Plane yaws/pitches/rolls slightly in air, even with SAS

[lewisd]

I'm trying to fly a long-distance flight, and it's proving to be a huge trial, because however much I might try to make my plane symmetrical, the yaw/pitch/roll continues to drift slightly once it's in the air.

Adding a couple SAS units (and plenty of power via RTG and solar panels) make it much more stable to actively fly (to the point that I can barely fly it without them), but they don't seem to do much, if anything, to counter the continual slow drift.

I've tried using MechJeb2's Spaceplane Autopilot to maintain a fixed heading and altitude, but every time I turn it on, it goes nuts and puts my plane into a spin.

Searching for other autopilot mods didn't turn up anything, other than a scriptable one, which seems way more complicated than I need.

What options do I have for a long flight, other than constantly babysitting it?

Edited October 7, 2013 by lewisd

[Vlk]

I dont think that you need too much sas. With the newest updates, the default sas works pretty good. At high altitudes and speeds, you have to experiment with it a little. It is not as straightforward as you might think. Try to set sas on at a certain angle, then dont correct it, see what its doing: It lets the plane raise a little, but then it holds it. With practice, you can make it so it holds exactly the desired angle.

One more thing: i know you said that you build symmetrical, but here is a few things you may missed: the airplane's thrusters has to be at the same height as the center of mass. If you build something on the top of the plane, or the landing gears are too heavy, then the center of mass is probably higher/lower than the center of thrust. And an other tip: lately i noticed that if i make some of the plane's wings a little angled (slight u shape from front view and/or side view), then it is more stable on low altitude as expected, but found that it messes up your plane a little at high altitudes/speeds.

[amo28]

Yea Mechjeb's spaceplane autopilot is garbage. The best use of that is to give you runway guidance -- never let it control your plane.

[lewisd]

Try to set sas on at a certain angle, then dont correct it, see what its doing: It lets the plane raise a little, but then it holds it. With practice, you can make it so it holds exactly the desired angle.

Not in my case. It keeps increasing the pitch until the plane is going straight up. (Now that I think about it, I have a probe core [this plane is unmanned {unkerbed?}] as well, so that's another set of reaction wheels I forgot I had, bringing the total to 3. Way more than I should need.)

One more thing: i know you said that you build symmetrical, but here is a few things you may missed: the airplane's thrusters has to be at the same height as the center of mass. If you build something on the top of the plane, or the landing gears are too heavy, then the center of mass is probably higher/lower than the center of thrust.

Interesting, I didn't really think about that. I was so focused on making sure the center of mass and center of lift were appropriately aligned, that I didn't even think about the thrust. I bet the RTG, solar panels, and tail fin, that are all on top of the plane, are raising the center of mass just enough that the thrusters are below it, making the plane want to pitch up, as it's doing.

And an other tip: lately i noticed that if i make some of the plane's wings a little angled (slight u shape from front view and/or side view), then it is more stable on low altitude as expected, but found that it messes up your plane a little at high altitudes/speeds.

That's interesting too. I'll see about giving that a try, but I think making sure the thrust is alligned vertically with the center of mass is probably the best place to start.

Thanks a lot for the great tips!

[Traches]

Instead of mechjeb's spaceplane autopilot, try using its smart A.S.S. in "surf" (surface) mode. Just remember when turning that just plugging in a new heading won't work very well, you've also gotta put in 25-30 degrees of bank in the correct direction and take it out when you make it to your desired heading.

When hand flying, Alt+WASD will trim your aircraft in that direction. In other words, you can adjust your pitch trim to where the nose stays where it is without requiring constant control input, make it far easier to fly.

[lewisd]

Instead of mechjeb's spaceplane autopilot, try using its smart A.S.S. in "surf" (surface) mode. Just remember when turning that just plugging in a new heading won't work very well, you've also gotta put in 25-30 degrees of bank in the correct direction and take it out when you make it to your desired heading.

Neat, I'll give that a try. I haven't looked closely at MechJeb's SASS, but it sounds like I should.

When hand flying, Alt+WASD will trim your aircraft in that direction. In other words, you can adjust your pitch trim to where the nose stays where it is without requiring constant control input, make it far easier to fly.

Oh, I forgot to mention, I did try that. It sort of worked to control the roll. I say "sort of" because I was able to slow down the roll, but not stop it completely. As for pitch, no matter how much trim I tried to apply, the plane would still always slowly pitch up. I'm not sure why the trim didn't help.

[MrPopcup]

it Looks like you have some weight on either side. check your Center of mass if it's correct ( at center ) or shifted a little, that can cause these things.

I face this problem usually while making a rover, it turns by itself , but I know that it's only a shift of mass problem.

so yea, try it out!

[-Velocity-]

I've had weird problems like this. I have rudder pedals and joysticks; sometimes I used to get "ghost" inputs, even AFTER I unplugged all of my controllers, and even if KSP did not have any assignment at all for anything beyond the keyboard!. Even if I unplugged them and restarted KSP, the ghost inputs would remain. Literally, KSP would have no controllers set up to use (and there never WERE any controllers set up for KSP to use!), there would be no controllers plugged into the computer anymore, and yet, every time I would jump into a spacecraft, it would slowly spin up.

Anyway, a solution to eliminate these ghost inputs that seems to work 100% of the time appears to be to simply unplug all my controllers, and restart the computer entirely. After that, my spacecraft would always stay still perfectly still after coming out of time compression until I actually applied an input via the keyboard.

KSP has some serious issues with controllers, IMO. The interface is awful (no way to set curves!!!), it's buggy, and the game just doesn't seem to respond well to actual flight simulation control hardware even if everything is working correctly. Hopefully, this area receives some attention in the future from the KSP devs.

