Spaceplane Wobbling

[Tantalus010]

Hi,

I've looked and looked for an answer to this, but have never been able to find one. I have an issue with flying space planes. Once I get them into the air, they wobble. Unrealistically and comically so. They wobble to the point that sometimes, the wings look like they're flapping as I pitch up and the parts of the fuselage bend up and down at their joints. Despite the wobbliness, they seem to fly okay (and somehow don't rip themselves apart), it's just too unreal to watch. If I strut every part to every other part, this behavior ceases and they fly beautifully. I have Kerbal Joint Reinforcement installed and I would have expected it to help with this, but apparently it does not. The only other mod that I have that I think might have anything to do with this is FAR.

Does anybody know what causes this and/or how to fix it permanently? I'd like my planes to fly realistically without needing to reinforce everything; it's like having to build the plane twice. I've watched Scott Manley's Interstellar Quest series on youtube, and he seems to be able to fly spaceplanes without tons of struts on them (or he's just really good at hiding them), but when I ask there, I never get a response. Any help would be greatly appreciated!

[LordFjord]

Depends a lot on the size of your plane. Small planes can get away easily without any strutting, but for larger designs it is inevitable.

Also note, that the more reinforced joints from 0.23.5 and the KJR mod do NOT cancel out wobbling completely. They reinforce the connections, but they can still break (which is fine).

A pic might help, maybe also one during flight.

You usually dont need to strut everything with everything, a few selected points do the job well.

[11Bravo]

It could be that there is too much weight on the wings. I notice it happens alot when you put big control surfaces on small wings as well

[Geschosskopf]

Does anybody know what causes this and/or how to fix it permanently? I'd like my planes to fly realistically without needing to reinforce everything; it's like having to build the plane twice. I've watched Scott Manley's Interstellar Quest series on youtube, and he seems to be able to fly spaceplanes without tons of struts on them (or he's just really good at hiding them), but when I ask there, I never get a response. Any help would be greatly appreciated!

As others have said, if your plane is much bigger than a 1-seat scooter, you're almost certainly going to need to strut the wings to the fuselage to prevent flexing. The reason you sometimes don't see struts on other people's planes is because they clip the struts inside the parts or plaster over them with structural panels, for aesthetics, or the struts are all on the bottom and they only show you pics from the top. Folks also use these same techniques to hide fuel lines. I OTOH leave them exposed because I think it's Kerbalish to have a sleek hypersonic jet with biplane-style bracing wires. All a matter of taste.

Since 0.23.5, you really don't need that many wing struts anymore. Certainly not 1 to every part, or to staple adjacent parts together. If you've got heavy stuff like fuel tanks and engines out on the wings, then you can just strut from these to the main fuselage and that will solve flexing at the wing roots. Then, depending on how far the wing extends out beyond the engines, and how much control surface area you have out there, you might also have to run a strut from the engine to the wingtip. And maybe on both top and bottom. But that's usually about it, other than sometimes some staples on the fuselage joints, especially between the fuselage part that the wings attach to and those adjacent to it, or if you're expecting to make very rough landings.

There are several ways to minimize the amount of wing struts you need. One is to get a mod (B9, Procedural Wings, Spaceplanes Expanded, etc.) that has much bigger wing parts than come with stock. The fewer parts in the wing, the fewer joints and the less it flexes. But OTOH, big wing parts often overlap multiple fuselage parts (unless you get big mod parts for the fuselage, too) but only attach to 1 of them. This puts all the torque from the wing control surfaces and engine gimbals on 1 fuselage part, so you'll probably have to staple that part to its neighbors. So if you use smaller stock-sized wing parts, you can attach them to different fuselage parts instead of just to each other, to spread the wing's torque over multiple fuselage parts.

Finally, for aesthetic purposes, you can the Firespitter mod. This contains a strut that's very thin, hardly noticeable at all. It was intended for use on WW1-style biplanes as a bracing wire, but can of course be used on anything. You can hardly see it at against the background of a spaceplane's wing panel.

