Comparison of CAMRAD II and RCAS Predictions of Tiltrotor Aeroelastic Stability

Tiltrotor whirl flutter in cruise flight is investigated using comprehensive rotorcraft analysis codes CAMRAD II and RCAS. A generic tiltrotor model with a 3-bladed gimballed rotor was systematically developed starting with a simple rigid rotor mounted on a rigid pylon and a more sophisticated model was built up by adding one design variable at a time. The rotor is also coupled with a flexible wing/pylon modeled from NASTRAN for aeroelastic stability analysis. The effects of pitch-flap coupling (δ₃), blade elasticity, precone, undersling, yoke chord and flap stiffness, pitch link stiffness, rotor rotational speed, density, speed of sound, inflow modeling, unsteady aerodynamics, and realistic airfoil tables on whirl flutter speed are thoroughly examined. With careful and thorough modeling/analysis, aeroelastic stability (frequency and damping) calculated by CAMRAD II and RCAS shows consistently excellent agreement with each other for wide variations of design variables and operating conditions. For the configurations investigated in this study, blade pitch-flap coupling, rotor lag frequency, rotor rotational speed, and density play an important influence on whirl flutter speed.