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 COMPARATIVE ANALYSIS OF MINIATURE INTERNAL COMBUSTION ENGINE AND ELECTRIC MOTOR FOR UAV PROPULSION(2017) Chiclana, Branden; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis compares the performance of an engine/fuel tank based propulsion system to a motor/battery based propulsion system of equal total mass. The results show that the endurance of the engine/fuel system at the same thrust output is approximately 5 times greater than that of the motor/battery system. This is a direct result of the fact that the specific energy of the fuel is 20 times that of the lithium-polymer batteries used to power the motor. A method is also developed to account for the additional benefits of fuel consumption (and hence weight reduction) over the course of the flight. Accounting for this effect can increase endurance exponentially. Taken together, the results also demonstrate the dramatic performance improvements that are possible simply by replacing motor/battery systems with engine/fuel systems on small unmanned air vehicles.Item Photogrammetric Reconstruction of Tandem-Wing Kinematics for Free-Flying Dragonflies Undergoing a Range of Flight Maneuvers(2017) Gabryszuk, Mateusz; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Photogrammetric methods are used to reconstruct the body and wing kinematics of free-flying dragonflies. A novel experimental setup was designed and constructed to allow for repeated untethered flights in a constrained flight arena. Kinematic data are presented for twelve individual flights and a total of 23 complete wing strokes, including unaccelerating, accelerating, climbing, and turning flight. High variability is observed in the wing motions employed by individual dragonflies, particularly in terms of stroke amplitude, pitch angle, and wingbeat frequency. Forewing and hindwing flapping is found to be neither in phase nor fully out of phase across all cases, with the forewings lagging the hindwings by an average of 90 degrees. Downstroke durations are observed to be shorter than upstroke durations except in highly accelerating flights. Migratory dragonflies are found to exhibit notably different wing kinematics than non-migratory species.Item RESILIENT STATE ESTIMATION FOR MICRO AIR VEHICLES UNDER SENSOR ATTACKS(2017) Prasad, Akshay; Chopra, Nikhil; Systems Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis proposes a solution to the problem of resilient state estimation and sensor fusion in an autonomous micro air vehicle. The setup comprises of redundant sensors that measure the same physical signal. An adversary may spoof a subset of these sensors and send falsified readings to the controller, potentially compromising performance and safety of the system. This work integrates Brooks-Iyengar Sensor fusion algorithm with a generic state estimator as a method to thwart sensor attacks. The algorithm outputs a point estimate and a fusion interval based on an assumed set of faulty sensors. Finally, the thesis illustrates the usefulness of the resilient state estimator with a case study on a MAV flight dataset.Item DESIGN, FABRICATION, AND PERFORMANCE CHARACTERIZATION OF MULTIFUNCTIONAL STRUCTURES TO HARVEST SOLAR ENERGY FOR FLAPPING WING AERIAL VEHICLES(2016) Perez-Rosado, Ariel; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flapping Wing Aerial Vehicles (FWAVs) have the capability to combine the benefits of both fixed wing vehicles and rotary vehicles. However, flight time is limited due to limited on-board energy storage capacity. For most Unmanned Aerial Vehicle (UAV) operators, frequent recharging of the batteries is not ideal due to lack of nearby electrical outlets. This imposes serious limitations on FWAV flights. The approach taken to extend the flight time of UAVs was to integrate photovoltaic solar cells onto different structures of the vehicle to harvest and use energy from the sun. Integration of the solar cells can greatly improve the energy capacity of an UAV; however, this integration does effect the performance of the UAV and especially FWAVs. The integration of solar cells affects the ability of the vehicle to produce the aerodynamic forces necessary to maintain flight. This PhD dissertation characterizes the effects of solar cell integration on the performance of a FWAV. Robo Raven, a recently developed FWAV, is used as the platform for this work. An additive manufacturing technique was developed to integrate photovoltaic solar cells into the wing and tail structures of the vehicle. An approach to characterizing the effects of solar cell integration to the wings, tail, and body of the UAV is also described. This approach includes measurement of aerodynamic forces generated by the vehicle and measurements of the wing shape during the flapping cycle using Digital Image Correlation. Various changes to