Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Revolutionary Flight Vehicle Based on Leonardo da Vinci Aerial Screw: A Paradigm Shift in VTOL Technology(2022) Prete, Austin Christopher; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerial screws are rotors that consist of continuous large solidity single surface blades, which are able to provide significant thrust and control authority at increased power consumption when compared to traditional rotor blades. By leveraging a unique bound tip vortex, also observed in delta-wings, aerial screws are able to attain figure of merit values nearing 0.7 or higher, comparable to a modern rotorcraft. To prove the function of aerial screws, physical models were fabricated and flight tested. The primary objective of this paper is to explore the performance of a 6-in (0.152 m) diameter aerial screw and compare its performance with a 6-in (0.152 m) diameter traditional rotor, to demonstrate its feasibility in a quadrotor configuration and to show its efficiency as determined in the student designs from the 2019-2020 VFS student design competition.Item Development of Hybrid Air-Water Rotor Transition Thrust Prediction and Control(2020) Semenov, Ilya Yevgeniyevich; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid vehicles are able to function in some combination of aerial, underwater, and terrestrial environments, which greatly expands the scope of missions a vehicle can perform. Hybrid aerial-water (HAW) vehicles are a promising subcategory that are designed to operate in two vastly different fluid mediums. Multirotor HAW vehicles configurations have advantages in maneuverability, but pose a challenge in the water entry or water exit transitions. The interaction of a powered rotor with the air-water interface and its performance in a mixed air-water medium are poorly understood. Previous HAW vehicle strategies avoid a powered rotor with additional propulsion and buoyancy systems, constraining the design space. A custom test stand was constructed to better understand rotor performance during the air-water transition. By recording powered rotor performance during controlled water entries and exits in a large tank, several novel observations were made. Previously unrecorded phenomenon such as the gradual height and RPM dependent transition and the underwater ceiling effect are determined. These observations inform the development of the Transition Index TI, a novel metric that indicates the transition state of the rotor, without the need for specialized sensors or computationally intensive modeling. TI is applied to experimental data to make further observations, and is also used in a novel thrust prediction formulation. The first known low-order prediction of thrust through the transition is validated against experimental data, and allows for the development of a TI based controller. A preliminary controller implementation shows promising results in maintaining constant thrust through the air-water transition. Finally, a HAW vehicle to apply this controller is built. Careful consideration to the waterproofing and motor choice is shown and preliminary flight tests are demonstrated. Future expansion on the application of the novel TI and thrust prediction has great potential to advance the capabilities of hybrid aerial-water vehicles.Item Fundamental Understanding of Rotor Aeromechanics at High Advance Ratio Through Wind Tunnel Testing(2016) Berry, Benjamin Otto; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this research is to further the understanding of rotor aeromechanics at advance ratios (mu) beyond the maximum of 0.5 (ratio of forward airspeed to rotor tip speed) for conventional helicopters. High advance ratio rotors have applications in high speed compound helicopters. In addition to one or more conventional main rotors, these aircraft employ either thrust compounding (propellers), lift compounding (fixed-wings), or both. An articulated 4-bladed model rotor was constructed, instrumented, and tested up to a maximum advance ratio of mu=1.6 in the Glenn L. Martin Wind Tunnel at the University of Maryland. The data set includes steady and unsteady rotor hub forces and moments, blade structural loads, blade flapping angles, swashplate control angles, and unsteady blade pressures. A collective-thrust control reversal---where increasing collective pitch results in lower rotor thrust---was observed and is a unique phenomenon to the high advance ratio flight regime. The thrust reversal is explained in a physical manner as well as through an analytical formulation. The requirements for the occurrence of the thrust reversal are enumerated. The effects of rotor geometry design on the thrust reversal onset are explored through the formulation and compared to the measured data. Reverse-flow dynamic stall was observed to extend the the lifting capability of the edgewise rotor well beyond the expected static stall behavior of the airfoil sections. Through embedded unsteady blade surface pressure transducers, the normal force, pitching moment, and shed dynamic stall vortex time histories at a blade section in strong reverse flow were analyzed. Favorable comparisons with published 2-D pitching airfoil reverse flow dynamic stall data indicate that the 3-D stall environment can likely be predicted using models developed from such 2-D experiments. Vibratory hub loads were observed to increase with advance ratio. Maximum amplitude was observed near mu=1, with a reduction in vibratory loads at higher advance ratios. Blade load 4/rev harmonics dominated due to operation near a 4/rev fanplot crossing of the 2nd flap bending mode natural frequency. Oscillatory loads sharply increase in the presence of retreating blade reverse flow dynamic stall, and are evident in blade torsion, pitch