Lateral body lift on Mk3 spaceplane

[A_name]

So I built a Mk3 SSTO spaceplane. My first one using Mk3 parts, mind you. Getting into orbit went normally. As usual when flying a spaceplane, I only used pitch control all the way up, never had to turn (pitch nor yaw).

When reentering I overshot the KSC and that's when the problems started. As I tried to turn around, the plane promptly entered a flat spin. I’m pretty sure my COM was in front of my COL because RCS Build Aid said so in the VAB, plus I put a small fuel tank right behind the cockpit and pumped all my remaining fuel into it for this very reason (why can’t we have a COM and COL overlay in-flight?).

Anyway, I turned on the aero forces overlay to investigate and discovered that every time I bank, the light blue arrows indicating body lift shift from pointing upwards (relative to the plane) to pointing sideways, parallel to the wings, in the opposite direction of my bank. I was able to somewhat remedy this by giving it a little rudder in the same direction of the bank, which would cause the light blue arrows to shift back to pointing upwards (still relative to the plane). Needless to say, keeping the arrows pointing upwards was real finicky, requiring very precise rudder input which neither I or the SAS module could manage.

I know a proper turn in a real aircraft requires a bit of rudder in the direction you’re banking but I’ve never had to do that in KSP before (normally just bank and pitch up to turn). In fact I very rarely even touch the rudder when flying a plane in KSP. And having the body lift arrows shifting around the longitudinal axis of the plane because of rudder input makes no sense to me. Note that I was doing a reasonable bank considering the size of this plane, not banking 90 degrees or anywhere near that, so it’s not a matter of the body lift keeping pointed up relative to the surface of Kerbin while I banked, they were pointing sideways relative to the plane, parallel to the wings as I mentioned before.

Has anyone seen this before? Is this how the Mk3 fuselage parts are supposed to work?

Thanks!

EDIT: Here are some screenshots http://imgur.com/a/sasHF

Edited October 21, 2015 by A_name

[tseitsei]

We need pics to be able to help. If we cant see your craft it's mostly guesswork

[Redhaze]

From the sound of it, it seems you need to increase the size of your tailplane(s). Don't worry, this is a a common thing in the real world as well. Especially aircraft with three tailplanes have three because they noticed this during flighttesting or initial simulations/calculations.

[Van Disaster]

Insufficient stabilizing surface is a common problem with KSP builders too - check some real airliners sometime & see how big their tails are, they are generally *huge*.

[A_name]

OK, I was able to grab some screenshots:

http://imgur.com/a/sasHF

I'll also edit the album into the original post.

I have my reserves that a small tailplane is the problem because (1) why would that induce lateral lift?, and (2) I put on the huge tailplane parts.

[Jimbimbibble]

There isn't a bug here. What's happening is you have a lot of sideslip in the wrong direction. The plane is flying crooked with the side of the plane exposed to the airflow and that's causing the sideways body lift. Look at your navball. You can see it yawing away from the turn a lot. You definitely don't have enough of a rudder. Try a larger rudder (like a large delta wing with control surfaces) and put it as far back as you can. That should prevent the sideslip.

[Van Disaster]

Just use the same part as the horizontal stabilizers. Yaw induces sideslip, and there's your lateral body lift.

[GoSlash27]

A_name,

This is why I recommend *not* having active control surfaces as rudders. You should have a vertical empennage to weathervane the fuselage into the wind, but active rudders only confuse the SAS. When you bank, the active rudder wants to resist changing heading when it's supposed to allow the turn. It dials in more and more cross- control trying to hold heading until you're side slipping and something like this happens.

You don't need active rudders in KSP. Just stick a passive wing in that position or if you want to use a control surface, disable it.

Give that a shot and I suspect it'll fly a lot better.

Best,

-Slashy

[tseitsei]

Yeah. Needs bigger rudder. It doesnt even need that much (if any like slash said) control authority but yiu need significantly bigger passive tailfin.

I usually still have some small control surface on my rudder but as slash really knows what he is talking about so....

and yeah delta wing is a good tailfin for bigger planes.

[A_name]

Thanks everyone for your help! Work hasn't allowed me the time to test with a bigger rudder but I'll report back with my results when I have a chance.