Crappy plane physics

[SmashingKirby148]

Almost all planes I make do this. The launch fine, but if I pitch the nose too high up, or go to the higher atmosphere, it does whatever crap is shown here. Please help.

[wasmic]

Can you show us a picture in the SPH with the CoL and CoM shown?

[SmashingKirby148]

Can you show us a picture in the SPH with the CoL and CoM shown?

[wasmic]

http://cloud-4.steamusercontent.com/ugc/538514066410222578/0EF9A6BAE6C4F6ECAC27BB812CA993165D61C8F1/

They're directly on top of one another. That will make the plane unstable. What probably happens is that as you burn fuel, the front of your plane becomes lighter. This causes the CoM to move behind your CoL, almost guaranteeing a flipout. The easy way to fix this is to move your wings further backwards, and if stability problems start during a flight you can just pump some fuel forwards. To pump fuel around, alt-rightclick on the two fuel tanks you wish to transfer fuel between.

[zarakon]

It's hard to tell exactly since I'm unfamiliar with the mods you're using, but it looks like you have your engines mounted below your wings, which puts your thrust out of line from your center of mass.

In the lower atmosphere, that's not a problem because your control surfaces can compensate for it. But when you go higher your control surfaces have less effect, so the misalignment of the thrust will cause rotation.

[capi3101]

Wasmic's got it. To be sure that's what's happening, install a mod called RCS Build Aid. Among its many useful features is a marker that will show you where the center of mass of your plane will be when the fuel tanks are empty. If it shows that the CoM will shift further aft, then wasmic has the problem dead on, and has given you the instructions you need to fix the problem. I think he's right anyway, but RCS Build Aid can verify it for sure.

[NASAHireMe]

You don't necessarily need RCS Build Aid. Just unload the fuel by right-clicking the tanks and you'll see where your CoM ends up.

[-Velocity-]

Is that plane yawing sideways? "Whatever crap" is a little vague. If it's yawing, I've had numerous problems like that. I will have perfectly stable aircraft that, at a certain altitude, just starts yawing sideways. I think it has to do with a thrust imbalance as engines begin to flame out or something. It's really annoying. The only way I "fix" it is to learn for each specific aircraft when it will usually start happening and switch to rocket engines before it starts. Oh, another way to fix it- and this only works for very small space planes, of course- is to just use a single jet engine

[SmashingKirby148]

Ok, thank's everyone.

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I put back the CoL, but it still does the stupid thing!

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Wait, I know what's going on. As I lose fuel, my CoM goes forward cause of a heaving thing in the cargo bay.

[cicatrix]

Consider your CoM a pivot. The following forces apply:

Gravity

Thrust

Lift

Drag

We won't be considering gravity here since it is bound to your CoM and pulls you down. Other forces are more interesting:

Each force applies torque to your craft. Simply imagine you hung your plane by its CoM and then the Drag force will push your intakes backwards (thus your nose will pitch up if you have positive AoA) and this is not very intuitive, the Lift force will pull your craft up by its CoL, and finally the Thrust force will push the center of thrust of your craft forward.

I can see you have a problem here - your engines are below and behind your CoM and thus their thrust will force your craft to pitch up, your intakes add to this as well and you do not have enough control authority to compensate this. Ideally, your CoT should be on the same axis with your CoM, your CoL should be behind it and if you want more stability - above your CoM.

It would help if you mentally hang your craft by its CoM and see which force will push where.

[SmashingKirby148]

Nope, it's still happening.

[How2FoldSoup]

I noticed you're not running FAR. As far(no pun intended) as I know, the b9 pack only works with FAR installed and I'm sure that could give you some aerodynamic instabilities. If that's not it, I believe the issue is the movement of CoM backwards during flight combo'd with your engines CoT being below the CoM making your plane want to backflip. It's probably some combination of those issues.

[percyPrune]

Zarakon may be right (post #5). Can you post a picture in the SPH with CoM and centre of thrust shown? If the engines are well below the dry CoM the nose will pitch up as the fuel mass decreases.

If you are having trouble with yaw (sideways drift), try increasing the size of the vertical tailfin or adding more of them.

Arrgh! Ninja'd by How2FoldSoup.