link, and hub load measurements. The blades exhibited torsion moment vibrations at the frequency of the 1st torsion mode in response to the reverse flow pitching moment loading.Item Effects of Body Shapes on Rotor In-Ground-Effect Aerodynamics(2012) Hance, Benjamin Thomas; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Videographic flow visualization and two-component particle image velocimetry (PIV) measurements were performed to examine the developing vortical wake produced by a two-bladed hovering rotor at a height of one rotor radius above a horizontal ground plane. The experiments were performed with an isolated rotor and with three different bodies placed in the wake below the rotor. The bodies examined had circular, elliptical, and rectangular cross-sections, respectively. Flow measurements were taken in planes that covered the nose and tail region of each body. The objective of the study was to gain a better understanding of the nature of the flow at the ground plane and to assess the overall effects of a body in the rotor wake, and in particular to document the nature of the unsteady, turbulent boundary layer flow over the ground. The flow visualization and PIV were performed using a Nd:YAG laser that illuminated a radial plane of the flow, with imaging performed with a CCD camera. Measurements of the spatial locations of the tip vortices as a function of wake age were obtained to quantify the wake distortion produced by each body shape. The outward flow over the ground plane was shown to have similar characteristics to a classical turbulent wall-jet; these similarities were especially apparent further downstream on the ground plane away from the rotor. The results showed that the flow over the nose of each of the bodies was similar to that of the isolated rotor, but with some minor differences in the flow at the ground. The slipstream boundary was shown to be severely disrupted by the tail of each body, and showed larger variations from that produced by the isolated rotor. Wake impingement on the body was shown to cause catastrophic bursting of the blade tip vortices. The body with a rectangular cross-section was generally found to produce the greatest differences in the overall flow characteristics near the ground plane compared to that of the isolated rotor. The work has relevance to the better understanding the problem of rotorcraft brownout, where the near-wall flow drives the mobilization and uplift of dust.Item A Coupled CFD/CSD Investigation of the Effects of Leading Edge Slat on Rotor Performance(2012) Mishra, Asitav; Baeder, James D.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A coupled Computational Fluid Dynamic (CFD) and Computational Structural Dynamics (CSD) methodology is extended to analyze the effectiveness of a leading edge slat (LE-Slat) for mitigating the adverse effects of dynamic stall on rotor blade aerodynamic and dynamic response. This involved the following improvements over the existing CFD methodology to handle a multi-element airfoil rotor: incorporating the so-called Implicit Hole Cutting method for inter-mesh connectivity, implementing a generalized force transfer routine for transferring LE-Slat loads onto the main blade, and achieving increased parallelization of the code. Initially, the structured overset mesh CFD solver is extensively validated against available 2-D experimental wind tunnel test cases in steady and unsteady flight conditions. The solver predicts the measurements with sufficient accuracy for test cases with both the baseline airfoil and that with two slat configurations, S-1 and S-6. As expected, the addition of the slat is found to be highly effective in delaying stall until larger angles for the case of a static airfoil and ameliorating the effects of dynamic stall for a 2-D pitching airfoil. The 3-D coupled CFD/CSD model is extensively validated against flight test data of a UH-60A rotor in a high-altitude, high-thrust flight condition, namely C9017, characterized by distinct dynamic stall events in the retreating side of the rotor disk. The validated rotor analysis tool is then used to successfully demonstrate the effectiveness of a LE-Slat in mitigating (or eliminating) dynamic stall on the rotor retreating side. The calculations are performed with a modified UH-60A blade with a 40%-span slatted airfoil section. The addition of the slat is effective in the mitigation (and/or elimination) of lift and moment stall at outboard stations, which in turn is accompanied by a reduction of torsional structural loads (upto 73%) and pitch link loads (upto 62%) as compared to the baseline C9017 values. The effect of a dynamically moving slat, actuating between slat positions S-1 and S-6, is thoroughly investigated, firstly on 2-D airfoil dynamic stall, and then on the UH-60A rotor. Three slat actuation strategies with upto [1, 3, 5]/rev harmonics, respectively, are considered. However, it is noted that the dynamic slat does not necessarily result in better rotor performance as compared to a static slat configuration. The coupled CFD/CSD platform is further used to successfully demonstrate the capability of the slat (S-6) to achieve upto 10% higher thrust than C9017, which is beyond the conventional thrust limit imposed by McHugh's stall boundary. Stall mitigation due to the slat results in a reduction of torsional load upto 54% and reduction of pitch link load upto 32% as compared to the baseline C9017 flight test values, even for an increase in thrust of 10%.Item Experimental Investigation of Shrouded Rotor Micro Air Vehicle in Hover and in Edgewise Gusts(2011) Hrishikeshavan, Vikram; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to the hover capability of rotary wing Micro Air Vehicles (MAVs), it is of interest to improve their aerodynamic