why do space planes drift?

[briansun1]

Question why do space planes drift forward on the runway. and why do they drift sideways during take off even if both sides are exactly the same

[Van Disaster]

The runway isn't flat, it dips towards the taxiway ( explained in more detail below - it is a straight line, though ). Drifting sideways... well I don't think it's actually properly 90 degrees so if you autopilot at 90 it's going to veer slightly left, but other than that it's probably more the aircraft design.

[Tex_NL]

You roll slowly forward as a result to the curvature of Kerbin. The runway is perfectly flat while Kerbin is curved. The start and end of the runway is further from Kerbins center than the middle, as a result you roll 'downhill' to the middle of the runway.

You drift sideways on the runway because your plane is NOT perfectly symmetrical. Even a single pixel difference can cause you to drift and/or flip.

A few degrees of caster won't cause and problems. Camber can cause problems but what you really want to avoid at any cost is toe-in or toe-out.

Edit:

The runway isn't flat, it dips towards the taxiway ( explained in more detail below - it is a straight line, though ). Drifting sideways... well I don't think it's actually properly 90 degrees so if you autopilot at 90 it's going to veer slightly left, but other than that it's probably more the aircraft design.

Actually, the runway is perfectly flat. That is exactly why this happens.

Edited September 24, 2013 by Tex_NL

[Hodo]

You roll slowly forward as a result to the curvature of Kerbin. The runway is perfectly flat while Kerbin is curved. The start and end of the runway is further from Kerbins center than the middle, as a result you roll 'downhill' to the middle of the runway.

You drift sideways on the runway because your plane is NOT perfectly symmetrical. Even a single pixel difference can cause you to drift and/or flip.

A few degrees of caster won't cause and problems. Camber can cause problems but what you really want to avoid at any cost is toe-in or toe-out.

Edit:

Actually, the runway is perfectly flat. That is exactly why this happens.

Tex love the explanation best post from a intro to alignments I have seen in all most 10 years.

[BubbaWilkins]

You roll slowly forward as a result to the curvature of Kerbin. The runway is perfectly flat while Kerbin is curved. The start and end of the runway is further from Kerbins center than the middle, as a result you roll 'downhill' to the middle of the runway.

You drift sideways on the runway because your plane is NOT perfectly symmetrical. Even a single pixel difference can cause you to drift and/or flip.

A few degrees of caster won't cause and problems. Camber can cause problems but what you really want to avoid at any cost is toe-in or toe-out.

Edit:

Actually, the runway is perfectly flat. That is exactly why this happens.

You can have a perfectly symmetrical craft and still have drift issues. Structural flex due to gravity can and will cause all those same misalignment issues. The larger your craft, the more likely structural flex will play a roll.

[Dispatcher]

Something which seems true is that just as there are random elements in real life, KSP to some degree imitates life. Take your same plane and let it fly without piloting (no manual, no auto, no SAS) and you should see differences in craft behavior from flight to flight. I think the above posters are right, plus I think you have random elements as well.

[SofusRud]

I find that if your landing gear aren't perpendicular to the ground it causes planes to flip out. I recommend that you check if landing gears are slanted.

[Vector]

A few degrees of caster won't cause and problems. Camber can cause problems but what you really want to avoid at any cost is toe-in or toe-out.

Great diagrams. Something else I'll point out, which took me a bit of figuring out, is camber can become tow-out or tow-in if your craft doesn't sit at exactly the same attitude as in the SPH. Even a seemingly mild camber with a small attitude difference can create a terrible tow-out or tow-in that tosses you off the runway and makes great explosions.

[ChevronTango]

if you have mechjeb installed and your using ASAS then if you set it to surface, with whatever pitch and you like, but a heading of 90.4 then you'll head straight down the runway. For somereason the plane is aligned to 90 degrees on launch but the runway is .4 degrees off this. Tow in and Tow out plays a big part in your corrective steering and your aircrafts beahviour at speed but if its just this slightly bizarre natural drift your worried about then its because the runway is not perfectly aligned east west. As far as I'm aware it is perfectly flat and doesn't slope forward or backward but I'm not sure, and I'm also not sure what circumstances that would truly affect your takeoff adversely.

[Zephram Kerman]

Here's another thing to consider. If you're talking about the plane swerving left or right when it gets close to takeoff speed, just pull up a bit earlier. What's going on here is the wings are making some lift and taking weight off the main wheels, but not the nose wheel. So if you pull back on the stick early, everything becomes more stable.