How to achieve a constant vertical velocity of zero in a spaceplane?

[Runezor]

So this is a question I've tried tackling for a while now, and I haven't been able to find the answer to it.

So I like flying long distances on airplanes in KSP, but it quickly gets annoying to correct the angle of the plane every 20th second to avoid the plane crashing or reaching too thin atmospheres. I've tried using the standard SAS mode to fix this, however, my plane wobbles a lot, so I'm not exactly sure which angle the SAS is trying to move to, and secondly, the fuel drain changes the center of mass. So have anybody in here faced this problem, and how did you solve it?

[Crzyrndm]

Pilot Assistant (link in signature) has an altitude control system. Mechjeb or kOS are alternatives depending on how much complexity you want to encounter

[capi3101]

Well, a lot of this issue is going to be dependent upon which aerodynamic model you're using. Short answer is that "level flight" is a foreign concept in KSP no matter how you choose to fly.

As far as fuel drain changing the center of mass goes, though, there is something you can do about it - there's this wonderful little mod called RCS Build Aid, which I'd recommend to anyone wanting to get serious about planes regardless of aero model. Among its many useful features is an indicator for the "dry center of mass", i.e. where the CoM will be when all the fuel tanks are empty. It's a good indicator of which way the mass will tend to shift as you fly, and you can use it to where you can balance your fuel load so that it won't shift around in flight when you're designing your planes.

EDIT: Mostly ninja'd.

[Runezor]

Thank you! That seems like the way to go!

[Vectura]

capi3101, with far I managed to build a cargo plane that followed the curve of kerbin by itself for around 15 minutes before having drifted a few meters up or down. It was stupidly stable. It is the one in the 'Going to KSC 2' video on youtube (my yt name should be Daniel Duncombe iirc)

[capi3101]

Thanks, I'll check it out. Stock or FAR?

[funk]

If you're using FAR determine your cruise altitude and velocity. Then go to the far Gui and enter the values. On the right upper side at level flight there is shown the AoA. So try to angle your plane as given or preangle your wings. The plane have to be stable at cruise altitude and velocity, wing shape determines mostly stability during level flight. Use TAC Fuelbalancer or goodspeed fuel pump, and assure the dry and wet COM are in the same place. I mostly use PilotAssistent SAS Assistant for smooth flight, if the plane is well balanced, you can physical timewarp up to x4. Another way could be using KOS.

Edited April 2, 2015 by funk

[Crzyrndm]

Short answer is that "level flight" is a foreign concept in KSP no matter how you choose to fly.

capi3101, with far I managed to build a cargo plane that followed the curve of kerbin by itself for around 15 minutes before having drifted a few meters up or down. It was stupidly stable

Larger vessels with a bit of trimming can match the curvature of a planet to their nose down rate over a longer period of time. The extra "stability" is mostly down to the reduced rate of CoM shift and often a bit of luck with trim/altitude/airspeed (the CoM shift could also be self stabilising, but that's fairly rare)