Symmetric placement not really symmetric?

[TheFlyingKerman]

I built this plane with symmetric placement tool so everything is supposed to be symmetric. But it seems one wing always has more lift than the other (see pic). This causes the plane to roll gradually over time. How can I correct this?

 

[Gargamel]

First off, that is goofiest awesomest plane I have ever seen.  That thing looks cool, and fast.   Like something out of the 50's...

 

I have not had any issues like that with symmetry. 

It looks like you have a bit of yaw input happening.  Either you are pulling slightly to one side, or your trim is set to yaw a bit.  The yawing would possibly reduce (or increase?) the lifting ability on one side of the plane. 

[bewing]

SAS cancels trim, so it's not trim. I think the physics problem is the sideslip in the AeroGUI display. But the fundamental problem is that symmetry is only approximate, AFAIK. I think I'd try to combat it by taking two of those symmetric engines and placing them one-at-a-time at slightly different positions, until the yaw/sideslip goes away.

 

[Kryxal]

I'd say to add some dihedral, that way a bit of roll means a lift imbalance and a countering force.

[XLjedi]

Although I cannot explain exactly why it happens...  I have noticed on occasion that something happens in the game and/or my PC/GPU that causes the following behavior:

Perfectly good craft designs begin to exhibit unwanted rolling behavior and/or have a tendency to want to nose-down after takeoff.

This has been most annoying on a couple of occasions where it began to happen in the midst of a new design that I was flight testing.  In which case, I didn't realize it was a game flight physics glitch and I spent a fair amount of time trying to correct the craft only to find out that it was happening on my older proven designs as well!   In which case, shutting down the entire PC and rebooting made the issue go away.  I think it might be linked to walking away from the PC (for hours) and then coming back to the game without rebooting the system.

Not saying that it's necessarily causing your issue, but just something to be aware of and maybe test another design or two just to make sure it's not a game related physics glitch before ripping your craft apart. 

Edit:  one other note... when it happened to me most recently (just this past week) I turned on the flight physics arrows and noticed that vertical control surfaces were showing that they were generating lift forces to the left or right causing unwanted yaw.  After rebooting and restarting the game, the flight physics arrows on my vertical stabilizers were back to normal.  My first indication that something was not behaving correctly was that the craft was pulling to the right or left on the runway before takeoff.

Edited May 31, 2018 by XLjedi

[Daniel Prates]

2 hours ago, Gargamel said:

First off, that is goofiest awesomest plane I have ever seen.  That thing looks cool, and fast.   Like something out of the 50's..

Like a soviet flying monster of the 50s! The nose resembles the caspian sea monster.

@TheFlyingKerman what is that screen full of data,  to the right of the pic?

 

 

29 minutes ago, XLjedi said:

My first indication that something was not behaving correctly was that the craft was pulling to the right or left on the runway before takeoff.

This is more likely to be a landing gear issue!

30 minutes ago, XLjedi said:

vertical control surfaces were showing that they were generating lift forces to the left or right causing unwanted yaw.

 Yaw certainly makes one side of the plane to travel faster than the other,  since it induces the outward side of the turn to go faster than the inward side. This could explain the slightly higher lift,  as the faster it goes,  more lift is generated.

Edited May 31, 2018 by Daniel Prates

[XLjedi]

3 hours ago, Daniel Prates said:

This is more likely to be a landing gear issue!

 Yaw certainly makes one side of the plane to travel faster than the other,  since it induces the outward side of the turn to go faster than the inward side. This could explain the slightly higher lift,  as the faster it goes,  more lift is generated.

No, it was not a landing gear issue...  the game flight physics were just bugged and I pointed out a few things I noticed and now watch for.  The planes tracked perfectly on the runway and in the air after rebooting.

Edited May 31, 2018 by XLjedi

[Daniel Prates]

21 minutes ago, XLjedi said:

No, it was not a landing gear issue...  the game flight physics were just bugged and I pointed out a few things I noticed and now watch for.  The planes tracked perfectly on the runway and in the air after rebooting.

Let me be a little more precise.

I too observe this issue of craft veering to the left or right during talkeoff. Its something that may look like a bug but may not be one.

Insuficient gear number/size for the craft weight, incorrect placement of the main gear in relation to the CoM,  or the thrust vector forcing the craft downwards during takeoff is a sure way to have your craft drift to the sides. It is usually solved by tweaking those things. 

Now,  granted,  I dont know what your specific issue was,  if any. Just seemed like a relevant comment to make since this forum is consulted by a lot of people with the same usual problems ans they seek these coments to find some light. Aircraft drifting out of the runway is a fairly common issue.

