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In the designer page, select the little blue icon that looks like a triangle balancing a board with three balls on it. You’ll then notice some lines appearing on your build. Make sure the red line, the Center of Mass (CoM) line, is more forward on your build then the Center of Lift (CoL) line. There are several ways to get these lines to move, the most straightforward way to move CoM forward is to add weight (either fuel or dead weight) to the front of your plane. Make sure the CoM is forward enough to prevent your airplane from tumbling, either immediately on takeoff or when flying and you pull back hard on the stick. You can also adjust the CoL position by changing the size of the wings or the position of the wings on your build. Good luck!

accidentally posted this on announcements so i had to repost it