Hello all. I have a question about a wing model... The model I have is an airplane wing (airfoil) in a wind tunnel. The wing is supported at the quarter-chord length and the wing body is free to move. My model is in the time domain with an oscillatory force at 20 Hz applied to the wing. When I look at the results, I see an oscillating signal for the displacement of the wing. Then I use this data in MATLAB for signal processing. The force on the wing has a frequency of 20 Hz, but the frequency of the vibration ranges from 48 to 68 Hz. Why do I get a range for the vibration, when the force on the wing is oscillating at 20 Hz? Thanks guys!