[XLjedi]

@Daniel Prates  ...my intent was to point out that it's not always a craft design flaw.

[Daniel Prates]

16 minutes ago, XLjedi said:

@Daniel Prates  ...my intent was to point out that it's not always a craft design flaw.

Oh you bet. Physics and aero in ksp will frequently bug out. 

Some more:

However in the issue mentioned in the OP, that is, one wing providing more lift than the other, I think that what you said above about control surfaces could be the answer to this. The control surfaces, although paired during craft design, work independently during flight. They are not 'tied' together, so that they move exactly the same in opposite directions. Now, assume one is moving slightly more than the other. That will generate slightly different torques, there is going to be a little yaw, there is going to be a slightly different draf profile in each side of the craft etc. If this generates an actual yaw motion, it's inevitable: one wing will be travelling faster than the other. Its what happen when you use rudders in a real life airplane: you start rolling to the side you are yawing, since the outward wing will be travelling faster, hence, generating more lift on the outward side. 

This could explain the difference in lift generated by the different wings in the craft shown in the OP.

[Laie]

I wonder if it might ultimately be coriolis forces. Will the plane still roll if you keep flying due east?

[bewing]

6 hours ago, Daniel Prates said:

what is that screen full of data,  to the right of the pic?

It's called the stock AeroGUI. You activate it in the "physics" tab in the debug menu.

 

[Daniel Prates]

1 hour ago, bewing said:

It's called the stock AeroGUI. You activate it in the "physics" tab in the debug menu.

 

Ok great,  I'll check it out. I use kerbal engineer redux for my data screens.bur that seems more complete!

2 hours ago, Laie said:

I wonder if it might ultimately be coriolis forces. Will the plane still roll if you keep flying due east?

I dont think ksp is THAT precise and realistic!

[bewing]

4 minutes ago, Daniel Prates said:

I dont think ksp is THAT precise and realistic!

Actually, coriolis forces are gravity-based. And if there is one thing that KSP does model carefully, it's gravity.

 

[AVaughan]

I think the root cause could be floating point precision.  In which case you can't do anything about it, and just need to live with it.  (Also don't forget that since you are rolling and yawing, the true angle of attack and true airspeed for each wing are different, so it's perfectly reasonable that they are producing different amounts of lift).

[CrashyMcCrashFace]

Not a plane but with super long inter-planet burns some of my ships will slowly roll. Once it starts, nothing I do will make it stop.

[pincushionman]

Slightly unsymmetric stiffness has been a longstanding bug in the game. The two wings deform differently under load (twist and bend), causing the difference in lift and a roll (usually to the left in my experience, but that’s not a certain thing). It’s also the source of the imfamous “why does my plane veer to the left on the runway?” behavior. Stiffer structure (struts, autostrut, KJR) helps, but you can’t get rid of it completely.

[TheFlyingKerman]

R

On ‎6‎/‎1‎/‎2018 at 8:31 AM, pincushionman said:

Slightly unsymmetric stiffness has been a longstanding bug in the game. The two wings deform differently under load (twist and bend), causing the difference in lift and a roll (usually to the left in my experience, but that’s not a certain thing). It’s also the source of the imfamous “why does my plane veer to the left on the runway?” behavior. Stiffer structure (struts, autostrut, KJR) helps, but you can’t get rid of it completel.

This is what I found. Changing autostrut mode seems to help, but cannot eliminate the problem...

[Laie]

On 6/1/2018 at 12:18 AM, Daniel Prates said:

[coriolis forces]

I dont think ksp is THAT precise and realistic!

Coriolis is real and quite noticeable on the small world that is Kerbin. When trying to move north or south, you have to constantly yaw in order to stay on track. It's easy to see how this could lead to roll.

[Daniel Prates]

1 hour ago, Laie said:

Coriolis is real and quite noticeable on the small world that is Kerbin. When trying to move north or south, you have to constantly yaw in order to stay on track. It's easy to see how this could lead to roll.

That could be due to the Coriolis effect, but could also be due to  something else, like a bug as @XLjedi mentioned above. You are precluding the possibility @Laie that they veer to one side of the other because perhaps the plane is not perfectly level, thought it may seem to be by glancing at the navball? Though I am not going to state categorically if in-game the effect 'is' or  'is not' implemented (since I don't know that), I would guess it probably isn't. I agree with @bewing that KSP models  gravity very well. But that is to say, it mimics SOME aspects of gravity very well, possibly the most relevant ones, such as gravity acceleration/pull and its inherent mathematics. But it is not a 100% precise, all-encompassing  laboratory replica or depiction of  what the real-life phenomena of gravity actually is. I think that would perhaps be impossible. So if the game programming replicates some or even most consequences of that thing we call gravity, that is not to say the mere existence of something akin to gravity in the game would automatically generate all effects of gravity that we see in real life! 

