UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    Urban Air Mobility: Effects of increasing three-dimensionality on fixed and rotary wings in unsteady aerodynamic environments
    (2024) Wild, Oliver Dominik; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The rapidly growing field of electric vertical takeoff and landing aircraft, air taxis, and urban air mobility vehicles promises transformative solutions to alleviate urban congestion, accelerate deliveries, and revolutionize transportation systems. Central to the successful integration of these futuristic modes of transportation is a comprehensive understanding of their aerodynamics, particularly in the context of unsteady airflow encountered in urban environments. This work explores the foundational aspects essential for achieving efficient and safe urban air mobility operation. The focus lies on the integration of rotary and translatory wings in gusty and unsteady flow environments since – unlike conventional fixed-wing aircraft – many urban air vehicles utilize rotor systems for both vertical takeoff and forward flight. The research framework is structured around three interconnected pillars: advancing rotary wings, fixed-wing-gust encounters, and the synthesis of rotary wings in gusty conditions. The combined results from these three pillars are fundamental in reaching the future goal of efficient and safe urban air mobility. The first pillar investigates the aerodynamic characteristics of advancing rotary wings, particularly concerning flow structures, blade loading, and the influence of the trailing edge geometry using experimental, numerical, and modeling techniques. A comparison between a standard NACA0012 airfoil profile and an elliptical profile is conducted at advance ratios ranging from 0.00 to 1.00 at pitch angles from 7 deg to 25 deg. Four main vortex structures were detected in reverse flow. At the aerodynamic leading edge, a strong interference of the tip vortex with the reverse flow dynamic stall vortex was identified when blade flapping was restricted. Dynamic stall vortices advect closer to the blade surface for the blunt elliptical airfoil, thus reducing the wake area in reverse flow. Overall, the vortex structures that form on the ellipse are more coherent than those on the NACA0012. A 29% pitching moment increase was measured in the reverse flow region with sharp trailing-edged blades compared to blunt blades. The blunt trailing-edged blade delayed flow separation and thus prevented the formation of a reverse flow dynamic stall vortex, reducing the pitching moment. The second pillar delves into the three-dimensional dynamics of fixed-wing-gust encounters, aiming to understand the formation of leading-edge vortices and their impact on lift generation. Emphasis is placed on exploring strong transverse gust encounters and the effects of sideslip angle on leading edge vortex formation, with the objective of devising predictive models for lift generation under varied gust scenarios. Experimental investigations in a towing tank and the employment of a strip theory Küssner model show a peak lift coefficient decrease with decreasing gust ratios and increasing sideslip angles. The model accurately predicts the experimental results at gust entry as well as within the gust. Flow reattachment is delayed due to the formation of a leading-edge vortex inducing reverse flow on the wing suction side, resulting in a non-zero wing forcing at gust exit. The third pillar examines the effects of gusts on both hovering and advancing rotors. It synthesizes the findings from the previous two pillars, mirroring real-world conditions occurring on urban air mobility vehicles. Gusts cause an increase in blade flapping and lagging moments, and a nose-down pitching moment in both hovering and advancing rotors. In forward flight, the moment response mirrors a wing-gust encounter. A lower advance ratio broadens the moment peaks. Reverse flow shows a smaller moment response but a wider azimuth angle impact. Increased gust and advance ratios amplify moment disturbances, with gust encounters on the retreating blade more sensitive to gust ratio changes. By integrating insights from rotary wings and gust encounters, this research provides a comprehensive understanding of aerodynamic phenomena crucial for the development of efficient and safe urban flight vehicles. Through this multidisciplinary approach, this thesis contributes to advancing the fundamental understanding of aerodynamic challenges in urban air mobility, paving the way for the development of innovative solutions to propel the future of urban air mobility.
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    ROBUST CONTROL OF AN EVTOL THROUGH TRANSITION WITH A GAIN SCHEDULING LQR CONTROLLER
    (2020) Thompson, Derek; Xu, Huan; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The advancements in electric motor propulsion and battery technologies have made the implementation of electric power in aerial transportation increasingly feasible. As such, the interest and development of electric vertical takeoff and landing (eVTOL) aircraft has become a greater portion of the market. This increase drives a need for research into control of the eVTOLs to ensure safe flight through the transition from hover to forward flight. This paper proposes a control strategy using the transition dynamics in a gain scheduling LQR attitude controller to robustly control the vehicle at any point throughout transition. The proposed control strategy is tested through implementation in nonlinear 3DOF and 6DOF simulations. The robustness of the controller is tested through simulating transition and virtual mission profiles.
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    An Experimental and Analytical Investigation of Hydrogen Fuel Cells for Electric Vertical Take-Off and Landing (eVTOL) Aircraft
    (2019) Ng, Wanyi; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The objective of this thesis is a comprehensive investigation of hydrogen fuel cells for electric vertical take-off and landing (eVTOL) aircraft. The primary drawback of battery powered eVTOL aircraft is their poor range and endurance with practical payloads. This work uses simulation and hardware testing to examine the potential of hydrogen fuel cells to overcome this drawback. The thesis develops steady state and transient models of fuel cells and batteries, and validates the models experimentally. An equivalent circuit network model was able to capture the waveforms and magnitudes of voltage as a function of current. Temperature and humidity corrections were also included. Examination of the results revealed that the transient behavior of batteries and fuel stacks are significant primarily shortly after startup of the fuel stack and at the limiting ranges of high and low power; for a nominal operating power and barring faults, steady state models were adequate. This work then demonstrates fuel cell and battery power sharing in regulated and unregulated parallel configurations. It details the development of a regulated architecture, which controls power sharing, to achieve a reduction in power plant weight. Finally, the thesis outlines weight models of motors, batteries, and fuel cells needed for eVTOL sizing, and carries out sizing analysis for on-demand urban air taxi missions of three different distances -- 50, 75, and 150~mi of cruise and 5~min total hover time. This revealed that for ranges within 75 mi, a light weight (5000-6000~lb gross weight) all-electric tilting proprotor configuration achieves a practical payload (500~lb or more) with current levels of battery specific energy (150~Wh/kg) if high burst C-rate batteries are available (4-10~C for 2.5~min). Either a battery-only or battery-fuel cell (B-FC) hybrid power plant is ideal depending on the range of the mission: For inter-city ranges (beyond approximately 50~mi), the mission is impossible with batteries alone, and fuel cells are a key enabling technology; a VTOL aircraft with a B-FC hybrid powerplant, an aircraft with 6200~lb gross take-off weight, 10~lb/ft$^2$ disk loading, and 10~C batteries, could be sized to carry a payload of 500~lb for a range of 75~mi. For this inter-city range, the research priority centers of fuel cells, as they appear to far surpass future projections of Li-ion battery energy levels based on performance numbers (at a component level), high weight fraction of hydrogen storage due to the short duration of eVTOL missions, and lack of a compressor due to low-altitude missions, with the added benefit of ease of re-fueling. However, for an intra-city mission (within approximately 50~mi), the B-FC combination provides no advantage over a battery-only powerplant; a VTOL aircraft with a battery-only powerplant with the same weight and disk loading as before, and 4~C batteries, can carry a payload of 800~lb for a range of 50~mi. For this mission range, improving battery energy density is the priority.