[1.2.2] How to fix a plane yawing during a roll?

[EVA_Reentry]

See titles. CoL is behind CoM. Yaws both ways. Not assymetrical in any way. LMK if pictures req'd.

[bewing]

Often this happens because you did not disable "roll" control on your tail. So the game is using your tailfin to help the plane roll when you hit Q/E. But a tailfin causes a lot of yaw when it is used (because that is its main purpose -- to cause yaw).

 

[Cunjo Carl]

@bewing's answer is simple and effective and realistic, but there's also a... well... Kerbal alternative.

If you happen to not want ailerons, you can still decouple yaw from roll by having tail fins both above and below the COM, whose yaw moments counteract eachother during roll (and vice-versa). The easy way would be just putting a symmetric tail fin pointing down, but to increase ground clearance asymetric versions can be made with the lower control surfaces raised up. As it turns out, when yaw and roll are decoupled in this way, flight feels a lot more crisp and SAS can keep you better on point during hard maneuvers! It's probably not worth the time expenditure, but it's an interesting thing to tinker on.

Just for a picture, here's an SSTO that uses this concept I'm trying to update for v1.2's SAS... It'll get there.

Just using ailerons and Bewing's advice is way easier though. In any case, have fun!

[AeroGav]

How much vertical stabilizer area does the plane have, and is it right at the back of the airplane?   The CoL/CoM indicator is only for pitch.   An aircraft can be unstable in yaw yet show the blue ball safely behind the yellow in SPH because it has positive pitch stability.

The thing is also,  when you roll an aircraft into a turn, the aileron on the side of the wing you're lifting up, has to generate more lift than the one on the side whose wing is dropping.   This creates extra drag on that side, which causes the nose to yaw in the opposite direction you're trying to turn.    That is why real airplanes have rudder pedals, and a "turn and slip" indicator, though FBW aircraft dial this out for you , as do commercial jets - even the non-fbw ones have a "yaw damper", a simple compensation circuit that applies enough rudder to correct for this.

In KSP, you can angle your wings with incidence so they make lift even if you've got SAS set to prograde assist. Because prograde takes care of yaw as well as pitch,  it will correct for any adverse yaw caused by your ailerons when turning.

Finally, you can lessen adverse yaw by fitting larger ailerons than you really need, then dialling down the "authority" in the tweakable.   This causes the control surface to deflect to lower angles.   A large aileron deflecting at a smaller angle creates less drag than a smaller one maxed out in order to create the same roll rate, as a result there is less yawing moment.