Getting Lateral torque on planes with thrust off center vertically

[Vorg]

Tried building a plane and ran into a problem I was using Karbonite radial mounted jets and RCS Build Aid was showing a lot of side thrust even if I switched to a single in line engine. I went ahead and tried to fly the plane and sure enough, it insist on turning to the left to the point it is unflyable. With some more experimenting in the hanger using RCS Build Aid, Any time thrust is above or below center mass, you get lateral thrust. I would expect vertical rotation and it does show the red circle area also. But if thrust is above center mass, you get a straight arrow to the left and if your thrust is below center mass, you get a straight arrow to the right even using stock parts. Could this be a mod screwing things up?

[Red Iron Crown]

If you could share a picture of your craft in the SPH with the CoM/CoL/CoT markers visible that would help immensely in figuring out what is happening.

[Vorg]

Can put pics up in an hour or two. In the mean time, It's simple to reproduce a craft that does it. Place ether of the aircraft cockpits or even a probe core might work (somthing to start placement), stick one of the stock jets on the back, (don't need a fuel tank to see the markers), then place something above or below center line that has mass, an aircraft landing gear, a fin, what ever to move COM up or down. RCS helper promptly shows the side ways arrow along with the correct vertical spin arrow. That is with RCS set to display engine thrust btw.

[Rhomphaia]

The orange arrow pointing from the COM?

That shows the axis around which the torque acts.

You haven't explained in much detail what is happening in flight, but if you are veering on the runway, it is likely that your off-center thrust is pushing the plane down onto a single nose landing gear which never ends well.

[TheMoonRover]

It sounds like he's trying to say that the centre of mass is in the same vertical plane as the thrust vector (but not along it). In that situation you would expect the aircraft to be stable in the yaw and roll axes, and to naturally pitch up or down. He instead experiences a torque which rotates the aircraft laterally (along the yaw axis).

[Vorg]

It sounds like he's trying to say that the centre of mass is in the same vertical plane as the thrust vector (but not along it). In that situation you would expect the aircraft to be stable in the yaw and roll axes, and to naturally pitch up or down. He instead experiences a torque which rotates the aircraft laterally (along the yaw axis).

Exactly, This plane pulls hard to the left:

Simple test with COM below COT:

Simple test with COM above COT:

Edited February 28, 2015 by Vorg

[strongest_2hu]

It's your gear.

[TheMoonRover]

Is there some assymmetry in the ship? (parts, different fuel levels, different thrust limiters

With the two tests, it could be due to the way lift is calculated in stock aerodynamics. Does the same thing happen with something else in place of the wings?

Does this ONLY affect that particular engine type, or all engines?

If it's only that engine, it's clearly a fault with the engine. Perhaps the thrust vector is defined incorrectly or something is messing with it. Or perhaps they've been placed in an odd way? Try redownloading the mod for the engine.

If it's all engines, and it's not an issue with the ship, something is seriously broken.

EDIT: Yay! Rocketry Enthusiast

[Wercho]

As Rhomphaia said, it is not telling you that it is producing lateral thrust. It is telling you that the axis around which your thrust would produce a rotation is that direction. The circular arrow shows the rotation. RCS build aid uses light blue arrows to show thrust.

As for why it does this, a vector along the axis of a moment (angular force equivalent) is in common use in engineering because it has computational advantages. In your case, the red arrow doesn't mean anything for you, just look for red circulars and blue straight arrows.

[Rhomphaia]

Again that orange arrow is not an issue, it simply indicates the axis around which the torque occurs. Your flight issue is something else.

If it is happening on the runway, once again I would suspect landing gear issues. I have found taildraggers to be a nightmare in KSP.

If it happens in flight I would suspect asymmetric thrust limiting due to limited intake resources. I don't have any experience of karbonite to tell when this would become an issue with those jets and intakes. just monitor the thrust output of both engines and see. There are build techniques you can use to prevent this.

other than that the plane has a couple of aerodynamic issues. (CoL should not be ahead of CoM, and in stock aero the front-most wing sections will provide no lift except to exacerbate any sideslip the plane encouters)

Edited February 28, 2015 by Rhomphaia

[Wercho]

Pulling to the left can also be caused by a bug where wings don't flex symmetrically. Try strutting up the wings (and also the landing gear) and see if that helps.

[Vorg]

This does happen in flight also.. It's not the gear because it is a tad easier to prevent the turn while on the ground. To me it acts as a combo of prop torque and a cross wind both pushing to the left. It's not the engine because in the test craft, it is all stock parts. I just stock the first level jet on the back to give it a thrust value.

I understand what you are saying abut the orange arrow being the rotation axis. I thought it was showing another force of some kind pushing sideways since that is exactly what the craft started doing. Then it starts turning to the left and rolling to the left, then nosing down. Could only get it a little bit in the air before there was no stopping it from crashing.

I and I though COL was supposed to be ahead of COM to keep it from wanting to nose dive.

Don't know what you mean about front wings not providing left. Should I put small dummy wings in front to make the main wings work?

Will be glad when they fix the lift system. It's hard to build planes when the wings don't follow known logic.