1,112 research outputs found
Towards the tumble resistant microlight
No full textThe tumble mode is a pitching departure from controlled flight which leads to a pitch autorotation that is generally unrecoverable – resulting in vertical ground impact, usually preceded by in-flight breakup (the mechanism for which, surprisingly, can sometimes prevent loss of life). This was identified in work led by the British Microlight Aircraft Association beginning in 1997 as a response to a number of fatal accidents in Rogallo winged microlight aeroplanes, although the tumble is also known to occur to hang-gliders. This paper explains how this class of aeroplane is controlled, and how it has been found that they can enter the tumble mode. The mechanism by which the tumble can be entered is described. This has led to work showing how flight testing can be used to establish and demonstrate resistance to tumble entry – particularly important with increasing number of very high performance flexwings. These flight tests will be explained, together with the significance of the results. Recent accident investigation work has also shown a new mechanism of tumble entry, through partial failure of the A-frame structure and the pitch-trimmer mechanism. Also described is a possible relevance to well known historical accidents to flying wing aeroplanes– specifically the YB-49 and dH-108, and discovered data on the characteristics of the BKB-1flying wing glider; are also described
Bivariate poisson-poisson model of zero-inflated absenteeism data.
No full textBimodal distributions of counts with one mode at zero are often seen in medical research. In a health survey parents were asked the number of days their children missed their activities (Y(1)) and the number of days their children spent in bed (Y(2)) due to illness in the past four weeks. Both variables exhibited zero inflation. We consider a bivariate Poisson-Poisson regression model, in which the two variables are regarded as indicators of an unobserved health status variable. Based on this, we further develop a bivariate Poisson-Poisson model that constrains Y(1)>or=Y(2). It is often claimed that there is a critical window of growth and nutrition in foetal life and infancy during which subsequent health status is affected. It is not clear whether the claim is true and whether childhood growth matters more. We analyse the bivariate data in relation to weight-for-age in infancy and weight gain from infancy to age 7 years. The findings do not support the existence of a critical window in infancy. There is some indication that childhood weight gain might affect health status
Calculation of surface tension using CALPHAD software as a Zero Phase Fraction line of ""surface"" phase
No full textA new approach for calculation of surface tension of solutions is presented. Based on the proposal of Butler (Proc. R. Soc. Lond. A: Math. Phys. Eng. Sci. 135 (1932) 348 [3]), a surface phase was assumed to be in equilibrium with a corresponding bulk phase. Following the proposal of Pajarre et al. (Calphad 30 (2006) 196 [71), a surface area element (X) was introduced, only in the surface phase. The surface tension can be obtained by Constrained Gibbs Energy Minimization (CGEM) under a constraint of constant surface. The equilibrium chemical potential of the surface element, X, was interpreted to be proportional to the surface tension. According to the present author (Calphad 50 (2015) 23 [9]), the surface tension calculation by the CGEM can be interpreted as if the bulk phase and the surface phase are in equilibrium with each other, and the chemical potential of the area element is obtained as a result of the equilibrium. It was shown that a geometrical relationship among the molar Gibbs energies of the bulk phase and the surface phase, the surface tension, and the surface concentration is found. In the present study, further improvement was made in such a way that the chemical potential of the area element can be simply obtained by combining the CGEM with Zero Phase Fraction (ZPF) line in Type II phase diagram, where the chemical potential of the area element is used as an axis variable. Such phase diagram can be easily calculated using conventional CALPHAD softwares. The proposed approach was validated for a number of binary and ternary systems. (C) 2015 Elsevier Ltd. All rights reserved.1122sciescopu
High power pulsed ytterbium doped fibre lasers and their applications
No full textThe aim of my project is to develop pulsed Ytterbium (Yb) doped fibre master oscillator power amplifier (MOPA) systems seeded by semiconductor lasers. I was principally focused on two specific projects aligned to sponsored programs of research within the ORC pulsed fibre laser group: the first project, TSB funded project LAMPS, aimed to develop an important class of next generation laser system capable of average output powers of more than 100 W when operating in both the nanosecond and picosecond regimes. The goal was to develop a fully fiberized, polarisation maintaining, single transverse mode system. The full project included the development of the necessary diode & micro-optic systems, fibre beam delivery technology and with application focused evaluations in collaboration with our industrial partners. The main project partners were BAE Systems, Selex, Ceram, Intense Photonics, ORC, Herriot Watt University, Power Photonics, OptoCap and Rofin Sinar. I contributed to the development of the single