Elevon issue ?

[Archibald]

Hey guys,
First time posting after 1000+ total hours, I must say the game's been pretty good to me so far.

I came across what seems like a bug yesterday. I just started a new Career playthrough (1.6 with mods, mainly visual mods, KER and Kerbalism) and went to build the first plane for atmospheric missions.

Problem is : the first elevons I unlocked don't behave as they should. When I pitch up with the elevon set on Normal deployment, the elevon pitches down. When I pitch down, it pitches up, but when I invert the way they deploy, it stays the same. Whether normal or inverted, pitching up always has the elevon pointing downwards.

What's weirder is that the canards at the back of the plane work just fine.

Any ideas for a fix?

Cheers!

[RoofCat]

your elevons are right next to your CoM and thus game engine can't decide should they work inverted like canards or regular like tail.

Having pitch elevons close to CoM is basically useless in KSP. In real life those "change" wing geometry increasing lift. In KSP wings have no lifting properties to start with. Just gliding surface at an angle if anything. There are no airspeed and pressure differences above and below.

So elevons at CoM could act as airbrakes or very slightly push plane upwards without pitching it in KSP. Which is slowing down or very inefficient at best (as either elevons or wings increase drag instead of smooth glide in this case). And game engine might not understand what are you trying to prove. Might also help to lift fuselage out of water for those landing and taking off from ocean, but in that case use deploy function instead of steering.

Pitch with tail (canards add front drag), yaw with vertical tail only, roll with elevons on sides of CoM only. Don't use same surface for more than one steering direction, that makes steering worse, chained up in a bad sense. Disable the rest.

[Archibald]

Originally posted by RoofCat:

your elevons are right next to your CoM and thus game engine can't decide should they work inverted like canards or regular like tail.

Having pitch elevons close to CoM is basically useless in KSP. In real life those "change" wing geometry increasing lift. In KSP wings have no lifting properties to start with. Just gliding surface at an angle if anything. There are no airspeed and pressure differences above and below.

So elevons at CoM could act as airbrakes or very slightly push plane upwards without pitching it in KSP. Which is slowing down or very inefficient at best (as either elevons or wings increase drag instead of smooth glide in this case). And game engine might not understand what are you trying to prove. Might also help to lift fuselage out of water for those landing and taking off from ocean, but in that case use deploy function instead of steering.

Pitch with tail (canards add front drag), yaw with vertical tail only, roll with elevons on sides of CoM only. Don't use same surface for more than one steering direction, that makes steering worse, chained up in a bad sense. Disable the rest.

Thanks for the info! I did not realize this could be an issue. Is that something that happens a lot? I made a bunch of planes on other save files and never got this issue. Most of them had delta-type wings so that could explain it. So you're saying if I disable pitch on the wings and/or move them away from the center of mass I should be good?

[RoofCat]

I'm saying pitch elevons at CoM don't make sense in the first place. They can't (hardly can?) make you pitch (rotate), because they are on that particular rotational axis. Pitch steering must be far ahead or behind CoM. Roll steering far on the sides of the CoM. Yaw far in back of CoM. Leverage.

If for this plane they got axactly at CoM with that particular fuel amount and stuff, it most likely just went more extreme (bugged out) unlike when they were just close.

Also I haven't seen your planes. So it's all rather guess.

Also delta wing does shet in stock KSP. The same sized wings (area) will have the same drag regular wings do. Just in case.
In my experience the best plane shape in stock KSP is balanced cross shape. But others will work too, just with some shift in favor of one or another steering direction/stability except you go extreme and it turns from special to disaster. Ones pitch better, others roll better... you got the idea. Choose your preferences/poison.

[Archibald]

Thanks for the tips! I went out and looked up this center-of-mass bug with the elevons and it's apparently something that happens since 0.9. I'll just move my wings backwards a little and I'll be fine.

[Manwith Noname]

It's not really a bug, it's just how the game determines which way to rotate them for the purpose of the intended key press. As has been attempted to make clear, there's little point in having pitch controls near the CoM in the first place. You said yourself...

Originally posted by Archibald:

What's weirder is that the canards at the back of the plane work just fine.

...this is because they are in the right place for pitch control. That is, the further the force is applied from the CoM, the less force is required to actually achieve the desired result. Not only does placing ailerons close to the CoM cause detection issues for pitch, it almost nullifies any effect they have when they do twist in the right direction.