A. James Clark School of Engineering
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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.