Aerospace Engineering Commons^™

A Comparison Of Molecular Vibration Modeling For Thermal Nonequilibrium Airflow, Kenneth J. Moran

Theses and Dissertations

The effects of thermal nonequilibrium on flows about blunt-bodies have not been studied numerically in isolation from chemical reactions. Typically, air is modeled as a perfect gas or as a chemically reacting mixture. In the former case, significant errors result at Mach numbers exceeding about 5. However, below Mach numbers around 8, the effects of chemical reactions are negligible. This study analyzes flow about a simple axisymmetric blunt-body at moderate hypersonic speeds (Mach numbers between 5-8). A first-order-accurate Roe scheme was used to compute solutions of the Euler equations assuming different gas models: perfect gas, thermal equilibrium, and thermal nonequilibrium. …

Investigation Of Damage Mechanisms In A Cross-Ply Metal Matrix Composite Under Thermo-Mechanical Loading, Joel J. Schubbe

Theses and Dissertations

Metal matrix composites (MMCs) are rapidly becoming strong candidates for high temperature and high stiffness structural applications such as the Advanced Tactical Fighter (ATF). This study systematically investigated the failure modes and associated damage in a cross-ply, [0/90]_2s SCS6/Ti-15-3 metal matrix composite under in-phase and out-of-phase thermomechanic fatigue. Initiation and progression of fatigue damage were recorded and correlated to changes in Young's Modulus of the composite material. Experimental results show an internal stabilization of reaction zone size but degradation and separation from constituent materials under extended cyclic thermal loading. Critical to damage were transverse cracks initiating in the 90° …

A Numerical Investigation Of The Series Reversion/Inversion And Series Reversion Of Lambert's Time Function, Michael P. Ward

Theses and Dissertations

An expression for the semi-major axis as a function of time may be determined by performing a series reversion and inversion of Lambert's Time Function. Since the resulting series contains a singularity, it is desirable to perform only a reversion on the original series to obtain an expression for the inverse of the semi-major axis. Using a Lagrange expansion to obtain the coefficients for this series is very computer intensive. Therefore, alternative methods are presented. Also, each series was expanded into a continued fraction which provided greater accuracy than the series using the same number of coefficients. The accuracy was …

Composite Structure Ultimate Strength Prediction From Acoustic Emission Amplitude Data, James Lewis Walker Ii

Master's Theses - Daytona Beach

The acoustic emission (AE) given off by a structure as it is stressed provides a passive means to characterize flaw growth activity in complex structures. This thesis demonstrates how the qualitative analysis of AE data can be refined to provide a quantitative tool for predicting the ultimate strength of composite tensile test specimens. From an original sample set of only six specimens, a multivariate statistical analysis was used to generate an ultimate strength prediction equation. The variables of the multivariate statistical analysis were obtained through the mathematical modeling of the specimen’s AE amplitude distributions produced during proof testing. Ultimate strengths …

Improving Reliability In A Stochastic Communication Network, William L. Gaught

Theses and Dissertations

This research investigated the behavior of increasing the reliability of components in stochastic communication networks. A stochastic communication network is one in which the components, links and nodes, are not 100 percent reliable. In addition the reliability value, each component also has a capacity value that limits the flow through the component. The throughput in a stochastic communication network can be improved by increasing the capacity or the reliability of the individual components. A set of linear and nonlinear models was developed to determine the best investment strategy for increasing the reliability of individual components. Once the models were developed, …

Use Of Grasp Force Focus Positioning To Enhance The Torque Resistance Capability Of Robotic Grasps, Stephen G. Edwards

Theses and Dissertations

Three-point-contact grasps are unique in that the homogeneous solution for the contact forces always produces a grasp force focus. Careful positioning of this focus point in the grasp plane can help avoid two things; slipping at the contact points, and violation of joint torque limits. The focus placement method is explored theoretically by examining two types of grasps on cylinders; 1) fingertip grasps using three independently operated fingers, and 2) single-finger power grasps with one contact point on each of three links. Constraint maps are generated for various fingertip grasps in order to show how proper placement of the grasp …

