Aircraft yaw in flight and spinning out

[BenH22]

I build an aircraft based on a EE lightning using procedural wings and parts. My CoL is behind my CoM and my trim is reset to 0, at least I pressed alt-x many times. This is strange since it always yaws to the left and it should be symmetrical. 

[Guest]

Question: when does it yaw left? Already on the ground, or in flight? (Edit -- re-read the thread title, in flight then.)

If it's on the ground, it's probably a landing gear issue -- something isn't quite straight, or your wings are flexing so much it makes the main gear go off straight, which then causes the craft to veer. (Why always left only Kraken knows.)

If it's in flight, then it's possibly something that's not quite as symmetrical as you think: probably a part that's surface attached when you think it's node attached. Enable the aerodynamic overlay (F12) and see if there's an asymmetrical drag vector somewhere; that'll give you an idea of what the problem part is.

It could also be because of a centre of pressure issue. Your tanks aren't node attached, which means the KSP aerodynamic model will treat them counterintuitively. It might be it "sees" a big flat surface instead of the pointy-ish nose you're trying to make. A draggy nose has the same effect on stability as moving the CoL forward. Solution to this would be to redesign with a pointier nose, with node-attached, non-rotated parts only near it.

Edited March 1, 2018 by Guest

[BenH22]

Problem solved, thanks. I used structural wings clipped into the fuselage to attach the wings and as soon as I turned they created a sideways force even though they were inside the fuselage, like putting a verticle fin at the front of the plane. 

[RizzoTheRat]

Dunno why you'd get the initial yaw, but it's it spinning out then it's because your side on centre of pressure is behind the CoM, ie as it starts to turn to the side there's more surface area, and therefore more drag, ahead of the CoM than behind, making it unstable.  Easiest fix is to fit a bigger vertical stabiliser.  The stabiliser on the F3 is actually longer than on the F1, presumably to compensate for increased fuselage length and/or CoM shifting between designs.

 

ETA:  D'Oh, dunno why it hadn't shown me your reply until after I posted

Edited March 1, 2018 by RizzoTheRat

[Snark]

56 minutes ago, BenH22 said:

I used structural wings clipped into the fuselage to attach the wings and as soon as I turned they created a sideways force even though they were inside the fuselage, like putting a verticle fin at the front of the plane. 

Ah, there you go.

It's important to understand that clipping isn't real.  It's eye-candy only.  The game makes no attempt whatsoever to determine whether one part is "inside" another part, in terms of its (X,Y,Z) location-- all it cares about is which parts are attached to which other parts.  Clipping something inside a part has zero effect on its aerodynamics, and the part will behave exactly the same as if you had attached it on the outside where you can see it.

KSP does a pretty good job, in general, of "WYSIWYG physics"-- i.e. if a thing looks like it should behave in a certain way, it generally comes reasonably close to that. That's great, and it's a big part of what makes KSP fun:  you don't have to be an aerospace engineer to build a working plane.

However... as soon as you start clipping any parts, you completely break that "WYSIWYG" behavior, and you will end up with a craft whose visual appearance does not match its physics.

Therefore, it's important to bear the options in mind.

• Option 1:  Don't clip.  This is simple, easy, and it makes playing the game straightforward because you generally won't get mysterious behavior that makes no sense to you.  (Also, if you have a problem, it's easy to get help from people because if you post a screenshot, they can instantly see what the problem is.)
• Option 2:  Become a guru-level expert at KSP physics so that you completely understand exactly how the physics model works, and then clip.
• Option 3:  If you mainly care about looks rather than in-flight performance, just go ahead and clip to your heart's content.    But be prepared for odd behavior.

None of the above options is objectively any "better" than the others-- it completely depends on what your priorities are as a player.  For example, I strongly prefer option 1 in my own gameplay, mainly because to me, KSP is a physics game and clipping just... bothers me, somehow.  And I'm not finicky about looks-- I'm fine with the physics determining the craft's appearance, rather than the other way around.  But a different player with a different set of priorities would come to a completely different conclusion about the right approach.