High Speed Whirl Flutter Tests of the Maryland Tiltrotor Rig

A 4.5-ft diameter proprotor was tested on the Maryland Tiltrotor Rig (MTR) in free-wheeling airplane mode reaching unprecedented speeds of up to 200 knots. Data was acquired at two wind-tunnels --- Navy Carderock tunnel and the Glenn L. Martin tunnel and for two hub configurations --- gimbal free and gimbal locked. The data consisted of frequency and damping of beam, chord, and torsion motions of the wing-pylon system at a Froude-scale RPM of 1050. The roots were extracted with two methods --- Moving-Block and Prony. The tests shed light on the nature of roots in high-speed tiltrotor flight. The key conclusions were: 1) the beam and chord damping always remained low to around 1-2%, 2) torsion damping was higher around 3-6%, 3) the gimbal-locked condition increased chord and torsion significantly and also changed their trends with speed, 4) the model remained flutter-free up to 200 knots, and 5) the chord damping showed signs of a sudden drop after 175 knots.