Like the post above I can answer this in relation to the Enstrom. when in the cruise at about 85 in the Enstrom it develops a similar waddle with a frequency of about 1.5 Hz. It happens when flying in perfect trim and resonates so gradually gets more severe. The Engineer told me it was some sort of interaction between the TR and the vertical stabiliser. It disappears if the aircraft is flown ever so slightly out of trim. It doesn't matter which way. Just a hint of pedal. Make sure its not enough to cause extreme flapping problems in a Robbie though!!
Altering the speed will also damp it out so it would appear to be a resonance problem.
Edit:
I just remembered what he said. It went something like this.
the TR acts like a vertical fin when in forward flight, the angle of which is determined by the pitch on the blades. The waddle happens when the 2 'fins' (TR and real fin) are at slight, critical, different angles to the airflow. Vary the angle of one and the resonance stops.
Last edited by Gaseous; 7th Jul 2005 at 22:37.