Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item INVESTIGATION OF COMPOUND ROTORCRAFT AEROMECHANICS THROUGH WIND-TUNNEL TESTING AND ANALYSIS(2022) Maurya, Shashank; Datta, Anubhav; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The aeromechanics of a slowed-rotor compound rotorcraft is investigated through wind-tunnel testing and comprehensive analysis. The emphasis is on a lift-offset wing compound with a hingeless rotor configuration. A new Maryland Compound Rig is developed and instrumented for wind-tunnel testing and an in-house rotor comprehensive code is modified and expanded for compound rotorcraft analysis. The compound rig consists of a lift compound model and a propeller model. The lift compound model consists of an interchangeable hub (articulated or hingeless), a fuselage, a half-wing of 70% rotor radius on the retreating side. The wing has a dedicated load cell and multiple attachment points relative to the rotor hub (16%R, 24%R, and 32%R and 5%R aft of the hub). The rotor diameter is 5.7-ft. The rotor has four blades with NACA 0012 airfoils with no twist and no taper. The wing incidence angle is variable between 0 to 12 degrees. The wing has a linearly varying thickness with symmetric airfoils NACA 0015 at the tip and NACA 0020 at the root. Sensors can measure rotor hub forces and moments, wing root forces and moments, blade pitch angles, structural loads (flap bending moment, lagbending moment, and torsional moment) at 25%R, pitch link loads, and hub vibratory loads. Wind tunnel tests are conducted up to advance ratio 0.7 for lift compound with half-wing at wing incidence angles of 4 and 8 degrees and compared with an isolated rotor. Hover tests are conducted up to tip Mach number of 0.5 to measure download penalty with the wing at various positions. The University of Maryland Advanced Rotorcraft Code (UMARC) is modified for compound rotorcraft analysis code. Aerodynamic models for the wing and the propeller are integrated. A recently developed Maryland Free Wake model is integrated, which can model the wake interaction between unequal and inharmonic speed rotor, wing, and propeller. The analysis is then validated with the test data. The validated analysis is used to analyze the US Army hypothetical full-scale aircraft. The compound rotorcraft is categorized into multiple configurations in a systematic manner to find the extreme limits of speed and efficiency of each. The key conclusions are: 1) slowing the rotor or compounding the configuration provide no benefit individually; they must be accomplished together, 2) Half-Wing is more beneficial if a lift-offset hingeless rotor is used, 3) hover download penalty is only 3% of net thrust, and this penalty can be predicted satisfactorily by free wake, 4) the main rotor wake interaction is more pronounced on the wing and less on the propeller, 5) the validated analysis indicates a speed of 240 knots may be possible with 20% RPM reduction along with a wing and propeller, if structural weights allow, and 6) the oscillatory and vibratory lag moments and in-plane hub loads may be significantly reduced by compounding.Item A Scalable Time-Parallel Solution of Periodic Dynamics for Three-Dimensional Rotorcraft Aeromechanics(2022) Patil, Mrinalgouda; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The principal barrier of computational time for rotorcraft trim solution using high-fidelity three-dimensional (3D) structures on real rotor problems was overcome with parallel and scalable algorithms. These algorithms were devised by leveraging the modern supercomputer architecture. The resulting parallel X3D solver was used to investigate advanced coaxial rotors using a notional hingeless rotor test case, Metaltail. This investigation included rotor performance, blade airloads, vibratory hub loads, and three-dimensional stresses. The technical approach consisted of first studying existing algorithms for periodic rotor dynamics --- time marching, finite element in time (FET), and harmonic balance. The feasibility of these algorithms was studied for large-scale rotor structures, and drawbacks were identified. Modifications were then performed on the harmonic balance method to obtain a Modified Harmonic Balance (MHB) method. A parallel algorithm for skyline solver was devised on shared memory to obtain faster solutions to large linear system of equations. The MHB method was implemented on a hybrid distributed--shared memory architecture to allow for parallel computations of harmonics. These developed algorithms were then integrated into the X3D solver to obtain a new parallel X3D. The new parallel X3D was verified and validated in hover and forward flight conditions for both idealized and real rotor test cases. A total of four test cases were studied: 1) uniform beam, 2) Frank Harris rotor, 3) UH-60A-like Black Hawk rotor, and 4) NASA Tilt Rotor Aeroacoustic Model (TRAM). The predictions of tip displacements, airloads, and stress distributions from the MHB algorithm showed good agreement with the test data and time marching predictions. The key conclusion is that the new solver converges to the time marching solution 50-70 times faster and achieves a performance greater than 1 teraFLOPS. The new parallel X3D solver opened the opportunity for modeling advanced rotor configurations. In this work, the coaxial rotor was the selected configuration. Two open access models were developed; 1) a notional hingeless coaxial rotor, and 2) a notional articulated UH-60A-like coaxial rotor. The aerodynamics, structural dynamics, and trim modules of X3D were expanded for coaxial modeling. The coaxial aerodynamics was validated with hover performance data from the U.S. Army model test. The coaxial solver was then used to study rotor aeromechanics in forward flight. The analysis was performed at a low-speed transition flight for which qualitative data is available for the Sikorsky S-97 Raider aircraft for comparison. The UH-60A coaxial airloads showed good agreement with the S-97 data as the twists are likely similar. However, the Metaltail model showed dissimilarities, and the cause was investigated to be its high twist. Vibratory hub loads with advance ratio were studied, and the maximum vibration occurred at the transition flight speed ($\mu = 0.1 - 0.15$), which was consistent with the S-97 data. The effect of the inter-rotor phase was examined for the reduction of vibratory hub loads. Three-dimensional stresses and strains were predicted and visualized for the first time