For most park jet designs, rudders are not really necessary for the bulk of flying a pilot might do, park jets are pretty much "yank and bank" in the pitch and roll. I have been flying with rudders for almost three years now and admittedly I only use them about 5% of the time, but now that I have been using them, I wouldn't do without them. I am constantly striving for smooth, scale like flying and having rudders that I can feather in for coordinated turns, more advanced aerobatics, high alpha, etc is definitely a necessity for me.
When it comes to trimming a plane, I never really considered applying any trim to the rudders to help with yaw control. I have always been pretty much locked in to trimming in the pitch and roll to keep my plane flying as straight as possible "hands off". For me, as long as my rudders were dead straight and I didn't have to trim against them, I never considered them in helping keep my plane straight or for compensating for bad habits caused by either the plane or by my build. I guess being "hard wired" into trimming just for pitch and roll, every time I saw my plane wanting to go left or right, I assessed it as a roll trim issue, never thinking that it could be a yaw issue that could be corrected with rudder trim.
I have been testing this Su-35 which is a modified version of an Su-37 from my good friends Bimo, Ian and Jeff from Parkflyers International. Please check out their website for more info on the plans they have available.
Strangely, ever since I maidened this plane, it seemed to require a lot of left trim in the roll. This is opposite to normal as most planes like a bit of trim to the right to counter torque roll. When I say a lot, about 8 clicks of left trim, almost half of what my transmitter will give me. I understand that trim is a fact of life once a plane is properly balanced, but I get concerned when I need to carry around that much trim.
As I would pull out of loops or split-S, the plane would take a "dirty dart" to the right at the bottom almost every time. Something I could anticipate, but I wanted to really see if there was something else that could be done to correct this behaviour. The other thing it would do is often in a right hand turn, it would "over bank" much further than the control inputs I was giving it should dictate.
So first I double checked everything, ensuring all my control surfaces were aligned properly with neutral trim, I checked motor and KF alignment to ensure nothing there was throwing off my plane's handling. I checked the alignment of the wings, horizontal and vertical stabilizers and everything seemed to be OK. Sometimes I guess it is just part of scratch building, you end up with planes that will pull in a certain direction for no apparent reason.
Anyway, I couldn't compensate with anymore left trim in the roll as then it wouldn't fly properly straight and level. So for some reason, I started to wonder if rudder trim would help. I put in a couple clicks of left rudder trim and suddenly the problems mentioned above started to lessen. So as I added more rudder trim, I was able to reduce the roll trim. I ended up with four clicks of left rudder trim and reduced my left roll trim down to two clicks. The problems went away almost entirely I was happy to discover.
The neat thing is that my rudder is only deflected over maybe 1-1.5 mm and now my elevons and ailerons are deflected about the same rather than about 5 mm as they were before. The rudders being smaller surfaces than the ailerons and elevons combined, I could feel that the plane handled better and was smoother in turns, I'm sure I had much less drag without the large elevons and ailerons deflected as much as they were. Gone was the problem of it wanting to dart to the right at the bottom of loops and split S, it no longer "over banked" in right turns.
In level flight, the plane stays dead straight, doesn't pull left or right now at all, so there must be some warp or curve in my plane that I can't see or check with a ruler or protractor.
So my biggest lesson learned here is that sometimes my plane wanting to go left or right is not always a function of needing more trim in the roll, it could be more of a yaw problem as it appears with this plane. In future, if I see the types of behaviour I mention above or find I am applying a large amount of trim one way in the roll, I will experiment more with rudder trim to help correct the problem and allow my plane to track and fly straighter and behave better in turns and aerobatics.
Another great day of discovery at the field in my continuing park jet journey!