I've spent the past three hours committing a genocide of historical proportions against my Kerbals, and it's making me quite sad.
Valentina would really appreciate if I could get my airplane off the ground, but it keeps exploding on the runway by the exact same failure mode: The plane veers to one side or another, either I or SAS correct it, which starts a left/right wobble, which builds until one of the wings hits pavement, at which point the craft quickly brings Valentina to her untimely end.
I've covered all my bases, as far as I can tell:
- All three wheels (one steerable in front, two fixed in back) are mounted with angle snapping, symmetrically
- Wheels are mounted to the fuselage, rather than to the wings (which flex under load)
- Wheel base is wide
- Center of lift is behind center of mass, and slightly raised
- Rear wheels are in front of tail, but not enough to make tail strikes easy
- The two engines have been mounted every imaginable way with the same result: Over the wings, on the fuselage, on girder segments, on the wingtips, etc.
- Wings have been flown both with and without struts
- Wheel friction was even decreased to 0.3 for front wheel and increased to 2.0 for rear wheels, as per a tip found online
It seems if I turn off SAS, I can prevent the oscillation from starting as it's initiated by some control input, whether from me or SAS, but then the plane veers off the runway until it hits something. Intriguingly, there is one thing I've found that does prevent the wobble from starting, and that is to use a single engine mounted directly on the back of the fuselage, which prevents using a tail and is ultimately a fairly significant restriction to build around.
I've spent some time scouring the internet for ways of solving this problem, and found a number of pieces of advice on preventing wobble and improving planes in general, some of which made it into the above list, but Valentina's still dying, and I'm tired of Kerbal funerals.
Best Answer
I've found that all my runway wiggling planes are caused by the wheels. This happens in two instances; initial placement and plane body deformation.
Wheels need to be touching the ground exactly as they were intended to, you cannot angle them in any direction other then their intended direction when placing them. So the ones that go straight up and down can only be straight up and down, no angling left, right, backwards or forwards such as when you place the little wheel on the angled body of the cockpit part. The ones that are at an angle must be placed at that exact angle, you can't angle them to make them wider.
Plane body deformations can cause the wheels to touch the ground at a slightly different angle. Body deformation can be caused by heavy parts, many parts connected together, or forces on the plane.
From your description of your plane I would check the following:
When your plane speeds up, it sounds like your lift could be pushing your nose into the runway causing deformation or an oscillation with the same effect.
Assuming nothing is blocking the exhaust of the engines, the relation of your thrust to your center of mass could also cause the same problems as above.
If your wheels are far apart there may be deformation caused by heavy parts or many parts between them, I would try attaching struts from each wheel to a single fuselage part. Also ensure that the wheels are placed "correctly".