An engineering model designed for the analysis of high angle-of-attack flight characteristics is developed and applied to the problem of aircraft poststall departure. The model consists of an aerodynamics package used interactively with a six-degree-of-freedom flight simulator. The aerodynamics are computed via a nonlinear lifting line theory with unsteady wake effects due to a discrete, nonplanar vortex system. A fully configured aircraft (main wing, horizontal tail and vertical fin) is mathematically constructed by modeling all lifting surfaces with bound, discrete vortex segments and associated control points; vehicle geometric influence on high angle-of-attack flight characteristics is included through complete variability in the relative locations, orientations and sizes of the flight surfaces. This aircraft model is “flown” through prescribed maneuvers by integrating the equations of motion. Selected results of trajectory simulations presented for a typical general aviation aircraft provide the following insights to wing-drop departure subsequent to stall. The abruptness of poststall roll-off depends on the presence of flight asymmetries at the stall break and the rate of stall penetration. Such out-of-trim flight conditions induce asymmetric wing panel unstall subsequent to deep stall penetration resulting in large wing-drop-producing roll moments. However, the abrupt departure from symmetric flight conditions is also found to be mathematically possible. This is a consequence of multiple lifting line solutions which exist for bound vortex systems assigned the lift properties of airfoils having stall discontinuities. The dynamic model is well suited to the prediction of departure resistance benefits realized through passive aerodynamic modifications, for example, drooped leading edge outboard wing panels. The model can also be applied to the generation of dynamic stability derivatives by analytically simulating forced oscillation test procedures. / Ph. D.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This thesis presents the development of a stall prevention flight control subsystem, which can easily be integrated into existing flight control architectures of fixed-wing unmanned aerial vehicles (UAV’s). This research forms an important part of faulttolerant flight control systems and will ensure that the aircraft continues to operate safely within its linear aerodynamic region. The focus of this thesis was the stall detection and prevention problem. After a thorough literature study on the topic of stall, a model based stall prevention control algorithm with feedback from an angle of attack sensor was developed. This algorithm takes into account the slew rate and saturation limits of the aircraft’s servos and is able to predict when the current flight condition will result in stall. The primary concern was stall during wings-level flight and involved the prevention of stall by utilising only the elevator control surface. A model predictive slew rate control algorithm was developed to override and dynamically limit the elevator command to ensure that the angle of attack does not exceed a predefined limit. The stall prevention control system was designed to operate as a switching control scheme, to minimise any restrictions imposed on the existing flight control system. Finally, software in the loop simulations were conducted using a nonlinear aircraft model and realistic sensor noise, to verify the theoretical results obtained during the development of this stall prevention control strategy. A worst-case performance analysis was also conducted to investigate the robustness of the control algorithms against model uncertainties. / AFRIKAANSE OPSOMMING: Hierdie tesis handel oor die ontwikkeling van ’n staak voorkomings-vlugbeheer substelsel wat maklik geïntegreer kan word in bestaande vlugbeheer argitektuur van onbemande vaste-vlerk lugvaartuie. Hierdie tesis vorm ’n belangrike deel van fouttolerante vlugbeheertegnieke en sal verseker dat die vliegtuig slegs binne sy lineêre aerodinamiese werksgebied bly. Die fokus van hierdie tesis is die staak opsporing en voorkomings probleem. Na afloop van ’n deeglike literatuurstudie oor die onderwerp van staak, is ’n model gebaseerde staak voorkomings-beheertegniek ontwikkel, wat terugvoer van ’n invalshoek sensor ontvang. Hierdie algoritme neem die sleur tempo en defleksie limiete van die vliegtuig se servos in ag en is in staat om staak te voorspel. Die primêre oorweging was staak tydens simmetriese vlugte en behels slegs die voorkoming van staak deur gebruik te maak van die hei beheer oppervlak. ’n Model voorspellings sleur tempo beheeralgoritme is ontwikkel om die hei-roer dinamies te beperk sodat die invalshoek nie ’n sekere vooraf bepaalde limiet oorskry nie. Die staak voorkomings beheerstelsel is ontwerp om te funksioneer as ’n skakel beheer skema om die beperkings op die bestaande vlugbeheerstelsel te minimaliseer. Laastens was sagteware-in-die-lus simulasies gebruik om die teoretiese resultate, wat verkry is tydens die ontwikkeling van hierdie staak voorkomings beheer-strategie, te kontroleer. Om die robuusthied van hierdie beheeralgoritmes teen model onsekerhede te ondersoek, is ’n ergste-geval prestasie analise ook uitgevoer.

