Landing gear problem. How to not explode on the runway?

[Colonel_Panic]

So I spent like 6 hours designing a spaceplane, finally ready for maiden flight... but as soon as I throttle up, she turns hard right, runs off the runway and smacks a wing into the ground and disintegrates. I retried the takeoff about a dozen times, always the same thing, regardless of control input or SAS.

So I took her back to the hangar, doubled up on the landing gear and took her back to the runway. Now she turns hard to the left.

So it's definitely the landing gear that's causing this. it's some kind of weird wobble feedback loop that always destroys the plane before it gets more than 50 feet down the runway. Thing is, everything is in symmetry and well reinforced, I can't for the life of me figure out why I can't get it off the ground. Anyone else ever have this problem, or have some tips and tricks to solving it? thanks.

P.S., it's a twin-fuselage plane, so only the front end is truly properly linked, despite all my bracing, and the back may be oscillating in and out enough to skew the gear. I'm not sure.

[Specialist290]

Are your landing gear perpendicular to the ground, or are some of them offset at an angle? The latter has been known to cause planes to veer wildly before.

[TelluriumCrystal]

The plane is bending, thus causing the gear to become angled, thus causing bad things to happen. What fixed it for me is to have an independent (not attached to the fuselage directly) set of "double wheels" (see pic below) that are reinforced enough that they won't wobble. Then put lots of struts between the two fuselages.

Also as a general rule, always include pictures of your craft so people can get a better idea of its design.

Image:

[Colonel_Panic]

Sorry, slipped my mind.

I did manage to 'fix' the gear problem by replacing the pylons they were mounted to with the much beefier trusses, and mounting double gear to a plate.

However I soon found that there's an underlying issue with the plane aerodynamics that makes it unflyable anyway. For some reason it will always yaw to the side harder than you can correct... and the funny thing is, it isn't always the same side. It will sometimes randomly switch which side it's pulling to mid flight. I can't figure out why. I checked and all the tanks are emptying at the correct rate, weight distribution isn't off, etc... it's just really boned. Any help would be great. :x

[SuperFastJellyfish]

Sorry, slipped my mind.

I did manage to 'fix' the gear problem by replacing the pylons they were mounted to with the much beefier trusses, and mounting double gear to a plate.

However I soon found that there's an underlying issue with the plane aerodynamics that makes it unflyable anyway. For some reason it will always yaw to the side harder than you can correct... and the funny thing is, it isn't always the same side. It will sometimes randomly switch which side it's pulling to mid flight. I can't figure out why. I checked and all the tanks are emptying at the correct rate, weight distribution isn't off, etc... it's just really boned. Any help would be great. :x

Obviously, it's a staged plane. My guess would be your CoM/CoL change dramatically when you stage the initial wings/engines/tanks. Always make sure the payload is up to code before attaching a launcher to it.

[Colonel_Panic]

It's not staged, it's SSTO.

The payload is mounted directly at the COM so that I can deliver it and return without changing the balance of the plane. It fits multiple payloads, but I'm just using the LV-Ns to test it since it's the longest payload I have to carry.

[bsalis]

Pics?

... and/or craft file.

EDIT : actually i see yo pics now! craft files also help though

[katateochi]

Are your landing gear perpendicular to the ground, or are some of them offset at an angle? The latter has been known to cause planes to veer wildly before.

The plane is bending, *snip*

What these guys said is probably your problem. On simple planes its generally a wheel not being on perfectly straight. One more complex ones, which yours looks to be, it's the bending that is often the problem. Where you have a weak connection, ie that docking port in the middle, it can cause issues. Even a single strut being asymmetrically placed can result in slightly more bend to one direction causing you hell on the runway. Generally not a problem once airborne, that is if you can get airborne!

[Colonel_Panic]

What these guys said is probably your problem. On simple planes its generally a wheel not being on perfectly straight. One more complex ones, which yours looks to be, it's the bending that is often the problem. Where you have a weak connection, ie that docking port in the middle, it can cause issues. Even a single strut being asymmetrically placed can result in slightly more bend to one direction causing you hell on the runway. Generally not a problem once airborne, that is if you can get airborne!

Well I actually found the cause of the problem. In my case the plane wasn't bending enough to be a problem (it's extremely rigid and reinforced. The docking port is only for holding the payload), and the wheels were perfectly in alignment (using snap-to geometry and symmetry settings), HOWEVER, it turns out that if the front wheels are on "backwards" (aka, they open down and forward instead of down and back), the front wheels will buckle and sway, causing the plane to tilt and veer to the side and crash.

At first I thought switching to the trusses from pylons had fixed it, but after rearranging the plane again the problem came back. Then I flipped the front wheels around and viola! problem solved.

As for the flight instability, it turns out that kerbal engineers haven't figured out how to make fly-by-wire vectored-thrust systems work nicely together... the front engines were gimballing the wrong way and turning the nose the opposite direction from where I wanted it to go. Locking the gimbals fixed it, and I achieved SSTO in the first flight after fixing them.

Edited May 17, 2013 by Colonel_Panic