Ghost rolling on aircraft (aero and symmetry related)

[Vegetal]

So, apart from the usual truckload of bugs, I always lived with a problem in KSP: Some of my planes, or maybe all of them (some just not as noticeable), have a tendency to naturally roll to either side uncommanded.

It always bugged me, I kept checking the symmetry, everything was all right. At least I thought so:

I just made this small plane, just screwing around, when I noticed this. See how the CoL isn't aligned longitudinally? WHY for the love of Kod? Everything was made with mirror symmetry and angle snapping, there is no way this is assymetric....anyone knows what is the problem? I tweaked the vertical stabilizers a bit with the rotation tool and it seemed to adjust it a bit, but not perfectly, the craft still has a tendency to roll.

[Tex_NL]

I've seen similar problems. Based on my experience double vertical stabilizers aren't always mirrored correctly. In normal flight one should generate lift to the left the other to the right cancelling each other out. If both 'point' in the same direction you'll start rolling.

What you could do is remove the entire engine-wing-tail assembly on both sides and reattach with mirror symmetry. With a little luck this will reset symmetry.

[Evanitis]

I'd suspect something having radial symmetry when it should be mirror. My trick to make sure it doesn't happen is to grab the root part of the engine-wing construction, make sure the symmetry is set to 'mirror' ('R' key switches that) and reattach it. If the problem presists do the same with the rest of the parts.

And ninja'd.

[Vegetal]

As I said, I made sure I was using mirror symmetry. The game doesn't seem to place some pieces precisely though, I'm gonna try reattaching the whole assembly tomorrow, now I gotta go sleep, thx guys.

EDIT: Oh, I tried installing them completely vertical, same problem

[FancyMouse]

I saw you more than one engine - check whether the thrust from two engines are the same. If not, then that's the problem (which means you'll need IntakeBuildAid mod if you don't want a manual tedious fix).

EDIT: never mind you said roll - if it yaws to one side then what I said could be relevant, but if rolls - I'm not sure.

Edited July 29, 2015 by FancyMouse

[steve_v]

There was an issue some time ago: unequal attachment strength / rigidity on mirrored parts causing uneven wing flex and therefore roll, I'm not sure if it was fixed or not. It would be easy to test with the addition of some struts though.

[Vegetal]

Well Steve, what you said makes sense. I have a tail dragger design which clearly flexes the wings asymmetrically under heavy loads.

I just don't think it's the case here, the craft is already asymmetric in the SPH, without any load.

[steve_v]

YAnother long shot: You don't have any doubled up parts (z-fighting is a dead giveaway) do you? I find that the symmetry system does get a bit janky at times, and can end up duplicating children of mirrored parts inside each other.

Failing that, I usually just pull bits off until I find the culprit. Do you see the same if you duplicate the design from scratch, or is it just this particular craft?

It also looks to me that your lift vector is somehow rotated anti-clockwise, however I've been running FAR for so long I have little idea what stock aero is up to these days.

[Vegetal]

YAnother long shot: You don't have any doubled up parts (z-fighting is a dead giveaway) do you? I find that the symmetry system does get a bit janky at times, and can end up duplicating children of mirrored parts inside each other.

Failing that, I usually just pull bits off until I find the culprit. Do you see the same if you duplicate the design from scratch, or is it just this particular craft?

It also looks to me that your lift vector is somehow rotated anti-clockwise, however I've been running FAR for so long I have little idea what stock aero is up to these days.

Lol the rotated lift vector is exactly what I wanted to show with that pic

A heavy lifter I created in my career game got the same problem, I kept removing and replacing stuff on it until it became bearable to fly, but the effect is still there. God this game is so alpha...

[steve_v]

Yeah. Dunno who decided to slap a big "1.0" on the side, but they must be playing a different game to the one I bought.

[MaverickSawyer]

I had come to the same conclusion a while ago that the parts were not lining up right... but I usually encountered this with three tails (two mirrored, one not).

I've never come up with a sure-fire test or solution for this, but it does seem to be more focused on the all-moving surfaces like the canards, and not elevons or the Delta Delux fin.