Trimming - Classroom Fighters

I have had a Zero for a few months now, and really enjoy flying it in the gym. Lately, however, I have noticed something, and am wondering how to correct it. When the rudder trim is set so the plane flies straight, turning to the right requires a big radius, almost half a basketball court, while left turns are nice and tight, about an 8 foot radius. The only way to get a tighter turn radius is to trim the rudder so the plane circles. The horizontal stab appears to be straight in relation to the wings, and looking from above, the vertical stab appears to be in line with the rest of fuselage. Any suggestions on trimming this so it will turn in a small radius in both directions?
Thanks

How To Trim the Classroom Fighters - Some Tips
You might just need a thrust line adjustment - give it 1 more degree to the Right. You do this by grabbing the motor with left hand finger and thumb while the right hand finger and thumb grips the fuselage just behind the rear of the motor. The motor is glued in place here, but there is some play - you can stretch and "persuade" a little more right thrust in this case.
Fly again and observe.
If it's still not turning to the Right as well, check the dihedral. Go ahead and pinch at the dihedral line - bend a wing UP a lot. Let it go and look at it, then fly it. If the problem is worse, you were supposed to do it on the other wing :) Try again on the other wing and undo the bend on the wing you just did, and fly again - just right! hopefully. If not repeat.
That trimming procedure should get you back on track - don't be afraid to put some bending on that foam - you can "will" all sorts of shapes into a wing - subtle little things can change a model's performance a lot, so have some fun trimming the model - if you make things worse - do the opposite - repeat until she's flying like a champ.
In between all of this, you can use your observation skills to notice if the undercamber looks symmetrical on both wings. Is the Fuselage straight, or does it have a "banana" look to it? Things like this might affect the way if flies. A combination of asymmetrical defects can actually make a warpy looking model fly well, or it can confound you in a trimming debug session as you adjust one variable and see no good results until you adjust 2 things at once. Those can get tricky. In general we'd like to see good geometry, but if that's not possible, put your focus on the performance in the air - where it really counts.
We might make some video and diagrams showing how to trim the Classroom Fighters - but in lieu of that for now the simple trimming algorithm described above will give you the same results. Stated another way - you don't have to know why you are making a trim adjustment - it's more important to just do some adjustments and learn from them.
Good luck with trimming and have fun with it - it's a great skill to pick up, and you'll enjoy your models even more when you can confidently make small adjustments when you are at your flying site.