Prediction and Validation of Whirl Flutter Data of the Maryland Tiltrotor Rig

University of Maryland's Maryland Tiltrotor Rig was tested in the Naval Surface Warfare Center Carderock Division 8-×10-ft subsonic wind tunnel. Flutter frequency and damping data were collected for wing beam and chord modes up to 100 kt. Eight configurations were tested. Baseline data are gimbal-free, freewheeling mode, wing fairings on with straight and swept-tip blades. Gimbal-locked, powered mode, and wing fairings off data were also collected, all with straight and swept-tip blades. The sweep angle is 20°, starting at 80%R. Details of the mathematical model are reported. Predictions were carried out for each configuration with the University of Maryland's new aeromechanics solver UMARC-II. Wing beam damping showed good agreement with the test data. Wing chord damping was underpredicted with a maximum of 0.9% difference. The trends for this mode for the gimbal-locked, straight blades configurations (freewheeling and powered) were not captured by the analysis. Swept-tip blades did not show a definitive increase in wing beam or chord damping for the gimbal-free configuration. However, wing chord damping increased (about 0.4% at 60 kt) due to swept-tip blades for the gimbal-locked, freewheeling configuration. Locking the gimbal increased the wing chord damping by 0.5%, which was picked up by the analysis. Powered mode also increased the wing chord damping by 0.5% compared to freewheeling mode, but the analysis did not predict this behavior. Wing beam damping test data showed an increase at higher speeds due to wing aerodynamics, although not as clearly as predictions due to scatter.