Spaceplane pulling to one side with high/low wing configurations

[Alshain]

I'm having some trouble with a plane I'm building using NEAR. I wanted to use a high anhedral wing design I had used before because I liked the way it flew and I noticed the plane pulled heavily to the right for some reason. So I went back to the SPH and turned on RCS Build aid in engine mode and sure enough it was in fact indicating a heavy force off the right of the plane. After playing around with it I realized it was the wing position. If I moved to a mid wing, level position it went away. I found this odd, so I went back to one of my other, smaller planes I did same configuration with and sure enough it had a pull too, just not as noticeable I guess.

My question is, is that normal? Apparently any high or low wing configuration causes this, even when center of thrust and center of lift are both in line with center of mass and the craft is otherwise symmetrical.

Second question, aside from moving the wings, what would be a good way to counteract this problem? Is there a canard or tail configuration to minimize the effect?

Edited August 27, 2014 by Alshain

[Alshain]

So no one can tell me why these planes pull to one side and how I can fix it? (Note, I'm not worried about the lift, that's really a good thing anyway). It turns out that it isn't the wings, I can put them in any configuration and it does it.

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Edited August 27, 2014 by Alshain

[Hodo]

Looks like your ladder is the cause of your drag to one side. Add a second ladder and it should even out your pulling issues.

[Wanderfound]

Got a non-anhedral pic for comparison?

The ladder idea is plausible, although I thought those were physicsless; not sure if Ferram applies drag to those. Could also be nont-quite-perfect auto-symmetry problems.

Anhedral will accentuate the issue; inducing roll instability is what it's for.

[Alshain]

Got a non-anhedral pic for comparison?

Removed the ladder and it and it didn't change a thing. I don't have a pic of the non-anhedral but it looked the same. I can get one later but it's late, I closed the game, and it takes way to long to restart it with B9 installed.

Edited August 27, 2014 by Alshain

[Alshain]

Here's a plane with a mid wing configuration, same issue. It's not NEAR either, I uninstalled it to make sure. I posted here to see if there was something I was doing wrong in my plane designs. No matter what I do it seems like it always wants to veer off the runway.

[r_rolo1]

Given your description, it has to be some kind of assymetry ... on first look, I would blame the winglet you have in the tail section ( it is hard as hell to put a dead center vertical surface in 1x mode on SPH and as long as you have even a slight displacement to the side in that part you will have side slip, especially if that part is fixed and it's angle of attack can't be controlled ). but the fact that it appears to have some relation with the wing placement makes me somewhat puzzled ...

[DaMichel]

I cannot answer you question. However i just want to add that this big arrow to the right does *not* indicate a force. Unless i have a moment of total brain deadness here, this does in fact indicate the torque axis. What is shown here is torque from the engines not being aligned with the CoM. It is pretty small though and will try to pitch the craft up.

Edit: Err i think r_rolo1 nailed it: No rudder, so no lateral control? You did activate steering for the front wheel, did you?

Edited August 27, 2014 by DaMichel

[Moonfrog]

u mention veering of the runway, is it possible its your wheels are slightly out of alignment? To explain why this may be the case.

When your plane lifts up it doesn't usually lift up everything at the same time. Your back end may lift up, pulling the two wheels of the ground first, means whole wait is on the first front wheel. Any slight movement which happens with sas or those control surfaces results in an angle for the wheel. This makes it veer.

Another possible reason with wheels is u have to align the front wheel exactly straight with centre of mass like you would do thrust. The two wheels at the back need to be perfectly horizontal with the front wheel. Depending on whether you use circle or hexagon to attach them you end up with the wheels at angles. Causes it to veer. Usually if attached to underside of a wing u use the circle. If you attach from the edge of the wing u use hexagon then 90 degrees rotate it to face down.

This is a common problem. You can also solve it 90% of the time with a tail wing rudder. If it is not the wheels for veering on runways then I am wrong.

More control surfaces you use will escalate this problem exponentially.

Edited August 27, 2014 by Moonfrog

[mythic_fci]

This is most likely because of the fact that you only have 1 rudder. Due to the way lift is written in-game (not affected by FAR/NEAR), only one side of the rudder will provide "lift". Therefore, there's a slight force pushing the plane sideways. Try redesigning it with dual rudders.

