Mk3 wobbles

[jordsta95]

Hey there,

I have been playing KSP for a while now, but I took a break for a few months while I was busy doing other stuff. I come back to messing around with spaceplanes to find that there is still something which I cannot master - Mk3.

Whenever I try and takeoff with something that is Mk3, it will speed up to about 50ms and then start wobbling on the runway (rolling left to right more and more violently each time). Does anyone know why this happens?

I put the wheels flat (as in perfectly vertical) this happens. Have them at a 45 degree angle, and the same happens. 3 wheels, 4 wheels, 8 wheels, doesn't matter, it's always the same.

Does anyone know of any tutorials, videos, etc. explaining why Mk3 is apparently so much different to Mk1 and 2 when it comes to takeoff. Seriously, if I launch the Mk3's off a rocket up a few hundred metres and detatch, it will fly and land just fine. But takeoff, it just won't have it.

[Daze]

Can we see a picture of the craft?

[jordsta95]

This one was one of the more annoying ones. This one wouldn't wobble, but it would turn (the other major issue I have with these crafts)

Veering left (always veer left) - note SAS is on, and I wasn't touching A at all:

http://steamcommunity.com/sharedfiles/filedetails/?id=401318928

I was actually pressing hard right, and the same still happened

On runway:

http://steamcommunity.com/sharedfiles/filedetails/?id=401318968

From the front:

http://steamcommunity.com/sharedfiles/filedetails/?id=401319004

From behind:

http://steamcommunity.com/sharedfiles/filedetails/?id=401319043

[theend3r]

It is probably too heavy for those puny wheels which makes them bend and wiggle. Get some stronger wheels from mods or wait for 1.0.

[Firedtm]

where is the center of mass ?

[rich1051414]

I have found that wobbling is usually from wing flex or wheel flex. Try adding struts in both directions(body to wheels, wheels to body), and same for the wings. That should stiffen it up.

[strongest_2hu]

Double nose gear side-by-side is a recipe for disaster every time.

[wolfedg]

Double nose gear side-by-side is a recipe for disaster every time.

Oh is it? I guess I wasn't aware of that when I built this.

http://s1075.photobucket.com/user/wolfedg/media/screenshot288.png.html

Also to the OP Symmetry and Struts are the keys to stability.

Edited March 2, 2015 by wolfedg

[Vanamonde]

Moved to Gameplay questions, since this is a how-to kind of a thread.

[eddiew]

Just to check, have you encountered Kerbal Joint Reinforcement? Helps reduce the flex and wobble that stock likes to give you for no valid reason whatsoever​

[Deathsoul097]

Double nose gear side-by-side is a recipe for disaster every time.

Designed many a thing with 2 gears on nose, never had a problem with it. Whats your reasoning here?

[Cirocco]

the left-to-right wobble you're refering to comes from "wheelbarrowing". It happens when you have a heavy aircraft and your wheelbase on your rear wheels is too narrow, especially if you have any sort of significant weight past the wheelbase. It means that small sideways pushes on your plane cause one wheel to support more weight than the others, which means more friction, which at take-off speeds means an unstable plane and lots of pretty explosions.

for illustration, imagine a round mass which stands on 3 toothpicks which form a very narrow traingle. Give it a sideways push. The mass will easily tilt sideways, then rock from side to side a bit before coming back to a standstill. Now try it with the same mass, but your toothpicks form a wide traingle. The sideways push will be far more likely to shunt the whole construction sideways rather than tilting it. Same thing happens to heavy airplanes: narrow wheelbase --> very unstable, will tilt side to side at the slightest provocation. Wide wheelbase: stable, will more likely be shunted side to side, but due to large mass the effect will be negligable.

This effect, combined with the fact that the wheels in KSP can handle super-heavy loads without buckling, it's better to use a wide wheelbase for the rear wheels and a single wheel on the nose.

Moral of the story: design your planes to have a wide wheelbase. Design possibilities to do this include (but are most certainly not limited to): attaching more tanks sideways on the back of the plane to have more attachment surface or attaching wheels directly to the wings. Just make sure you put some structural element in between the wings and the wheels to make sure they reach down low enough to support the plane.

Edited March 2, 2015 by Cirocco

[TokiTech]

Try attach the wheels to the fuselage. Is known to me the Mk3 cockpit doesn't have a solid flat surface to place wheels on especially when is not 100% midpoint. I suggest place the front wheels like on the rear fuselage.

[EtherDragon]

Wobbling on the runway is usually indicative of the Center of Lift (CoL) being too far behind the Center of Mass (CoM). As the ship gains speed, the wings try to lift off, placing a lot of pressure on the front wheels - lifting off of the back wheels, and this is unbalanced.

Does it wobble less if you apply full pressure on pitch-up? Also, because of your wing placement, your CoL is likely above your CoM. There's nothing inherently wrong with this, in fact, it can be quite useful for a stable build, but you want to make sure that your CoL is directly above your CoM.

Can you post any side-views of this build in the Hangar with CoL and CoM turned on?

EDIT: Also - 1.0 will include different kinds of wheels for various Splane sizes, can't wait!

[Frozen_Heart]

Double nose gear side-by-side is a recipe for disaster every time.

I've not had that problem. My longest used SSTO had double nose gears and never had a problem with it.

Helps reduce the flex and wobble that stock likes to give you for no valid reason whatsoever​

My sweet summer child... If you think its bad now you should have played something pre 0.23.5!

[BudgetHedgehog]

My sweet summer child... If you think its bad now you should have played something pre 0.23.5!

Ah, the 'Wet Noodle' years..

[Ion Man]

This a very simple method to attach landing gears to an mk3 body. You can also use "wing parts" on the wings, strengthen them with struts, one left, one right and mount a gear on then. They're lighter and have an aerodynamic function. With the offset function, you can place mostly everything where you want it.

Too much weight on a gear makes it move and wobble, which will result in a nice fireball.

Edited March 4, 2015 by Ion Man

[eddiew]

My sweet summer child... If you think its bad now you should have played something pre 0.23.5!

Lol, I arrived on the scene with 0.25, and frankly feel quite glad I didn't get into KSP before then

Without KJR, even my smallest mk2 spaceplanes bend in the middle when under timewarp, and the big ones do actually fold back on themselves! Amazed that Squad haven't brought KJR into stock, tbh - but maybe it will turn up with the new aerodynamics, since they seem to have seen the light with regard to planes