Showing papers by "Vaughn College of Aeronautics and Technology published in 2002"

Prediction of stiffness and stresses in z-fibre reinforced composite laminates

Abstract: The mechanical properties of z-pinned composite laminates were examined numerically. Finite element calculations have been performed to understand how the through-thickness reinforcement modifies the engineering elastic constants and local stress distributions. Solutions were found for four basic laminate stacking sequences, all having two percent volume fraction of z-fibres. For the stiffness analysis, a micro-mechanical finite element model was employed that was based on the actual geometric configuration of a z-pinned composite unit cell. The numerical results agreed very well with some published solutions. It showed that by adding 2% volume fraction of z-fibres, the through-thickness Young's modulus was increased by 22–35%. The reductions in the in-plane moduli were contained within 7–10%. The stress analysis showed that interlaminar stress distributions near a laminate free edge were significantly affected when z-fibres were placed within a characteristic distance of one z-fibre diameter from the free edge. Local z-fibres carried significant amount of interlaminar normal and shear stresses.

The more electric aircraft—assessing the benefits:

Abstract: The concept of the more electric aircraft (MEA), where electrical power is used for at least some of the on-board functions at present powered by hydraulics and pneumatics, has many claimed benefits and is far from new. However, almost all larger aircraft retain hydraulics and pneumatics.Over recent years there has been a great deal of interest in the MEA concept, with developments in relevant technologies and their adoption in other areas of engineering. In support of UK efforts on this and related concepts, the College of Aeronautics has been performing studies since the mid 1980s to assess the benefits of changes. This paper describes the background to these studies and the relationships between them as well as some of their findings, concluding that it is important that such assessment work should be continued in the future.

Simulation of Deployment of a Flexible Solar Array

Abstract: The deployment of a solar array is simulated three-dimensionally using the multibody program SIMPACK. The analyses are performed for 500 real-time seconds, which contain the three deployment phases, (I) jump-out, (II) steering phase and (III) deployed phase. The goal of the simulations is to check the influence of the flexibility of the solar array on the solar generator motions during these three phases against results obtained by a rigid body model simulation. The modelling of flexible bodies is based on the widely used method of floating frame of reference formulation applying global shape functions (Ritz method). The preparation of a proper set of shape functions to represent the flexibility of the yoke and the six solar panels is one of the main objectives of thispaper. For each of the components, eigenmodes and static modes forvarious boundary conditions are computed using the finite elementprogram NASTRAN. For a good convergence of the Ritz approximation with a smallnumber of shape functions, the shape functions are selected usingmodal participation factors, that are computed for various load casesprior to the time simulations. The load cases are obtained, for example,by a rigid body simulation of the deployment phases. The proposed methodof shape function selection using modal participation factors isdemonstrated by examples.

Study by MOA of extrasolar planets in gravitational microlensing events of high magnification

Abstract: A search for extrasolar planets was carried out in three gravitational microlensing events of high magnification, MACHO 98-BLG-35, MACHO 99-LMC-2 and OGLE 00-BUL-12. Photometry was derived from observational images by the MOA and OGLE groups using an image subtraction technique. For MACHO 98-BLG-35, additional photometry derived from the MPS and PLANET groups was included. Planetary modelling of the three events was carried out in a supercluster computing environment. The estimated probability for explaining the data on MACHO 98-BLG-35 without a planet is < 1 per cent. The best planetary model has a planet of mass ∼ (0.4-1.5) X M E a r t h at a projected radius of either ∼ 1.5 or ∼ 2.3 au. We show how multiplanet models can be applied to the data. We calculate exclusion regions for the three events and find that Jupiter-mass planets can be excluded with projected radii from as wide as about 30au to as close as around 0.5 au for MACHO 98-BLG-35 and OGLE 00-BUL-12. For MACHO 99-LMC-2, the exclusion region extends out to around 10 au and constitutes the first limit placed on a planetary companion to an extragalactic star. We derive a particularly high peak magnification of ∼160 for OGLE 00-BUL-12. We discuss the detectability of planets with masses as low as Mercury in this and similar events.

Improving the prospects for detecting extrasolar planets in gravitational microlensing events in 2002

Abstract: Gravitational microlensing events of high magnification have been shown to be promising targets for detecting extrasolar planets. However, only a few events of high magnification have been found using conventional survey techniques. Here we demonstrate that high-magnification events can be readily found in microlensing surveys using a strategy that combines high-frequency sampling of target fields with on-line difference imaging analysis. We present 10 microlensing events with peak magnifications greater than 40 that were detected in real-time towards the Galactic bulge during 2001 by the Microlensing Observations in Astrophysics (MOA) project. We show that Earth-mass planets can be detected in future events such as these through intensive follow-up observations around the event peaks. We report this result with urgency as a similar number of such events are expected in 2002.

Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud

Abstract: ABSTRA C T 146 long-period red variable stars in the Large Magellanic Cloud (LMC) from the three-year Microlensing Observations in Astrophysics (MOA) project data base were analysed. A careful periodic analysis was performed on these stars and a catalogue of their magnitudes, colours, periods and amplitudes is presented. We convert our blue and red magnitudes to K-band values using 19 oxygen-rich stars. A group of red short-period stars separated from the Mira sequence has been found on a (log P,K) diagram. They are located at the short period side of the Mira sequence consistent with the work of Wood & Sebo. There are two interpretations for such stars; a difference in pulsation mode or a difference in chemical composition. We investigated the properties of these stars together with their colour, amplitude and periodicity. We conclude that they have small amplitudes and less regular variability. They are likely to be higher-mode pulsators. A large scatter has also been found on the longperiod side of the (log P,K) diagram. This is possibly a systematic spread given that the blue band of our photometric system covers both standard B and V bands and affects carbon-rich stars.

Normal shock wave-turbulent boundary-layer interactions in the presence of streamwise slots and grooves

Abstract: The effect of streamwise slots and grooves on a normal shock wave-turbulent boundary-layer interaction has been investigated experimentally at a Mach number of 1.3. The surface pressure distribution for the controlled interaction in the presence of slots featured a distinct plateau. This was due to a change in shock structure from a typical unseparated normal shock wave-boundary-layer interaction to a large bifurcated lambda type shock pattern. Velocity measurements downstream of the slots revealed a strong spanwise variation of boundary-layer properties, whereas the modified shock structure was found to be relatively two-dimensional. Cross flow measurements indicate that slots introduce streamwise vortices into the flow. When applied to an aerofoil, streamwise slots have the potential to reduce wave drag while incurring only small viscous penalties. In the presence of grooves the interaction was initially found to be significantly different. A bifurcated shock structure was observed but the trailing leg appeared stronger and featured a second lambda foot. Oil flow visualisation also revealed differences in the interactions, with the region of suction and blowing being limited to a smaller extent of the grooved control surface. The amount of crossflow present was reduced compared to the slotted control surface. By varying the internal geometry of the grooves it was found that the interaction could be modified to be similar to that in the presence of slots indicating that a more practical control device can be designed.

The case for flight simulation in general aviation

Abstract: This paper sets out the case to extend the role of flight simulation in general aviation. It outlines the benefits that flight simulation has brought to airline safety but observes that the majority of pilots worldwide receive no simulator-based training or checking. The paper reviews the potential benefits of synthetic training and provides an analysis of accidents in general aviation to show that a significant reduction in fatal accidents is achievable by instituting recurrent pilot checks in a flight simulator, similar to the simulator checks undertaken by airline pilots. The paper summarises the advances in simulator technology which have occurred in recent years and outlines a scheme to fund the introduction of flight training devices in general aviation, based on compulsory recurrent checks for PPL and CPL pilots. The paper surmises that flight simulation offers a practical means to reduce the accident rate in general aviation and concludes by outlining a financial case to fund the introduction of mandatory simulation-based recurrent checks in general aviation. Language: en

Nano Satellite Propulsion using FEEP Thrusters

Abstract: A Field Emission Electric Propulsion (FEEP) system for nano-satellite propulsion is discussed. The feasibility of the technology is considered, taking into account both the nano-satellite and the FEEP thruster. In order to meet short-term nano-satellite applications, the emphasis is to understand performance capabilities of the total system in a LEO mission. This is achieved by considering the available technology, development and cost of launch for nano-satellite(s) as well as FEEP propulsion system requirements in order to meet those demands. FEEP propellant (Cesium and Rubidium) contamination is identified as one of the major problems with this propulsion system.

Linearised identification of the longitududinal flight dynamics of the solar powered airship Lotte

Abstract: This paper relates to a recent project in which the aim was to devise a procedure that could be used to identify the linearised transfer functions of the longitudinal motion of the solar powered airship Lotte from flight test data. Using a non-linear mathematical model of the Airship, a linearised state space model was derived numerically and this was used to derive the response transfer functions, which formed the theoretical results. A test procedure was then developed to identify the transfer functions experimentally from frequency response data obtained in simulated flight tests. Initially, the linearised model was used and the test procedure was refined until a suitable level of coherence between the experimental and theoretical results was observed. The test procedure was then applied to simulated flight test data from the non-linear model, and finally sensor noise was added in order to represent actual flight test conditions as closely as possible