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 ! :)