on lift offset coaxial rotors in the blade and the hub.Item A MODERN AEROMECHANICAL ANALYSIS OF HINGELESS HUB TILTROTORS WITH MODEL- AND FULL-SCALE WIND TUNNEL VALIDATION(2022) Gul, Seyhan; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A new aeromechanics solver was developed, verified, and validated systematically to explore how whirl flutter might be eliminated to achieve significantly higher cruise speeds with future tiltrotor aircraft. The hub explored is hingeless, more advanced than the gimballed hub of current generation tiltrotors. The major finding is that whirl flutter is not the barrier at all for hingeless hubs, instead air resonance, which is another fascinating instability particular to soft in-plane rotors. A possible design change to achieve high cruise speeds with thin, low-profile wings is blade tip sweep. The key mechanism is the aerodynamic center shift. The trade-off is the increase in blade and control system loads. A fundamental understanding of the physics for soft in-plane hingeless hub stability was provided. The induced flow model showed no effect on high-speed stability, as the wake is quickly washed away and insignificant for airplane mode flight. Predictions in powered mode are necessary. At least the first rotor flap, lag, and torsion modes need to be included. Rotor aerodynamics should use airfoil tables; wing aerodynamics is not essential for air resonance. Periodic solution before stability analysis is necessary for powered mode flight. Details of the mathematical model were reported. The solver was built to study high-speed stability of hingeless hub tiltrotors; hence the verification and validation cases were chosen accordingly. The stability predictions were verified with U.S. Army's CAMRAD II and RCAS results that were obtained for hypothetical wing/pylon and rotor models. Soft in-plane, stiff in-plane, hyper-stiff in-plane, and rigid rotors were studied with a simple and a generic wing/pylon model. A total of nine cases were investigated. A satisfactory agreement was achieved. Validation was carried out with Boeing Model 222 test data from 1972. This rotor utilized a soft in-plane hingeless hub. Good agreement was observed for performance predictions. Trends for the oscillatory blade loads were captured, but differences in the magnitudes are present. The agreement between the stability predictions and test data was good for low speeds, but some offset in the damping levels was observed for high speeds. U.S. Army also published stability predictions for this rotor, which agreed well with the present predictions. A further parametric validation study was carried out using the University of Maryland's Maryland Tiltrotor Rig test data. This is a brand new rig that was first tested for stability in October – November 2021. Eight different configurations were tested. Baseline data is gimbal-free, freewheeling mode, wing fairings on with straight and swept-tip blades. Gimbal-locked, powered mode, and wing fairings off data was also collected, all with straight and swept-tip blades. Wing beam mode damping showed good agreement with the test data. Wing chord mode damping was generally under-predicted. The trends for this mode for the gimbal-locked, straight blade configurations (freewheeling and powered) were not captured by the analysis. Swept-tip blades showed an increase in wing chord mode damping for gimbal-locked, freewheeling configuration. Locking the gimbal increased wing chord damping, which was picked up by the analysis. Powered mode also increased the wing chord damping compared to freewheeling mode, but the analysis did not predict this behavior. Wing beam mode damping test data showed an increase at high speeds due to wing aerodynamics, and the analysis agreed.Item AEROMECHANICAL BEHAVIOR OF TWIST-MORPHING, HIGH-SPEED, SLOWED RPM ROTORS(2017) Ward, Elizabeth; Chopra, Inderjit; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes the first comprehensive analysis of a composite coupled edgewise rotor in high-speed forward flight. The design objective was to use composite coupling, namely extension-torsion coupling, to morph the built-in twist of a UH-60A-like rotor in slowed RPM flight. As a part of this work, this study included the first analysis of a morphing rotor using full 3-D analysis coupled with aeromechanics. The use of 3-D FEM along with an integrated trim solver and aerodynamic modeling was shown to have been key in developing a fundamental understanding of how composite coupling effects rotor performance and the aerodynamics in different flow conditions. This research shows that extension-torsion composite coupling in the spar of a UH-60A-like rotor can provide a significant increase in the efficiency when the RPM is reduced. This was achieved through a combination of delayed stall drag along the retreating side of the rotor and reduced negative lift along the advancing side, providing an overall improvement in rotor efficiency. A comprehensive analysis was performed using a full 3-D FEA based aeroelastic computational structural dynamics (CSD) solver with the inclusion of a freewake aerodynamics model. A reduction of RPM down to 85% of the nominal hover RPM (which is well within the operational capacity of current turboshaft engines) showed an improvement in the lift-to-drag ratio, 𝐿/𝐷𝑒, over all blade loadings, 𝐶𝑇/𝜎. The maximum improvement in efficiency occurred at the peak blade loading, 𝐶𝑇/𝜎≈0.1. A further RPM reduction to 65NR (65% of nominal RPM), an RPM that future rotorcraft could potentially achieve with improvements in variable drive train design, showed general efficiency improvement at blade loadings below 𝐶𝑇/𝜎=0.08, with no change in the peak efficiency when compared to an uncoupled rotor. A hygrothermally stable Winckler layup was shown to perform just as well as a nominal coupled layup at 85NR, and marginally better at 65NR, in addition to contributing to practical manufacturability of the rotor design. Close study of the strains in the rotor showed that a rotor with an extension-torsion coupled composite spar would be within the realm of practical manufacturability as the axial strains around the azimuth fell well within IM7/8552’s allowable tensile strain of 6000 𝜇𝜀. Tensile strain is directly related to the amount of twist change in the rotor and is reduced when the RPM is slowed and the rotor untwists towards its original cold shape.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 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.