Aircraft and winged-launch vehicles must be free from aerodynamic instabilities such as flutter and divergence to ensure safe operation. Divergence is a static aeroelastic phenomenon where the aerodynamic forces and moments are greater than elastic restoring forces of the structure. The aerodynamic forces are a function of the airspeed while the elastic forces are independent of airspeed, so there is a speed above which a small deformation due to the aerodynamic forces will lead to a large deflection in the structure.

Flutter is a dynamic instability that involves coupling of aerodynamic forces and elastic and inertial forces of the structure. As an elastic vehicle increases in flight speed, the aerodynamic forces interact with and alter the structural vibrations of the vehicle. In a flow, an oscillating structure generates unsteady aerodynamic forces. These unsteady aerodynamic forces introduce coupling into the structure and cause phase shifts between the motions of the structure (degrees of freedom). The speed of the flow affects the amplitude ratios and phase shifts between the various degrees of freedom in such a way that energy is extracted from the airstream. At the critical airspeed, the energy dissipated is exactly equal to the available structural damping. At speeds greater than the critical speed, the extracted energy dissipated is less than available structural damping and the motion is divergent.

Case Studies

• OSC X-34

Technical Papers

Brillhart, R., D. Hensley, and M. Stabb, "Pegasus XL Fin Dynamic Characterization for Flutter Evaluation," 14th International Modal Analysis Conference, Dearborn, Michigan, February 12-15, 1996. [folder 372]

Stabb, M., R. Brillhart, C. Flanigan, and R. Pinkham, "Aeroelastic Analysis and Test of the Pegasus XL Launch Vehicle," 16th Aerospace Testing Seminar, Manhattan Beach, California, March 12-14, 1996.

Flanigan, C., M. Stabb, and R. Pinkham, "Aeroelastic Design and Analysis of Winged Launch Vehicles," AIAA Dynamics Specialists Conference, April 18-19, 1996. [folder 377]

Antal, G., Brillhart, R., Hensley, D., and Freudinger, L., "Implementation of an Automated Approach for Spatial Filtering of Flight Flutter Test Data," 13th International Modal Analysis Conference, Nashville, Tennessee, February 13-16, 1995. [folder 370]

Mazzoni, I. A. and I. M.Pizzamiglio, "GP 180 Wind Tunnel Flutter Test," 4th International Modal Analysis Conference, 1986. [folder 273]