A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Aeroacoustic Analysis of Asymmetric Lift-Offset Helicopter in Forward Flight(2021) Arias, Paulo; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years, the University of Maryland has worked on an asymmetric lift-offset compound helicopter. The configuration consists of a single main rotor helicopter with the addition of two key ways to increase the forward speed: a stubbed wing on the retreating fuselage side, and a slowed down rotor. Experiments and simulations have shown that the novel concept provides improved thrust potential and lift-to-drag ratios in high-speed forward flight. This study aims to determine whether the asymmetric lift-offset configuration also provides aeroacoustic benefits in forward flight in addition to its aerodynamic advantages. The aerodynamic results from previous computational and experimental studies are recreated using the Mercury framework, in-house Computational Fluid Dynamics solver based on Reynolds-Averaged Navier-Stokes (RANS) coupled to a comprehensive rotor analysis for structural deformations and trim. The acoustic analysis is performed using an acoustic code based on the Ffowcs William-Hawkings equation to solve for the tonal noise propagating from the surfaces of the aircraft. The BPM model is used for broadband noise prediction. It was found that for an advance ratio of 0.5 the wing-lift offset configuration can produce 56.8% more thrust at the same collective angle without any penalties in total noise. When the configurations produce equal thrust it was found that the wing-lift offset case has a 4 dB reduction in maximum overall sound pressure level. At an advance ratio of 0.3 with trim for equivalentthrust between configurations, a 3 dB maximum OASPL reduction was obtained with the inclusion of the wing. The rotor of the wing-lift offset case was also slowed down while maintaining equal thrust to find a 6 dB reduction at an advance ratio of 0.55. Blade flap and lag bending moments near the root were also significantly reduced for the wing-lift offset configuration with equal thrust.Item FUNDAMENTAL UNDERSTANDING OF HELICOPTER AEROMECHANICS ON MARS THROUGH CHAMBER TESTING AND HIGH-FIDELITY ANALYSIS(2020) Escobar, Daniel; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fundamental aeromechanics of rotary-wing flight on Mars is explored. The exploration is based on chamber testing of Mars-like low Reynolds number rotors and the development of comprehensive analysis and comprehensive analysis coupled with computational fluid dynamics for systematic investigation of aeromechanical phenomena--critical for weights and packaging for Mars. The investigation includes rotor airloads, structural loads, and control loads, comparison of hingeless and articulated hubs, hover and forward flight, and the impact of fuselage aerodynamics. The coaxial configuration is the baseline platform for this work. The use of a helicopter on Mars would dramatically increase the speed, range, and coverage of exploration by providing access to caves, craters, over polar ice, along icy scarps and recurring slope lineae that are just plain inaccessible or too dangerous for rovers. Many factors go into the design of a Mars helicopter from launch/entry loads to power to controls to packaging. Aeromechanics is only one factor, but the principal factor for efficient and effective flight that impacts everything else. This work is focused on this principal factor. Current knowledge extrapolated from Earth would allow for short hops into the Mars atmosphere. Deeper understanding of Martian aeromechanics is needed to design larger more capable aircraft. Accurate predictions are needed for performance, blade loads, control loads, and blade strike behavior. True high-fidelity is needed for unlike on Earth decades of data sets do not exist on Mars. In fact there is not even a single data set. Thus clever and innovative means of verification and validation must be found. The objective of this thesis is to carry out all of these tasks. The key conclusions are: (1) the design of aircraft, hub, blades, and controls are substantially different on Mars because of its unique aeromechanics, (2) an articulated hub can in fact have lesser danger of blade strike, (2) a hingeless hub can experience lower or only marginally higher (6-7%) flap bending moments, (3) control / pitch link loads are dramatically impacted more by choice of Mars airfoils than rotor hubs, (4) lifting-line analysis does not even begin to capture the precise magnitudes of blade passage impulsive loads, and (5) fuselage aerodynamics is irrelevant in preliminary design. These, and other interesting phenomena will be the topics of this dissertation.Item CFD/CSD STUDY OF INTERACTIONAL AERODYNAMICS OF A COAXIAL COMPOUND HELICOPTER IN HIGH-SPEED FORWARD FLIGHT(2020) Klimchenko, Vera; Baeder, James; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work presents a computational study of the aerodynamic interactions that arise between the components of a high-speed lift-offset coaxial compound helicopter in forward flight. The objective of this study is to develop a computational methodology that would enable fundamental understanding of the complex aeromechanics of a modern lift-offset coaxial compound rotorcraft configuration in it's entirety. The modeling of a helicopter is a coupled aeroelastic problem, in which the aerodynamics is highly dependent on the structural dynamics, and vice versa. Therefore, the