Why does my SSTO flip through the air like crazy?

[omgimonfrie]

At first I was thinking it was because it was using the fuel towards the front of the plane first, so I installed the fuel balencer. And it still starts flipping theough the air like mad. The higher and faster I go the more the separation between the direction its pointing is, and the direction its traveling. At about 35-40 degrees of separation the nose just goes up and up and up and then past dead straight up, and keeps going and going. I cannot figure out why this is happening.

I have had this happen with other planes as well, some of them very stable designs. And also it happens occasionally with a rocket as I am passing into orbit sometimes.

Edited October 10, 2013 by omgimonfrie

[SandMan13]

Check your center of thrust. Sounds like it is below your center of mass. If that happens, even slightly, it will push your nose up.

[I_Killed_Jeb]

post a picture of your plane in the SPH with the center of mass and center of lift turned on. sounds like you might have the two too far off. i think within the game physics you'll always have higher angle of attack as you get higher due to reduced wing lift

[Moar Boosters]

Did you make sure the center of lift is behind the center of mass?

[briansun1]

don't you mean in/in front of?

[omgimonfrie]

The center of thrust is directly on the center of mass. And my center of lift is directly on my center of mass as well. At normal speeds and altitudes it is a very stable plane.

I'm starting to think its an issue in my KSP folder itself; Just a few minutes ago I was sending up a refueling rocket carrying 1.5 orange tanks worth of fuel and it started flipping through the air as well. About an hour ago it started wobbling in a circle as I neared orbit, but not doing complete circles. Then I got annoyed and went to the SPH. My planes were flying ok then. Didn't try that SSTO this time. But as time went on and on they started flipping through the air again. Its like the longer I play this game the more issues (specifically this one) arise. If I shut it down and start it back up they all go on the back burner for at least an hour.

Now when I closed out the game like 2 seconds ago, I went back to the space center where you can see all the buildings and I could not get a single building to highlight and become selectable. The only thing I could click was the quit option.

Then when I was back at the main log in scree, again nothing would highlight. The only thing I could select was "check for updates." I had to force quit the game. I am using a MAC OSX. I'll go post up the SSTO on my spaceport profile and link it here in a few moments so you guys can see if the thing works on your game. If there is interest I'll also post the refueling tanker I had the flipping issue with as well. Its a solid design, I just keep running into this software issue. At least I think its a bug in the game???

-Thanks for the input/help fellas!!!!

[Hodo]

Ok if your CoM and CoT and the CoL are all on top of each other you will end up with an aircraft that is riding on the head of a pin. If it so much as thinks of moving an ounce of dust from the nose to the tail it will shift the center of mass far outside of what the craft was set for when built. So your best bet is to shift the CoM a bit in front of the CoL. So that the arrow pointing up is just through the back of the yellow and black ball that is the CoM.

[omgimonfrie]

Here is a link to the plane in question: http://kerbalspaceprogram.com/ssto-experimental/

While I was trying to upload it the page wasn't loading so I spammed the upload button. So it popped up on my profile 5 times. I am now trying to figure out how to delete 4 of them.

I'm not so sure thats the issue. I always build my planes with the lift and mass centered on each other like real planes are built. If the mass is in front of the lift it will want to nose dive. If I ues the trim feature to kill the nose diving, well all that does is move the center of lift right on the center of mass. Why not build it that way from the start?

[enki]

I'm missing a large selection of the parts included in that .craft file so I was unable to peruse it. Perhaps a screenshot would be better, but I did have some concerns. In order for your plane to maintain its altitude how pitched up is the nose? If the prograde marker is more than maybe 5 degrees below your orientation you might want to re-think how much wing is on the plane. I've also had good luck with pitching a bit of the wing surface up to counteract a little of the CoM being in front of the CoL and so that when I'm pointed at the horizon I can maintain level flight. If the separation there is too high you're basically climbing on engines with some pretty wings tacked on for decoration. Also, wings generally don't perform well when there is little to no air. Did you include a SAS module in the build? I found that they help a ton to keep my plane pointed the right direction once the air is too thin to be of much help.

[Saberus]

Ok if your CoM and CoT and the CoL are all on top of each other you will end up with an aircraft that is riding on the head of a pin. If it so much as thinks of moving an ounce of dust from the nose to the tail it will shift the center of mass far outside of what the craft was set for when built. So your best bet is to shift the CoM a bit in front of the CoL. So that the arrow pointing up is just through the back of the yellow and black ball that is the CoM.

So much this. If you're all centered up in the SPH, as soon as you start burning fuel your CoM is going to creep fore or aft.

Another thing to consider is stall angle. Even in KSP's default drag system, if your nose lifts too high and the angle of attack on the wings is too great, the CoL changes, the amount of lift generated changes or drops off completely.

having not seen your plane I would recommend adding reaction wheels to the rear of the craft to give you a more centered "magic torque" that will help you pitch the plane even if you go into a stall. If your craft is nose heavy and all the torque is in the front, its hard to lever the rest of the craft. It's just as bad if you're tail heavy and all the torque's in the front.

[Hodo]

Here is a link to the plane in question: http://kerbalspaceprogram.com/ssto-experimental/

While I was trying to upload it the page wasn't loading so I spammed the upload button. So it popped up on my profile 5 times. I am now trying to figure out how to delete 4 of them.

I'm not so sure thats the issue. I always build my planes with the lift and mass centered on each other like real planes are built. If the mass is in front of the lift it will want to nose dive. If I ues the trim feature to kill the nose diving, well all that does is move the center of lift right on the center of mass. Why not build it that way from the start?

I tried downloading it, but you are using some mods I don't use. It would be helpful if you listed the mods you are using.

Here is a pretty basic picture of how my fastest SSTO is set up.

This is the "Jackal".

Notice how the CoL is just behind the CoM. And that the CoT runs right through the CoM. It doesn't matter if you are running FAR or the basic KSP without mods, it holds true for both.

[RoboRay]

don't you mean in/in front of?

No. CoL in front of CoM guarantees negative dynamic stability and the plane will actively try to turn itself around backwards in flight.

CoL and CoM in the same place yields neutral dynamic stability, where the plane has no preferred orientation in respect to the airflow and will easily tumble out of control.

Only with the CoL behind the CoM do you get positive dynamic stability, where the pointy end of the plane tries to keep itself out front.

Lift is a form of drag. An object moving through a fluid (like air) will rotate on it's own to seek a position where the drag is behind the center of mass. It's like a weathervane, or an arrow shot from a bow... the feathers are on back, producing drag, so the point always turns to face the wind.

Edited October 10, 2013 by RoboRay

[3_bit]

I had this problem a lot when I was playing around with SSTOs a while ago, but never bothered to ask. I guess I owe you my thanks, omgimonfrie, for making this thread.

[omgimonfrie]

Well you're welcome. I'm starting to see the logic in having the center of mass in the front. I'll try that on a new design.

In the mean time, you guys should try out my 2 stage to space plane. It works great. It does require the drop fuel tank mod. I recommend leveling off at 15,000 meters and gaining speed slowly rising till you reach .2 intake air. Defiantly stage it at that point at aim the nose directly up. You might need RCS to keep this angle of attack at those wind speeds and stay stable.

http://kerbalspaceprogram.com/space-plane/