transverse mode Ytterbium (Yb)-doped fibre system and achieved the full target specifications of 100 W of output power with single mode and single polarisation operation in both the nanosecond and picosecond regimes. In addition, second harmonic generation pumped by the fundamental beam at 1.06 µm was also achieved. In order to transfer from picosecond pulses to nanosecond pulses it is only necessary to switch the seed laser, the power amplifier system remaining unchanged making for a highly flexible system. Both fundamental and second harmonic beam were successfully used to do material processing and various high power frequency conversion experiments (visible, broadband supercontinuum and mid-IR). The second project, called HEGAC (also funded by the TSB), was a collaboration with the University of Cambridge and SPI Lasers Ltd. The aim of the HEGAC project was to develop a high power nanosecond fibre laser with an active pulse shaping capability suitable for cutting metals. This project targeted mJ pulses with more than 100 W average power at the final output – with a 200 W stretch objective. We first achieved more than 310 W using a free space seeding and pumping configuration in our laboratories proving power scaling of our proposed approach. I subsequently rebuilt and improved this system and developed a fully- fiberized version (including all pump launches). The laser was capable of generating >100 W of output power and pulse energies up to 2.5 mJ. This project also involved spatial mode as well as temporal pulse shaping. Using a pair of axicon lenses the normal Gaussian beam profile was converted to a ring shaped profile as required and the system tested up to average powers of 100 W. In addition to the normal temporal pulse shapes required using our pulse shaping system (square, triangle and step), I also achieved high average power pulses with smooth shaped pulses (Parabolic and Gaussian) using an adaptive pulse shaping technique. The laser was transported and successfully used in materials processing experiments at Cambridge, proving the robustness of the design and implementation. I also did some novel experiments on high efficiency Raman conversion exploiting the square shaped pulses possible using this laser
Near-IR Luminescent Yb-III Coordination Polymers Composed of Pyrene Derivatives for Thermostable Oxygen Sensors
No full textThis work synthesized oxygen sensitive and near infrared (NIR) luminescent Yb(III) coordination polymers incorporating ligands based on pyrene derivatives: Yb(III)-TBAPy and Yb(III)-TIAPy (TBAPy: 1,3,6,8-tetrskis(p-benzoate)pyrene, TIAPy: 1,3,6,8-tetrakis(3,5-isophtalic acid)pyrene). The coordination structures of these materials were characterized using electrospray ionization mass spectrometry, X-ray diffraction and thermogravimetric analysis, while the porous structure of the Yb(III)-TIAPy was evaluated based on its adsorption isotherm. The NIR luminescence properties of the Yb(III)-TBAPy and Yb(III)-TIAPy were examined by acquiring emission spectra and determining emission lifetimes under air, argon and vacuum. The Yb(III)-TIAPy exhibited high thermal stability (with a decomposition temperature of 400 °C), intense luminescence (with an emission quantum yield under argon of 6.6%) and effective oxygen-sensing characteristics. These results suggest that NIR luminescent Yb(III) coordination polymers made using pyrene derivatives could have applications in novel thermo-stable oxygen sensors
Achieving aerospace standard porosity levels when welding thin and thick-section aluminium using fibre-delivered lasers : executive summary
No full textEnvironmental and commercial pressures have forced the aerospace industry to look at
alternatives to riveting for the manufacture of aluminium aircraft structures. This resulted, at
the end of last century, in an extensive study by Airbus into the possiblities of using CO2
lasers, which led to the process being implemented for a (small) number of stringer-to-skin
fuselage panels in the newer Airbus models. Since this initial commercial success, new laser
sources have become available that are more suitable for the welding of aluminium than CO2
lasers, in the form of Nd: YAG and Yb-fibre lasers. Both produce a wavelength that is
absorbed more efficiently by aluminium alloys than the CO2 laser wavelength, resulting in an
improved keyhole stability, as demonstrated in the late nineties for Nd: YAG lasers. In addition,
Yb-fibre lasers have become available at output powers higher than available for Nd: YAG
lasers, allowing thicker sections of aluminium to be welded in a single pass. However, despite
their claimed advantages, no efforts were made to demonstrate the potential of these lasers
for (aluminium) aircraft manufacture. For this reason, the author initiated a series of studies in
2001, with the overall aim to develop procedures to laser weld both thin (3.2mm) and thicksection
(12.7mm) aerospace aluminium alloys using these fibre-delivered lasers to a weld
quality, in particular related to weld metal porosity, suitable for aerospace service. The focus
in this research was on weld metal porosity, because this is a particular problem when laser
welding aluminium, either in the form of fine (hydrogen) porosity or larger porosity associated
with an unstable keyhole behaviour. The benchmark weld metal porosity for this study was
obtained from the stringent weld quality classes defined in BS EN 13919-2 and AWS D17.1.