Material Properties Of Silicon Carbide Fibers With Continuously Applied Sol-Gel Alumina Coatings, Christian C. Daehnick

Theses and Dissertations

An investigation was conducted to determine the effect of non-alpha- alumina coatings applied under varying conditions on some of the properties of silicon carbide fibers. Limited characterization of the coating by optical, scanning electron, and transmitted electron microscopy as well as energy- dispersive x-ray analysis was performed. Tensile tests were used to determine changes in elastic modulus, ultimate strength, and strain to failure of the coated fibers relative to uncoated fibers. The coatings were found to lower the mean value of the measured properties as their thickness increased, but the measurements were not accurate enough to determine if this corresponded …

Multi-Input Multi-Output Flight Control System Design For The Yf-16 Using Nonlinear Qft And Pilot Compensation, Russell B. Miller

Theses and Dissertations

Nonlinear Quantitative Feedback Theory (QFT) and pilot compensation techniques are used to design a 2x2 flight control system for the YF-16 aircraft over a large range of plant uncertainty. The design is based on numerical input- output time histories generated with a FORTRAN implemented nonlinear simulation of the YF-16. The first step of the design process is the generation of a set of equivalent linear time-invariant (LTI) plant models to represent the actual nonlinear plant. It has been proven that the solution to the equivalent plant problem is guaranteed to solve the original nonlinear problem. Standard QFT techniques are then …

An Experimental Study Of A Sting-Mounted Single-Slot Circulation Control Wing, Michael E. Pelletier

Theses and Dissertations

This wind tunnel study investigated the feasibility of using a sting and force balance to measure the aerodynamic forces and moments on a circulation control wing. A 20% this, 8.5% camber, single blowing slot, rectangular wing was designed, built, and tested in the AFIT 5-ft wind tunnel. Lift and drag coefficients were referenced to the stability axis. The Reynolds number for all tests was 950,000; angle of attack was varied from -6 to +6 degrees. Trends in the data were similar to two dimensional data, with the exception of high drag coefficients with increased blowing. Results show it is feasible …

Effect Of Riblets On Pressure Recovery In A Straight-Walled Diffuser, Michael K. Reagan

Theses and Dissertations

The objective of this thesis was to investigate the effect of riblets on the pressure recovery in a straight-walled diffuser. Previous work has shown that riblets were effective in reducing the viscous drag over surfaces subjected to a turbulent boundary layer. More recently, riblets were shown to delay the flow separation within a subsonic, straight-walled diffuser by as much as 200%. The purpose of this investigation was to determine the effect on the pressure distribution within a diffuser that has had the flow separation point favorably altered by the application of riblets. Investigation results revealed that riblets not only delayed …

Failure Mechanisms In A Quasi-Isotropic Ceramic Composite Laminate Under Tensile Fatigue Loading, Gregory D. Tracy

Theses and Dissertations

A study was conducted to experimentally investigate the tensile fatigue damage mechanisms in a quasi-isotropic ceramic matrix composite (Corning's Nicalon/CASIII). The goals of this study were to (1) develop a grip fixture for a servo-hydraulic MTS test machine and (2) conduct a series of tests with (0/±45/90)s specimens subjected to tensile fatigue loading to study their fatigue damage growth mechanisms. Results showed that when the material was subjected to stresses below 90 MPa, it lasted over 1000000 cycles but when stressed at or above 90 MPa, it failed in less than 1000 cycles.