In eubacteria, ribosome stalling on broken messenger RNA transcripts can lead to cell death. The trans-translation quality control mechanism rescues many of these stalled ribosomes. In this process, tmRNA enters stalled ribosomes by mimicking a transfer RNA, accepting the stalled nascent peptide. The ribosome then releases the broken mRNA and resumes translation on a coding region within tmRNA itself. Translation of tmRNA marks the nascent peptide for destruction by the addition of a short proteolysis tag and the ribosome is released at a stop codon within the tmRNA open reading frame. An intriguing aspect of trans-translation is that the ribosome synthesizes one protein from two RNA templates. How is the proper site chosen on tmRNA to resume translation? Do the conserved pseudoknot structures help set the reading frame? Using a genetic selection to assay libraries of tmRNA mutants, we found that stable hairpin structures can functionally replace pseudoknot 1. We conclude that the role of pseudoknot 1 in tmRNA function is purely structural. Our results demonstrate that the inactivity of an RNA mutant designed to destroy a given structure should not be interpreted as proof that the structure is necessary for RNA function. Such mutations may only destabilize a global fold that could be formed equally well by an entirely different, stable structure. Broken mRNAs are not the only cause of ribosome stalling; stalling can also result from nascent peptide interactions with the ribosomal exit tunnel that inhibit peptidyl-transferase activity. SecM, TnaC, and ErmCL all stall ribosomes to regulate the expression of downstream genes. What other peptide sequences can cause ribosome stalling? We modified our tmRNA-based selection to screen libraries of random peptides and identified a number of novel stalling peptides, including the sequence FxxYxIWPP. This sequence interacts with the exit tunnel differently than SecM and TnaC as seen in studies using mutant ribosomes. Like SecM, stalling occurs on this sequence with the next aminoacyl tRNA trapped in the A site but unable to react with the nascent peptide. These results show that a variety of peptides can interact in the exit tunnel and peptidyl-transferase center to regulate ribosome activity.

Unsteady flow separation represents a highly complex and important area of study within fluid mechanics. The extent of separation and specific time scales over which it occurs are not fully understood and has significant consequences in numerous industrial applications such as helicopters, jet engines, hydroelectric turbines and wind turbines. A direct consequence of unsteady separation is the erratic movement of the separation point which causes highly dynamic and unpredictable loads on an airfoil. Current computational models underestimate the aerodynamic loads due to the inaccurate prediction of the emergence and severity of unsteady flow separation especially in response to a sudden change in the effective angle of attack. To capture the complex flow phenomena over wind turbine blades during stall development, a scaled three-dimensional non-rotating blade model is designed to be dynamically similar to a rotating full-scale NREL 5MW wind turbine blade. A time-resolved particle image velocimetry (PIV) investigation of flow behavior during the stall cycle examines the processes of stall development and flow reattachment. The flow fields are examined through the application of Eulerian techniques such as proper orthogonal decomposition and empirical mode decomposition to capture unsteady separation characteristics within the flow field. Then, for a higher order description, coherent structures such as vortices and material lines are resolved to fully characterize the flow during a full pitching cycle in a Lagrangian framework. The Eulerian and Lagrangian methods described in the present analysis is extended to investigate the spanwise characteristics within the root section of a three dimensional airfoil. Furthermore, statistical information of the separation point is pursued along four spanwise positions during two cases of unsteady separation. The results of the study describe a critical role of surface vorticity accumulation in unsteady separation and reattachment. Evaluation of the unsteady characteristics of the shear layer reveal evidence that the build-up and shedding of surface vorticity directly influence the dynamic changes in separation point. The quantitative characterization of surface vorticity and shear layer stability enables improved aerodynamic design, but also has broader implications on the larger discipline of unsteady fluid dynamics.

