A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Development of the Maryland Tiltrotor Rig (MTR) and Whirl Flutter Stability Testing(2022) Tsai, Frederick; Datta, Anubhav; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tiltrotor aircraft encounter an aeroelastic instability called whirl flutter at high speeds. Whirl flutter is caused by the complex interaction between the aerodynamics and dynamics of the rotating proprotor blades, hub, and the wing. Current tiltrotors are limited to about 280 kt in cruise. While many computational analyses have been performed to assess potential improvements in whirl flutter stability, few have been validated by test data. There is a scarcity of publicly available test data along with documented model properties. A new tiltrotor rig is developed in this work to address this gap. The new rig, henceforth called the Maryland Tiltrotor Rig (MTR), is a semi-span, floor-mounted, optionally-powered rig with a static rotor tilt mechanism, capable of testing 3-bladed proprotors of up to 4.75-ft diameter in the Glenn L. Martin Wind Tunnel (7.75- by 11-ft section with 200 kt maximum speed). The objective is to experimentally characterize the parameters that affect the onset of whirl flutter which is vital to validating computational models and analyses. The MTR supports interchangeable hubs (gimballed and hingeless), interchangeable blades (straight and swept tip), and interchangeable wing spars, to allow a systematic variation of components important for tiltrotor flutter and loads. The vision for this rig is to conduct research towards flutter-free tiltrotors capable of achieving 400 kt and higher speeds in cruise. This dissertation lays the groundwork toward that vision by describing the test and evaluation of a baseline gimballed hub model. The features, controls, instrumentation, data acquisition, and all supporting equipment of the rig are described. A simple whirl flutter analysis model is developed, verified, and used for pre-test stability prediction of the MTR. The damping measurement methods are detailed. The first whirl flutter tests of the MTR were carried out at the Naval Surface Warfare Center-Carderock Division wind tunnel between 26 October - 2 November, 2021. Frequency and damping data were measured for four parametric configurations of wing on versus wing off, gimbal free versus gimbal locked, freewheel versus powered rotor, and straight versus swept-tip blades. The tests were conducted up to 100 kt windspeed, restricted only by the tunnel precautionary measures. Since the baseline model is loosely a 1/5.26-scale XV-15, 100 kt translates to a full-scale speed of 230 kt. It was observed that the baseline rig was stable up to 100 kt with an average wing damping lower than 1% critical in beamwise and 1.5% critical in chordwise motion. The effect of wing aerodynamics was insignificant up to 100 kt. Locking the gimbal affected mostly the chord mode and increased is damping significantly. Powering the rotor also affected mostly the chord mode and increased its damping significantly. The swept-tip blades showed interesting trends near 100 kt but higher speeds are needed for definitive conclusions. Overall, the MTR allowed the controlled variation of parameters that are important for fundamental understanding and analysis validation, but are impossible to carry out on an actual aircraft.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 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 Stability and Control Modeling of Tiltrotor Aircraft(2007-06-05) Kleinhesselink, Kristi; Celi, Roberto; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis develops a simple open-source model of a tiltrotor using the basic equations of motion. The model focused on stability and control aspects of the XV-15 aircraft using simple linear analysis and, in general, did not add in correction or scaling factors to obtain a better match with flight data. Subsequent analysis performed included a trim and time history solution. A linearized state space model was also developed and analyzed using state space matrices, Bode plots, and eigenvalue analysis. The results were validated against generic tiltrotor simulation model results and compared to flight test where available. The model resulted in was able to show inherent tiltrotor characteristics, however, further model refinements are needed. Helicopter and airplane mode flight data was used for comparisons. In order to make a true assessment of how well a simple model can approximate a tiltrotor, comparison with conversion mode flight data is required.Item An Experimental Investigation of Ground Effect on a Quad Tilt Rotor in Hover and Low Speed Forward Flight(2006-11-06) Radhakrishnan, Anand M; Schmitz, Fredric H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The performance of a Quad Tilt Rotor (QTR) in helicopter mode was experimentally studied in ground effect (IGE) and out of ground effect (OGE). A 0.03 geometrically scaled fuselage/wing model of the QTR was tested in hover and very low speed forward flight. Fixed-pitch propellers were used to model the rotors. In order to avoid the boundary layer problems associated with wind tunnel testing of rotorcraft IGE, a unique moving setup was developed for testing in forward flight. The effect of ground proximity was tested by varying the height of the model above the ground. Download on the airframe; thrust, torque and rpm of the rotors, and pressures along the centerline of the bottom of the fuselage were measured. The downwash distributions of the rotors were measured and found to compare well with V-22 rotor measurements. Tuft flow visualization was used to identify the physical processes causing changes in the download and pressure measurements. An uncertainty analysis was performed on the measured quantities to determine the 95% confidence levels. A strong download (9% of the rotor thrust) was observed in hover, OGE. The download reduced substantially IGE and become an upload (9% of the rotor thrust), when the wheels of the QTR were on the ground. The upload IGE was found to be caused by the entrapment of the rotor wakes under the fuselage. The upload was observed to persist in forward flight IGE, but reduced slightly at certain low skew angles. The measured downloads, coupled with power measurements, indicate that for a given power, the available vehicle thrust greatly increases IGE. Therefore, the QTR displays a potential for significant increase in payload carrying capacity by operating IGE.