Question about Col and CoM for Spaceplanes

[Cranium]

I have been really getting into spaceplanes, and loving the SPH. I have noticed that the CoM and CoL needs to be balanced a certain way, and after reading ont he forums, a general concensus is that a CoL slightly behind(towards the tail) is the most stable.

However, what about above(sky) or below(ground)? I have wondered if that could be what is causing my spaceplane to go crazy once I turn on the rocket engines? It really likes to suddenly pitch up, and do flips as soon as I turn the jets off, and turn the rockets on.

The only thing I can think of, is that my CoL is quite a bit above my CoM. It is still slightly behind, but more than half of it is above.

Does anyone have any ideas for this? I am trying to make a large, heavy refueling plane that I can dispatch anywhere on Kerbin, but since I have to carry so much weight, instead of building my wings out, which is ugly and less stable, I built them UP.

[rodion_herrera]

I built them UP.

Post a snapshot.

[Cranium]

Post a snapshot.

Can't until tonight. I'm at work right now, but I'll show you when I get home.

[Tex_NL]

Have you looked at your line of thrust yet. If your jet engine thrust isn't exactly lined up with your CoG it isn't much of a problem but rocket engines are way more powerful and could flip your plane.

[bsalis]

I have noticed that the CoM and CoL needs to be balanced a certain way, and after reading ont he forums, a general concensus is that a CoL slightly behind(towards the tail) is the most stable.

However, what about above(sky) or below(ground)?

Yes, slightly behind. But make sure that the CoM does not drift behind the CoL after fuel drain when you want to re-enter and fly again.

Ideally CoL should be a bit above the CoM as well for stability, but it's not a big issue if it is not. It should not be too far above, since then the Center of Drag (CoD) will probably be higher above the CoM. This drag will pull back, from above, causing the plane to pitch up. Wings also have a little mass, so high CoL means high placed wings, so this will raise the CoM above the CoT (unless balanced with dry mass below)... so theres more pitching up from that too.

I have wondered if that could be what is causing my spaceplane to go crazy once I turn on the rocket engines? It really likes to suddenly pitch up, and do flips as soon as I turn the jets off, and turn the rockets on.

Without seeing a pic I would say that the CoM is above the CoT, and that the rocket thrust is higher than the jet thrust, so you don't notice the problem until you switch. There can be other reasons and contributory factors though.

[Cranium]

I now have pics.

I did move some wings underneath and to the front of the plane.

[Cranium]

I just tried getting it into orbit, and it just went absolutely crazy....

As soon as I switched off my jets, it started flipping, and I hadn't even turned on my rockets yet.

[Tex_NL]

Engines tend to take fuel from the most distant tank first. From your images I guess that would be the module directly behind the cockpit. If a tank in front of the CoG is getting lighter the CoG will shift to the rear putting it in your case behind the CoL.

A possible solution could be to move the wings slightly more to the rear putting the CoL a bit more behind the CoG. This way you'll make sure the CoG never gets behind the CoL.

An in-flight solution could be to ignite your rocket engines before you disengage your jet engines