Hello, if you go to my account you can try to fix my C-17. The main problem is that if you fly it for long enough, you will see that it will start to yaw a certain direction until the plane is basically uncontrollable. It is due to the lack of Center of Lift on the plane, but I haven't found a very good solution to this. If you want bonus Brownie Points, you can try to fix why my primary wings are flexing so badly and try to make them completely concrete stable.
@HuskyDynamics01 thanks, this really helps a lot!
I took a look and couldn't figure out how to properly set up the CoL so it would fly stably, but I think at least part of the yaw problem is due to the fact that there is no actual vertical stabilizer "wing". You should be able to hide one inside the existing tail and it'll work fine, and should improve lateral stability significantly.
To fix the wing flexing, select the wing part, open Overload (the button near the bottom right that looks like </>), and change the part type to
Wing-2. You might need to resize control surfaces a little after doing this, and you will need to re-attach the wing (the Auto Reconnect button should work fine for this) but this will stop the wing from flexing.