Why is this plane very unstable?

[Kulebron]

I try this plane on the runway, and just in few seconds it goes off-course and starts tipping over to the side. This happens both in Kerbin atmosphere, and if I set Eve atmosphere properties with HyperEdit. (It's intended for Eve) This is the craft file

What am I doing wrong?

I also noted that pylons tend to bend, so what is a better part to attach them with?

[Psycix]

For starters, your CoM (center of mass) is not inline with your thrust (I assume you do not fire the 909 until after separation), though that would cause an upward tilt.

Aside from that, you could try adding some struts between the two bodies, maybe the top one wobbles and pulls it to the side.

[boomerdog2000]

Yeah that fuel/ crew section on top will pull the whole thing over as soon as it starts to tip at all.

[lammatt]

thrust doesnt line up with the center of mass, you will be spinning like no tomorrow.

[Aphox]

thrust doesnt line up with the center of mass, you will be spinning like no tomorrow.

Pretty much. With that much off-set balance on the thrust, you're either going to need a high degree of thrust vectoring or a lot of SAS torque to keep it stable, unless you can somehow offset the thrust to match up with your CoM.

[allmhuran]

The learned posters above me all make a very good point about CoM not being aligned with CoT, so heed their words.

However, you mention in your original post that the plane is tipping "to the side", and that this is happening on the runway.

Your problem is probably that the rear wheels are not perfectly aligned with the fuselage. This is a common problem when attaching wheels to wings. What you could do in this case is to attach the pylons to the outside edges of the wings (the delta wing geometry results in a perfectly straight alignment if you attach here with angle snap), and then rotate them 90 degrees so they're facing down. Another way is to change your attachment mode.

But you have another problem too: Too much weight! Those rocket fuel tanks are much heavier than jet fuel tanks. The wings and gear will flex on the runway since there is nothing directly under the fuel tanks to support their weight, and this flexing is going to result in further misalignment.

You can fix this by putting landing gear directly under the fuel tanks at the back. Now, this alone will create a problem because your craft will no longer have any nose-up on the runway and it will make it difficult to launch. You can solve this by, again, attaching two landing gear to the sides of the tank as rear landing gear, and using shift-rotate to angle them until they are straight up and down.

Leave the wing gear on as well, otherwise your gear tricycle will be much too narrow and your plane will tip over!

Now - one final thing. Once you do get airborne, this plane is going to be unstable. Part of the reason will, of course, be because of the centre of thrust problem mentioned by others here. But you have another source of instability: your centre of lift is also far below your centre of mass. A way to solve this is to give the wings some dihedral... that is to say, angle them up towards the tips.

For some easy to follow (GREAT PICTURES!) aircraft building information, see this thread http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

Edited October 13, 2013 by allmhuran

[lammatt]

O...

if it tips on the runway, it's that the wheel and the plane body are not relatively stationary enough (aka, the plane is not rigid enough)

to fix this, add more struts.

[Kulebron]

Thanks to all, and, yes, the problem happens yet while on the runway, it tips to one side or another.

I know the concepts of aircraft well enough since years ago, but tried to make something rather unusual here.

allmhuran: thanks for the tips. I aligned things as much as I could, and the center of lift is perfectly aligned with center of lift. Note: the wings start above the center of the bottom fuselage. CoL is just behind the CoM, so it is stable and yet does not try to dive, so I won't need to pull it with canards all the time. I made similar designs for tests, and they took off well with the canards in the front.

Thrust vector is a bit off center, of course, but I still need to take off.

[JumpsterG]

In addition to other tips, you possibly just need more lift (more wing pieces) overall, since this is pretty heavy. I also read somewhere that sliding left or right on the runway is a symptom of the rear of the plane lifting before the front, so you could try moving CoL forward more to see if it improves takeoff.

[NeMeSiS]

My wheels always get wobbly when mounted on those things, try strutting them.

[Kulebron]

Good points, thanks!