Do wings have got a "up" and a "down"?

[Signo]

First of all, I apologize if this is already a known and widely discussed issue. My install is almost stock, x64 app on Win 7 with just a few graphic stuff and MJ. 

Lately I tried to give my planes more yaw stability and I tried to enlarge the tail using "fixed appendices" like Big-S wing strakes, swept wings and/or the structural wing type B.

I noticed that on the landing strip the craft (actually the crafts, at least 3 different iterations) was steering left w/o direct input and I supposed it was due to the common wheels issues (double checked the CoL, wheels orientation, friction and so on). 

 

The issue was still present while flying, ruling out any possible wheels connection. I can describe the experience like "flying with a deployed airbrake".

Still the craft was able to overcome the steering force during the flight envelope, ruling out clipping phantom forces.

This time I double checked for any asymmetrical strut or fuel line but I did not find any that could be guilty of such a behaviour.

 

Finally I checked the aero overlay and I found that apparently fixed appendices used as tails receive a lift "from left to right", making the craft unstable. 

Prograde direction apparently does not influence the vector of the lift. (it always goes from left to right disregarding craft attitude)

Doubling the appendix just doubles the problem (lift comes from the same side for both with both appendices right at 90°).

"Built up" appendices (i.e. swept wing + random elevon) retain the issue with the moving part trying to oppose the left/right lift.

Common tailfins, canards and moving appendices in general seem not affected.

It is "contagious" - a not affected part (i.e. big-s tailfin) coupled with an affected part (big-s wing strake) will generate the issue.

Below an example with a Big-S wing strake used as tailfin.

 

Did anybody notice this before?

 

Edited July 30, 2016 by Signo
grammar

[bewing]

No, I had never noticed such a thing before. I will have to look carefully now. I use structural wing parts constantly -- sometimes mounted vertically, the way you have for those BigS strakes. I have never seen any "pulling to one side" action. There is still a chance that MJ is doing this to you.

[Val]

 

Wings in KSP are simulated as having symmetrical airfoil. So they do not have an up or down.

It also means they do not produce lift unless they have angle of attack.

The small sideways force while flying is more likely to be caused by not flying wings level. Even banking a few fractions of a degree will cause a small side slip. In my experience it's enough to create the force you see in the screenshot.

But the Navball shows you slightly to the opposite side, though. So maybe you're on to something. 

Side slipping slightly on the runway is due to the wheels, though. And not related in my experience.

Edited July 30, 2016 by Val
Accidentally post and autocorrect

[klesh]

I noticed the other day that some wing I had put on with snap were showing a downward lift arrow.  It had to have been a bug?  Maybe a symmetry bug.  I dont believe the part I was attaching to was reversed.  Angling the wings ever so slightly with the rotate tool made the arrow pop upward.

[Signo]

18 minutes ago, bewing said:

No, I had never noticed such a thing before. I will have to look carefully now. I use structural wing parts constantly -- sometimes mounted vertically, the way you have for those BigS strakes. I have never seen any "pulling to one side" action. There is still a chance that MJ is doing this to you.

I tried w/o MJ too, but you can really feel there is indeed something wrong while piloting - as I wrote above feels like flying with a deployed airbrake.

11 minutes ago, Val said:

It's a very small force. It's more likely to be caused by not flying wings level. Even banking a few fractions of a degree will cause a small side slip. In my experience it's enough to create the force you see in the screenshot. 

Side slipping slightly on the runway is due to the wheels, though. And not related institutions my experience.

It is pretty small in this picture however pretty strong in my experience - I had my fair share of "slight runway slipping" but this is definitely different: the craft goes off track really quickly and feels really hard to steer back. 

 

Thank you for your answers guys.

[Signo]

4 hours ago, klesh said:

I noticed the other day that some wing I had put on with snap were showing a downward lift arrow.  It had to have been a bug?  Maybe a symmetry bug.  I dont believe the part I was attaching to was reversed.  Angling the wings ever so slightly with the rotate tool made the arrow pop upward.

I had a simmetry related thought too, but I was thinking to a radial/mirror matter instead. I did not have the chance to try yet. 

I will experiment a little bit more tonight.

I'll report back the science if I succeed. 

[Signo]

Well, after a few extra flights it seems that Val is absolutely right and it is just dependent on AoA. 

I was mocked by the overlay: if a spike is not there but it should it does not mean it does not exist. They should add an extra blue spike to yaw devoted moving surfaces too.

Thank you for all your answers.

(last but not least, we still have the "slippery runway mistery" - the same craft with a different tail proved very different at take off, pics below - the first one is a lawnmower while the second one is straight as a sword - I deduced that the longer the lifting surface involved the higher the effect AoA will have on the craft by simple leverage: we maybe accidentally discovered "sailing ships" w/o wind)

http://imgur.com/a/VH7Ah

(could any mod please fix this? Today it seems I fully derped and I can not find the imgur button anymore; thank you.)

 

Edited July 31, 2016 by Signo

[pincushionman]

 There is an old bug that prevents symmetry from properly…"symmetrizing" stiffness.  This leads to uneven deflection of wing  surfaces, which leads to uneven forces on wings, which leads to roll, which leads to turning.  This same bug also contributes to the tendency for  wheels to pull to the left on the runway, but in that case it's not the only factor at play.

Edited July 31, 2016 by pincushionman

[Signo]

On 31/7/2016 at 4:39 PM, pincushionman said:

 There is an old bug that prevents symmetry from properly…"symmetrizing" stiffness.  This leads to uneven deflection of wing  surfaces, which leads to uneven forces on wings, which leads to roll, which leads to turning.  This same bug also contributes to the tendency for  wheels to pull to the left on the runway, but in that case it's not the only factor at play.

About wheels pulling left I made a few interesting experiments with rovers - with the proper downforce the go absolutely straight, so it is indeed a traction/applied weight matter.

 

[foamyesque]

I've long suspected that something was off, but I blamed asymmetric forces on tailfins.

[pincushionman]

2 hours ago, Signo said:

About wheels pulling left I made a few interesting experiments with rovers - with the proper downforce the go absolutely straight, so it is indeed a traction/applied weight matter.

Which is why I noted that it "contributes" to the problem, and there are other factors at play. Moving gear to the center (reducing flexibility) can help somewhat, in certain cases.

Remember that the whole point of the takeoff roll is to produce enough lift to takeoff, and the wobble becomes prominent at speed - after the wings are producing lift, uneven lift, and resulting in uneven load on the wheels.

There is a lot of physics going on in the takeoff roll.