How the aircraft unstable?
1. Make sure COM is in front of the CoL. The closer Col COM then the plane will be more agile. But if too close to the air going back flip and can not be controlled.
2. The aircraft tends to fall if the possibilities are:
  a. load in front is too heavy, and COM CoL distance is too long. The solution is to reduce the load on the front.
  b. wing position is under the machine. the solution is positioned parallel between the wing and machine.
  c. less engine thrust. should replace the engine more power.

  d. In addition to these three things we can change the type of a model wing into semi-symetric, symetric, or flat bottom. Symetric wing aircraft makes a very agile but do not have the ability to lift the wing while the semi-symetric have the ability to lift and maneuver stable, while the flat bottom wing has a high lift but makes the aircraft is not too agile.

1. The shape of the wing also greatly affect the position CoL. The swept wing forward caused CoL will moving forward too.
2. width of elevators also determines the position of CoL.

Let's try.