My plane starts to roll left and right whenever I turn SAS on. (contains video)

[m1sz]

The tittle says everything. I have this small, simple plane, using B9 parts and having FAR installed. I was trying to make a highly maneuver plane. Although it looks stable in the VAB, and behaves good while flying, whenever i turn SAS on, it starts to roll uncontrollable.

Any idea of what could make this happen?

I made a quick video showing the plane and the problem:

Thanks in advance!

Edited November 30, 2013 by m1sz
Answered!

[Taki117]

This is Normal. What's happening is your SAS is making micro adjustments to keep you on heading, and in doing so is causing your plane to roll because of the large control surfaces (and engine vectoring) as well as the small size of your planet. If you want a maneuverable plane that doesn't do that use smaller wings with smaller control surfaces. Once you get to a higher altitude it will stop because there is less air, which means that you can maintain your heading better.

[DMagic]

Yeah, it's too much control available. You can do the same thing with reaction wheels, or at least you could in 0.21.1, even a one-seat command pod would get these kind of shakes when it was by itself with SAS on. They fixed the command pod, but any time you have enough torque or control surfaces with a small mass you can get this kind of thing happening.

[m1sz]

Thank you both!, I'll try to change the wings for a smaller ones, then I'll post the result! ^^

[Taki117]

if you use the Stabilators (i belive thats what they are called) they have a built in control surface. Also, depending on the mass of your plane you may never get rid of the wobble.

[gtmattz]

With FAR you don't have to replace the control surfaces. In the hangar open the FAR control panel and on the first tab you can tweak all your control surfaces. You will want to restrict the deflection and which movements are effected by the specific surfaces. For instance take the rudders and restrict them to only control yaw, then take the long control surfaces and lower the deflection and restrict them to roll and pitch. It may take a few trys but you should be able to tweak the setup and get a relatively stable plane.

[Hodo]

You really want to have two separate control surfaces for pitch and roll. Otherwise every time your aircraft needs to roll it will pitch slightly or if you need to pitch it will roll slightly.

And there is some moderate flex in your wings which is throwing off your control surfaces angle, this is also adding to the roll. I would use some more struts on the wings to keep them in place. B9 has some wonderful invisible struts just for that use.

[m1sz]

1st: My first attempt was to lower the angle of the control surface from 15 to 5. This way, the rolling stoped being noticeable, but was still occurring. The problem was, that the plane lost most of its acrobatics capabilities.

2nd: Tryed to change the angle from 15 to 7. Rolling was again noticeable, and the plane was not flying smooth enough.

3rd: Keeping the angle on 7º, I set some struts. Definetly, they helped increasing the critical speed when the plane starts to roll under SAS control. Plane was way more stable and his turning capabilities were very good.

4th: 2 different control surfaces for pitch and roll on the same wing. Keeping struts. Both control surfaces set at 15º. HEAVY rolling again.

5th: Keeping struts, I lowered roll CS down to 5º. Stoped from uncontrolled rolling under SAS, but crazy pitch stuff happent.

6th: Lowering pitch to 10º: FINALLY, it works good enough.

Total adjustments: two pairs of struts above and under the tip of the wings, 2 different control surfaces, roll surfaces down to 5º and pitch surfaces down to 10º.

Final test + some stunts here! (dont miss the end, the plane is capable of getting away of a very bad situation)

Thanks again!