Multibody Analysis of Whirl Flutter Dynamics on a Tiltrotor Wind Tunnel Model

Tiltrotor aircraft are equipped with large rotors, usually driven by heavy engines in nacelles located at the tip of the wings. This combination of large rotors and high masses on wings which form a relatively elastic support, makes whirl flutter a critical phenomenon for the design of the aircraft and its possible performance. At DLR, a multibody based simulation model of a tiltrotor wind tunnel model has been set up, combining a mixed finite element / multibody representation of the support, a detailed kinematic model of the rotor hub, and a strip-wise definition of air loads on the blades. Investigations on the effect of parameter changes and non-linearities on the calculation of the stability boundary are performed. The paper will present the work and some comparisons with wind tunnel experiments performed by a consortium of partners in the course of the European ADYN project. The regarded wind tunnel model is a half-model of a tiltrotor configuration, simplified for the use in whirl flutter investigations. Thus, the wing is represented as an elastic tube with the nacelle on the top, as it could be shown that the aerodynamics of the wing surface can be neglected for whirl flutter analysis for the given case. The tube is connected to the wind tunnel floor by a rotating degree of freedom around the vertical axis, restricted by a pair of rotational springs with variable stiffness. The spring stiffness is highly non-linear. Values for resonance frequencies and spring stiffness are known from detailed experimental investigations. The rotor hub is of a gimbal type, the four bladed rotor is modelled assuming stiff blades. Stationary simulations for various wind speeds show good agreement of simulations and experiment for rotor speed as a function of blade pitch angle. Investigations of the dynamic behaviour of the model are performed in the frequency and in the time domain. Results show considerable differences for linear and non-linear models. The main influence is seen in the non-linear properties of the end stops in the rotational springs. The paper describes the simulation approach, the model set-up and the validation of the data with experimental results. Simulation results are compared to wind tunnel measurements, and the differences are discussed