Fins are counter productive.

[bakanando]

I was having some trouble with different ships, where all of them had an inclination bias towards west-north-west. It was so consistent across many ships that I thought that the launchpad was the culprit. After many many tests, I realized that all of the ships had tail fins for 'stability' so I promptyl removed them and the rocket happily went (almost) directly upwards.

I have to say that this was noticeable because my ship had very little reaction wheel torque, but even with control surfaces the ships where still veering WNW with such force that I couldn't counter the tilting.

Many users have mentioned that using a gimballed engine and/or other methods of control make this a non-issue. While this is mostly true, if the lift from the fins is too high (every wing section except for the smallest one, actually) you'll be fighting the fins' lift all the way up to orbit. This is the problem I'm trying to highlight with this tests: that the fins are basically useless, and in extreme cases will make your ship turn so fast you'll never reach space.

This tests are indeed made with a really bad designed rocket on purpose: a long rocket without any form of stabilizing themselves apart from the fins. This way the problem is isolated to demonstrate its effects.

--update--

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--update+--

Some more info from other users:

gogozerg and another

herbal space program

Necrobones

SAI Peregrinus

TL;DR: any fin with more than a wee bit of lift will tilt your rocket if not countered with other types of control. Unexpectedly, ships without fins can -with proper care- fly more stable than rockets with them.

Edited May 11, 2015 by bakanando
made the op a little bit clearer.

[Alshain]

Normally your fins will hold you steady. But your fins are blocked by boosters so the wind is only hitting one side of each one.

The air coming off the nose cones of the booster doesn't take a sharp turn and go straight down once the booster goes away. It keeps going out at an angle and then curls back (called turbulence). So the air is getting split around only half the fin, the fin needs air on both sides equally to be effective.

Edited May 9, 2015 by Alshain

[bakanando]

There are four fins with those two boosters in-between so it should balance the forces out. As I mentioned though, even with the boosters blocking the airflow the ship veers WNW. As soon as I removed the fins the ship magically stopped tilting in that direction.

This was still happening with vessels that had no radial boosters, so it's not really that. I'll do another test ship later to see if it fixes it.

[bakanando]

So I did a thorough testing of this problem. I'll update the OP with the relevant information.

[Laie]

There used to be a problem where symmetrically attached parts had differing attachment strengths. The part you place is slightly more rigid than it's auto-generated peers.

I had the same problem recenty, but only if the vessel had little to no control authority. MOAR STRUTS have helped, that' why I thought of that old bug.

[Roflcopterkklol]

I was having some trouble with different ships, where all of them had an inclination bias towards west-north-west. It was so consistent across many ships that I thought that the launchpad was the culprit. After many many tests, I realized that all of the ships had tail fins for 'stability' so I promptyl removed them and the rocket happily went (almost) directly upwards.

I have to say that this was noticeable because my ship had very little reaction wheel torque, but even with control surfaces the ships where still veering WNW with such force that I couldn't counter the tilting.

--update--

http://imgur.com/a/CvmMN

This is one of those ships:

http://images.akamai.steamusercontent.com/ugc/26238017169251186/6343AA3643DEFC54B487698A52DCF8E3B78F1F3E/

Are you 100% sure its not Gimbal on your rockets engines?

[sal_vager]

So why are my finned rockets behaving? Something is wrong here, I suggest you upload these craft so others can test them.

What I've found so far with the fixed winglets is that they can add too much stability, rockets like to stay straight much like an arrow and for the same reasons, so it takes more gimbal or reaction wheels to steer.

[hellion13]

All I can say is the new tier 0 fins have been a boon for me. Been putting them as low as I can on the second stages. The other older model winglets made flight too stiff feeling, but the small ones keep the business end of my rockets pointed the right way while making flight smooth(proper-ish gravity turn required). Id almost say they feel OP.

Strange they send you in a consistent direction. Sure its not just your rocket leaning over just a tad on the pad before launch?

Ive noticed there is a lot more sag in the upper stages of taller rockets. Mine tend to sag a little to the north/northwest. its enough to have to really wrestle with anything with solid fuel boosters in the launch stage.

[Eric S]

It was so consistent across many ships that I thought that the launchpad was the culprit.

I'm still of that opinion. The KSC facility is built on a large flat area of the ground, except that it's really flat, not flat relative to the center of gravity of Kerbin, so anything far enough from the center winds up having a slight tilt towards the center.

The reason I think you don't notice this without fins is because the initial tilt is so slight that it isn't noticeable without being magnified by a gravity turn starting at ground level and largely controller by aerodynamic forces. Without the aerodynamic forces, craft won't follow the prograde but rather maintain the same heading.

[bakanando]

I made a rocket without control surfaces, gimbal or reaction wheels to test this. Everything I tested pointed towards the fins being the problem, with rockets tilting harder the higher the amount of lift the fins had. This can be hardly seen in pictures, but when you launch the same rocket only changing the fins you notice the difference.

I encourage other people trying this, a long rocket with and without fins to peer test the game.

@Eric_S It does have a slight tilt, but the finless rocket actually started turning East instead of the other rockets' Westward tendency. This still needs testing, but there is definitely a problem here that can become game breaking is some circumstances.

[herbal space program]

I made a rocket without control surfaces, gimbal or reaction wheels to test this. Everything I tested pointed towards the fins being the problem, with rockets tilting harder the higher the amount of lift the fins had. This can be hardly seen in pictures, but when you launch the same rocket only changing the fins you notice the difference.

I encourage other people trying this, a long rocket with and without fins to peer test the game.

@Eric_S It does have a slight tilt, but the finless rocket actually started turning East instead of the other rockets' Westward tendency. This still needs testing, but there is definitely a problem here that can become game breaking is some circumstances.

