This aircraft refuses to fly properly!

[RocketPilot573]

I've been trying the make a space shuttle... again... But this time, the cargo bay is not made out of those pesky wings.

Now, I know the placement of those indicators looks be weird, and you probably expect the ship to pitch down with great force. However, this thing is without a doubt DETERMINED to pitch up. In fact, originally the center of lift was right under the CoM, but the plane pitched up, so I moved the control surfaces to be pitched down. But even with entire wings in a pitched down position this craft refuses to do any thing but back-flips...

Sooooooo, can anyone explain the aerodynamics of this aircraft? Because it makes no sense, even for KSP!

DOWNLOAD

And here are some more pics:

Edited November 4, 2013 by RocketPilot573

[Deathsoul097]

Your engines are placed below the center of mass. I assume this is a SSTO, so change the engines out for TurboJets and move them up higher and Problem Solved!

[arq]

It looks like the problem could be that your CoT is below your CoM?

[TheKerbinator]

I'd say there's a good chance it might be because your center of thrust is below your center of mass. I'm assuming you're only using the atmospheric engines and perhaps the boosters for take-off?

Can you add another picture with the center of thrust showing as well?

[RocketPilot573]

Err, by the way, this happens even with the engines off...

[TheKerbinator]

If it pitches up without your engines firing, then you should look at your landing gear. If your center of mass is behind the rear landing gear, the landing gear will act as a fulcrum and lift up your nose.

[Hodo]

Simple answer is this.

Your center of thrust (CoT) is lower than your Center of Mass (CoM).

Bring those inline and you will have no problems flying this craft.

[RocketPilot573]

If it pitches up without your engines firing, then you should look at your landing gear. If your center of mass is behind the rear landing gear, the landing gear will act as a fulcrum and lift up your nose.

This is in the air, I don't even use the gears. Instead, I have it held up by launch clamps...

[Skyrunner27]

At what speed do you begin to lose control. Because if you shut down the engines it could still be picking up speed. I would suggest testing your shuttle on the runway by taxiing it to see if it can take off. If it does not take off you have to much downforce or too little thrust which could also be a problem with all that weight.

[RocketPilot573]

Alright, I have added a download if anyone is interested. NOTE: I have changed the ship a bit, it can now fly straight (sort of) but it still becomes extremely unstable during maneuvers.

Also, do NOT apply more then 16 pounds of force to the landing gear, as it WILL break!

[Traches]

Intakes and parachutes (even un-deployed) generate more drag than other parts, so if they are placed forward of the CoM they will cause a lot of instability. Can we see a few more pictures?

[xandalis]

Your main problem is that the CoL is way off for your CoM. Your CoT looks off as well. Just like for rockets, you want all those as close to being exactly on the same axis as possible, only for winged vehicles you also need to ensure the CoL has close to a 90 degree pitch compared to the center line of the craft. Also it's best to have it either dead-centered with your CoM, or just slightly behind it (generally, actually if it's not perfectly centered you want it to be closest to where the CoM will shift as you burn off fuel mass to maintain stability). It's tricky, as just about every tweak can change one of the other two Centers you need to account for.

As for design notes, use the reaction wheels (SAS) in-line, not where you have them. One, they won't help very much for craft stability if you're having lift problems. Two, if you need the torque for maneuvering in vacuum, you're probably better off going with RCS nozzles (which will also help for maneuvering at higher altitude as control surfaces start stalling). Again, it's a tough balance designing a viable SSTO in the beginning.

Also, about your CoT: just because the builder shows it at one location when using mixed propulsion, does not mean that is where it will be in effect when using only one at a time in flight. Yank them off and see where it is for each type as you put them on (one type at a time), you may have to adjust placement to maintain a CoM-aligned CoT, even though the editor will tell you differently.

[Sirine]

Your main solution...

http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures