep rapid climb; 2) lifting the airplane from the ground as soon as possible and then climbing at a relatively large angle of attack. These cases, as well as all the other conceivable combinations, can be expressed with a single basic formula, which is derived from the energy equation for rectilinear flight