Whirl flutter stability is a critical limitation for tiltrotor aircraft. This paper investigates whirl flutter predictions for the Wing and Rotor Aeroelastic Test System (WRATS) using comprehensive analysis, focusing on the comparison of the whirl flutter stability predictions between Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRAD) II and the Rotorcraft Comprehensive Analysis System (RCAS). The analytical models were created using a modular approach to systematically validate the modeling process of the WRATS tiltrotor. Comparison of nonrotating frequencies for blade, flexbeam, flexbeam and cuff, and blade with flexbeam and cuff shows excellent agreement between CAMRAD II and RCAS. The assembled model is then used to predict the whirl flutter stability boundary for various configurations with varying levels of fidelity. Results show near exact agreement between the two analyses for a rigid rotor and linear aerodynamics, and good to fair agreement when an elastic rotor is used. Predicted wing beam mode frequencies and damping values are also compared against the wind tunnel test data, and the frequencies are shown to be reasonably well-predicted. However, damping values, and thus stability boundaries, are not accurately predicted.

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Tiltrotor whirl flutter in cruise flight is investigated using comprehensive rotorcraft analysis codes Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRAD) II and Rotorcraft Comprehensive Analysis System (RCAS). A generic tiltrotor model with a three-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 (δ 3), 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 have an important influence on whirl flutter speed. Nomenclature a speed of sound k x , k y , k z pitch bearing translational stiffness k θx , k θy , k θz pitch bearing rotational stiffness R blade radius V speed X, Y, Z translational NASTRAN mode shape at rotor hub β flap angle θ blade pitch angle δ 3 pitch-flap coupling θ X , θ Y , θ Z rotational NASTRAN mode shape at rotor hub ν ζ blade fundamental lag mode frequency ρ freestream density rotor rotational speed

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