Aircraft Takeoff and Landing Wobble

[Wrecker013]

I'm certain this is quite a frequent subject, but I've not found information that pertains accurately to the situation I seem to be having. I have an aircraft that is quite stable in-flight, however, at speeds over 30 m/s on the ground, it will waddle from side to side on its wheels. While I can manage a take-off with that, landing is quite challenging, nevermind the fact that I'm trying to fly a plane that acts like that!

Conventional wisdom seems to say it has something to do with the wheels being misaligned, however, I would swear up and down that the wheels are straight and flat. It would seem like to me that it has something to do with lift pulling the aircraft up but not enough to push it into the air without elevators nor at that speed.

Notably, I use the FAR aerodynamic model. I'm aware the post is lax on explicit details to the design of the aircraft in question, but I don't have time to supply them at this exact moment. I shall endeavor to give them later. As a final note, the aircraft is constructed, structurally (nevermind the science instruments and storage) from vanilla parts that are supplied from the 'aviation' science node. Any help is appreciated, thank you.

[DrLicor]

@Wrecker013 welcome to the forums, would you be so kind to provide us some screenshots? 

1. Press F1 ingame. (screenshots will appear in ...\Kerbal Space Program\Screenshots\...
2. Upload the screenshots to Imgur/Dropbox/[any image site where you can get a link]
3. Copy paste the link (must end with .jpeg/.pgn/.gif/.......) into the text section here or click on 'Insert other media'  in the right bottom' .
4. ...?...
5. profit.

My guess is either the gear is to close to the middle, or your spring and damping isn't sett to the right vallue in the editor

Edited September 28, 2017 by DrLicor

[bewing]

My guess is that the craft is wheelbarrowing.

[Vanamonde]

2 hours ago, bewing said:

My guess is that the craft is wheelbarrowing.

Yes, the most common cause of this is that the plane starts to rise from the ground, leaving most of the weight on the single front wheel, and then you're trying to ride a unicycle.

[DrLicor]

16 hours ago, bewing said:

My guess is that the craft is wheelbarrowing.

That's also possible indeed. @Wrecker013 Always try to have the gear on back as close a possible to the CoM, just behind it and the front gear as far as possible in the front, another thing could be to lower the nose relative to the back, this way you're sure the front gear will lift first. 
Also, try to use flaps, this increases your lift, but also the needed AoA. When landing, you can maintain nose up, and slow down slowly that your nose gear touches the ground gently.

[DrLicor]

I looked up some newish screenshots of a plane in RSS/RO/RP-0/FAR instal, here you can see what ahappens when landing with the right flap setup. 
A higher AoA is needed to maintain vertical speed near 0 m/s, this way you can softly land the plane with it's butt first.
When braking the nose will slowly come down and touch down without any *boing* back in the air if your damper and spring are setup are right.
Last picture shows the flap/elevons/etc placement, far is a very good tool to configure those.

Spoiler

 

[shdwlrd]

A lot of the stock suspension issues lay in the fact that there is too much spring and not enough damping. (Much like when you see a car bouncing several times after hitting a bump in the road. The springs are good, but the shocks (dampers) are bad.) There is very little you can do about it if you run a stock install. If you are using Module Manager, you can write a patch to increase the damping or you can use the one I wrote for 1.2 and is still valid.

Spoiler

@PART[SmallGearBay]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 7.5

}

}

@PART[GearSmall]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 8.5

}

}

@PART[GearLarge]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 10.5

}

}

@PART[GearMedium]

{

　

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 9.5

}

}

@PART[GearFixed]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 8

}

}

@PART[GearFree]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 12

}

}

@PART[roverWheel1]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 3.5

}

}

@PART[roverWheel2]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 2

}

}

@PART[wheelMed]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 3.5

}

}

@PART[roverWheel3]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 2

}

}

@PART[landingLeg1]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 3

}

}

@PART[landingLeg1-2]

{

@MODULE[ModuleWheelSuspension]

{

@damperRatio = 3

}

}

@PART[miniLandingLeg]

{

@MODULE[ModuleWheelSuspension]

{

@springRatio = 4

@damperRatio = 1.5

}

}

 

[Daniel Prates]

Indeed what does it to me is to move the main gears as close as possible to the CoM and leave little weight to act over the nose (or tail) wheel. But there are other things worth investigating in your design as well:

1. Is your thrust vector going through the CoM?  If the thrust line goes above it,  there is a down-wise tortion and that concentrates more momentum over the nose wheel, pressing it against the ground.

2. Heavier aircraft require larger wheels. If a larger part is not available,  consider multiple pairs of them.

3. FAR is an increeible mod that really forces you design more efficiently. It does not let you get away with what stock would have. Move your wings around to have the CoL closer to the CoM and perhaps increase wing area to have more lift with less speed. It might help, as a better geometry and less weigh on the wheels as you run along helps decrease wobble. For that matter,  all wobble has to do with excessive,  incorrect or unbalanced weight on the wheels (in my experience).

4. Still on the lines of '3' above,  the same goes for the control surfaces,  and specially so for the elevators. Are they positioned in such a way that they push the nose towards the ground?

5. Use some autopilot,  as a "trim" of sorts. Atmospheric autopilot is my favorite. Pressing P makes it go in a straight-keeping stance and it takes the load of having to stabilize the craft running in the runway with the awkward keyboard.

6. Different wheels from different mods do not dialogue very well together. Say,  kerbal foundries for main wheels and stock for a tail wheel. Always a hassle.

 

[Spricigo]

8 hours ago, Daniel Prates said:

3. FAR is an incredible mod that really forces you design more efficiently. It does not let you get away with what stock would have.

I really don't recommend this from players struggling with planes, more likely it would make the failure more spectacular instead of helping to solve the issue.  Is one of those mods that one should stay away until become confident enough he can deal with what it bring to the game and can easily overwhelm a less prepared player. Don't get me wrong, FAR is a great mod, just that is better to learn to walk before attempting to run, and the more forgiving stock aero may help with that.

Other than this, spot on advice.

[Daniel Prates]

On 16/10/2017 at 4:54 PM, Spricigo said:

I really don't recommend this from players struggling with planes, more likely it would make the failure more spectacular instead of helping to solve the issue.  Is one of those mods that one should stay away until become confident enough he can deal with what it bring to the game and can easily overwhelm a less prepared player. Don't get me wrong, FAR is a great mod, just that is better to learn to walk before attempting to run, and the more forgiving stock aero may help with that.

Fully agree!  I still struggle a lot with it,  even though using it for a long time. ...

EDIT: I wasnt really recommending FAR for him. In the OP he mentioned that he uses it already! Odd choice though, it is really a "pro" mod. 

EDIT2:  another good thing to do is install RCS BUILD AID. It has a dry center of mass tool. Unless your design is very unique,  usually the CoM shifts aa you consume fuel. So landing may turn out very differently from taking off. Using this tool lets you plan your LG placement to encompass the whole range of a moveable CoM.

Edited October 21, 2017 by Daniel Prates