Why is it that we check CG without gas in the tank rather than with gas in the tank? There is always a logical explanation , but I usually am unable to see it. I suppose that the plane flies with various amounts of gas in the tank, so it would seem reasonable to check CG with either NO gas in the tank , or with the tank FULL. But that would mean that the plane would be unbalanced with any amount in between? Obviously this is not the case! Any explanation would be greatly appreciated.

Basically, when you fly, you burn fuel and the CG will change as you burn off fuel (weight changes). So, you check it under worst conditions, which is empty fuel.

CGr.

For starters tail heavy planes are difficult to impossible to fly.
Landing is one of the hardest things to do.
Taking those two things into account, most landings are done when the tank is low to empty.
Dead stick landings are often done with an empty tank.
We set our airplanes up to be flyable in all conditions and empty tank is one of those conditions. Again a tail heavy plane is not good so we set up our most tail heavy condition, empty tank, to our safest balance point.

What CG said is right, but let me add on to it. First of all there is a saying that needs to be remembered. A nose heavy plane flies poorly, a tail heavy plane flies once! Now with that being said.... If you balance the plane with a full tank of fuel as the fuel is burned off the plane will become progressively more tail heavy, and the performance will worsen (and possibly be unflyable). On the other hand, if you balance the plane with an empty tank then at take off the plane will be a bit nose heavy (but still flyable) on take off, but as the fuel is burned off the plane will become balanced. In the end, if you run out of fuel you now have to dead stick a plane that is easy to handle because it's balanced. If the plane was tail heavy on a dead stick (because you balanced with a full tank) it's a pretty good bet that you won't get the plane back down in one piece.

In the end, a slightly nose heavy plane is a lot easier to deal with than a severly tail heavy one.

Ken

Great. Thanks!

Perhaps beating a dead horse, but here goes . . . Everything previously said is right on target. Also, because of the location of the tank not too far in front of the CG, changes in the amount of fuel do not have a drastic affect on balance. This fulcrum principle is why you have to add much more weight to the front of a plane to balance out a tail heavy plane than you have to add out by the tail to balance a comparably nose-heavy plane.

If you start flying larger planes with gas engines and fuel pumps in the carb, you can put the tank right on the CG because the pump will draw the fuel. With a standard non-pumped glow engine, it won't draw the fuel that far and the engine won't run well.

I know, TMI.

Yeah, I know too.. having to add over a pound of lead weight to the nose of my Tiger 120 ARF with an OS 1.20 AX engine. So, balance.. cg.. is important. I thought I had it all wrong, but flew it nonetheless with all that weight. Well, it flew fine.. just really upset me to have to add all that lead up in the nose.

CGr

With some "pusher" aircraft the engine and fuel tank are located BEHIND the CG. In these cases you want to balance with a FULL tank. As the fuel burns off the balance will turn to nose heavy which is preferred. ONLY balance with a full tank if the tank is located BEHIND the desired CG.