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Planes yawing on runway


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Too much drag at the front, and not enough at the back. :)

And now foamyesque and I will argue about whether to fix the problem by adding more aerodynamic stability (his answer, which is probably good in your case), or by manually reducing the friction on the front landing gear and increasing it at the back (my answer), which may work for you or it may not. Or a combination of the two, of course.

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There's two fixes.   The first one, invented by @bewing , is to use friction control override and turn the traction of the front wheel right down and the back wheels up.   It's like putting fatter tyres on the rear of a car so the handling tends towards understeer.  Yes, this works on rovers too !

Second, your gears might not have attached perfectly straight.   This is less common than it used to be,  but the fix for that is to go into SPH, select the rotation tool, make sure you're in angle snap mode, then click on your gear.  Before rotating, press F to toggle between local and absolute rotation modes.  Make sure you're in absolute, then rotate the gear one notch left or right then snap it back again, this ensures it points true north.

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Even with friction control tweaking, every single craft I make will pull left on acceleration. I can correct it with enough yaw control, but it's completely invariable by type of craft. No matter what I make, or how its wheels are attached, or how many wheels it has (up to 8), it still slowly drifts left while accelerating along the runway.

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Recently I had a plane doing this...

I added aerodynamic stability, changed wheel friction, even rebuilt parts of the craft that would be a hair out of alignment and nothing solved it...

Then, after a couple hours messing with the design, I noticed my ruder and yaw indicator in a unexpected position. No idea what caused it.

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The third (more fun, IMO) way to stay stable on the takeoff roll, is to trim up.
Take weight off the nose-wheel, or in general off the front wheels, by pulling a bit up, trimming up with alt-S, or deploying flaps/canards to pull the nose up a just bit.

Nose-wheel, or tricycle, aircraft do tend to wheelbarrow once they build up speed if you don't trim up enough, which I expect based on where the wheels have to be.  The weight of the craft is distributed on the gear so that the sum of their forces against the ground is directed straight through the CoM.  A first approximation of rolling friction would give each wheel rolling drag proportional to is force against the ground, so there you have a drag force acting at ground level just below the CoM.  In absence of aerodynamic forces, this would pull the nose down, putting a bit more weight on the front gear, making their contribution to drag a little higher, until eventually the net drag force acts from a center ahead of the center of mass, making the craft unstable in yaw.

@Spricigo, probably you know (but just being sure) about trim, and alt-X to re-center the trimmed position of the controls.

Edited by OHara
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Somewhat overlapping what others have said: as the craft starts to lift off, it may end up riding the front wheel as a unicycle and start wobbling. Applying some pitch-up as you roll down the runway can prevent that. 

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It can also happen if the gear isn't completely center front.

If it is angled to the left or right, the craft will veer to the side where the nose gear is pointed. If it sits where the wheel housing is at an angle, such as the end of a nosecone or in a slight aerodynamic curve, it will also cause issues on the runway and veer to the side, also...

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On 24/03/2017 at 7:20 PM, OHara said:

@Spricigo, probably you know (but just being sure) about trim, and alt-X to re-center the trimmed position of the controls.

Yes, I know it. But I was expecting the trim to be zeroed since I didn't touch any control after launching the craft. AFAIK is not possible to set trim at SPH/VAB (or I’m wrong?) so something else need to be the cause, no gamepads/joystick also. Maybe some mod or a different program (e.g. Steam overlay? Skype?)

Anyways, after noticing the unexpected  trim, the solution was automatic (alt-X)  and, since happened only with this craft, I just cleared it from my mind.

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Isn't there inherent instability with a craft starting on the runway simply due to Kerbin's rotation being so... pronounced...? I mean, assuming all things being equal and all balance, drag and aerodynamic factors being perfect, isn't there inherent instability because

1. How Kerbin's rotation programming behaves at a core game level and

2. Nothing actually starts flush and static on the runway

?

 

Or have such things been corrected? I know these were factors in the past.

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6 hours ago, Spricigo said:

AFAIK is not possible to set trim at SPH/VAB (or I’m wrong?)

Not trim -- but you can put a little extra friction on the right side to make it pull just a little bit .....

Quote

Isn't there inherent instability with a craft starting on the runway simply due to Kerbin's rotation being so... pronounced... ?

That seems like an extremely good point that I haven't thought about. However, the fact that the runway is on the equator should minimize all the coriolis problems. If it were just a case of instability, then 50% of the time, the planes should drift off to the right, and they never do. They always drift left.

 

Edited by bewing
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The causes of a plane yawing / veering on the runway are many and varied, and these are the ones Ive come across myself.

In no particular order:

Wheel misalignment in VAB (align them using rotation in absolute mode)

Wheel misalignment when mounted on wings caused by wing flex (wheels "splay out" - try autostrutting wings in VAB)

Wheelbarrowing or unicycling (rear wheels / body lifts before the nose - COL / COM issues)

Fuselage flex.

Asymmetric thrust (rare one, found once or twice on bugged craft, but worth checking thrust isnt blocked either)

No of wheels vs mass of craft. If the craft is too heavy for the wheels youre using, it can cause veering. Try adding a couple more around the COM / COL pivot point. For example, ideal weight for the lowest gear is around 5 to 6 tons, more than they they stress / explode / veer. Adding more landing gear spreads the mass, reduces the wheel stress. I suspect that each landing gear has an ideal mass range associated with it.

 

 

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BTW: even with a yawing plane, it is possible to get it into the air.

The main trick is to be very gentle with the steering, and follow each nudge of the yaw controls with a tap of roll in the same direction so as to counteract the tendency to tip over.

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