wing, body, and tail design are investigated and changes in performance for each design are measured. The electrical performance from the solar cells is also characterized. A new multifunctional performance model was formulated that describes how integration of solar cells influences the flight performance. Aerodynamic models were developed to describe effects of solar cell integration force production and performance of the FWAV. Thus, performance changes can be predicted depending on changes in design. Sensing capabilities of the solar cells were also discovered and correlated to the deformation of the wing. This demonstrated that the solar cells were capable of: (1) Lightweight and flexible structure to generate aerodynamic forces, (2) Energy harvesting to extend operational time and autonomy, (3) Sensing of an aerodynamic force associated with wing deformation. Finally, different flexible photovoltaic materials with higher efficiencies are investigated, which enable the multifunctional wings to provide enough solar power to keep the FWAV aloft without batteries as long as there is enough sunlight to power the vehicle.Item EXPERIMENTAL INVESTIGATION OF A QUAD-ROTOR BIPLANE MICRO AIR VEHICLE(2015) Bogdanowicz, Christopher Michael; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Micro air vehicles are expected to perform demanding missions requiring efficient operation in both hover and forward flight. This thesis discusses the development of a hybrid air vehicle which seamlessly combines both flight capabilities: hover and high-speed forward flight. It is the quad-rotor biplane, which weighs 240 grams and consists of four propellers with wings arranged in a biplane configuration. The performance of the vehicle system was investigated in conditions representative of flight through a series of wind tunnel experiments. These studies provided an understanding of propeller-wing interaction effects and system trim analysis. This showed that the maximum speed of 11 m/s and a cruise speed of 4 m/s were achievable and that the cruise power is approximately one-third of the hover power. Free flight testing of the vehicle successfully highlighted its ability to achieve equilibrium transition flight. Key design parameters were experimentally investigated to understand their effect on overall performance. It was found that a trade-off between efficiency and compactness affects the final choice of the design. Design improvements have allowed for decreases in vehicle weight and ground footprint, while increasing structural soundness. Numerous vehicle designs, models, and flight tests have proven system scalability as well as versatility, including an upscaled model to be utilized in an extensive commercial package delivery system. Overall, the quad-rotor biplane is proven to be an efficient and effective multi-role vehicle.Item AN INVESTIGATION OF A MAV-SCALE FLEXIBLE FLAPPING WING IN FORWARD FLIGHT: FLOW FIELD AND AIRLOADS EXPERIMENTS WITH COUPLED CFD-CSD(2014) Mayo, David Benjamin; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experiments were systematically executed with a coupled computational fluid/structural dynamics aeroelastic analysis for a micro air vehicle scale flexible flapping-wing undergoing pure flap wing kinematics in forward flight. 2-D time-resolved particle image velocimetry and force measurements were performed in a wind tunnel where the free stream velocity was set to 3 m/s, Re = 15,000. Chordwise velocity fields were obtained at equally spaced spanwise sections along the wing (30% to 90% span) at the mid-downstroke and mid-upstroke of the flap cycle. The flowfield measurements and averaged force measurements were used for the validation of the 3-D aeroelastic model. The computational fluid/structural dynamics analysis combined a compressible Reynolds Averaged Navier Stokes solver with a multi-body structural solver. The objective of the combined efforts was to understand the force production of a flexible wing undergoing an avian-type flapping motion. The temporal and spanwise variation of wing pitch angle affected lift and drag, and primarily aided in producing positive thrust during both upstroke and downstroke.Item Unsteady Low Reynolds Number Aerodynamics of a Rotating Wing(2012) Kolluru Venkata, Siddarth; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Micro air vehicles (MAVs) are small, unmanned aircraft useful for reconnaissance. The small size of MAVs presents unique challenges as they operate at low Reynolds numbers O(10^4), and they share a flight regime with insects rather than conventional aircraft. The low Reynolds number regime is dominated by poor aerodynamic characteristics such as low lift-to-drag ratios. To overcome this, birds and insects utilize unsteady high lift mechanisms to generate sufficient lift. A leading edge vortex (LEV), one of these unsteady lift mechanism, is