Edited February 6, 2015 by percyPrune
Can't type fast enough

[keptin]

If you're flying into space, then you're going to want your center of thrust in line with your center of mass. Asymmetric thrust will ruin your day unless you can counter it with enough RCS control.

Another thing to keep in mind is that you have a pusher-canard setup with a lot of mass aft. Canard designs are inherently sensitive to changes in mass, i.e. fuel burn, because their fulcrum is far aft instead of center.

Basic Aircraft Design - Explained Simply, With Pictures

[Hodo]

Nope, it's still happening.

If the craft is rolling then falling to one side or the other, it sounds like a stalling issue. Not sure about the stock physics if it calculates that but in FAR it will cause one wing to lose lift and make your craft roll to one side then fall out of the sky.... case and point, the Taiwan plane crash this week.

The only other guess I have is you are having issues with your thrust becoming uneven and forcing your craft to yaw one way then it will roll then spin out of control due to uneven thrust.

[SmashingKirby148]

It's doing these weird phantom forces without the engines even on...

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Also, when I get high up, the engines cut out at different times than each other.

[moronwrocket]

Your engines are being starved of intake air. KSP doesn't distribute intake air evenly, it depends on the build order. Get the Intake Build Aid Mod, or delete all your intakes and engines, and add them back like this

All intakes for engine 1 -> engine 1

All intakes for engine 2 -> engine 2

etc

Really, just get Intake Build Aid and hit F7 to autobalance

http://forum.kerbalspaceprogram.com/threads/104704-0-90-Intake-Build-Aid

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re: wierd phantom forces

At what altitude? Most likely, it is because CoL and CoM are not aligned. With the engines off, the CoL is pulling you one way and the CoM (gravity) pulling you another. If you don't have enough RCS/RW authority, your plane will start pitching (most likely down?). If you are turning sideways, maybe the plane is not horizontally balanced? Are all the tanks draining the same side to side? Are there any parts accidentally clipped into one side but not the other unbalancing mass or lift?

[ratchet freak]

That last is a known issue and depend oddly enough on the order you placed the intakes and engines in, interleave them so you first place some intakes and then the engine that they feed and then the nex set of intake and the engine that they feed etc.

[SmashingKirby148]

Your engines are being starved of intake air. KSP doesn't distribute intake air evenly, it depends on the build order. Get the Intake Build Aid Mod, or delete all your intakes and engines, and add them back like this

All intakes for engine 1 -> engine 1

All intakes for engine 2 -> engine 2

etc

Really, just get Intake Build Aid and hit F7 to autobalance

http://forum.kerbalspaceprogram.com/threads/104704-0-90-Intake-Build-Aid

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re: wierd phantom forces

At what altitude? Most likely, it is because CoL and CoM are not aligned. With the engines off, the CoL is pulling you one way and the CoM (gravity) pulling you another. If you don't have enough RCS/RW authority, your plane will start pitching (most likely down?). If you are turning sideways, maybe the plane is not horizontally balanced? Are all the tanks draining the same side to side? Are there any parts accidentally clipped into one side but not the other unbalancing mass or lift?

I Only have one, and thats at the front. It's build in with the S2 to Wide cargo bay.

[Alshain]

I Only have one, and thats at the front. It's build in with the S2 to Wide cargo bay.

I'm not entirely certain what issues do and do not affect stock aero but that could be your other problem. I know with FAR intakes at the front of the plane and far from the center of mass will create drag forces that pull the plane off in strange directions. Generally speaking you don't want high drag parts far ahead of the center of mass, intakes are high drag parts. That said, based on my understanding of stock, I don't think it should affect you the same way... unless you are in fact using NEAR which I cannot tell from the screenshots since it has no toolbar icon.

[moronwrocket]

How many jet engines do you have? KSP does not balance intake air among all engines, it gives it out excess air first come-first served based on build order. So as intake air decreases, with one intake, one engine will eventually get starved before the rest. Figure out that altitude, and close your intake with an action group and switch to rockets there, or add more intakes and balance them as above.

[How2FoldSoup]

I'm going to reiterate this point again, b9 parts do not play nice in stock aero. When you talk of "phantom forces" it makes me think more and more that it's the conflict between b9 parts and stock aero. Either install FAR(or near, but FAR is easier imo) or don't use b9 parts. Try using OPT plane parts or MkIV for stock aero without having to deal with mod incompatibilities.