[Hodo]

B9 has invisible struts which are handy for making good aircraft that dont flex. The other option is to run them between the parts by "clipping" them. Your wobble is because the reaction wheel is causing the front or back of the plane to move indipendantly of the other half, making it flex in the center like a slinky.

[xcorps]

Heh, I build a plane the other day that had some very large wings. When it flew it looked like a large bird flapping its wings. It was quite humorous until I started getting very high drag effects and the wings came off.

[RexKramer]

In the real world, all aircraft structures bend and flap to some extent. There is a weight penalty to making structures stiffer. The bending and flapping is known, and the materials used can withstand the bending.

In KSP, there is NO weight penalty for stiffening up structures- just add struts, they don't add weight to your craft. One method I've used is to make 'biplanes', with two wing structures, reinforced by struts. This may seem like a giant leap backwards in technology, but the way KSP handles drag causes no negative effects from creating strutted biplane structures. In fact, you get double the lift if your upper and lower wings are roughly the same!

In KSP practice, if the bending and flapping events are so large that aircraft control or integrity is an issue- then add struts or structures such as biplane wings (truss style with struts). If the only issues are visual only, I generally don't worry about a little bending and flapping. I generally build smallish spaceplanes, so the flapping is limited and tolerable.

The next time you take a commercial airline flight, take a look at the wing during flight. It flexes. It's normal and expected, and thoroughly tested.

In KSP, it is certainly possible that large structures could bend and flex to the point where control issues or structural problems (parts falling off..) can become a problem. Add struts or a truss type structure. It isn't a perfect aerodynamic model- the usefulness of external struts and truss structures went away in the real world about 70 years ago. However, before materials and engineering were able to manufacture full cantilever wing assemblies, trusses or struts were a very effective way to support long wing structures. The penalty was weight and drag, those penalties aren't evident in KSP.

[Tantalus010]

Hi everybody,

Thanks for the replies so far. I actually have all the mods that you guys have mentioned, but I haven't ever seen any invisible struts anywhere in my part catalog. I'll look harder next time. This happens to all my planes - big, small, heavy, light, doesn't matter. The only things I ever put on wings are airbrakes, wheels, RCS ports, sometimes aviation lights and radial intakes from B9. I also try to reproduce some of Scott Manley's designs, and get the same results. I arrived at the solution of strutting every part to every other (adjacent) part through trial and error, so it does seem I need to. If I just strut the wings to the fuselage, then the fuselage parts still wobble and flex.

So apparently strutting is inevitable. Bummer. Okay, so about the clipping - when I try placing struts in some places, the strut doesn't connect to the other part I want; it snaps to the origin point. I've always assumed that was a failed strut attempt, then I move the origin point somewhere else and try again until I get a strut that crosses the boundary between the two parts. Do I need to enable part clipping from the cheat menu to hide the struts via clipping?

Again, thanks for the help!

[O-Doc]

I arrived at the solution of strutting every part to every other (adjacent) part through trial and error, so it does seem I need to. If I just strut the wings to the fuselage, then the fuselage parts still wobble and flex.

You don't need to. Unless you want to post images of your craft(w/ CoM, CoL, CoT) there's no way of telling what the problem is.

[LethalDose]

I had this issue when I started using FAR as well. In my case, the "wobble" would start as an oscillation around one axis (e.g. a roll) that I think SAS would try to correct, and could translate into other axes as well. I found two issues help correct the problem:

1 - Placing the wings over the fuselage so the CoM is beneath the CoL for stability.

2 - Keeping my airspeed under control in the low atmosphere (basically below ~ 300 ms under 5 km altitude).

But like O-Doc pointed out, without knowing what you're flying and how you're flying it, there's not much the community can do to help.

EDIT: also this. May or may not help, but discusses the wing stability issue, amongst others.

