[FAR] Pitch forces roll on a completely symmetric plane

[theend3r]

In many of my designs I seem to experience a clockwise roll when pitching up. The (space)planes are still flyable with this although I have to correct for this the whole time. It also drifts right on the runway leading me to believe that the aerodynamics are not the reason this is happening. The wheels come with an alignment guide and are perfectly aligned. All my craft are 100% symmetric and I can't fathom the cause of this so any help/ideas would be appreciated.

The only thing that comes to my mind is that the intakes (that are thankfully getting reworked in the next patch) asymetrically supply the engines and make them produce slightly different thrust. All the relevant FAR flight data should be in the gallery:

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Edit: Also, posts like Your design is stupid. You don't need that much lift. The wing sweep is useless etc. are... welcome. A good advice is always welcome.

Edited February 19, 2015 by theend3r

[cantab]

At Mach 0.5 and Mach 1, that red L-Beta number I think means your plane is unstable in roll. (But I'm not 100% sure on the stability derivatives, so take this with a pinch of salt). A slight perturbation, probably just due to floating point error, will increase instead of self-correcting, producing the behaviour you experience. A slight dihedral angle on the wings can help this, as can raising them.

Workarounds include using FAR's wing leveller, or dialling in a little roll trim. As far as the build goes, if you do suspect the engines are producing unequal thrust, have you tried the intake placement trick? (Place intakes, engine, intakes, engine instead of all the engines then all the intakes.)

[pincushionman]

I had the exact same issue, except to the left. Apparently it has to do with joint strength and flexibility; installing Kerbal Joint Reinforcement mod did wonders for me. Still not completely gone, but helped. Others say struts will help too, but extra struts only made it worse for me. And yes, pitch-up caused it to happen more than anything else. Some bug with the flexibility code.

[Stargate525]

This might be a no-brainer, but does the flap on your vertical stabilizer have pitch authority? That'll cause a roll.

[NathanKell]

Stargate525, I rather think you meant control surface. A flap is something a control surface can be set to be, just like elevator, aileron, rudder...

For one reason why: When you pitch, you are adding a downward force on the rear of your wing. That will push the wing down. Due to this ancient bug the joints will not respond symmetrically. So one wing will have less (or more) anhedral than the other, thus adding some roll.

[theend3r]

Thanks for all the replies. I already have KJR and some struts on but I'll try to strengthen the wings further and see what happens.

Edit: Pincushionman and NathanKell were right about the cause, strutting wings closer to the tips to reduce wobble improved the flight characteristics and eliminated the problem. The plane still drifts off on the runway for mysterious reasons but otherwise it's all fine now.

Edited February 19, 2015 by theend3r

[capi3101]

As far as the build goes, if you do suspect the engines are producing unequal thrust, have you tried the intake placement trick? (Place intakes, engine, intakes, engine instead of all the engines then all the intakes.)

I would suggest to anyone who has had headaches with this particular potential issue to use the Intake Build Aid mod. It does the rearranging for you automatically at the push of a single key. It's is extraordinarily useful in that regard.

That's really the only thing I can add here; I don't know enough about FAR to say one way or the other. I know enough to know that red numbers are bad; that's about it.

[Renegrade]

By the way, one way of diagnosing flex issues is to look at the joints carefully when maneuvering. If no flex is visible, try it again at x2 physwarp...and then x3..and finally x4. The higher warps will really bring out the flex, which can help you figure out where things might not be firmly attached.

[Rokmonkey]

As for the drifting on the runway, I've had this problem a number of times as well. If your wheels are perfectly in line, then it actually IS an aerodynamics issue. As you go faster and generate more lift, you will have an uneven force on your wheels. I find that having wheels that are straight down, not angled like yours, helps alleviate this. Another option is to not pitch so quickly until all of your wheels are off the runway.

I'm not sure if this helps, but your wheels are really far back. I typically place my wheels just behind the center of mass. This will make it much easier for your plane to lift off, as the lever arm is that much shorter.

[theend3r]

As for the drifting on the runway, I've had this problem a number of times as well. If your wheels are perfectly in line, then it actually IS an aerodynamics issue. As you go faster and generate more lift, you will have an uneven force on your wheels. I find that having wheels that are straight down, not angled like yours, helps alleviate this. Another option is to not pitch so quickly until all of your wheels are off the runway.

I'm not sure if this helps, but your wheels are really far back. I typically place my wheels just behind the center of mass. This will make it much easier for your plane to lift off, as the lever arm is that much shorter.

I normally don't place them so far back but in this case I have to due to the LV-N's.

[Rokmonkey]

I normally don't place them so far back but in this case I have to due to the LV-N's.

I've seen folks put a landing gear on the back clipped in to be kind of a fail safe in case of tail strike. Either way, I think making your rear landing gear perpendicular might help with the drifting.

[cantab]

Yeah, a safety wheel or skid under the tail that you don't normally roll on can be useful. A fair few real aircraft have them.