[lewisd]

it Looks like you have some weight on either side.

That really puzzles me, because everything on the plane was either added as a single unit along the centerline, with "snap angles" on, or with symmetry.

I've had weird problems like this. I have rudder pedals and joysticks; sometimes I used to get "ghost" inputs, even AFTER I unplugged all of my controllers, and even if KSP did not have any assignment at all for anything beyond the keyboard!. Even if I unplugged them and restarted KSP, the ghost inputs would remain. Literally, KSP would have no controllers set up to use (and there never WERE any controllers set up for KSP to use!), there would be no controllers plugged into the computer anymore, and yet, every time I would jump into a spacecraft, it would slowly spin up.

Anyway, a solution to eliminate these ghost inputs that seems to work 100% of the time appears to be to simply unplug all my controllers, and restart the computer entirely. After that, my spacecraft would always stay still perfectly still after coming out of time compression until I actually applied an input.

Hmm, that is interesting indeed. I *do* have a joystick hooked up, but I assumed the center dead zone would prevent this sort of thing from happening. Any idea why that wouldn't be the case? Does KSP not configure any deadzone by default? Is there a way to configure a dead zone?

[Traches]

V, is it always the same direction? Maybe you put some trim in and don't realize it-- alt+x will reset it.

[Somerled]

The dead zone sometimes doesn't register. For mine, I just tap the stick a few times in different directions whenever I see a joystick-assisted-tumble. It usually picks up the dead zone after that mini calibration.

[-Velocity-]

That really puzzles me, because everything on the plane was either added as a single unit along the centerline, with "snap angles" on, or with symmetry.

Hmm, that is interesting indeed. I *do* have a joystick hooked up, but I assumed the center dead zone would prevent this sort of thing from happening. Any idea why that wouldn't be the case? Does KSP not configure any deadzone by default? Is there a way to configure a dead zone?

Unless you can program the joystick with external joystick software, no you can't set curves or deadzones in KSP, at least to my knowledge. KSP has the worst joystick support of any game that supports joysticks that I have ever seen. True, KSP isn't truly finished yet, and that's why it gets a pass in this category... at least FOR NOW.

Keep that joystick unplugged during the entirety of your computer's operation, and see if your problem goes away.

[lewisd]

Damn, that's frustrating. I got a joystick for the sole purpose of making the planes in KSP easier to fly. Oh well. I'll see if unplugging it (and rebooting) helps any. Thanks.

[ChevronTango]

Not in my case. It keeps increasing the pitch until the plane is going straight up. (Now that I think about it, I have a probe core [this plane is unmanned {unkerbed?}] as well, so that's another set of reaction wheels I forgot I had, bringing the total to 3. Way more than I should need.)

If it's pitching up with SAS over a long distance flight then that's because the sas is trying to keep your heading from an orbital point of view, not a surface one. The practical upshot is that if you travel horizontally with SAS, when your 1/4 of the way round the planet you'll be pointing vertically up, similar to what you'd notice if you had a satelite orbiting the planet.

[Geschosskopf]

I've noticed on my planes that with SAS on, the control surfaces never seem to sit quite straight in a neutral position. They always seem to have a fraction of a degree of deflection 1 way or the other. And they sit this way not moving for several seconds, then SAS gives them a tiny flick to maintain the set attitude, repeat. The net result is that, over long flights, the plane retains the desired pitch angle but gradually drifts off in yaw and/or roll, almost always to the right, even at normal time speed. This seems an unavoidable part of the game.

I have made planes that compensate for this, however. My usual solution is to put 1 asymmetrical radial air scoop on the left side of the fuselage. The drag from this pulls the nose left, the unavoidable slight control deflections pull the plane right, and the result is that the plane maintains the desired heading although flying at a slight angle in yaw. This is fine with me because on long flights, I'm interested in the heading, not so much the attitude of the plane. I'd rather have the plane crab a bit as it flies along, staying on course forever so I can use time warp, than drifting off course and me having to correct it every few minutes.

There's another tip that might not be applicable to your situation but which is pretty useful anyway. Just reset the SAS. Turn it off for a couple of seconds while holding the plane where you want it manually, then turn SAS on again. It seems like SAS dozes off eventually and you have to wake it up .

[merlinux]

Wow, very happy I have found this thread! So the last week I have been playing with spaceplanes finally. Ive come out with a few design and was wondering why they always want to pitch down with SAS on. Spent hours trying to fix those issues and I now feel like I know how to make good SSTO. However I am playing with a xbox 360 gamepad, and I didnt know this was an issue In the mean time I have played with designs that I got from spaceport and they sometimes had the same issue.

So after reading this thread and rebooting my pc without the gamepad plugged in suddenly everything works and my design are flawlessly getting into orbit, with sas working as it should....

But I cant see any goal in flying planes/spaceplanes without my pad as it takes away most of the fun, for me anyway...

So from what I understand the pad is giving false input that makes the planes wanting to go a certain direction at all times, for me its pitching down...

How the hell do I fix this ? I am using a xbox 360 wireless controller btw

[lewisd]

How the hell do I fix this ? I am using a xbox 360 wireless controller btw

Cool! What are you using to do that? I have an xbox 360 wireless steering wheel I want to use on my PC, but couldn't find something that would let me.

[lewisd]

I don't know what fixed it, whether it was unplugging the joystick, or, I think more likely, making sure my center of thrust is vertically aligned with my center of mass, but I'm having a bit more luck with planes. Thanks for all the advice.