do you mean with different weight the CG mentioned by Josh 2" from leading edge cannot be used? sorry for this noob question. this is my first rc plane and first time building this

My plane flies with CG approx 5-7cm (1-1.5") from the leading edge, and it feels perfectly stable.

thanks for the input..yes i guess if tail heavy i can replace to heavier tyre. btw my tyre is already 3/4oz hehe diameter 90mm

Changing the wheels will not make almost any difference. What Tirick had in mind is to go to regular car workshop/ or car parts shop) and ask for the rim weights - the ones used in regular cars to balance the rims/wheels. Those weights are small, but quite heavy; and usually have double-sided adhesive tape already sticked to the back. You can use one or two of them (glued inside the nose of a plane, as far forward as you can) to move the CG into proper place.

I was expecting faster and more power with the gee bee motor I have but I never thought the behaviour of the plane "draggy" would eventually effect the way it fly. yeap I still need how to find the CoG.. should I use 2" behind leading edge in the video?

what is 1:2 t/w, i think t/w standfor thrust and weight? how you calculate or know you are flying 1:2t/w?

thanks so much guys for your help.. I definitely need to build new airframe

some part are broken.

By "draggy" we meant the plane creates a lot of air drag, and thus don't want to fly fast. But in your case the drag is not a problem. It's the CG position. When the CG is too far back, the plane cannot be controlled and increases AoA (Angle of Attack) by itself. Exactly as your plane did on the video.

1:2 t/w stands for "1 to 2 thrust to weight ratio". That means the plane plane creates one "unit" of thrust while it weights two "units" of mass. In case of my Storch, it's exactly 1:2 t/w: my plane weights 800g, but is capable of producing 400g of thrust only. Your plane weights 1.2kg, and the motor/prop combo you have should be capable of producing 1.2kg of thrust - that's 1:1 t/w.

1:2 t/w can usually be found in trainer/scale looking planes; and some gliders (especially the bigger ones). 1:1 t/w can usually be found in classic aerobatic planes. Anything more than that (for example 1.5:1 or 2:1) means you can generate more thrust than the plane weights - and that is a must for overpowered, 3D capable, 200Mph+ flying motors with tiny wings.