I see them rolling, I hatin'

[Agost]

Hi all

I'm playing a bit of KSP these days and I tried to make my first SSTOs in 1.0.4. I probably still don't have enough tech ( aerospikes would help ) but my problem involves a strange rolling effect in both of the two SSTO designs I made.

The effect is stronger in mark 1 and it's noticeable even with SAS on. Without SAS they roll A LOT. I also noticed that it only happens when I pull up... and they both roll in the same direction ( to the right )

I removed some struts that could induce strange drag, checked all control surfaces, but still nothing. Here there are some photos. BTW I forgot to open the cargo bay: in both version it's filled with 4 1.25m RCS tanks ( attached on the rear side of the cargo bay ), 2 big radial batteries, some static solar panels attached on the bottom ( with simmetry enabled ) and the KER module.

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Do you spot any possible issue? D:

Thanks in advance

[InterCity]

your rudders are trying to pitch the plane. Since they are rudders (and not exactly vertical), it causes disastrous results.

Right-click each of the rudders in SPH, then find the blue button next to "Pitch" and "Roll", and press it. Description should change from Active to Inactive.

[Agost]

your rudders are trying to pitch the plane. Since they are rudders (and not exactly vertical), it causes disastrous results.

Right-click each of the rudders in SPH, then find the blue button next to "Pitch" and "Roll", and press it. Description should change from Active to Inactive.

As I wrote, I checked the control surfaces. Elevons are set only for pitch and roll, while rudders for yaw.

Moreover that couldn't explain why they only roll while I pitch up ( and only to the right )

[Tex_NL]

I've seen this issue a few times before and it always happens with a double vertical stabilizer. Mirror symmetry doesn't always work as it should and the vertical stabilizers might not get mirrored correctly. (During flight enable the aerodynamics display F11. If I am correct both tail fins will display arrows in the same direction instead of opposite.)

Try removing the complete wing/engine/tail section and reattach with mirror symmetry. This will usually solve the problem although the fix is not always permanent.

[ajburges]

I also find that angled rudders introduce unwanted roll even when all the tips are followed. SAS is a horrid substitute for a fly by wire system.

Having the rudder plane tangent to CoM should give it no roll authority, yet it still rolls for me.

[Fearless Son]

I also find that angled rudders introduce unwanted roll even when all the tips are followed. SAS is a horrid substitute for a fly by wire system.

Having the rudder plane tangent to CoM should give it no roll authority, yet it still rolls for me.

Yeah, this would be my guess as well. Your rudders are slightly inclined inward. While this is accurate to the Blackbird, my experience has been that any rudders that are not perpendicular to the lifting surfaces tends to negatively impact stability. Call this a quirk of the game's control mechanics, a bit of a glitch. But one we can work around.

[cantab]

Planes can be unstable in roll. In FAR that instability can change with speed, not sure about stock. A little dihedral on the wings usually helps, and wing sweep can also be important.

As for why it always rolls right, that might be a subtle bias due to floating point error.

[Agost]

KSP is strange. Yesterday and this morning I had the problem, now it looks like it solved by itself. I also noticed, after a 4x physics time warp in a test flight, that the plane was stuck pitching up ( and I didn't use the "deploy" function of the elevons )

I took some screenshots of the aerodynamic forces but they are parallel. Yeah. No issue right now ( except for that single event of "always-pitch-up" stuff )

Kerbal Bug Program, like it has always been.

Could it possibly be caused by time warps? I used other ships before testing those SSTO prototypes, both yesterday and this morning, but not in the latest startup of KSP ( in which I didn't encounter the problem )

p.s.: which version do you guys prefer? Don't mind the rocket engines, I'll probably switch them for aerospikes when unlocked

[GoSlash27]

I almost always disable my rudders completely so that they don't respond to yaw inputs.

With SAS on, any roll that's not precisely wings level will result in an increasing automatic "crab" yaw command to the rudders. Since they are not mirrored below the plane's centerline, the yaw correction will also induce roll. If the nose is sufficiently offset in crab yaw, it will induce a roll that is unrecoverable.

Since the yaw command itself can't be turned off, the only recourse is to either a) fly without SAS or disable the yaw response in the rudders and steer with pitch and roll only. So long as you have sufficient vertical stabilizer area and leverage, it will still naturally track in commanded turns and these problems tend to disappear.

Full disclosure: I have noticed that some other pilots don't like how my planes fly when I do this. It makes them feel a bit vague in roll and they prefer to have more positive yaw response. So caveat emptor, ask your doctor if disabling the rudders is right for you, etc.

Best,

-Slashy

*edit* Addendum: I also tend to use a lot less active control surface than most pilots in my spaceplanes. I hate the laggy, excessive response that KSP's stability assist gives and I find that it interferes with the stability, making it twitchy and erratic IMO.

Reducing (or in some cases eliminating) the active control surfaces gives a lot less control authority, but IMO makes it much more stable, reassured, and pleasant to fly. YMMV.

As an example, my "Shuttle Kourageous"

Shuttles are notoriously ill-tempered. While this one is festooned with all the normal control surfaces, the only ones that are actually *active* are the little outboard elevons.

