Wind Tunnel Test Results for a 0.2 Scale 4-Bladed Tiltrotor Aeroelastic Model

A 0.2 scale 4-bladed stiff inplane tiltrotor aeroelastic model was tested at the NASA Langley Transonic Dynamics Tunnel (TDT) in October and November 2011. This aeroelastic model included a novel stepover control mechanism designed to improve whirl flutter stability boundaries for a 4-bladed tiltrotor. The stepover control mechanism provides the rotor kinematic control coupling necessary for stability but is designed to fit within the limited space of a 4-bladed rotor hub. Airplane mode stability boundaries with and without the stepover control mechanism installed were measured during the wind tunnel test. Stability test results show that the stepover mechanism provides as much or more stability to the 4-bladed rotor as the conventional control system provided for the 3-bladed rotor. With the stepover control mechanism removed (i.e. reverting to a conventional control system design), the 4-bladed configuration was found to be unstable at significantly lower airspeeds. In addition, a GPC (Generalized Predictive Controller) active stability control algorithm was tested and shown to significantly increase the subcritical damping of the 4-bladed aeroelastic model with a conventional control system installed. Rotor loads and vibrations were measured at various nacelle angles and trim conditions. For direct comparison with the 4-bladed rotor, rotor loads were also measured on a 0.2 scale 3-bladed V-22 rotor. Differences in the dynamic behavior and loads of the two rotor configurations are explored.