Again, I'll say I AM NOT SURE if Coriolis was embedded in the programming (and I would guess you guys aren't either). I am just saying that if the programming mimics some effects of gravity, it does not automatically implies that all real life characteristics of gravity automatically would arise too. We would have to study this (I searched the web and found nothing) or ask the devs. 

In any case... it should be easy to find out, or at least, to exclude Coriolis altogether. The experiment made by @TheFlyingKerman could be done first flying north/south, then south/north. Since coriolis pulls to the same side, the effect would have to change sides and be experimented on the other wings. If they don't.. its not Coriolis. If they do, it's still not proof, though (although I would start to suspect it might be)! 

@TheFlyingKerman why don't you try it and post it here for us?

Please nobody take me for being a stubborn type. AFAIK, it could be or couldn't. I just think it's not showing much lover for the scientific principle to reason like that: a) i see a phenomena and I have an idea of the cause; b) ergo, IT IS the cause! Consider this: if the laws of gravity were fully implemented in KSP as they are in real life, we should be able to see matter attracting matter, shouldn't we? Place a couple of parts in space and see if they attract each other. See what I mean? Its not because we see some of the manifestations of gravity in-game, that the whole enchilada is there working as it does in real life. The devs would be much more than good to achieve this, they would have to the like 'lucy' from that Scarlett Johanson movie!

Edited June 2, 2018 by Daniel Prates

[Laie]

1 hour ago, Daniel Prates said:

Again, I'll say I AM NOT SURE if Coriolis was embedded in the programming (and I would guess you guys aren't either). I am just saying that if the programming mimics some effects of gravity, it does not automatically implies that all real life characteristics of gravity automatically would arise too. We would have to study this (I searched the web and found nothing) or ask the devs. 

Oh boy... this is getting more of an argument than it has any right to. Coriolis forces is nothing that needs to be put into the game separately, it follows from the physics we have. Just play a round of Kerbal Artillery Program and fire something on a ballistic trajecory north or south, then watch what happens.

On Kerbin, coriolis forces are not negligible. Even a subsonic plane can cover ten degrees of latitude in matter of minutes.

I am by no means certain that it is the reason for the OP's troubles. It's a hunch, no more,  a thought that struck me when I noticed that the plane goes almost, but not quite, due south.

@TheFlyingKermanA test, taking the plane on another course, would take very little time compared to wrangling with symmetry issues. And if my hunch is correct, this means that your problem is not due to engineering.

Edited June 2, 2018 by Laie

[Daniel Prates]

1 hour ago, Laie said:

it follows from the physics we have

Yeah but does it, though? I am not sure of it. 

Look, its too bad you're not liking this conversation, its those kinds of discussions that make this forum worthwhile - as long as they are friendly, which from my part you can be sure it is. No need for 'oh boy' me, allright? 

1 hour ago, Laie said:

Just play a round of Kerbal Artillery Program and fire something on a ballistic trajecory north or south, then watch what happens.

I would say this is not Coriolis effect, but rather, the object moving against an also-moving background. This is not what the Coriolis effect is, iirc. Planes drifting in bearing is something else indeed, but I have my own explanation for this, given above.

 

EDIT. Wait. ... maybe there's a disparity of terminology here, which is my fault. I'll be clearer.

There are two different things, coriolis FORCE and coriolis EFFECT. the force is due to inertia, gravity etc. The effect is the perceived drift. The first is due to force vectors and the second is what you see when you compare two differently traveling objects. I still maintain that if the coriolis FORCE is depicted in-game, that is something to be seen, but the coriolis EFFECT, yeah, I guess in that sense you can indeed see it. It's the artillery shell example you mentioned above. But that would have nothing to do with gravity, or how well the game models it. 

Edited June 2, 2018 by Daniel Prates

[OHara]

3 hours ago, Laie said:

Coriolis forces is nothing that needs to be put into the game separately, it follows from the physics we have

It would follow, except for the bug (or sloppiness, or maybe needed simplification) that KSP does physics in the rotating frame of the planet, for everything inside the 'low orbit' altitude.  So Foucault pendulums don't work on Kerbin (even if put far enough from the equator that they should).

It bothered me that the left wing shows − lift while the right wing +, but presumably KSP uses just one model of the symmetric-shaped wings for each of left- and right-side wings in mirror symmetry, so the left wing might have its up/down coordinate inverted, but physically acts (roughly) symmetrically to the right wing.  The magnitudes of the lift usually match to better than the 0.3% in the initial post; bad luck there.  You can use alt-E to trim out the roll.