Thesis (Ph.D)--Aerospace Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Dr. Lakshmi N. Sankar; Committee Member: Dr. Alex Stein; Committee Member: Dr. J.V. R. Prasad; Committee Member: Dr. Richard Gaeta; Committee Member: Dr. Suresh Menon. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The studies reported in this thesis describe the work done in the area of translation initiation where a previously unknown role of multiple copies of initiator tRNA in E. coli has been reported. Also the role of SsrA resume codon in resumption of translation, until not clearly known has been reported here. Chapter -1 discusses the relevant literature in understanding translation and initiator tRNA selection on the ribosome during initiation. It also discusses the literature pertaining to the aspect of release of stalled ribosomal complexes by SsrA. This is followed by the next chapter (chapter- 2) which discusses the materials and methods used throughout the study. Chapter- 3 describes the studies leading to the role of multiple copies of initiator tRNA in E. coli in governing the fidelity of initiator tRNA selection on the P site of the ribosome. This is followed by Chapter-4 which describes the role of the resume codon of the SsrA in governing the efficiency of trans-translation in releasing the stalled ribosomal complexes. The summaries of the chapters 3 and chapter 4 are briefly described below. i) Role of conserved 3GC base pairs of initiator tRNA in the initiator-elongator tRNA discrimination. Translation initiation is the first step in the very important and highly conserved biological process of protein biosynthesis. The process involves many steps, a wide array of protein factors at each specialized step and a large ribonucleoprotein particle; the ribosome to decode the information of the mRNA template into biologically active proteins. The process of initiation is still unclear largely due to fewer reports of available structural data. One of the very interesting questions that people have been trying to address is how the initiator tRNA is selected on the P- site of the ribosome and what is the importance of the conserved three GC base pairs in the anticodon stem of the initiator tRNA. Here in this study, I have studied this question by using the classical genetic technique of generating and characterizing the mutant initiator tRNA defective at the step of initiation. I have identified and analyzed the suppressors which are capable of rescuing this defect in initiation. The study involves two such E. coli suppressor strains (named D4 and D27). These suppressors can initiate translation from a reporter CAT mRNA with amber codon, independent of the presence of the three consecutive GC base pairs in the anticodon stem of initiator tRNAs. Mapping of the mutations revealed that the mutants are defective in expression of the tRNA1fMet (metZVW) gene locus which encodes the initiator tRNA. Both the suppressors (D4 and D27) also allow initiation with elongator tRNA species in E. coli. Taken together, the results show that E. coli when deficient in the initiator tRNA concentration can lead to initiation with elongator tRNA species. ii) The Role of SsrA/tmRNA in ribosome recycling and rescue. Occasionally during the process of translation, the ribosomes stall on the mRNA before the polypeptide synthesis is complete. This situation is detrimental to the organism because of the sequestration of the tRNAs as ‘peptidyl tRNAs’ and the ribosomes. In E. coli one of the pathways to rescue stalled ribosomes involves disassembly of these stalled complexes to release peptidyl tRNAs which are then recycled by peptidyl tRNA hydrolase (Pth), an essiential enzyme in E. coli. The other pathway which is not essential in E. coli but is conserved in all prokaryotes involves SsrA or tmRNA (transfer messenger RNA). The tmRNA is charged with alanine and recognizes the stalled ribosomal complexes and acts as tRNA to bind the A-site. It also functions as mRNA by adding a undecapeptide (which is actually a tag for degradation by cellular proteases) to the existing polypeptide and there is normal resumption of the translation. In most sequences of SsrA ORF, the first codon of the ORF, called as resume codon, is conserved. I wanted to understand the importance of the conservation of the resume codon. Towards this end I randomly mutated the resume codon and studied the effect of the altered resume codon in the rescue of stalled ribosomal complexes. The effect of over-expression of these mutants was investigated in the rescue of the Pthts defect since it is known that the overexpression of SsrA rescues the temperature sensitive phenotype of the Pthts strain and so causes less accumulation of peptidyl–tRNA in E. coli .The effect for these mutants has also been studied by the growth of hybrid λimmP22 phages. I also used AGA minigene system to study the effect of various mutants which has been shown to sequester tRNAArg (UCU) in the ribosomal P-site, translation of this minigene causes toxicity to E. coli. I have tried to study the effect of the SsrA mutants in rescue of toxicity caused by the minigene. Overall, the observations indicate that the conservation of the resume codon is important in E. coli and having mutated resume codon probably leads to deficient trans-translation during one or the other growth conditions.

