Spaceplane building, extremely unstable?

[Reinoud]

Heya!

I've been trying to build a spaceplane, but for some reason I cannot build anything that's remotely stable or steerable at all. I've been looking at some tutorials on youtube, but those mostly use parts I don't have access to.

I've attached a pic of a plane I built, the problem with this one is that it is very unstable. Before it even has any serious speed, it veers off to the right strongly and is almost impossible to stabilize. So that's all before take off. I haven't been able to have a single plane take off properly yet. What am I doing wrong? Is there any guide on how to build a plane with just the basic plane parts?

Thanks a lot!

 

http://imgur.com/ojmDi54

Edited February 5, 2016 by Reinoud

[RizzoTheRat]

Veering off on the runway is quite a common problem.  Sometimes having the front wheel attached to the cockpit can cause it so try moving it back to just behind the cockpit.

You've also go your rear wheels quite a long way behind the Center of Mass, this will make it quite hard to unstick from the runway unless you have it in a nose up attitude already, as the tailplane has to try and lever the CoM around the wheels, plus it means there's a lot of weight on the front wheel, moving the rear wheels forward and leaving the front wheel where it is would reduce the weight on the front and might solve the problem.

 

Edited February 5, 2016 by RizzoTheRat

[Starhawk]

It's almost certainly caused by your rear wheels not being perfectly straight.  If you attach the wheels to a curved surface, they will be slightly angled one way or the other.

To fix the problem go into the SPH and select the rotation gizmo, then select the rear landing gear.  Pressing F will toggle the rotation to absolute mode.  Adjust the rotation of the gear slightly and it will snap into absolute alignment.  Make sure you have it snapped straight forward and straight up and down.

Your plane should now roll straight down the runway.  There are other problems that can affect this, but the one described seems to be the culprit in almost every case.

Happy landings!

[Evanitis]

If the above doesn't work, I'd try attaching the lower gears to the fuselage. Just to have them on the wings could cause veering.

[Reinoud]

Thanks for all these tips guys, worked like a charm!

Now I'm onto my next problem, do you have any tips on how to improve manoeuvrability? Right now it seems quite difficult to first roll over to one side and then awkwardly make a turn. I find it very hard to go where I want to go (for example, land back on the runway). Maybe this just takes a lot of practice though?

Cheers!

[steve_v]

31 minutes ago, Starhawk said:

It's almost certainly caused by your rear wheels not being perfectly straight.  If you attach the wheels to a curved surface, they will be slightly angled one way or the other.

Yeah, this. Particularly when attached to that tail-connector part, they'll have a slight toe-out. Wheels in KSP (currently) don't really behave much like wheels... even a slight out-of-true causes very strange things to happen.

You may have better results attaching the rear gears to the straight fuselage, then offsetting them back to where they need to be.

[Boris-Barboris]

17 minutes ago, Reinoud said:

it seems quite difficult to first roll over to one side and then awkwardly make a turn

You have very little to roll with, place some ailerons.

After that, lots of practice and experimentation.

[Snark]

56 minutes ago, Boris-Barboris said:

You have very little to roll with, place some ailerons.

+1 to ailerons.

Another thing that should help with both roll authority and stability:  Make sure you have disabled roll authority on your vertical stabilizer.  (In the VAB, you can right-click any control surface and toggle roll, pitch, and yaw authority on it.  They're all turned on by default.)

You have a tail fin for one reason and one reason only, and that's to give yaw stability.  Turn off pitch and roll.

Turning off roll is especially important because failing to do so will not only make it harder for you to roll, but will also reduce the tail fin's ability to give you yaw.  Here's why:  Because the tail fin doesn't have a symmetrical twin down below, every time it deflects left or right, it will not only yaw the craft, it will also induce roll.  It's unavoidable.  If you haven't disabled roll authority on the vertical stabilizer, then SAS will try to use it to correct for the roll... and that correction is in the exact opposite direction of the yaw.  Therefore it ends up fighting itself and is much less effective.  Furthermore, if you don't disable roll on it, then any time you tell the craft to roll, it'll try to use the tail fin to do that, which will induce yaw too, and hilarity ensues.

So, you'll do yourself a big favor if you turn off roll authority on the vertical stabilizer.  Adding ailerons will also help, for both pitch authority and roll authority.  You'll get the most roll authority if you put the ailerons farther out towards the wingtips, rather than close in near the fuselage.