Here's a video of a flutter test-flight that is fairly well known in the gliding community.
The aircraft is a DG-300/17, a specially-built research glider. The wingspan was increased from the 15m of a production DG-300 to 17m, by adding new wing panels at the the wing root and the water ballast tanks were increased in size.
The flutter tendency with an increased amount of water was predicted, based on flutter calculations and static flutter tests.
The video shows the flutter occurring at airspeeds between 140 and 150 km/hr (75-81 kts)
More information here on the DG website:
https://www.dg-flugzeugbau.de/en/lib...of-the-dg-1000
PS megan, thanks for the link to the NASA paper. Exactly what I was searching for.