The dynamic response of the compression system is a key factor in determining the operability characteristics of an aircraft gas turbine engine subjected to various transient environmental and control inputs. Computer models have been developed to simulate this response. The primary inputs to these models are the wide-range, steady-state compressor stage characteristics. To reduce the dependence of these dynamic models on experimental performance data, significant effort has been devoted to the development of stage characteristic prediction techniques. As part of this ongoing effort, a model to simulate rotating stall inception and development in axial-flow compressor stages was constructed. This model was applied to an isolated rotor build to investigate the sensitivity of the predicted stall behavior to the shape of the high-incidence portions of the blading relative total pressure loss and turning angle characteristics, as well as to the rotor speed. In addition, the predicted steady-state, stalled rotor performance was compared with corresponding low-speed, experimental data. By superimposing small flow perturbations on the rotor flow field over a range of initial operating conditions, it was demonstrated that stall inception occurs only for initial relative flow incidence near some critical value, defined as the incidence for which the relative total pressure losses incurred in the blade passage increase sharply. For initial operating points away from the critical one, no propagating disturbance was predicted. Also, a strong sensitivity of the predicted stall behavior to the shape of the high-incidence portion of the relative total pressure loss characteristic was observed with increased-slope curves resulting in earlier stall inception and larger amplitude stall disturbances. The effect of increased-slope loss curves on the predicted steady-state rotor performance was to cause a more abrupt drop in the flow and total pressure rise coefficients at the stall limit. Comparatively, varying the shape of the turning angle characteristic or the rotor speed had only a slight effect on the simulated rotating stall phenomena. Finally, the predicted install total pressure characteristic for a selected low-speed case was compared with experimental data with favorable results. / Master of Science

Flow visualization movies of flow through a cascade of compressor blades showed propagating stall at stagger angles of 36.5 and 45 degrees for angles of attack of 20 degrees and higher. At a stagger angle of 25 degrees, the development of a steady, separated boundary layer occurred with no propagation. The observed propagating stall process was the development of a vortex in the boundary layer and its subsequent shedding. The shedding mechanism was observed to be the interference by the reverse flow from the previously stalled passage with the vortex flow in the stalled passage. This dissipated the vortex in the blade passage and the incoming flow then flushed the stagnated vortex out of the passage. Measurements of propagation speeds showed that the propagation speed is related to the blockage of the passage, that stagger angle has an insignificant effect on propagation speed, and that propagation speed is proportional to the relative velocity. Based on the observations, a geometric model was developed to predict the onset of propagating stall. This model showed that increased solidity, decreased stagger angles, and operation at low angles of attack make a cascade more resistant to propagating stall inception. The model shows the relation of the operating point of a compressor to the stall inception point. When expanded to include all significant aspects of blade geometry, the model may provide a basis for controlling propagating, and hence, rotating, stall inception based on the blade row·geometry. / Master of Science

Separated flows about single and multi-element airfoils are featured in many scenarios of practical interest, including: stall of fixed wing aircraft, dynamic stall of rotorcraft blades, and stall of compressor and turbine elements within jet engines. In each case, static and/or dynamic stall can lead to losses in performance. More importantly, modeling and analysis tools for stalled flows are relatively poorly evolved and designs must completely avoid stall due to a lack of understanding. The underlying argument is that advancements are necessary to facilitate understanding of and applications involving static and dynamic stall. The state-of-the-art in modeling stall involves numerical solutions to the governing equations of fluids. These tools often either lack fidelity or are prohibitively expensive. Ever-increasing computational power will likely lead to increased application of numerical solutions. The focus of this thesis is improvements in numerical modeling of stall, the need of which arises from poorly evolved analysis tools and the spread of numerical approaches. Technical barriers have included ensuring unsteady flow field and vorticity reproduction, transition modeling, non-linear effects such as viscosity, and convergence of predictions. Contributions to static and dynamic stall analysis have been been made. A hybrid Reynolds-Averaged Navier-Stokes/Large-Eddy-Simulation turbulence technique was demonstrated to predict the unsteadiness and acoustics within a cavity with accuracy approaching Large-Eddy-Simulation. Practices to model separated flows were developed and applied to stalled airfoils. Convergence was characterized to allow computational resources to be focused only as needed. Techniques were established for estimation of integrated coefficients, onset of stall, and reattachment from unconverged data. Separation and stall onset were governed by turbulent transport, while the location of reattachment depended on the mean flow. Application of these methodologies to oscillating flapped airfoils revealed flow through the gap was dominated by the flap angle for low angles of attack. Lag between the aerodynamic response and input flap scheduling was associated with increased oscillation frequency and airfoil/flap gap size. Massively separated flow structures were also examined.

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