[Wanderfound]

Ninja'd a few times before I got here, but yeah: the mention of runway veering is a red flag. Does this happen in the air as well, or just on the runway?

If it's just runway, it's almost certain a landing gear issue. Wing-mounted gear are particularly troublesome: as the wing flexes, the steering geometry goes out of whack.

[Kasuha]

Veering off the runway is often issue of insufficient lift.

If you place a rocket on launchpad with TWR = 1 and start its engines, it will slide along the launchpad surface in random direction. With Unity physics it may not be completely random but it depends on very subtle details up to rounding errors.

If the same happens to a plane, it has only two directions in which it can randomly slide - to the left, or to the right.

Vertical stabilizer has usually way too little wing area to stop that.

You can try to concentrate on getting more speed before you pull up.

[Alshain]

Edit: Err i think r_rolo1 nailed it: No rudder, so no lateral control? You did activate steering for the front wheel, did you?

My understanding is planes do not require an active rudder. I did activate steering on the front wheel but you can't really steer mid-take off, at those speeds the plane will flip. Steering is really for taxiing. Really the plane should go down the runway straight. Some of my planes do, some do not. The two smaller planes above go straight enough for take off but I did notice some slight side movement. The larger one (the SSTO) veers off too quickly and I'm in the grass.

u mention veering of the runway, is it possible its your wheels are slightly out of alignment?

I activated the wheel alignment guides from firespitter in the images above. I can't get them straighter than that. Some of them are off by a point, but I don't think that would cause it. Even if it does, the editor simply won't let me fix it without over compensating. I just don't believe that can be the cause or this would be a bigger issue for everyone.

Veering off the runway is often issue of insufficient lift.

If you place a rocket on launchpad with TWR = 1 and start its engines, it will slide along the launchpad surface in random direction. With Unity physics it may not be completely random but it depends on very subtle details up to rounding errors.

If the same happens to a plane, it has only two directions in which it can randomly slide - to the left, or to the right.

Vertical stabilizer has usually way too little wing area to stop that.

You can try to concentrate on getting more speed before you pull up.

It has enough lift to take off rather quickly (if steer it over to the grass first), I can't get more speed on the runway, speed is the trigger for it's turning. If I get more speed, I just turn faster.

Are you saying the vertical tail wing is too small? I hadn't considered that but that does make sense if my understanding of what the vertical tail is for is correct.

Edited August 27, 2014 by Alshain

[Kasuha]

It has enough lift to take off rather quickly (if steer it over to the grass first), I can't get more speed on the runway, speed is the trigger for it's turning. If I get more speed, I just turn faster. Are you saying the vertical tail wing is too small?

No I don't mean your tail wing is too small, I just say that it's not big enough to stop the plane sliding.

Okay, my theory:

Anhedral wing is a positive-feedback configuration. If the plane rolls ever so slightly to the left or right, the lift on the side which went up increases and lift on the other side decreases, both acting in the direction of increasing the roll. Since you still have wheels on the ground it can only roll a little but the difference in how the plane is sitting on its wheels affects where it is going. The side with more lift is going to be a little faster than the other side.

If it starts veering to one side, try to roll it to the other side (Q/E). If it helps (or sends the plane to the other side) it was that.

[Alshain]

No I don't mean your tail wing is too small, I just say that it's not big enough to stop the plane sliding.

Okay, my theory:

Anhedral wing is a positive-feedback configuration. If the plane rolls ever so slightly to the left or right, the lift on the side which went up increases and lift on the other side decreases, both acting in the direction of increasing the roll. Since you still have wheels on the ground it can only roll a little but the difference in how the plane is sitting on its wheels affects where it is going. The side with more lift is going to be a little faster than the other side.

If it starts veering to one side, try to roll it to the other side (Q/E). If it helps (or sends the plane to the other side) it was that.

Ok, well I'm at work right now but I will definitely give that a try this evening.

[DaMichel]

Fair enough.

Perhaps it is a weight distribution issue. IIRC it can happen that basically the rear experiences serious lift forces, putting the weight on the front wheel which makes it veer off. Have to tried to take off without SAS and with some pitch-up trim, to lift the nose first?

[Claw]

I could venture several guesses too, but it would be a lot easier to troubleshoot if you could provide a craft file. It's easier to figure out if I can fly instead of guessing.