Investigation Of Direct And Indirect Optimization Algorithms For Aerospace Structures, Harry Hopkins Iii

Theses and Dissertations

This thesis investigated the performance of two direct and two indirect methods of structural optimization. A gradient calculator and overall design system was created with the finite element code FRAME. Three of the methods were employed using this system with the fourth being a commercial available code. The algorithms were applied to two different sized trusses using static constraints and were measured for accuracy, reliability, and efficiency. Results for the problems tested showed the Direct methods were sensitive to starting designs and their performance depended on the proper selection of internal parameters. They were also shown to have a high …

An Analysis Of The Effects Of Phenytoin In Treating Motion Sickness And The Effects Of Motion Sickness On The Human Electroencephalogram, Todd M. Banducci

Theses and Dissertations

This research had two goals: to continue the ongoing research at AFIT to determine the efficacy of the anticonvulsant drug phenytoin in combating the onset and progression of motion sickness; and to note whether or not there is a discernable similarity in the brains of the test subjects to indicate a common point of origin (epicenter) and propagation pattern of the effects of motion sickness in the brain. Eight male DOD personnel were used as subjects to complete twelve trials. Four subjects completed the phenytoin verses placebo double-blind crossover experiment. These four subjects experienced a 99% mean increase in their …

Application Of Influence Diagrams In Identifying Soviet Satellite Missions, Cary C. Chun

Theses and Dissertations

This thesis investigates the feasibility of applying influence diagrams to capture expert knowledge and incorporate decision theory to construct a Soviet satellite mission prediction model. Before a Soviet launch occurs, formulations of the model's prior probability distribution allows the input of expert knowledge and any available information. As the predictive variables of the model become known, the information is used to successively refine the estimate of the probable mission, thereby, reducing the uncertainty in the model. Discretizing is used to approximate continuous variables as discrete variables which successfully allows the combination of both variable types in a single influence diagram …

Bifurcation Analysis Of The Longitudinal Dynamics Of A Simple Powered Lifting Hypersonic Vehicle, Eric E. Fox

Theses and Dissertations

Bifurcation analysis was used to investigate the nonlinear behavior of a simple powered lifting hypersonic vehicle in circular orbit about a spherical nonrotating Earth with gradients in atmospheric density and pressure and an inverse square law for gravity. Vehicle motion is constrained to a vertical plane so only longitudinal dynamics were modeled. Bifurcation analysis was conducted using the AUTO software package. A simple five-state model with three different thrust laws was derived to describe an unaugmented vehicle whose geometric and aerodynamic characteristics follow those of the literature. A parameter representing a body flap deflection was used to conduct one set …

Investigation Of The High Angle Of Attack Dynamics Of The F-15b Using Bifurcation Analysis, Robert J. Mcdonnell

Theses and Dissertations

Previous studies predicted the F-15B high angle of attack and flat spin behavior using bifurcation analysis. These studies varied control surface deflections to find equilibrium and periodic solutions. The purpose of this research was to use bifurcation analysis to predict the F-15B high angle attack and flat spin behavior as a result of variable thrust, asymmetric thrust, and thrust vectoring. Using a previously developed model of the F-15, bifurcation analysis and continuation methods were used to map out the equilibrium and periodic solutions of the model as a function of the thrust parameters. A baseline bifurcation diagram, as a function …

Modelling And Analysis Of Kernel Function And Development Of Equivalent Theodorsen Function For Three-Dimensional Aeroelastic Analysis, John K. Ryder

Theses and Dissertations

A fractional calculus model is developed for the kernal function under incompressible subsonic flow conditions for rectangular planform airfoils with small aspect ratio. A model valid for restricted regions of the kernel function for compressible subsonic flow conditions is also developed. Additionally, a method for numerically solving the pressure-downwash integral equation for rectangular planform wings of aspect ratio two through ten in incompressible flow is developed. An equivalent Theodorsen function for three- dimensional unsteady flow is developed, enabling the use of simpler two- dimensional aeroelastic equations of motion to fully capture the more complicated three-dimensional effects.

