A 4.75-ft-diameter, dynamically scaled, swept-tip proprotor was tested up to 200 kts. The stability roots (frequencies and damping) were collected for wing-pylon beam, chord, and torsion motions. The impacts of both gimbaled and gimbal-locked hubs were measured. Predictions from an in-house rotor aeromechanics solver were compared with data to understand the stability behavior and identify gaps in prediction capabilities. The data showed that swept-tip blades increased the damping of the wing chord and torsion modes, but only when the gimbal was locked through perturbation of hub pitch and roll moments. There was no significant effect on the beam mode. The comprehensive analysis was able to predict the stability trends generally, except for torsion, particularly when the gimbal was locked.
Delgado et al. (Mon,) studied this question.