SSTO Wiggle at 12,000-17,000 meters?

[TronX33]

So, using the OPT spaceplane parts mod, I attempted to build a not-yet-confirmed SSTO (by that I mean I don't know if it'll make it to orbit. Anyway, at around 12km to 17 km, it starts to wiggle, not side to side, but like small minute roll commands, but this happens ith SAS and without it. I'm playing with (don't kill me) stock areodynamics. This eventually gets so bad that I lose control, and spin out. I checked my thrust on my engines, and they are the same. I would appreciate any help. I ill provide .craft file if needed.

[capi3101]

What you're describing sounds a bit like Dutch Roll. I'd have to do more research to figure out ways to counter it - meantime, does that article accurately describe what your plane is doing?

[TronX33]

It's similar, with yawing making it worse, but without any yaw, at that altitude, it just starts rolling. Its slightly like the diagram in the article, but the central body of the plane still roughly remains in that red circle, until I try and counter the problem, then it goes out of control. Doing nothing still eventually leads to spinout though. It just takes longer that way.

[AaronLS]

1) If you have any wings or control surfaces which are large enough to flex at all, then you can get roll or other anomalies. Perhaps if you have large wings or heavy objects mounted on them.

Struts can sometimes help relieve that problem.

2) If you have angled control surfaces then yawing can cause roll and vice versa. This can make STS ineffective, and with STS off attempts to counter roll may make it worse.

I used to have issues with yaw causing rolling and vice versa. Dedicated control surfaces that don't serve dual purposes helped.

Essentially same thing you have on real planes:

Ailerons: These are the controls on wings. Disable Yaw and Pitch, leaving only Roll.

Elevators: These should be far back on the plane. Disable Yaw and Roll, leaving only Pitch.

Rudder(s): These should be far back on the plane. Disable Roll and Pitch, leaving only Yaw.

Elevators should not be angled, and rudders should be perfect 90 degree angle.

You can probably find a way to work outside these guidelines, but you got to know what you're doing. If you try to put two rudders on top at 45 degree angles so they can double as roll/yaw, then a roll will generate some yaw, and yaw will generate some roll. In such a situation STS will fight itself, and with STS off you will be fighting yourself as you correct for yaw you will then have to correct roll, etc.

[Nansuchao]

It can happen for the fuel consumption that cause your nose to be lighter and yaw in one direction or another. Or not sufficient yaw authority from the control surfaces as said from others.

[AaronLS]

It can happen for the fuel consumption that cause your nose to be lighter and yaw in one direction or another. Or not sufficient yaw authority from the control surfaces as said from others.

Yeh good point, worth checking. I've had this happen where I had side tanks that became uneven, between each other, but I only would lose control once I got to upper atmosphere(30k+) where control surfaces had less effect.

[TronX33]

Yeah, the plane in questio did have angled control surfaces and heavy ings, bu I strutted them together. I also made a version with straight wings, tails, and control surfaces. The effect still happens. I use TAC fuel balancer BTW, so I don't think it's the CoM moving to a unstable position. I appreciate all the replies though.

[juanml82]

How many engines are you using? Check, by using KER, your intake air supply and demand. It is possible, once intake air becomes low, for engines to receive a different amount and produce different thrust

[TronX33]

I'm using two Screamjet engines from OPT, but I checked, they have symmetrical thrust

[cybersol]

You are basically at the point where we are going to need pictures. Especially helpful would be one with the aero drag arrows turned on at the point just after it starts happening.