I just want a plane that goes down the runway straight

[ibanix]

I've got a number of successful *flying* planes, but every single thing I try to make a plane launch down the runway without veering off, fails.

1) Assure landing gear is correctly aligned? Check. Gear are 90 deg to ground, pointing straight down the flight axis of the vehicle.

2) Landing gear overloaded? Nope. Multiple gear and braced with struts. Smallish craft with multiple landing gear.

3) SAS causing overcorrect? Nope. SAS turned off.

4) Unbalanced lift? Nope. Editor shows lift correctly aligned to the axis of the craft.

So what the hell is going on here? Is it really so hard to get a plane to go straight down a runway?

Edited July 18, 2015 by Claw

[HoloYolo]

Is your landing gear aligned with your center of mass? That's usually what causes.

[Wolfox]

Screenshot of example craft would be helpful. Also, any mods installed or stock?

Other than that, common causes of this effect are:

- Imbalanced craft, make double sure that CoM is exactly in the middle, with CoL aligned aswell.

- Too many landing gears. Using more than two pairs tend to make craft very unstable if not absolutely accordingly aligned. Use two bigger ones instead of four small.

- Steering. Make sure that steering on (steerable, of course) wheels is inverted if they are placed backwards. Some wheels also have to be inverted if placed behind CoM as tail drager (like LY05).

You might also try to enable Aerodynamic forces overlay (F11) to see if there are any unwanted air dragging elements when speeding up.

[ibanix]

I have KAX installed, but (most) of my planes are not using those parts at the moment.

Here's a pastebin of the .craft file for one of many craft that go wild on the runway. http://pastebin.com/yJ8WdBiN

[Xannari Ferrows]

That is true, actually. The center of friction should be aligned with the center of mass. Although, just like how aerodynamics work with the center of lift, having them close together can make a vehicle sensitive to applied forces.

Unfortunately nothing tells you where the center of friction is, so that's something left to eyeball.

[Wolfox]

Unfortunately nothing tells you where the center of friction is, so that's something left to eyeball.

Thats why i hate tail-draggers, they can easily slip out of control. Disabling brakes from front wheels and enabling a little bit at rear, then barking during start can help. You can tweak breaks for landing in flight, but this method is also a bit tricky, better go with simply trike.

Here's a pastebin of the .craft file for one of many craft that go wild on the runway. http://pastebin.com/yJ8WdBiN

Will try as soon as I get home to test it. I´m curious if I can find anything out.

[maltesh]

Use the Rotate Gizmo.

Press 'F' to switch on absolute rotation (instead of the default relative rotation)

Click on the angle snap.

Allows for adjusting to produce straight and true landing gear, regardless of the slope of the parent fuselage.

[Val]

The tail wheel was definitely not put on straight, even though it may have looked it.

I replaced all the wheels, to make sure they had no inherent adjustments errors.

And except for the tail wheel, mounted them differently.

The main load bearing wheels need to be very close to CoM, because if any noticeable load is placed on wheels far from CoM, then you get veering.

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Craft file

Edit: Forgot to mention. For stability, the main wheels are actually attached to the fuselage, not the wing. They've been moved with the gizmo to look like they're mounted on the wings, so the wheel base is a little wider, than if they were flush against the fuselage.

- - - Updated - - -

Press 'F' to switch on absolute rotation (instead of the default relative rotation)

...

Allows for adjusting to produce straight and true landing gear, regardless of the slope of the parent fuselage.

Oh, I did not know that. Thanks. Edited July 18, 2015 by Val
Clarification

[Kryxal]

Disabling brakes from front wheels and enabling a little bit at rear, then barking during start can help.

Barking during start? Something seems a bit odd about that.

Set brakes - throttle up - start engine - BARK BARK BARK!

[Val]

Barking during start? Something seems a bit odd about that.

Set brakes - throttle up - start engine - BARK BARK BARK!

I laughed so hard my cats ran.

[ibanix]

The tail wheel was definitely not put on straight, even though it may have looked it.

How did you determine that? Just eyeballing it?

I replaced all the wheels, to make sure they had no inherent adjustments errors.

And except for the tail wheel, mounted them differently.

The main load bearing wheels need to be very close to CoM, because if any noticeable load is placed on wheels far from CoM, then you get veering.

http://imgur.com/a/5Oj38

Craft file

Edit: Forgot to mention. For stability, the main wheels are actually attached to the fuselage, not the wing. They've been moved with the gizmo to look like they're mounted on the wings, so the wheel base is a little wider, than if they were flush against the fuselage.

Thank you for the craft file, that was really helpful. I feel silly for not having thought of using two wheels next to the body under the wings.

I don't get the wheels by the engines, they don't touch the ground?

- - - Updated - - -

And SAS really makes a difference. Without it, I veer off at ~60m/s and crash badly. With it, the plane just takes off nicely on it's own, straight down the runway. Awesome.

[Wolfox]

Barking during start? Something seems a bit odd about that.

Set brakes - throttle up - start engine - BARK BARK BARK!

Oh god, took me a good while till I got it My best typo in a while!

[Val]

How did you determine that? Just eyeballing it?

When I replaced it with a fresh wheel from the part list, the craft didn't veer left any more.

Thank you for the craft file, that was really helpful. I feel silly for not having thought of using two wheels next to the body under the wings.

...

And SAS really makes a difference. Without it, I veer off at ~60m/s and crash badly. With it, the plane just takes off nicely on it's own, straight down the runway. Awesome.

You're welcome

I don't get the wheels by the engines, they don't touch the ground?

They aren't needed, but can be helpful, if you dip a wing during landing. Uneven terrain or because the craft is a bit hard to fly.

[ibanix]

The craft is already a bit hard to fly. It's twitchy; I wonder if I have the CoM too close to the CoL.

[Val]

The craft is already a bit hard to fly. It's twitchy; I wonder if I have the CoM too close to the CoL.

When I flew it, it felt more like, too much wingspan, relative to the vertical stabilizer.

So you can try one these 3 things:

• Move the vertical stabilizer further back.
  
• Put a bigger vertical stabilizer on.
  
• Make the wingspan shorter.
  

[moogoob]

I've put wing gear on planes before... like my really bad idea one where the jet engines were way out on the tips of the wings. Turns out that makes roll maneuvers super slow. The wheels help if you come in anything other than super level so you don't lose an engine nacelle or something and get REALLY off-balance (and un-flyable) as a result.

However! If the wing wheels touch during takeoff, they can and will cause your plane to swerve. Not enough to foul up a landing, but enough to send you off the side of the runway on takeoff, depending on design. Something to look out for.