Hi, i have an alpha trainer .40 size with a os .46 engine and i checked my cg lately and i was wandering if it needs to be done with a fuel tank full... i checked it with a fuel tank... and it's still sitting a bit behind... what happens if i would be flying my plane with the fuel tank running low because it would be very tail heavy then?

thanks

G'day Marcus,
You need to set your CG with the fuel tank EMPTY, move batteries, or add weight, whatever, to have it on the advised point, usually on the spar with a trainer wing.
When flying, with a full tank, it will be slightly nose heavy, & so be a little less efficient on the elevators & as the fuel burns off, the CG will come back closer to the advised point.
After a few flights, your instructor may advise you to move the CG back to find the sweet spot, but that will determined after flight testing.

always check CG with an empty tank...the reason is, you can fly an airplane slightly nose heavy, you can fly an airplane tail heavy to the crash site.
so with an empty tank and the airplane balanced,as you fuel your slightly nose heavy, as you burn fuel you come more into the balance range. Now don't worry if you had to make a landing right away with a full tank, you'd be ok

The others are absolutely correct. With a plane that has the fuel tank forward of the CG need to be balanced with the tank empty. This needs to be done because if you balance the plane with the tank full the plane will become incresingly tail heavy as fuel is burned off. Please remember this saying "A nose heavy plane flies poorly, a tail heavy plane usually flies just once."

Ken

If the tank is mounted aft of the CG (a very rare occurrence) then you would balance with a full tank. With a tank located EXACTLY at the CG it can be done either way. In all cases follow the instructions in the manual assuming you have used stock locations for equipment.