Edited August 12, 2015 by GoSlash27

[ajburges]

Disable rudders eh? Interesting concept and a good way to stop novices from trying to change heading with yaw.

Still, the ability to vary AoA means you need less stabilizer if it is also a control surface. It's also nice for correcting inclination as you transition to ballistic flight. Even a passive stabilizer will induce roll while slipstreaming if it's unbalanced.

I find that you need surprisingly little yaw stabilization for most craft if you are careful during reentry and avoid high slipstream manuvers. A single winglet is enough to keep my 60-80 t craft stable. This can be mounted on the centerline with a very small proportion of asymmetry.

[Agost]

I don't always use full authority on control surfaces, only if I need it.

I'm not so familiar with KSP 1.0+, is control surface authority tweaking a stock thing or does it come from stockplus plugin?

[cantab]

I almost always disable my rudders completely so that they don't respond to yaw inputs.

With SAS on, any roll that's not precisely wings level will result in an increasing automatic "crab" yaw command to the rudders.

My solution to that problem is not to use SAS for planes FAR's Flight Assistance tools are very useful, or there are things like Pilot Assistant or Mechjeb's SmartASS that are better for planes. Stock SAS is really designed for spacecraft, and I will turn it on when my spaceplanes are the spacecraft-aircraft transition or in space.

[GoSlash27]

My solution to that problem is not to use SAS for planes FAR's Flight Assistance tools are very useful, or there are things like Pilot Assistant or Mechjeb's SmartASS that are better for planes. Stock SAS is really designed for spacecraft, and I will turn it on when my spaceplanes are the spacecraft-aircraft transition or in space.

cantab,

That's a solid solution for people who use mods. I run 100% stock, so that wouldn't work for me.

Best,

-Slashy

[Starhawk]

I don't always use full authority on control surfaces, only if I need it.

I'm not so familiar with KSP 1.0+, is control surface authority tweaking a stock thing or does it come from stockplus plugin?

The control surfaces are tweakable in stock. Both in the SPH and in flight.

Happy landings!

[Kobymaru]

There's another thread that describes the exact same problem, and apparently this guy has a different explanation than double-rudders:

http://forum.kerbalspaceprogram.com/threads/131461-Plane-rolling-when-pulling-up?p=2136299&viewfull=1#post2136299

Lemme guess. You have multiple wing sections. There's a well-documented bug where symmetric part attachments have different stiffnesses, and the flex and twist caused by pitch-up tends to cause uneven deflection in the wings. Try adding more struts between the wing sections and/or install the KJR mod, and see if that helps.

[Agost]

There's another thread that describes the exact same problem, and apparently this guy has a different explanation than double-rudders:

http://forum.kerbalspaceprogram.com/threads/131461-Plane-rolling-when-pulling-up?p=2136299&viewfull=1#post2136299

I thought about it too, but now I can't replicate the problem after installing KJR. Probably that, but I also had that always-pitch-up bug. I'm still thinking it was a bug caused by high warp speeds

[Claw]

Some of the posts here are slightly confusing because modded and unmodded discussions are being mixed. But I did want to point out this one...

I'm not so familiar with KSP 1.0+, is control surface authority tweaking a stock thing or does it come from stockplus plugin?

Stock KSP allows turning on/off surfaces in each axis via the tweakables. Stock KSP does not have a tweakable for control authority. StockPlus does have this option.

Also, the amount of yaw authority needed will depend on a combination of how resposive you want the plane to be, and how statically stable you want. You could easily make a plane that is stable in yaw with zero vertical stabilizers. But then it might not be super responsive.

As for the "pitch up" timewarp thing, make sure you aren't accidentally inputting trim. It's easy to do when flying, especially under warp. If it's not trim, then I would be interested in seeing more.

Cheers,

-Claw

[Tex_NL]

Anybody willing to provide an actual link to this StockPlus mod?

There are no thread titles on the forum here that contain the phrase 'StockPlus' nor can it be found on Curse, Kerbal Stuff or Kerbaktek.

[Kobymaru]

Anybody willing to provide an actual link to this StockPlus mod?

There are no thread titles on the forum here that contain the phrase 'StockPlus' nor can it be found on Curse, Kerbal Stuff or Kerbaktek.

http://forum.kerbalspaceprogram.com/threads/97285-KSP-v1-0-4-Stock-Bug-Fix-Modules-%28Release-v1-0-4b-2-1-Aug-15%29

[Tex_NL]

http://forum.kerbalspaceprogram.com/threads/97285-KSP-v1-0-4-Stock-Bug-Fix-Modules-%28Release-v1-0-4b-2-1-Aug-15%29

Thanks

[Claw]

Anybody willing to provide an actual link to this StockPlus mod?

Ironically it's all over the place, sine it is linked in my sig as part of the bug fixes. Perhaps I should indicate that fact by adding "StockPlus".

Cheers,

-Claw

[Agost]

As for the "pitch up" timewarp thing, make sure you aren't accidentally inputting trim. It's easy to do when flying, especially under warp. If it's not trim, then I would be interested in seeing more.

Cheers,

-Claw

Do you mean alt+WASD ?