prediction of the rotorcraft airloads and blade deformations must be performed with sufficient fidelity to accurately model both aspects of the problem. A high-fidelity computational fluid dynamics framework, HPCMP CREATE$^{TM}$-AV Helios, was used in conjunction with an in-house comprehensive analysis solver, to simulate a lift-offset coaxial compound helicopter in forward flight. A notional X2TD helicopter consisting of a lift-offset coaxial rotor, airframe and an aft-mounted propeller, was modeled in this work. An in-house comprehensive analysis solver, PRASADUM, performed trim calculations and the structural modeling using low order aerodynamics. Conventionally, the comprehensive analysis rotor airloads that are computed from the built-in low order aerodynamic models, would be corrected with the high-fidelity CFD airloads using delta coupling procedure. In this study, the conventional rotor delta coupling methodology was used to study the interactional aerodynamics of a coaxial rotor system in forward flight at a range of flight speeds (50 knots to 225 knots). This study also focused on extending this methodology to perform high-fidelity airloads corrections for airframe and the propeller. The low order rotor, airframe and propeller aerodynamic loads were corrected with the high-fidelity CFD airloads, using a full vehicle loose delta coupling methodology. The two CFD/CSD coupling approaches, rotor and full vehicle, were compared. The results showed that correcting the low fidelity CSD airframe airloads with high-fidelity CFD airloads affects the rotor trim solution. The converged trim state from the full vehicle delta coupling procedure was utilized to study the fundamental interactional aerodynamics between various components of the coaxial compound helicopter. The CFD simulations were performed for isolated helicopter components and component combinations.Item CAD-based Modeling of Advanced Rotary Wing Structures for Integrated 3-D Aeromechanics Analysis(2017) Staruk, William; Chopra, Inderjit; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes the first comprehensive use of integrated 3-D aeromechanics modeling, defined as the coupling of 3-D solid finite element method (FEM) structural dynamics with 3-D computational fluid dynamics (CFD), for the analysis of a real helicopter rotor. The development of this new methodology (a departure from how rotor aeroelastic analysis has been performed for 40 years), its execution on a real rotor, and the fundamental understanding of aeromechanics gained from it, are the key contributions of this dissertation. This work also presents the first CFD/CSD analysis of a tiltrotor in edgewise flight, revealing many of its unique loading mechanisms. The use of 3-D FEM, integrated with a trim solver and aerodynamics modeling, has the potential to enhance the design of advanced rotors by overcoming fundamental limitations of current generation beam-based analysis tools and offering integrated internal dynamic stress and strain predictions for design. Two primary goals drove this research effort: 1) developing a methodology to create 3-D CAD-based brick finite element models of rotors including multibody joints, controls, and aerodynamic interfaces, and 2) refining X3D, the US Army’s next generation rotor structural dynamics solver featuring 3-D FEM within a multibody formulation with integrated aerodynamics, to model a tiltrotor in the edgewise conversion flight regime, which drives critical proprotor structural loads. Prior tiltrotor analysis has primarily focused on hover aerodynamics with rigid blades or forward flight whirl-flutter stability with simplified aerodynamics. The first goal was met with the development of a detailed methodology for generating multibody 3-D structural models, starting from CAD geometry, continuing to higher-order hexahedral finite element meshing, to final assembly of the multibody model by creating joints, assigning material properties, and defining the aerodynamic interface. Several levels of verification and validation were carried out systematically, covering formulation, model accuracy, and accuracy of the physics of the problem and the many complex coupled aeromechanical phenomena that characterize the behavior of a tiltrotor in the conversion corridor. Compatibility of the new structural analysis models with X3D is demonstrated using analytical test cases, including 90° twisted beams and thick composite plates, and a notional bearingless rotor. Prediction of deformations and stresses in composite beams and plates is validated and verified against experimental measurements, theory, and state-of-the-art beam models. The second goal was met through integrated analysis of the Tilt Rotor Aeroacoustic Model (TRAM) proprotor using X3D coupled to Helios¬¬ – the US Army’s next generation CFD framework featuring a high fidelity Reynolds-average Navier-Stokes (RANS) structured/unstructured overset solver – as well as low order aerodynamic models. Although development of CFD was not part of this work, coupling X3D with Helios was, including establishing consistent interface definitions for blade deformations (for CFD mesh motion), aerodynamic interfaces (for loads transfer), and rotor control angles (for trim). It is expected that this method and solver will henceforth be an integral part of the Helios framework, providing an equal fidelity of representation for fluids and structures in the development of future advanced rotor systems. Structural