thought to be responsible for the high lift generated by these natural fliers, but the factors which contribute to the formation, stability, and persistence of LEVs are still unclear. The objectives of this study are to: 1) qualitatively understand the formation of the LEV by evaluating the effect of wing acceleration profiles, wing root geometry, Reynolds number, and unsteady variations of pitch, 2) quantify whether high lift can be sustained at low Reynolds numbers on a rotary wing in continuous revolution, and 3) determine the effect of wing flexibility on the unsteady lift coefficient. Experiments were performed on a rotating wing setup designed to model the translational phase of the insect wing stroke during hover. Experiments were performed in a water tank at Reynolds numbers between 5,000 and 25,000, and the flow was investigated using dye flow visualization, as well as lift and drag force measurements. A rigid wing and a simple one degree-of-freedom flexible wing were tested. Dye flow visualization on a rotating wing showed the formation of a coherent LEV near the wing root. The LEV became less coherent further outboard, and eventually burst. As the wing continued to rotate, the location where the LEV burst moved inboard. Dye injection within the burst vortex showed the formation of multiple small scale shedding vortices that traveled downstream and formed a region of recirculating flow (i.e., a burst vortex). Parameter variations in this experiment included velocity profiles, acceleration profiles, and Reynolds numbers. High lift and drag coefficient peaks were measured during the acceleration phase of the wing stroke at Reynolds numbers of 15,000 and 25,000. After the initial peak, the coefficients dropped, increased, and eventually attained a ``steady-state" intermediate value after 5 chord-lengths of travel, which they maintained for the remainder of the first revolution. When the wing began the second revolution, both the lift and drag coefficients decreased, and leveled out at a second intermediate value. Force measurements on a chordwise flexible wing revealed lower lift coefficients. For all of the cases tested, high lift was achieved during the acceleration phase and first revolution of the wing stroke, though values dropped during the second revolution.Item Open Loop System Identificaiton of a Micro Quadrotor Helicopter from Closed Loop Data(2011) Miller, Derek; Humbert, James S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quadrotors are a favorite platform amongst academic researchers. Yet there is limited work present on the identification of Linear Time Invariant (LTI) state space models for quadrotors. This thesis focuses on the development of a 70 gram quadrotor with a maximum dimension of less than 6 inches. An MAV this small is extremely agile and in this case, dynamically unstable. Therefore it requires an active control scheme to stabilize the vehicle's attitude. To develop an effective controller using the vast array of available linear control tools, model knowledge is required. The scope of this thesis is to identify an LTI state space model of the 70 gram quadrotor mentioned about hover. Because the quadrotor is unstable, it cannot be flown without some form of feedback control present. To determine the bare airframe dynamics of a closed loop system, the feedback gains must be turned down as low as possible so as to not distort the natural response. Then the bare airframe dynamics can be calculated from the identified closed loop dynamics if the feedback gains and control law are known. Using the derived open loop system this thesis presents options for controller development for both a stabilizing inner loop, and a station keeping outer loop controller.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 Biomimetic Insect Flapping Aerodynamics and Controls for Micro Air Vehicles(2011) Seshadri, Pranay; Chopra, Inderjit; Benedict, Moble; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of the current research is to develop a hover capable, fully autonomous, flapping wing micro air vehicle with a thorough understanding of the associated aerodynamics and aeroelasticity. The approach followed in this work is to look at this problem from three different perspectives: 1) Instantaneous Rigid Wing Aerodynamics, 2) Time Averaged Flexible Wing Aerodynamics, and 3) Vehicle Integration and Control. Unlike prior studies that have focused on one of these aspects, this study will encompass each aspect using a different methodology. The commonality between these three elements in this study is the flapping mechanism. At the core of this work is a simplified, elegant flapping actuation system that is capable of emulating insect wing kinematics in all its degrees of freedom. The mechanism is shown to be novel compared to existing flapping mechanisms and is easily scalable.