Japa’s 2nd suggestion is the what I’ve found to be the number one failure of plane designs. Due to unity constraints, wheels MUST be at perpendicular to the ground. This is why adjustable landing gear is awesome, because you hit a button and it takes care of that for you. If you’re having trouble lifting off, try placing your gear just behind the CoM, so that it can rotate about the CoM like a fulcrum. Otherwise, you’re putting too much force on the wheel joints when they are farther back, causing them to flex.
I’m using the adjustable gear and auto adjusting, so the wheels ought to be flat to the ground. I’ll try placing them as close to the CoM as I can to see if that’s the problem.
I’m only speaking from stock experience, but try to be moderate about the throttle when taking off. I usually find that if I go balls-to-the-wall on the accelerator from the get go, plane spins out and crashes as you describe. Take it slow, find the minimum speed you need to take off, and stick to that.
I have had the same problem. Try using the FAR stability tools.
Sweep mach through 0 to 1 for various AoA between 0 and 10. You especially want to minimise any yaw or roll acceleration which will have a knock-on effect via the wheels, as the others have said.
Regarding wheel placement, unless your plane is massive, position relative to the CoM isn’t as important as their position relative to the elevators. Eg if your rear gear is behind or not far enough in front of the elevators, they’re not going to be able to push the tail down.
Anyway, I doubt that’s the issue here.
Ok I still can’t solve this, so here’s some pictures and maybe one of you can:
The back wheels are just behind the CoM. The front wheels are slightly higher than the back (which assists with launch, if I’m remembering correctly). There are lots of struts, so things shouldn’t be bouncing around.
I also tested with some stock planes to make sure I’m not doing my launch sequence all wrong, I can generally get them into the air just fine.
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Did you do the far analysis?
All the stability values have the right sign. I haven’t swept for different speeds because I’m not certain what I’m looking for there, I’ll have to read up on that a little and try again tomorrow.
Camkin decided against the Bop landing, preferring instead to return home. His woefully outdated Sunfish I was sent to a museum and Camkin retired to a beach house somewhere.
The mission controllers are never satisfied, though. The new Sunfish III has just been dispatched, with a more… perilous… target in mind: Moho.
Though weighing 1000 kilograms more than the Sunfish I (for a total of 5.5t), the III is a much more capable spacecraft. Added features include:
- Triple the thrust of the Sunfish I: Maximum acceleration of ~0.3g.
- 37 (!!!) times the charge storage of the Sunfish I, enabling full-power burn times exceeding 6 minutes.
- A detachable lander, capable of seating a kerbal and sporting a 3:1 Kerbin TWR and >2700 m/s dv, RCS equipped and capable of autonomous operation. It can do Mun landings and returns from parking orbits of 100km with fuel to spare, and can probably do Moho landings as well.
- Antennas, a dish and an Omni, enabling control of the lander even in the remote reaches of the solar system.
- Greater RCS efficiency.
- Emergency landing legs, enabling reliable landings on minmus-scale bodies and not-so-reliable landings on mun-scale bodies.
- All this and it still packs more than 35 km/s in the tanks, keeping the original’s ability to travel almost anywhere in the Kerbol system, return, and do it again.
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On the stability page, set mach to 0.05 and calculate. Increase in steps until mach 0.2 (245kmph)
I’ve just checked and sweep doesn’t have yaw output.
You didn’t ask the question. 
Write what it does and what you need it to do.
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He did…
I think we can assume he needs it to go in a straight line and take off.
@martindevans
We can’t see from those screens if the tail fins are on the wings or the fuselage. If they are on the wings, try taking the wings off and placing them again using symmetry. That will ensure the tail fins and intakes are all symmetrical.
Additionally, try just taking everything off and placing it again - just click it and then click it back on again with symmetry.
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The tail fins are on the body, right above the engines. They’re rotated slightly so they’re vertical (parallel to one another).
I’ll do some more fiddling and post some more pictures later.
Maybe move the rear gears a little back?
I wouldn’t be able to lift this thing off the runway without clipping it and ripping off the engines.
Also it looks like the craft is tilted when set on the runway from the get go … this may create a situation where the front wheels leave the runway, but the plane isn’t fast enough to lift of yet. In such a situation small drag in one direction flips you out. Maybe even the instant the front wheels disconnect from the ground.
From what martin said and from my experience I believe the issue is the plane veers off to one side while still having all wheels down and no where near enough speed for any part to start lifting.
Not if the landing gear is too close to the center of mass. If it is, verry little force is needed to tilt the ship up, turning it into a two wheeler for a split second.
Just my guess. Would like to know what the problem is, especially because others don’t have this problem.
Further testing!
