Ship veering sharply at takeoff, even at low speeds

[Hyomoto]

I'm completely confused by this, I've literally tried everything I can possibly think of to fix it and no matter what I do, the ship veers to the left during take off. Always the left. High speed, low speed, just rolling with ion engines. I've looked over every piece, I can't find any asymmetry. I've tried moving pieces to adjust weight, etc... but no joy. I even built a custom truss for the wheels, aligning them in default space -before- placing them on the ship to ensure they were absolutely 100% lined up. Nothing. So please, can someone take a look at this? I honestly don't care if it can fly, I just want to know why it can't move forwards ten feet without turning. There's only two possibilities that I can see, I'm and idiot or it's a bug. I'll accept either one, but I have to get a second opinion before I tear my hair out.

Craft File

Unmodded .craft

The original uses KSPX pack and RW Improved Trusses and Girders. The unmodded one has the same problem but no modded pieces.

Edited May 11, 2013 by Hyomoto

[HoY]

I'll give it a shot when I get home in an hour, but I'll say this now, it looks pretty cool

I don't have FAR however, I'll get it in the air without it, and if it still doesn't work for you it's beyond me lol

[pebble_garden]

Unfortunately without that custom strut, I can't load the craft file. Could you save a version with only stock parts?

[Cal'Mihe]

Without knowing precisely how FAR handles a plane like that (looks cool by the way!) I would guess that it might be the moveable winglets that you are using for rudders that cause it grief. Try maybe replacing them with a more traditional fixed rudder with attached control surface ?

[pebble_garden]

So here's a thought. Do you have a joystick plugged into your computer? I found that even if I wasn't using the stick to fly my KSP aircraft, any stick deflection would affect the plane's control surfaces. Long shot, but hey.

Or perhaps your input device (if you're using one) needs to be calibrated?

[Stochasty]

If you are veering during takeoff, that's a landing gear problem. What it means is that, at some point during your takeoff, all of the weight of your craft is balanced on your single front gear. Jebediah is not a very good unicyclist, so you fall over. The reason you always fall left is due floating point precision errors in the game; you are never perfectly balanced even if everything is absolutely symmetric.

There are several possible way to fix this, depending on exactly what is causing the weight shift during takeoff. If it's an angle of attack issue, try a tripod arrangement with two gear up front and one in the back. If it's a center-of-lift versus center-of-mass issue, adjust your center of lift. If it's simply that your rear end wants to take off first, adjust your trim before launch.

[SecondGuessing]

It seems like the issue very well might be that you've design the plane to look good but not paid much attention to actual aerodynamics. I can't load it due the mod parts, so could you upload an image with a view of the landing gear and one with a view of the centers of mass, thrust and lift?

[42undead2]

What I think I can see is some turned landing gear.

[Jason Patterson]

What I think I can see is some turned landing gear.

Good point. It does look like the front gear is turned to the left a bit. Can't load the plane to find out for sure without the mods though.

[Hyomoto]

You guys, I'm an ass. Of course I'll do that. Honestly I was so frustrated I didn't even consider that I had modded pieces, and I'm really sorry for not mentioning that. It's the KSPX pack and RW Improved Trusses and Girders.

Here's are some screen shots of the bottom and the weight/lift/thrust distribution. Also, I updated the main post with a link to an unmodded file and the two mods used in the regular craft. Thanks for helping me out with this, I'm really sorry to have wasted some of your time

Unmodded .craft

Edited May 11, 2013 by Hyomoto

[Cal'Mihe]

Downloading now to take it for a spin and see.

Edit: Ok, it looks like its the front landing gear that is not 100% vertical. On take-off it looks like the front gear buckles ever so slightly, which causes a turn, and on compensating for that, its easy to over compensate and crash it. Its probably caused by having the front gear attached to the seperator, which off-sets it slightly. Try moving the front gear back. Also, I've had bad experiences with attaching struts to the land gears, usually that causes the gears to be pushed and pulled in unpredictable ways during take off, leading to crashes. Also, attach the gears directly to the wings, I think you get stability issues with having them attached to a set of struts buried inside the wings.

I was able to get it up though, once it is airborne it is quite stable and flies nicely, it has a tendency to be jittery in the roll, and the pitch is sluggish, but the Yaw control is excellent. I could touch-back down on the runway at 40 m/s.

On a side note, I think you'll have problems getting it into orbit. The air intakes, while cool-looking, won't give you enough air at 10k+ to enable you to go into orbit.

Edit Again: What Stochasty says is true, its a balance problem. When holding slightly back during the take-off roll, it is completly stable and stays straight on the runway all the time up to 80m/s where it lifts easily

Edited May 11, 2013 by Cal'Mihe

[Hyomoto]

Thanks Stochasty and Cal'Mihe. The technical explanation will certainly help me the most as it is info I can apply to other craft in the future. I'll give this information a go. I take it, then, Cal'Mihe that you are using a Joystick?

Follow up question: is that ideal? I have a gamepad I can use, and no lack of disposable income to invest in a joystick. My question is, assuming I absolutely love KSP, is that an investment I should make?

[pebble_garden]

I was able to take off at 2/3 power and fly, but the plane feels very nose-heavy. I needed a lot of up-elevator to maintain level flight.

At full power the plane gets very squirrelly, and there's a strong pitch-down as if the center of thrust was above the longitudinal axis. I'd shift some things around at the very least.

Also having so much wing ahead of the center of mass is probably not helping matters.

[Rune]

Wow. That flies? I'm, ashamed at the aerodynamics of KSP. You should never put the center of lift ahead of the center of mass. That is the recipe for an unstable plane.

Rune. "Planes with the center of lift behind the center of mass fly poorly. Planes with the center of mass behind the center of lift fly once."

[Cal'Mihe]

For some strange reason his screenshot is not showing the CoL correctly. When I load it into a Vanilla install, then the CoL is properly behind the CoM.

Hyomoto, yep I use a joystick. Cheap old Logitech but it serves me well enough.

I would say yes depending on if you play flight sims, or play KSP with a strong focus on spaceplanes. You can train yourself to fly flight sims using only keyboard and mouse, but I'd recommend a joystick for the flexibility.

[miro]

I had this problem a while ago, and fixed it by moving the centre of lift closer to the centre of mass. I have no idea how to do that without compromising the glorious beauty of the ship itself.

I really hope you can make it work, looks stunning.

[Guest]

NECROPOSTING //WARNING // NECROPOSTING

Did you resolve this? My problems have often stemmed from landing gear that were perfectly straight, but attachied to items that moved under the stress of weight even standing still. Use struts either directly on the gear or stabilize the component they're mounted on. The other thing I have noticed is the wheelbarrow effect which had been mentioned. Either thru aerodynamic forces or just plain old weight, the main gear (the rear tires) must be firmly planted on the pavement.

I make craft with a "looks first" mantra such as you, I suppose. I end up editing CFG files. To move your CG aft, you could edit the mass of your engine.

Edited August 5, 2013 by Guest

[Deejay2000]

I don't think it was necessary to revive this thread. Let it die, the problem has probably been resolved, or the plane has been forgotten/deleted.

-Deejay

[Guest]

It was only two months ago. Enlighten me the need for such heavy policing on this site.