Why do my plane controls sometimes switch?

[delta wee]

I built a plane and when I flew it and pressed the Down arrow key the control surfaces went up and the nose went up. Then I changed the shape of the wings and now when I push the Down arrow key the control surfaces go down and the plane's nose goes down. Is this a bug, or am I not understanding something?

[bewing]

Most likely you have swept wings on your plane.

Imagine a canard-wing system, with control surfaces at the very front of your plane. If you want the nose to go up with canards, which way do the control surfaces need to push? They need to push UP. OK, now you have the control surfaces at the back end of the plane. To make the nose go up, which way do the control surfaces need to move? They need to push down.

So what's the rule? The rule is: control surfaces need to move in opposite directions depending on which side of the CoM they are on. A control surface that's at the CoM has no control authority at all, and should not move.

And that's what the game tries to do. Figure out where each control surface is, relative to the CoM, and then make it move in opposite directions (or not at all) depending on that CoM position. But it doesn't always do a perfect job of locating the CoM or the control surface when it does this quicky calculation -- swept wings cause all sorts of difficulties for the game. It takes its best shot and guesses. If it consistently guesses wrong, then you can use the "control authority" slider for that control surface, to make it move in the opposite direction.

But if your control surface is close enough to the CoM to cause it to move in the wrong direction for pitch control -- then you should probably turn pitch control off for that control surface in the first place.

Edited March 22, 2019 by bewing

[Boris-Barboris]

22 minutes ago, delta wee said:

Is this a bug, or am I not understanding something?

Yes, and an old one, you'll have to invert such surfaces if you want to keep the sweep.

[delta wee]

1 hour ago, Boris-Barboris said:

Yes, and an old one, you'll have to invert such surfaces if you want to keep the sweep.

Thanks, I had to invert the roll controls also, but I finally got it to work so the SAS kept the plane stable.

[pincushionman]

On 3/22/2019 at 4:44 PM, delta wee said:

…so the SAS kept the plane stable.

Tip (not pro tip, I can’t claim that): Don’t do this. SAS makes a really bad autopilot, and you’re much better off learing how to use trim (alt+control) and fine control mode to fine-tune your plane in flight.

[Reinhart Mk.1]

On 3/23/2019 at 7:26 PM, pincushionman said:

Tip (not pro tip, I can’t claim that): Don’t do this. SAS makes a really bad autopilot, and you’re much better off learing how to use trim (alt+control) and fine control mode to fine-tune your plane in flight.

im torn because of pride but i feel like if you make a plane very aerodynamically friendly (with a full fuel loud and without) sas DOES assist you... but if you made a plane PERFECTLY aerodynamic SAS might hinder you