performance, and hence hover endurance (or payload capability). In this research, a shrouded rotor conguration is studied and implemented, that has the potential to oer two key operational benets: enhanced system thrust for a given input power, and improved structural rigidity and crashworthiness of an MAV platform. The main challenges involved in realising such a system for a lightweight craft are: design of a lightweight and stiff shroud, and increased sensitivity to external flow disturbances that can affect flight stability. These key aspects are addressed and studied in order to assess the capability of the shrouded rotor as a platform of choice for MAV applications. A fully functional shrouded rotor vehicle (disk loading 60 N/m2<\super>) was designed and constructed with key shroud design variables derived from previous studies on micro shrouded rotors. The vehicle weighed about 280 g (244 mm rotor diameter). The shrouded rotor had a 30% increase in power loading in hover compared to an unshrouded rotor. Due to the stiff, lightweight shroud construction, a net payload benefit of 20-30 g was achieved. The different components such as the rotor, stabilizer bar, yaw control vanes and the shroud were systematically studied for system efficiency and overall aerodynamic improvements. Analysis of the data showed that the chosen shroud dimensions was close to optimum for a design payload of 250 g. Risk reduction prototypes were built to sequentially arrive at the nal conguration. In order to prevent periodic oscillations in flight, a hingeless rotor was incorporated in the shroud. The vehicle was successfully flight tested in hover with a proportional-integral-derivative feedback controller. A flybarless rotor was incorporated for efficiency and control moment improvements. Time domain system identification of the attitude dynamics of the flybar and flybarless rotor vehicle was conducted about hover. Controllability metrics were extracted based on controllability gramian treatment for the flybar and flybarless rotor. In edgewise gusts, the shrouded rotor generated up to 3 times greater pitching moment and 80% greater drag than an equivalent unshrouded rotor. In order to improve gust tolerance and control moments, rotor design optimizations were made by varying solidity, collective, operating RPM and planform. A rectangular planform rotor at a collective of 18 deg was seen to offer the highest control moments. The shrouded rotor produced 100% higher control moments due to pressure asymmetry arising from cyclic control of the rotor. It was seen that the control margin of the shrouded rotor increased as the disk loading increased, which is however deleterious in terms of hover performance. This is an important trade-off that needs to be considered. The flight performance of the vehicle in terms of edgewise gust disturbance rejection was tested in a series of bench top and free flight tests. A standard table fan and an open jet wind tunnel setup was used for bench top setup. The shrouded rotor had an edgewise gust tolerance of about 3 m/s while the unshrouded rotor could tolerate edgewise gusts greater than 5 m/s. Free flight tests on the vehicle, using VICON for position feedback control, indicated the capability of the vehicle to recover from gust impulse inputs from a pedestal fan at low gust values (up to 3 m/s).Item Optimal Propulsion System Design for a Micro Quad Rotor(2011) Harrington, Aaron Michael; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Currently a 50 gram micro quad rotor vehicle is being developed in collaboration with Daedalus Flight Systems. Optimization of the design at this scale requires a systematic study to be carried out to investigate the factors that affect the vehicles performance. Endurance of hovering vehicles at this scale is severely limited by the low efficiencies of their propulsion systems and rotor design and optimization has been performed in the past in an attempt to increase endurance, but proper coupling of the rotor with the motor has been lacking. The current study chose to investigate the factors that had the greatest effect on the vehicle's endurance through analysis of the propulsion system. Therefore, a coupled aerodynamic and structural analysis was carried out that incorporated low Reynolds number airfoil table lookup in order to predict micro rotor performance. A parametric study on rotor design was performed further determine the effect of different rotor designs on hover performance. The experiments performed showed that airfoil camber had the biggest impact on rotor efficiency and other factors such as leading edge shape, number of blades, max camber location, and blade planform taper only had negligible influence on performance. Systematic studies of the interactions between micro rotor blades operating in close proximity to each other were performed in order to determine the changes in rotor efficiency that might occur in a compact quad rotor design. Tests done on the effect of rotor separation demonstrated that there is a negligible interaction between rotors operating near each other. Brushless motors were also tested systematically and characterized by their torque, rpm, and efficiency. It was found that the maximum efficiency of the motors tested was only 60%, which has significant effects on the efficiency of the coupled system. A method for rotor and motor coupling was also established that utilized the motor efficiency curves and the known torque and rotational speed of the rotors at their operating thrust. Through this, it was found that propulsion system efficiency could be increased by 10% by simply using the proper motor and rotor combination. Further, coupled design would have additional benefits and could increase vehicle efficiency further.