Structural Dynamic Behavior of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 660

According to this analysis, the 2 nd flapwise mode combines with the 1 st torsional mode
to form the flutter mode. This combination of modes was also observed by Hansen [2004] in
predicting classical flutter in HAWTs. In contrast, for fixed wing aircraft, it is the 1 st flapwise
mode that combines with the 1 st torsional mode.
Experimental Verification
Unfortunately, no experimental data exist to corroborate these results. However, the
flutter analysis technique described herein has been verified by comparison to the observed
flutter in a VAWT [Lobitz and Ashwill 1985]. The experimental flutter frequency was mea-
sured at 745 rpm [Popelka 1982], reasonably close to the predicted frequency of 680 rpm.
The inclusion of a small amount of structural damping in the model greatly improved the
agreement. The flutter mode shape was also well predicted using this technique.
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