Effects of internal friction on the dynamic behavior of aeroelastic systems

This paper focuses on the effects of friction, internal to an aircraft, on the level and characteristics of preexisting limit cycle oscillations. Two different models are investigated in this context. First, a simple two-degree-of-freedom mass-spring system is considered in which the aerodynamic forces are globally represented through a van der Pol force acting on the largest mass while friction takes place during relative motions of the two masses. The second system considered involves an airfoil pitching and plunging in a low speed flow. A full aerodynamic - structural feedback is considered and friction is introduced through a torsional degree-of-freedom internal to the airfoil. The results obtained with both models demonstrate the strong potential of internal friction to reduce existing LCO provided that the torsional degree of freedom acts as a "tuned damper" for the system. The existence of multiple stable solutions is demonstrated when the coefficient of friction is small and the nature of the system response, i.e. single/multiple frequency and slip-stick/continuous slip motions is discussed. In the aeroelastic model, it is finally seen that the amplitude of response of the system can be increased by the addition of the torsional degree of freedom.