RE: Issues with a Sig Riser 78
How much throw is "full" rudder?
I have never had a 2 channel sailplane that doesn't turn. You did put in the correct polyhedral, right? Or is this an ARF? Never had a Riser but it should be in the Spirit/Gentle Lady class of 2 channel sailplane.
Try to get as much throw in the rudder as possible. If you have a sloppy pushrod, firm it up. Air pressure could be pushing the rudder back to neutral (although this is unlikely)
If you have a large gap in the hingeline, seal it up.
If all else fails, make the rudder surface bigger.
Don