First off, here’s some more pictures of the plane. This is how she sits on the runway, nose slightly up above tail.
I’ve moved the rear wheels further back (per Lomsor) but that hasn’t solved the problem.
Here’s one from above, showing where the tails are attached (and the silly amount of struts on this plane):
I watched carefully to see when the plane begins to veer off course. Set the throttle to 70%, turned off the brakes and watched; she veers at about 20m/s (i.e. very soon after starting to move, and definitely not enough speed to generate life).
Are there any options on the wheels I should fiddle with? For example dynamic steering on/off?
Just did this and turned up an interesting issue. With the default mach number she’s stable, but at 0.01 I have:
- Zw: 1637.05 (should be negative)
- Mδe: -0.0204 (should be positive)
Up to 0.1 some other values become bad:
- Zw: 159 (better, but still the wrong sign)
- Zu: 2.21 (wrong sign)
- Xu: 0.12 (wrong sign)
- Mδe: -2.05 (wrong sign, even worse than before)
Various values wobble around become correct and no, until everything finally stabilises at Mach 0.5
I’m going to write off this design completely and try something new, paying closer attention to the stability derivatives. Thanks for the help guys!
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It shouldn’t be that difficult to get something that just takes off. Have you tried the tricycle setup for wheels? Nip one of the forward wheels and just use 1 in the center. I’m not sure how much it would change, but maybe something is happening where the fuselage flexes and the wheels misalign. Less problems with 1 up front than 2. The tilted design is very good for easy takeoff, so I would keep it tilted up. Moving the wheels forwards or backwards shouldn’t affect a wobbling of the plane in any significant manner(less you do something drastic like having all your wheels in front of the CoM, etc). Could you drop a link to the craft file and I’ll see if I can take a look. I still have to update so I probably won’t get to it until later tonight. I’ve got to catch up on other work first.
Never give up! I just read this part. Honestly, I’ve never paid too much attention to the stability stuff and my planes usually fly good 'nuff for me. I always test empirically for better or for worse. Try the wheel thing and/or please drop the craft file. I like solving a good mystery
This is what’s been so vexing, I’ve designed SSTOs before and they flew just fine! I’ve never had a problem as mundane as wobbling a little on the runway that was so damn hard to solve 
Anyway, I’m designing a shiny new plane now. So here’s the craft file for those who want to solve the mystery.
oh sorry i was lazy to read back… sorry 
i see.
This is almost always caused by two things - wheels (mostly) or wings
Let me guess you put only one wheel in front at first, but it always did what you describe, right?
So you tried to fix it with two of em.
You see parts in KSP (or in any game - as you surely know) aren’t perfectly round.
They are composed of pixels.
And circles are complicated in computer imagery.
On most parts (fuel tanks and fuselages) it would look like this up close
as you can see there is flat place at top bottom and sides.
If you put your front wheel there it will work just fine.
But you are not putting it on such surface.
You are putting it on cone.
And that cone along its length is changing its cross-section.
Imagine it like placing it on tip of very obtuse angle
but the game has a rule that it cannot place items on tips… so it has to decide one side - and the result is that your plane is wearing of course at start
(there is technique, when you disable angle snap and put two mirrored wheels next to each other, tricky but doable too)
Even though you are placing it with Angle Snap on, on some parts it is not possible to place it exactly straight.
First of all get rid of that double wheel in front.
Grab a one wheel (with angle snap on)
try moving it back and forth perhaps you can find a sweetspot on that cockpit where it will be straight.
Or rotate the wheel so it faces the back wheels and put it on fuselage section.
Try to put it as much forward as possible, but still on the fuselage.
Also it is nice you try to put back wheels close to CoM, but craft has canards and plane on its wheels is aiming up .
I don’t think it is necessary it should be able to lift off just fine.
You can move em back, so it’s not that close if you moved the front wheel.
Another thing that can cause what you describe are wings
I am not sure, but it looks to me that those canards are in anhedral angle is that right?
if they are that is your second cause.
(third cause could be overloaded wheel - that is not your cause trust me 
)
Damn i am giving away my know-how tsk tsk tsk
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More ksp stuff! 1.0 coming in… ~10 days, I think it is?
http://m.imgur.com/VjYaPql,
http://m.imgur.com/ujlQmNY,
http://m.imgur.com/7gfvOKf,
http://m.imgur.com/jNFCnBN,
http://m.imgur.com/c4Jub1i,
http://m.imgur.com/NDFBvXW,
http://m.imgur.com/6pAlfA5,
http://m.imgur.com/XL1VKUx,
http://m.imgur.com/jrKynIF,
http://m.imgur.com/tby5gyY
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