pilot assistant target heading question

[Pandemic]

I'm trying to circumnavigate kerbin at about 315 degrees. I've noticed that before long the plane is at 270 degrees and falling. I tested with a plane that will hold 90.44 degrees all the way around kerbin at 16,000 meters and experienced the same issue. Is this normal for a straight flying plane?

[Alshain]

Pilot assist doesn't fix sideslip. You've gotta build a better plane.

[Highguard]

315 degrees, as in Northwest? Guess what - you'll soon be going Southwest, just like a miniature orbit. Equatorial headings - 90 or 270 degrees - are constant, polar ones - 0 or 180 - flip as you pass the poles (North, 0 degrees to the pole, then South 180 and back again at the South pole). Anything in between will oscillate +/- around 90/270, so; 315 - 270 = 45, 270 - 45 = 225. Your compass heading will vary +/- 45 degrees, across the range 225 to 315.

[Pandemic]

Thanks High Guard. I think that's what I didn't know.

Also, I have a bunch of fins for my test plane but how else can I mitigate side slip?