Effects Of Specific Heat Ratio On A Simulated Chemical Laser Cavity Flow, Curt Dewayne Botts

Theses and Dissertations

Mixing of primary cold flow air and secondary helium to control the ratio of specific heats for the medium flowing through a simulated chemical laser nozzle/lasing cavity was accomplished. The effects of a range of mixture specific heat ratios on flowfield behavior were examined using static pressure ports in the test cavity. Schlieren photography and high speed filming aided description of the flow dynamics. Results indicated that boundary layer effects became evident in the nozzles as specific heat ratios increased. Large pressure fluctuations were observed in the cavity when helium was introduced into the flow to raise the specific heat …

Damping Of Layered Beams With Mixed Boundary Conditions, Eugene T. Cottle

Theses and Dissertations

For many years, viscoelastic materials have been used in damping treatments to control vibration. In order to optimize a damping treatment for maximum damping, a design engineer must be able to predict frequencies of vibration of damped normal modes and loss factors as a function of the design parameters. Theoretical models of layered beams have been developed, but to date, exact solutions have assumed boundary conditions to be the same for all layers. In this thesis, equations of motion for a damped, layered beam are solved for the longitudinal displacement of both elastic layers, as well as the lateral displacement …

Analysis Of Hypersonic Blunt-Body Flows Using A Total Variation Diminishing Scheme And The Maccormack Scheme, Montgomery C. Hughson

Theses and Dissertations

An explicit second-order accurate, total variation diminishing (TVD) scheme and the MacCormack scheme are applied to the Euler equations in axisymmetric form to study hypersonic blunt body flows. The modified flux approach of Harten, with modification by Yee, for two-dimensional flows is extended to treat axisymmetric flows. Calculated flow properties for the steady state, blunt body problem such as shock standoff distance, bow shock shape, surface pressure distribution and entropy jump conditions are compared to theory, results from the MacCormack scheme, and experimental data for Mach numbers of 3.0, 4.03, 5.06, 6.03 and 8.1. Additionally, the TVD and MacCormack schemes …

Numerical Analysis Of Critical Inclinations About The Planet Mars, Kenneth J. Hyatt

Theses and Dissertations

This study investigated critical inclinations about the planet Mars. Averaged equations of motion were numerically integrated over a time span of ten Earth years for various inclinations, eccentricities, and perigee heights. Each orbit (i.e. set of initial conditions) produced a standard deviation of the variations in eccentricity (SDE) and inclination (SDI). These were calculated using a polynomial approximation to the variations. Surface plots of SDI and SDE vs the initial conditions are then examined to ascertain the critical inclinations.

Minimum Orbits About The Planet Venus, Jeffrey J. Smith

Theses and Dissertations

An investigation is conducted into low orbiting satellites around the planet Venus with drag limited lifetimes. It is possible to specify combinations of orbital elements which result in orbital lifetimes in excess of some desired value. These combinations can be assembled into Critical Curves and Critical surfaces. Critical Curves and Surfaces are defined as curves and surfaces in orbital element space above which initial element sets will result in orbits that meet or exceed lifetime requirements. A numerical method is implemented for finding these combinations, and three Critical Surfaces are examined. A decay threshold is selected for bounding satellite lifetime. …

Dynamic Analysis Of A Combat Aircraft With Control Surface Failure, Marc Roy

Theses and Dissertations

An investigation was performed to analyze the dynamic stability characteristic of an aircraft which has sustained damage to a primary control surface. The analysis was performed using the existing functional form of actual wind tunnel data taken on an F-16 model. Two control schemes are used for trimming an F-16 that has sustained damage to its rudder. The first control scheme represent the basic aircraft, while the second allowed the horizontal tail ailerons to move independently from the flaperons. The investigation was conducted for one flight condition representative of the aircraft at cruise speed. Region in α/β (angle of attack)/(sideslip …

Analysis Of Parking Orbits For A Sts External Tank In Low Earth Orbit, James E. Cross

Theses and Dissertations

A study was conducted of a single external tank in low earth orbit. Criteria for a parking orbit are then defined. Various orbits are then selected by varying the semi-major axis, eccentricity, and inclination. The resulting equations of motion for each of the above orbits (including atmospheric drag and a 2x0 gravity model) are numerically integrated over a time span of 90 earth days. An examination was made of the lowest initial altitudes which allowed the external tank to remain in the orbit window.

