Understanding High-Speed Aeroelastic Stability of a Gimballed Proprotor

A 4.75-ft-diameter dynamically scaled proprotor was tested on a semispan wing pylon in airplane mode up to high speeds of 200 kt. Aeroelastic stability data was acquired at two wind tunnels: Navy Carderock tunnel and the University of Maryland Glenn L. Martin tunnel, and for two hub configurations: gimballed and gimbal locked. The data consisted of frequency and damping of beam, chord, and torsion modes of the coupled rotor-wing-pylon system at a Froude-scale rpm of 1050. The eigenvalues were extracted with moving-block and Prony methods and were compared. The tests shed light on the nature of roots in high-speed tiltrotor flight. The key conclusions were the following: 1) the beam and chord damping for the gimballed rotor remained low, around 1-2%; 2) torsion damping was higher, around 3-6%; 3) the gimbal-locked condition increased chord and torsion damping significantly and also changed their trends with speed; 4) the model remained flutter-free up to 200 kt, which is equivalent to 458 kt in full scale; and 5) moving block and Prony are methods equally effective for extracting damping from time-series data for these test conditions.