Based on the info given to me by Dr. Niewoehner, and posted on Dubd's thread, it seems like a rudder "violently" fluttering at 30 mph has not been properly engineered. Dr. Niewoehner stated that flutter will happen, but proper engineering places the flutter speed at least 15% above Vne. 30 mph doesn't sound like 15% above Vne to me. Heck, 30 mph sounds like 15% below rotation speed...if that.
My bet would be the arm broke fighting to keep the rudder from "violently" fluttering.
In any case, glad you are having good results with your FEJ jets, Bart.
Sluggo
ORIGINAL: Aussie Bart
Next day I went for a drive and held the fin outside the window into the airstream to see if the rudder would oscillate, even at a low speed of 30 kph, it became so violent that I struggled to hold onto it.