Why does this spaceplane flip around?

[SmiteZero]

Even with minimal pitch inputs, at speed it will flip over almost without warning. I've noticed this behavior a lot in varying degrees lately but can't figure out why it happens to some designs and not others with similar CoM/CoL. Can someone please explain how to make sure spaceplanes behave with the new aero?

Edited October 3, 2015 by SmiteZero

[TheMoonRover]

All the control surfaces - and wings in general really - are right at the back, which would probably make it harder to control than if you had even two small winglets at the front to compensate, particularly with the CoM so far back - especially as the fuel runs out.

Also the direction of lift appears to be pulling back slightly rather than straight up (unless that's just the camera) which would tend to make it pitch up.

[Red Iron Crown]

It looks like there is more fuel mass ahead of the CoM than behind it, I think the CoM is moving rearward as the fuel is burned and getting behind the CoL, causing instability.

There's a great mod that shows CoM when fuel is expended, RCS Build Aid. Strongly recommended for diagnosing this sort of thing.

[SmiteZero]

Thanks for the replies. I can confirm that the problem is that despite overall balance being okay, the position of all the lift points is close to or behind the CoM as TheMoonRover was suggesting. Adding these small canards and re-balancing solved the problem.

[mattssheep4]

It looks like there is more fuel mass ahead of the CoM than behind it, I think the CoM is moving rearward as the fuel is burned and getting behind the CoL, causing instability.

Correct me if I am wrong, but I was under the impression that fuel (liquid fuel at least) was pulled evenly from all tanks now, so I'm not sure that would have an effect.

[Red Iron Crown]

Correct me if I am wrong, but I was under the impression that fuel (liquid fuel at least) was pulled evenly from all tanks now, so I'm not sure that would have an effect.

The CoM can still move. Imagine you have a vessel with more fuel ahead of the CoM and more dry mass behind it, as the fuel is burned off the CoM will shift rearward since the fuel mass is disappearing but the dry mass is not.

[Scarecrow88]

RCS build aid also has a display that shows not only where your CoM is but also where the empty CoM is. It's amazing how far apart the two can be sometimes.

[SmiteZero]

RCS-Build Aid is fairly awesome. I used to use it often. You are right in that as I start to run out of fuel my balance will change, too, however it should remain good until at least one tank runs dry

PWB Fuel Balancer doesn't seem to be updated and I haven't noticed anything else like it.

Edited October 3, 2015 by SmiteZero