ExcaliburVT

@Stormie I Had the same issue but I had it with the vertical cargo bay J module. I was not able to find a solution.

@JadeOfMaar I only have OPT Main and was getting it with an entirely OPT-J plane.

@ss8193 Lol usually I’m a stickler for getting terms right but you’re perfectly correct about all that. The issue is definitely Yaw not Roll and it was usually occurring I guess as I was transitioning into supersonic (to remove the ground speed and alt from the equation). I tried FAR and had more success but I’m glad you mentioned that the CoM and CoL aren’t accurate as I had a lot of difficulty with that I was finally able to get things to work without FAR last night and I think the problem may have had something to do with me enabling the advanced settings and making parts rigid attachment. I removed rigid for everything but the wings as I previously enabled it because I was getting seriously floppy wings that were shearing off. I still had significant Yaw while taking off but it evened out and flew reasonably well for the rest of the orbital insertion. I may try FAR again and see if things improve more. Thanks!

All, I'm hoping someone recognizes my issue and what I need to do about it. First off I absolutely love the OPT parts, they make for a perfect spaceplane. My problem unfortunately is that no matter what I do (Including stripping everything down to bare essentials) my planes seem to ALWAYS be trying to Yaw to one side or the other once I get up to a certain speed (Generally ~350m/s). If I have SAS on the plane starts to use the tail more and more to one side in order to stay on course and if i turn SAS off the plane starts to bank (Yaw only) to one side. If i don't turn off SAS it eventually gets so bad that the entire plane spins out of control. I have also focused on CoM and CoL. I have ensured that CoL is always behind CoM and the amount of fuel burned is less than 10% when the Yaw gets really bad. I've also completely restarted the aircraft from scratch and focused on balance of parts to ensure I didn't put something on the wrong side etc with no change. I've tried several different designs but nothing seems to work. I even re-installed Kerbal from scratch and the mods I use. Any ideas to try? -Thanks in advance