dynamics analysis of the TRAM model show accurate prediction of the lower natural frequencies, demonstrating the ability to model advanced rotors from first principles using 3-D structural dynamics, and a study of how joint properties affect these frequencies reveals how X3D can be used as a detailed design tool. The CFD/CSD analysis reveals accurate prediction of rotor performance and airloads in edgewise flight when compared to wind tunnel test data. Structural blade loads trends are well predicted at low thrust, but a 3/rev component of flap and lag bending moment appearing in test data at high thrust remains a mystery. Efficiently simulating a gimbaled rotor is not trivial; a time-domain method with only a single blade model is proposed and tested. The internal stress in the blade, particularly at its root where the gimbal action has major influence, is carefully examined, revealing complex localized loading patterns.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 Analysis of Helicopter Flight Dynamics through Modeling and Simulation of Primary Flight Control Actuation System(2015) Nelson, Hunter Barton; Celi, Robertro; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A simplified second-order transfer function actuator model used in most flight dynamics applications cannot easily capture the effects of different actuator parameters. The present work integrates a nonlinear actuator model into a nonlinear state space rotorcraft model to determine the effect of actuator parameters on key flight dynamics. The completed actuator model was integrated with a swashplate kinematics where step responses were generated over a range of key hydraulic parameters. The actuator-swashplate system was then introduced into a nonlinear state space rotorcraft simulation where flight dynamics quantities such as bandwidth and phase delay analyzed. Frequency sweeps were simulated for unique actuator configurations using the coupled nonlinear actuator-rotorcraft system. The software package CIFER was used for system identification and compared directly to the linearized models. As the actuator became rate saturated, the effects on bandwidth and phase delay were apparent on the predicted handling qualities specifications.Item Performance and Loads of Variable Tip Speed Rotorcraft at High Advance Ratios(2015) Bowen-Davies, Graham Michael; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a lifting-line, comprehensive approach to predicting the performance and loads of high advance ratio rotorcraft. At high advance ratios, the reverse flow region is large and its unique aerodynamics impacts the rotor performance and dynamics more than at conventional airspeeds where they are often ignored. The analysis is refined and augmented with improved modeling of the nearwake in reverse flow, a new aerodynamic model of the fuselage and the root cutout region and corrections to the airfoil properties for highly yawed flow. The analysis is correlated and evaluated against a full-scale UH-60A rotor test to an advance ratio of 1.0 and against an in-house Mach-scaled rotor to an advance ratio of 1.2. High advance ratio performance is predicted satisfactorily for both tests, including predicting the onset of thrust reversal. Despite the high advance ratio, correctly modeling the wake is most important for predicting airloads and the resulting blade bending loads, while yawed flow, nearwake inflow and the fuselage flow disturbances are important for predicting high advance ratio thrust and power. The validated analysis is used to investigate the effect of reverse flow stall, blade twist, root cut-out and shaft angle on high advance ratio performance.Item Measurement and Scaling Analysis of Rotor-Induced Sediment Mobilization(2014) Perrotta, Gino; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow visualization and particle imaging velocimetry (PIV) experiments were conducted in a water tank to investigate the effects of rotor wake and sediment properties on rotor-induced sediment mobilization during hover in ground effect. The two-phase flow was separated into the carrier phase and the dispersed phase for characterization. The carrier phase was studied using PIV to acquire time-resolved planar velocity measurements for a field of view within the rotor wake. The rotor-induced flow was confirmed to be dominated by blade tip vortices and was characterized primarily in terms of the vortex characteristics. Vortices were identified using a tracking function, and were compared to the Lamb-Oseen vortex velocity profile to evaluate their size and strength. The rotor-induced flow was also characterized in terms of wall-jet velocity and turbulent kinetic energy. The dispersed phase was separated using image filtering procedures and was quantified by identifying mobilized sediment particles visible in the field of view. Characteristics of the rotor-induced flow and quantification of sediment mobilization were each averaged over time for several rotor rotations to reduce the effects of wake aperiodicity and asymmetry. New parameter groups were created by combining rotor-induced sediment mobilization system characteristics and each was inspected for correlation with sediment mobilization. Three parameter groups which correlated for all cases measure here are identified and discussed.Item Contributions Towards the Detailed Understanding of Rotor Flow Fields in Ground Effect Operations(2014) Milluzzo, Joseph; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-speed