Aero Elastic Instability:
When aeroelastic instabilities occur, the phase difference between the motions of the structure and the aerodynamic forces becomes such that the structure absorbs energy from the air. The aerodynamic forces pump energy into the structure, leading to vibrations with increasing amplitude. In the worst case scenario this can lead to failure of the structure. The most famous accident due to aeroelastic instability is the collapse of the Tacoma Narrows Bridge, also known as “Galloping Gertie” in Washington, US, in 1940.
Stall flutter is an aeroelastic instability which occurs when the structure is in stalled conditions.
Classical flutter is an aeroelastic instability involving coupling of the torsion and the deflection of the body. When classical flutter occurs, it rapidly creates violent vibrations, and potentially failure of the structure. Classical flutter is known from aircraft and civil engineering and may also occur in the human body. No commercial wind turbines have so far been reported to fail due to classical flutter. Subsequently classical flutter will be denoted as “flutter”, which is not to be confused with stall flutter.