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.