flow visualization and high-resolution particle image velocimetry experiments were conducted on a two-bladed rotor that was operated in a hovering state, both out of ground effect (well away from the ground) and in ground effect at several rotor heights. Recent advances in flow diagnostic instrumentation allowed measurements of the rotor wake to be performed with unprecedented levels of resolution. In particular, the goal of the present work was to gain a better understanding of the fluid dynamics of the wake sheets (and the blade tip vortices) that were trailed from the blades. The present work examined the effects produced by two blade sets: 1. A baseline untwisted blade, 2. A twisted blade with $-17^{\circ}$ of linear twist, and has revealed fluid dynamic details of the wake sheet that were hitherto unknown. For the measurements made with the rotor operating out of ground effect, the blade sets were tested at two blade loading coefficients of 0.053 and 0.08, although only the higher loading condition was tested with the rotors operating in ground effect. For the rotor operating out of ground effect, a helicoidal tip vortex was shown to form at the blade tip and the associated wake sheets were initially laid down as small-scale counterrotating vortical pairs. However, this initial vorticity quickly diffused and the sheet was then convected as a concentrated bands of turbulence, including several dominant eddies. Several types of sheet dynamics were documented in the rotor wake, including sheet interactions with the tip vortices, the detailed behavior of this interaction depending on both the blade twist and the rotor thrust. At earlier wake ages, a sinusoidal wave-like perturbation was seen to be formed on the wake sheets, although the growth in wave amplitude was limited as the sheets were convected and stretched in the velocity gradients in the downstream wake. When the rotor was operated in ground effect, the vorticity in the wake sheets persisted to much older wake ages. Wave-like perturbations did not form on the wake sheets when the rotors were operating in ground effect because the outward radial stretching of the rotor wake in the presence of the ground suppressed their development. The wake sheets were found to convect to the ground and introduce significant vorticity into the near-wall flow field closer to the rotor, contributing to fluctuations in the local flow velocities. The flow field near the ground was also observed to be significantly affected by the use of twist on the blade, with the wake impinging on the ground further inboard and closer to the rotor, which also resulted in higher flow velocities being produced further downstream.Item SIMULATION MODELING OF FLIGHT DYNAMICS, CONTROL AND TRAJECTORY OPTIMIZATION OF ROTORCRAFT TOWING SUBMERGED LOADS(2014) Sridharan, Ananth; Celi, Roberto; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work presents the mathematical modeling and analysis of helicopters towing submerged loads using long cables for sub-surface object detection when surface-based vessels cannot operate safely. A geometrically exact model of rotating beams is derived, and used to represent both the cable dynamics and rotor blade dynamics. Flight dynamics and trim conditions for an axially flexible straight cable and a curved cable are separately formulated for a general case of helical climbing turns, and used to cross-validate each other. In steady flight, the trim longitudinal dynamics of the submerged load produces down-forces from towed body fins, increasing the apparent weight of the tow system. Cable and towed body drag manifest as increases in the effective equivalent flat- plate area, necessitating excessive nose-down helicopter trim pitch attitudes (-6◦ ) and causing pilot discomfort. Excessive pitch attitudes can be avoided using aft offset of the helicopter tow point, or the deployment of longer cables in combination with pitching fins to regulate towed body depth. In steady level turning flight, cable and towed body drag result in the submerged load turning with a consistently smaller radius than the helicopter. Depth regulation in turning flight using pitching fins is less effective than in forward flight due to increased cable drag opposing larger down-forces. Analysis of linearized models showed that the helicopter frequency response to pilot inputs is unaffected by the addition of the cable and towed body above 1 rad/s. The low-frequency response magnitude reduces with increasing hydrodynamic drag on the cable and towed body, and is unaffected by cable structural properties due to over-damped stabilization from hydrodynamics. The swashplate inputs required to guide the towed body along a "tear-drop" shaped trajectory are obtained using a two-stage process. The motions of the tow point that guide the submerged load along the target path are obtained using an optimization process. The system target states are generated based on these tow point motions, and an LQR controller is used to guide the helicopter along its target path. Trim rotor inflows from the vortex wake model are obtained at the various equilibrium points used to construct helicopter target states, interpolated and applied as "delta" corrections to the dynamic inflow model. Blade elastic twist has a significant effect on rotor power predictions and the steady hub loads, while flap bending elasticity acts as a vibration absorber to attenuate the oscillatory component of hub rolling and pitching moments.