Dynamics And Control Of Two Suspension Mechanisms For Ground-Based Testing Of Space Structures, Joseph E. Wilson

Mechanical & Aerospace Engineering Theses & Dissertations

Two suspension systems which effectively simulate a gravity-free environment are presented. A single degree-of-freedom suspension system utilizing a specially profiled band mechanism in conjunction with a torsional spring is presented to show its feasibility and applicability for use as a ground-based testing device for flexible space structures. The second device, a two degree-of-freedom suspension system consisting of two springs and a folding linkage is presented to show the feasibility as a testing device for suspending space structures on earth.

Problems resulting in unwanted dynamic behavior due to deviations from the design parameters for both systems are addressed. A nonlinear controller …

Integrated Control Of Thermally Distorted Large Space Antennas, Robert H. Tolson

Mechanical & Aerospace Engineering Theses & Dissertations

Studies on controlling the thermal distortion of large space antennae have generally investigated a single orbital position and have optimized actuator locations based on minimizing the RMS surface deviation from the original parabolic shape. One study showed the benefits of directly using far zone electric field characteristics as the performance measure; but, this approach resulted in a nonlinear programming problem. The objective of the current study is to develop an approach to designing a control system that (1) recognizes the time dependence of the distortion and (2) controls variables that are directly related to far field performance in a quadratic …

Development Of A Perturbation Generator For Vortex Stability Studies, John Edward Riester Jr.

Mechanical & Aerospace Engineering Theses & Dissertations

Theory predicts vortex instability when subjected to certain types of disturbances. It was desired to build a device which could introduce controlled velocity perturbations into a trailing line vortex in order to study the effects on stability. A perturbation generator has been designed and manufactured which can be attached to the center-body of an airfoil type vortex generator. Details of design tests and manufacturing of the perturbation generator are presented. The device produced controlled perturbations with frequencies in excess of 250 Hz. Preliminary testing and evaluation of the perturbation generator performance was conducted in a 4 inch cylindrical pipe. Observations …

On The Maximum Range Of Flying Wings, Peter J. Torvik

AFIT Documents

The classical equations for determining the maximum range of aircraft with propeller and jet propulsion systems are reviewed, along with previous work conducted to determine the optimal division of aircraft volume between fuselage and wing components. That the jet powered flying wing configuration produces optimal range only for limited geometries is confirmed. The optimal range of aircraft employing high bypass jet engines is explored, and found to lead to a broader range of design parameters for which the flying wing design produces maximum range than is the case when a pure jet system is used.

The Development And Application Of A Trapezoidal Shear Panel For Use In Finite Element Codes, Glenn P. Greiner

Master's Theses - Daytona Beach

This thesis documents the efforts of the writer and his colleagues over the past several years to improve the theoretical foundation of the arbitrary quadrilateral shear panel used in structural analysis codes to model aircraft structures. An equilibrium stress-based element with pure shear resultants on its sides was developed using the principle of complementary virtual work. The internal stress field was derived from a complete polynomial Airy stress function. The element was numerically tested as a pure stress element and a hybrid element to assess the deflection properties for highly distorted planar panels. Linear-stress and quadratic-displacement rods were used, as …

Extension Of A Noninteractive Reliability Model For Ceramic Matrix Composites, Stephen F. Duffy, Robert C. Wetherhold, Lalit K. Jain

Civil and Environmental Engineering Faculty Publications

In the next decade ceramic composites will increasingly be used as high-temperature components in advanced gas turbines. Ceramic components will allow an increase in the operating temperature of the gas stream (measured by the turbine entry temperature), thereby resulting in greater fuel economy in aerospace and automotive turbine applications. Since these materials will be produced from abundant nonstrategic materials, computational structural mechanics methods are evolving to keep pace with this technology. As a result, establishing protocols for sound design methodology, which is the subject of this technical note, is the focus of much current analytical research.

Our objective was to …