Plane balancing

[N3xusPr1me]

Hello guys,

I would like to ask you a question. In stock KSP, some of the stock planes have their fuel being balanced everytime, and so their center of mass stays exactly where their center of lift was previously before using fuel, so it keeps balancing correctly. I would like to know : how do they do that ? I tried some balancing mods but they just balance using % of the fuel, and that isn't weight-based balancing. I like very much planes, and this is something that I need to know. I recently made a plane and when it got nearly empty, it was unstable as hell, so I needed to balance manually and not accurately using alt+rightclick.

So is there any tip to keep the fuel balanced like the stock planes do ? Or a way to build planes so they have the same balance when they are all full and empty ?

Thx for your help !

Edited December 31, 2014 by N3xusPr1me
Missing cap

[Anquietas314]

The trick is to build your plane so that the center of lift is just ahead of the center of mass. That way, as fuel drains it moves slightly towards the back of the plane (usually), but not so far back that it makes the plane unstable. There's not really a way to build the plane so it stays perfectly balanced, at least if you use more than one fuel tank.

You can get an idea of how CoM will shift by manually draining the fuel tanks in the SPH . Just don't forget to fill them up again before launching

EDIT: derp. I meant put CoM ahead of CoL

Edited December 31, 2014 by armagheddonsgw

[RexKramer]

Putting the COL ahead of the COM is usually a recipe for disaster, I'd recommend not doing that.

Regarding the original question of keeping your COM from moving greatly during fuel burn, there are several approaches that can help.

Using fuel lines to keep fuel from simply draining from front to back can work. Sometimes fuel lines work in unexpected ways however.

It also helps to keep all your fuel tanks roughly laterally aligned, not stacked from front to back. If the tanks start out with the COM near their center, then as fuel burns the COM will not move much.

If you need more fuel, choose a single larger tank instead of stacking more smaller tanks on. The game considers each fuel tanks COM to be constant regardless of how much fuel is in the tank- only the weight of the tank changes.

Some trial and error in the SPH is required, moving engines, tanks, and components around. Again, if you start with the planes COM in the middle of your fuel tank, then burning fuel from that tank will not affect your COM much. Fuel tanks which are forward or aft of the COM will cause the COM to move significantly as fuel (mass) is removed from them.

[N3xusPr1me]

Hello again,

Thank you for your help. I read carefully what you both said, and I have now some useful tips in SPH ( thx for that ). I would suggest also that my both side tanks (front one and back one) are locked and I change their fuel "fullness" to move the center of mass up to the center of the main tank. This way, the weight would change but not the center of mass, and THEN place my wings. I think I'm doing it the wrong order, because I'm placing the wings without carrying about the wright after burn. Let me try this with the aid of your useful tips and I will keep you informed. Thank you again I'll maybe submit my plane after if this works well

[GoSlash27]

Your fuel tanks need to be in the middle of the plane.

That way as the fuel drains, your CoM won't shift.

If you have multiple tanks, stack them laterally, but not longitudinally.

If you must stack tanks longitudinally, place the center of the junction of tanks in the center of the plane. The CoM will shift as the fuel drains, but will wind up back where it started when the fuel's gone.

For 3 longitudinal tanks, (A-B-C from front to back) you can use fuel line to keep them balanced. A->B, C->B, and B-> engine.

Use weight and balance to get the CoM right in the middle of the tanks. A large mass such as an engine can be counter-balanced by a lighter mass on the other side by moving the lighter mass farther away from center.

Center of lift is right where your wings attach. Place them either exactly in the middle of CoM *at it's furthest aft point of travel* or just behind.

If you follow these simple rules, you'll have a perfectly balanced plane every time.

Best,

-Slashy

[N3xusPr1me]

Ok guys,

I just did what I said : balancing the CoM up to the center of the main tank using both front and back locked tanks, THEN placing wings (so moving again the CoM a bit due to their weight) and THEN rebalancing a littlebit with the locked tanks again. This way I came up with a design which always keeps its CoM at the same place, no matter how my fuel is drained up, 'cause the CoM is at the exact center (visually) of the main tank.

Thank you guys for your help, I know now how to build properly a balanced plane !

But one question remains : how do the stock planes balance themselves (some of them actually), I looked for fuel lines, but nothing visible so far...

Thank you soooo much, you can use my "way-of-doing-it" and tell me if it works ! :)

[Janos1986]

Also, if you run out of fuselage and your CoL is slightly in front ot too far back, you can fiddle with the rotate gizmo on wings to balance it.

I usually place the wings on the bottom of the fuselage with the tips slighly pointing up, and then I increase the AoA.

If I have rear elevators in a design, I tilt them in the opposite direction of the wings to balance the CoL's height, which is important too.

[Anquietas314]

So I accidentally made a plane where you want CoL in front of CoM, at least at launch:

That little beauty flies better than anything else I've built, ever (stock aero).

Not sure about the RAPIERs though; I might swap them for turbos and use some radial engines.

I should probably add that it's designed mainly for launching a bunch of satellites in one launch

Also, the way fuel drains, CoM moves forward for the middle tank, and then back to roughly its current position as the rear tank drains.

EDIT: The same plane in orbit, after adding a few intakes (turns out it needed 10 structurals per engine):

Edited December 31, 2014 by armagheddonsgw

[WanderingKid]

But one question remains : how do the stock planes balance themselves (some of them actually), I looked for fuel lines, but nothing visible so far...

Fuel Flow Rules. If you build your fuselage Engine->Fuel A->Fuel B->Fuel C->Cockpit, and hang the wings (and engines on them) off Fuel B, the engines on the wings will pull from the furthest tanks first that have fuel flowthrough. So, Fuel A and Fuel C get emptied first (and simultaneously, which is the real benefit), before Fuel B gets touched. However, if you've got engines hanging off the back of the plane, you'd pull all the Fuel from C, then B, and finally A. This throws the CoM all over the place. At that point, stick two gas cans equilateral across the CoM, and let it drain off both tanks at once, and use something else as your fuselage.

[N3xusPr1me]

Ok, posted my plane : http://forum.kerbalspaceprogram.com/threads/105589-Windbreaker-Mk3-1

hope you'll like it !

Thx again guys