Flutter derivatives from free decay tests of a rectangular B/D=10 section estimated by optimized system identification methods

The present paper suggests a hybrid system identification method to estimate the flutter derivatives from coupled free vibration tests, An optimized covariance-based method is used as the initial guess in the modified unifying least squares method. This combination optimizes the accuracy described by the coefficients of determinations between measured and synthesized signals. Flutter derivatives are identified at high wind speeds, close to and even above the critical flutter wind speed. Results for a sharp-edged rectangular section with a width-to-depth ratio B/D = 10 are presented for two different torsional-to-vertical frequency ratios. In one case the torsional frequency are lower than the vertical, due to a high mass moment of inertia, which makes it possible to estimate the flutter derivatives at very high reduced wind speeds. This reveals that the torsional aerodynamic damping derivative A(2)(*) reaches a positive maximum followed by a continuous decreasing tendency and eventually negative A(2)(*) values are identified. This implies that torsional flutter for the B/D = 10 section can be avoided if the structural damping is designed to balance the negative torsional aerodynamic damping expressed by the positive peak value for A(2)(*).