Proprotor Loads and Whirl-Flutter Stability of a Tiltrotor Wind Tunnel Model

This paper studies the blade loads and whirl-flutter stability of a three bladed stiff-inplane tiltrotor wind tunnel model mounted on the Wing and Rotor Aeroelastic Test System (WRATS). Proprotor loads are predicted and compared with the WRATS model at different pylon conversion angles. The tiltrotor whirl flutter stability is predicted and tested in airplane mode. The analytical models are developed with three rotorcraft codes: RCAS, CAMRAD II, and Dymore. The Dymore and RCAS models contain the structural model of the wing/pylon fixed system whereas the CAMRAD II model for this study uses an isolated proprotor. The study indicates that the analytical models capture the overall vibratory flap and chord bending moments, however, miss the higher harmonics of the blade loads. The steady flap and chord bending moments predicted by CAMRAD II and Dymore differ in the torque tube and flexbeam region but agree along the span of the blade. RCAS shows a good prediction of the damping ratio of the chord and torsion modes of the WRATS stiff-inplane tiltrotor. Dymore captures the relative trends of these modal dampings but over-predicts the wing chord mode damping and under-predicts the wing torsion mode damping. Both analyses slightly over-predict the wing chord frequency.