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Transonic Dip & Phase lag

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Old   April 24, 2009, 15:52
Post Transonic Dip & Phase lag
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Hi Guys,
Welcome to my first post!

I'm reading about transonic aeroelasticity at the moment; it is well known that the flutter speed of an airfoil decreases substantially in the transonic regime (a phenomenon known as the transonic dip). I was wondering if someone would please shed some light as to what causes this?

I understand that it has something to do with the characteristically large timescale of transonic flows - there is a large phase lag between the motion of an airfoil, say, and the resulting change in airloads. Apparently this can give rise to a negative damping of the motion - which contributes to flutter.

I am not sure why a phase lag between the airfoil motion and the airloads would increase the sensitivity to flutter - could someone please advise?

Many thanks,
T
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