FWIW I had exactly the same problem. When I started a career game in 1.0 I had a big tall rocket made of all tier 1/2 stuff, and when I put four fins either on the bottom or in the middle, it would start tipping on me as it gained speed and altitude no matter what I did. There were no side boosters. Taking the fins off left me with a sluggish but totally flyable rocket. I tried taking them off and re-placing them a bunch of times and still no matter what the fins made the rocket unflyable. I have no idea why some people are having this problem and others not, but I never had this issue with fins before the new aero model took effect.

Edited May 9, 2015 by herbal space program

[Sky_walker]

Try putting your fins on top of the rocket. Above center of mass.

[herbal space program]

Try putting your fins on top of the rocket. Above center of mass.

Well if that works then it's pretty clear they're not modeling the fins' drag very well.

[Alshain]

Well if that works then it's pretty clear they're not modeling the fins' drag very well.

Don't worry, it doesn't work. Center of lift needs to be behind center of mass.

[cybersol]

Just a hunch, but I'm guessing what you are seeing with the slight westward tendency is related to the sidereal rotation of the KSC eastward under you.

[DoToH]

When using fins, you are adding drag (also lift, but drag is the important one here) to you rocket. Placing them on your rocket is like placing feathers on an arrow, you want them behind the CoM.

[bonyetty]

The oberth effect difference between the top and bottom of the rocket may be contributing but doubt it. It would need to be a very tall rocket, I would guess for a visible difference.

[gogozerg]

The oberth effect

?!

@bakanando

I just tried many launches with a rocket like yours (three FL-T800, LV-T30 90-95% thrust, no torque). Trying to find what was wrong with your setup.

I was very surprised, but yes, you are right.

Adding four AV-T1 makes the rocket fall toward west very significantly most of the time!

When climbing slowly, it's normal if fins are useless. But they seem to change the rocket balance in a curious way.

but the finless rocket actually started turning East instead of the other rockets' Westward tendency.

That's what I noticed too.

Edited May 10, 2015 by gogozerg

[Alshain]

I haven't seen an issue. Is it possible this is a symmetry glitch? Try putting the fins on one at a time. Of course I always use symmetry too and still I haven't seen it, but give it a try.

[gogozerg]

Try putting the fins on one at a time.

4 new launches with the same four fins, assembled one at a time:

1) Crashed west

2) Crashed west

3) Crashed north

4) Crashed west

Then removed fins:

1) Climbing straight, then falling east.

2) Same.

3) Same.

4) Same.

Craft file

You may not see the problem at 100% thrust. It's maybe clearer with a LV-T45 with gimbal locked.

Edited May 10, 2015 by gogozerg

[T.A.P.O.R.]

I found that the earliest fins are useless and caused more flipping than a finless design.

The AV-R8 winglet seems to do the trick, though I find that I need fins on my upper atmospheric stages, otherwise any rocket with a decent payload tends to flip over before gimbals can correct it.

[bakanando]

It's a problem that fins are not working as they should, as finless rockets tumble if they are ever so slightly misaligned with prograde, but turn too quickly when fins are included.

As of now, I have noticed the problem with three and four way symmetry. I'll try alshain's suggestion of putting each fin individually (bad workaround), and test other symmetry modes (namely, 6 and 8 way simmetry).

[Temeter]

Maybe the fins are to large? Otherwise: Press F12 for the overlay, then you'll get a visualisation what those Fins do during flight.

Otherwise I don't have any of these issuse. When add fins to my rockets they only get more stable. There is only a tiny amount of bias towards east when uncontrolled, which can be explained with the launchpads alignment.

edit: So, tried your rocket, it's an effect i've already noticed. Looks easy enough to explain: Your fins are basically acting as weak lifting surfaces, especially because of your low speed. And since they are so far behind the COM, they give your rocket a minimal bias to slowly turn. As said, the direction of the turn is caused by the slight misalignment of the launchpad.

Normally a rocket get's enough speed to counteract that lift, but your rocket is just above hovering speed, so the slightest imbalance builds up really fast. Everyone who tried an assymetrical lander or vtol's can tell you stories about that effect. The rocket is also extremly long, which makes it even more instable when it start's to tip.

It's an engineering issue. You need a better T/W ratio and actual control authority.

Edited May 10, 2015 by Temeter

[gogozerg]

I see what you mean, but it does not explain the Fins->West/No-Fins->East issue.

Under FAR 0.15, it does not happen.

It's an engineering issue. You need a better T/W ratio and actual control authority.

Of course. Building an optimal rocket was not the point.

Edited May 10, 2015 by gogozerg

[Ival70]

I tested it with different wings. Canards and other control surfaces did not cause tilt. But if control on all three axis was disabled for the control surfaces, then the rocket did tilt towards the 270. Furthermore, the bigger the wings and/or the further back they were, the quicker it tilted. By using a launch clamp to start the rocket slightly tilted off to the 90, it exhibited the same behavior, but in the opposite direction. This has led me to conclude that the issue is simply that the center of lift is too far below the center of mass (too stable) for the available control forces to correct. Now, you may be asking why does it seem to have a tendency to tilt towards the 270 much of the time? I believe it is caused by the fact that if you point a ship straight upwards and try to fly with no input, the ship will appear to be rotating westward ever so slightly, when actually the planet is rotating eastward, and the ship is maintaining an absolute heading. (This effect is much more pronounced in low orbit). And as the ship tilts slightly westward (relative to Kerbin), its engine will start giving the ship a tiny westward velocity, which the extremely low center of lift will only exacerbate. On top of this, the launchpad is slightly tilted towards the 270. There are probably other factors at play though, as this does not happen 100% of the time...