Edited June 28, 2014 by LethalDose
Added aircraft tutorial

[hafevil333]

I've been having some real issues with this as well. No matter what design I build, I cannot cancel out the flex and bend. It only happens after I try taking off, and it with larger planes, it happens on its own. I've watched Scott Manley build a spaceplane from scratch, and he didn't really reinforce anything. I'm thinking it's something to do with KSP's joints. I think I've heard Manley mention a joint strengthening mod, so that's what I'm gonna try next. I was hoping to enjoy B9, but I'm just been continually frustrated with even simple designs.

[UnusualAttitude]

I've been having some real issues with this as well. No matter what design I build, I cannot cancel out the flex and bend. It only happens after I try taking off, and it with larger planes, it happens on its own. I've watched Scott Manley build a spaceplane from scratch, and he didn't really reinforce anything. I'm thinking it's something to do with KSP's joints. I think I've heard Manley mention a joint strengthening mod, so that's what I'm gonna try next. I was hoping to enjoy B9, but I'm just been continually frustrated with even simple designs.

Hi Hafevil,

If you having the same problems as the OP of this (very old) thread ie: wobble with FAR, in my experience it is more of an SAS problem than a structural one. SAS is constantly trying to over-correct. I would suggest you try a PID tuner, such as the one included in Pilot Assistant. Try reducing the different values until the wobble goes away.

However I would certainly recommend Kerbal Joint Reinforcement also. I use it simply because my tired old Mac can't support the extra part-count of heavy strutting. With this, even my Mk3 designs don't require a single strut.

Hope this helps you out.

http://forum.kerbalspaceprogram.com/threads/100073-0-90-Pilot-Assistant-Atmospheric-piloting-aids-1-4-1-(Mar-11)

http://forum.kerbalspaceprogram.com/threads/55657-0-90-Kerbal-Joint-Reinforcement-v3-1-1-1-15-15

[Wesreidau]

I recently had very good results by building wing spars; I-beams built into the fuselage that run down the length of the wing. Mount engines directly to those beams and strut a few panels to the beam. It made my rather heavy four-engine jet bomber fly at x4 time and there's only minimal, dare I say realistic, wing flex. You could probably build the wing panels directly onto the beam or even construct the whole aircraft, fuselage and all, on an I-beam keel and spar array.

[Space Cadet]

You can of course tweak the strength of Kerbal Joint Reinforcement. See the readme for details.

[capi3101]

Since you're using B9 (and therefore FAR by extension), I think LethalDose may have already indicated the root cause of the wobbliness (namely that what you're seeing is SAS continually overcorrecting itself). Putting the CoM ahead and below the CoL will help with the issues (at the very least, it'll make your plane more dynamically stable anyways). Might also want to check what your FAR simulations are telling you at various speeds and altitudes - specifically, you should test the altitude and speed where the oscillations are first occurring. If you see red, run a simulation for the axis - if the sim shows you an oscillation, it's something you can live with. If the graphs diverge, you've got a major design flaw and will be lucky to make space today.

You might also try using less wing. That's something I've had hammered into my head since I made the switch to FAR - you don't need as much wing as you think you do (certainly not the 1:1 ratio that's recommended in stock anyways). In general, if you've got as low as 0.75 TWR on the runway and can still take off going less than 100 m/s without flaps, you've got too much wing.

Since you've got FAR, you might also try turning down the strength of the lifting parts to see if that helps any - you'll still generate just as much lift, but you'll save a fair amount of mass in the process. I generally go with 0.7.

Other than that, I can't offer you much help.

[impyre]

...I haven't ever seen any invisible struts anywhere...

...*invisible*... You just have to know where they are.

[InterCity]

Struts.

Or if you don't want that, distribute your control surfaces more evenly. Use tail fins and canards (in their aerospace, not KSP meaning) and don't put everything on wings. In fact, there should be only ailerons on wings, nothing more.