Wobbly SAS in planes - workaround

[Norcu]

I think the problem with the SAS  shaking the plane apart is based in the dual use of large surfaces as both roll and pitch control.

As soon as there is a little roll induced, those make overcorrections when having input in both axis (and there is nearly always some Pitch Input) that destabilise pitch and roll simultaneously, leading to the runaway of control.

 I’ve reduced this problem by a large margin in just splitting those in two distinct surfaces, the big elevators solely for the pitch, and on the wingtips a separate small wing with a small aileron for roll (cave: if that is too big/has too much authority, it will reintroduce the problem) (the roll on the rudder should be switched off too) 

tldr: Three separate surfaces for the three axis of control, roll as small as possible and on the wing tip. 

Edited March 5, 2023 by Norcu

[Mutex]

Basically you can mitigate the issue by reducing pitch authority, which is effectively what you're doing here. You can also reduce the maximum angle of control surfaces.

[Sequence]

You can also press capslock to enable fine control, it affects SAS as well as manual controls. Don't forget to turn it off after leaving the atmosphere though. But you're right,  it's best to separate control authority onto different wings anyway. 

[Norcu]

1 hour ago, Mutex said:

Basically you can mitigate the issue by reducing pitch authority, which is effectively what you're doing here. You can also reduce the maximum angle of control surfaces.

I’d say limiting roll authority is the most important, I never had problems with large pitch authority as long as the same large surface didn’t input roll. Sometimes the tiny ailerons keep flapping, but because they are so small the plane keeps flying straight and only rolls with a somewhat sustained input on the surface. 

[Mutex]

25 minutes ago, Norcu said:

I’d say limiting roll authority is the most important, I never had problems with large pitch authority as long as the same large surface didn’t input roll. Sometimes the tiny ailerons keep flapping, but because they are so small the plane keeps flying straight and only rolls with a somewhat sustained input on the surface. 

That's possible yes, roll requires less energy usually so it's probably more sensitive.

[Daniel Prates]

Maybe this is being taken for granted as nobody is mentioning it,  but I would also add, beware if one of your command parts has reaction wheels on. The reaction wheels and the control surfaces will be conflicting with eachother and that may cause the wobble.

[Wheehaw Kerman]

On 3/4/2023 at 7:24 PM, Norcu said:

I think the problem with the SAS  shaking the plane apart is based in the dual use of large surfaces as both roll and pitch control.

As soon as there is a little roll induced, those make overcorrections when having input in both axis (and there is nearly always some Pitch Input) that destabilise pitch and roll simultaneously, leading to the runaway of control.

 I’ve reduced this problem by a large margin in just splitting those in two distinct surfaces, the big elevators solely for the pitch, and on the wingtips a separate small wing with a small aileron for roll (cave: if that is too big/has too much authority, it will reintroduce the problem) (the roll on the rudder should be switched off too) 

tldr: Three separate surfaces for the three axis of control, roll as small as possible and on the wing tip. 

I agree.  I’ve been using advanced controls and setting ailerons to roll only, elevators to pitch only, and rudder to yaw only, and getting zero flap.  Then I built a very 1950s single engine delta winged design, with ailerons set to roll and pitch, and got flap.

[Wheehaw Kerman]

Also, something I just remembered: the red out-of-fuel aircraft sneak peek we got had a second set of control surfaces on the wingtips.  This might have been a one-axis-per-set of control surfaces workaround for the SASflappies.

[didkodidko]

It's so called PID gain. P is hight, D is low in current state. If you have flown planes or quads it's easy to spot bad PID setup.

[shdwlrd]

You can turn off the reaction wheels. This will help a lot with the flappy planes. Also, trim your planes or design them with where they require little trimming. 

This is related to the SAS torque bug. It seems that the reaction torque is being applied 90° from prograde.