One thing I will say is that you should not need two tails to offset some stock lift problem. Control surfaces are buggy, but they produce lift on both sides where appropriate.

I will say that at face value, it looks like your wheels are too far back. That makes me think you need excessive speed before you are able to rotate. Excessive speed can make a plane jittery, easy to cartwheel, or puts added stress on the gear making it flex. If you are using angled wings, the added downforce can make these things worse.

It's also possible that the wings are flexing, which will cause gear flex issues with the wide mounted gear you are using.

(.craft file?)

Cheers,

-Claw

Edited August 27, 2014 by Claw

[r_rolo1]

My hunch on the rudder was not about the rudder being too small to prevent side slipping, it was about the single fixed rudder you have being a possible creator of that same issue. If you put it even slightly out of center ( and due to the way the game works, that is almost certain ) you will get a resulting force due to the lift created from the rudder beinng outside of the center axis ... and this will get worse with increasing speed. OTOH it might be also a landing gear issue, as others pointed ...

[Alshain]

Fair enough.

Perhaps it is a weight distribution issue. IIRC it can happen that basically the rear experiences serious lift forces, putting the weight on the front wheel which makes it veer off. Have to tried to take off without SAS and with some pitch-up trim, to lift the nose first?

Another good suggestion to try tonight.

I could venture several guesses too, but it would be a lot easier to troubleshoot if you could provide a craft file. It's easier to figure out if I can fly instead of guessing.

One thing I will say is that you should not need two tails to offset some stock lift problem. Control surfaces are buggy, but they produce lift on both sides where appropriate.

I will say that at face value, it looks like your wheels are too far back. That makes me think you need excessive speed before you are able to rotate. Excessive speed can make a plane jittery, easy to cartwheel, or puts added stress on the gear making it flex. If you are using angled wings, the added downforce can make these things worse.

It's also possible that the wings are flexing, which will cause gear flex issues with the wide mounted gear you are using.

(.craft file?)

Cheers,

-Claw

I can say the wheels are in the correct place but looking at the images I see what your seeing. It's an optical illusion caused by the angle in the image. The leading tip of the gear bay is directly on the center of mass putting the wheels just behind it. It actually lifts pretty easily if I take off from the grass where I don't have to worry about running off a hill. I might try moving the gear on the SSTO to the rockets.

Just to be clear I was using the little plane as an example, it drifts to the side a little but not enough to be a problem. The SSTO is the concern, I'll get you a craft file when I'm back at that machine.

My hunch on the rudder was not about the rudder being too small to prevent side slipping, it was about the single fixed rudder you have being a possible creator of that same issue. If you put it even slightly out of center ( and due to the way the game works, that is almost certain ) you will get a resulting force due to the lift created from the rudder beinng outside of the center axis ... and this will get worse with increasing speed. OTOH it might be also a landing gear issue, as others pointed ...

I'll look at it again, I know what you mean I did have trouble getting it right but I'm fairly certain it is straight. I put that one on with angle snap but I didn't have the "lines" like I did on the liquid fuel tank texture as a guide so that is possible.

Edited August 27, 2014 by Alshain

[Moonfrog]

I've given your design a look over a bit more closely from the imagine. Just a suggestion but try it and see if it works. place struts on your fuel tanks with the lvl909 engines and attach them to the main body. Do the same for the wing. I think the guy who said its the wings flexing might be right.

[Wanderfound]

It's not just about wheels in the correct place, centre- and symmetry-wise. Even perfectly symmetrical gear can send you astray if the thing it's mounted to moves, and it's hard to completely eliminate wing flex without extreme strutting.

Angled gear is similarly problematic; even momentarily uneven suspension compression during takeoff wrecks your steering geometry and can create a nasty positive feedback loop.

[Alshain]

It's not just about wheels in the correct place, centre- and symmetry-wise. Even perfectly symmetrical gear can send you astray if the thing it's mounted to moves, and it's hard to completely eliminate wing flex without extreme strutting.

Angled gear is similarly problematic; even momentarily uneven suspension compression during takeoff wrecks your steering geometry and can create a nasty positive feedback loop.

We have a winner. I had put struts on the wings but I did such a great job hiding them I failed to notice symmetry mode didn't connect the other side, as it tends to do with the struts. Problem solved.

Thank you to everyone who helped +Rep all around!