The approach to this research was in three parts, with work in the first aimed at demonstrating
that a 3kW Nd: YAG laser was capable of producing low-porosity welds in 3.2mm thickness
2024 aluminium alloy, and thus can be considered for replacing the CO2 laser currently used
for the stringer-to-skin fuselage application. Prior to the final part of the research, in which a
7kW Yb-fibre laser was used to demonstrate that these benchmark porosity levels could also
be achieved in thicker section (aerospace-grade) aluminium, a comparison study was carried
out to quantify the difference in welding performance between the Nd: YAG and the Yb-fibre
laser. At an output power of 4kW focused in a 0.4mm diameter spot, the Yb-fibre laser was
capable of a 30% higher welding speeds in 4mm (5083) aluminium alloy, or a 20% increase in
depth of penetration for welding speeds between 1 and 15m/min, compared with the Nd: YAG
laser. This improvement in welding performance, together with an output power of 7W,
produced full penetration in 12.7mm thickness (aerospace-grade) AI-Zn-Mg-Cu aluminium
alloy using the Yb-fibre laser autogenously, or in a hybrid configuration with a MIG arc. Both
the autogenous laser and hybrid laser-MIG process were capable of producing welds with a
weld metal porosity in line with the BS EN 13919-2 and AWS D17.1 benchmark conditions, at
welding speeds of 0.55 and 0.75m/min, respectively. At these production rates, the 248
metres of stringer incorporated in a typical aluminium wing structure can be welded in 7.5 and
5.5 hours, in case of autogenous laser and hybrid laser-MIG, respectively, compared with
37.6 hours currently needed for the riveting process
Isolation, characterization, and expression of the rat liver glutathione s-transferase yb-, subunit gene, 1989
No full textIn situ hybridization using a cDNA probe complementary to glutathione S-transferase Yb-j mRNA was utilized to isolate a glutathione S-transferase Yb-| gene from a rat liver genomic library. DNA sequence analysis incorporating "gene walking" strategies revealed that the Yb-j gene spans approximately 5.5 Kb, is comprised of eight exons separated by seven introns and is 99% homologous to the Ybi cDNA clone pGTA/C44. The transcription start site, which maps 29 bp downstream of the TATA element, was determined by primer extension analysis. Gene fragments containing the Yb-| promoter (and various deletions of the promoter region) were inserted upstream of the structural gene encoding chloramphenicol acetyl transferase (CAT). When these chimeric genes were transfected into mammalian cells, CAT assays revealed that the Yb^ minimal promoter is contained within an 80 bp DNA fragment which contains a TATA element, a transcription initiation site and 50 bp of sequence upstream of the TATA element. Contrary to in vivo studies which have shown that Yb-| gene activity is induced in response to phenobarbital and 3-methylcholanthrene administration, no elevation in activity was observed when mammalian hepatoma cells which had been transfected with the chimeric genes were treated with these xenobiotics. These data suggests that the regulatory element which confers xenobiotic inducibility is absent from the first 1700 bp of the Yb-j gene
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