Aircraft Help??

[robogungt8]

Hello, I have finished scanning Kerbin for Kethane, and I wanted to go try mining some. Instead of doing a sub-orbital hop with a mining lander, I wanted to use a plane to get there and back. Cue the Goose.

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With a wingspan of over 15 meters, she's the biggest plane I've ever made. The problem: for some reason, it tries to turn to the left or right at seemingly random times. At first, it was yaw, so I removed the dynamic vertical stabilizer and replaced it with a static one. That fixed the yaw problem, but now it was rolling! Everything should be stable, with the CoM in front to the CoL, and the CoT in line with the CoM. Does anyone know what the problem is?

Here's the .craft file. I've never tried uploading one, so hopefully it works. Let me know if it doesn't.

CRAFT: https://www./?h8ma5acp9a41sgu

Mods used on the craft: B9 Aerospace and Kethane

Thank you for any help in advance!

Edited March 6, 2014 by robogungt8
Added picture

[Taki117]

You could add more rear wheels to spread the load. (you might also want to post a pic so that others don't have to download mods that don't already use them) Also ensure your wheels are in a straight line. The easiest way to do this is turning off angle snap for the rear wheels and turning it on for the front wheel.

[robogungt8]

I'll get a picture now. The problem isn't on the runway, but in the air. That's when it starts to turn. Picture on the way.

*EDIT* Pictures added!

Edited March 2, 2014 by robogungt8

[Vanamonde]

The center of lift is even with the center of mass in the vertical plane. That has no inherent stability and the vehicle is as likely roll as not. Try making the wings shoulder-mounted so that the weight is hanging from the wings, and the plane will have a slight but helpful tendency to stay upright. I don't see an obvious cause for unwanted yaw, though. Perhaps make the tail boom longer so that the control surfaces are working over a longer lever?

[robogungt8]

Okay, I'll try that. Thanks for the help!

[Taki117]

You might also try adding yaw authority. Do you play with FAR? That will change how the thing flies. I agree, you may want to raise the wings so that your aircraft is inherently stable. You could also try a Di-hedral design. (There is a tutorial around here somewhere about aircraft building.)

[Claw]

How much roll/yaw are you talking about? And are you flying with SAS on?

I would fly your craft but I don't have B9 installed. So if you have more details about how much roll/yaw, what you're doing before/after, that sort of thing.

Dihedral will help out, but if you have SAS on it shouldn't really be rolling at all unless you happen to be in the very fine NULL zone where SAS messes up and makes your plane "twitchy." If it's bobbling around a lot then you can try disabling some of the control surfaces via tweakables. This keeps them from fighting each other.

Alternatively, if your plane is still experiencing a bit of yaw, you might be getting roll because of the upturned/swept wingtips.

(Just depends on what kind of roll you're getting.)

As for your yaw problems, sometimes airplanes that have long wings will have yaw stability issues. As you see, adding a more effective tail helps this out. You can make the tail area bigger (as you did), or put the tail back further (as Vanamonde suggested).

Edited March 2, 2014 by Claw