I'm a little confused by what happens to the design CG (balance point) when I move the fuel tank from its front location back to over the CG. You adjust location of weights with an empty tank to get to the designated balance point. Now, with a forward tank, this location presumably takes into account the effect on the flight CG of some percentage of the fuel in the tank. In other words, the designated location would be aft of the desired flight CG. It seems to me that with the tank "on the CG", you would end up being tail heavy if you balanced at the design point. Or am I missing something?

If you move the tank back, the balance will move back too. If you do nothing else, then add just enough weight forward to give the same balance point as before. Generally we don't consider the fuel weight when balancing our airplanes, they are balanced without fuel, as this would be the 'worst case' as far as the airplane is concerned. 'CG' as in Centre of Gravity' is a common, but incorrect term we modellers use when we mean 'balance point'. CG is a specific physical property, not just a convenient place to support the model for balancing.
Evan, WB #12.

If you start out from the get go with your tank positioned over the CG, then balance the plane accordingly, you will have no CG change as you burn off the fuel.

If you start out with your tank behind the firewall, you should balance with the tank empty, as this will be the condition when you are low on fuel when flying.

It is preferred to maintain a constant CG throughout the flight for obvious reasons. So a CG mounted tank is a good way to go. This was not as easy in the past as most glow engines were draw feed and not pumped. With the popularity of the gas engines, it is much more viable as the carbs are pumped, making tank location less critical.

Guess it wasn't clear what I was asking! The plane is a Venus II with the tank forward, close to the engine, and the balance point is given as 6.5" from the leading edge of the wing. When I move the tank to the "CG," should it still be balanced at 6.5"?

Yes - the fuel tank location is a compromise -- finding a place where it will fit AND the engine will draw fuel as required
ideally -the fuel tank is spot on the CG- this is impossible on many models -so --we compromise.

Yes. Your airplane "wants" a certain CG to fly properly. This is a fixed dimension. Once the CG is set, you want to leave it there.

So if you move your tank back to the CG, you might have to add nose weight to get your 6.5" CG location back.

Thanks for the advice.

Just to throw in my 2 cents. As a P-3 Flight Engineer my job was to compute the weight and balance (CG) of the plane for each and every flight. What I learned was when you move anything you change the CG. When we had changes at the last minute I preferred to put the added weight over the wings so the CG wouldn't change. The same goes for RC. If you move the fuel tank over the balance point you will still balance the plane at the same point. You may have to add weight to the nose because the weight of the tank has moved aft, but it will still balance the same.

As for the comments about balancing the plane with or without fuel. As the plane burns off the fuel the CG of the plane changes. If the fuel tank is forward the the CG will move aft. If the fuel tank is aft of the balance point the CG will move forward. If the fuel tank is directly over the balance point there should be no change in CG. So if your tank is located in the front of the plane balance with the fuel tank empty because this causes the most aft CG (which is the more dangerous limit to be over). If the tank is aft of the balance point, balance the plane with the fuel tank full. A full fuel tank will make the CG the most aft it will be in this condition.

If it wasn't clear from Richard's post then let me stress the idea that fuel draw is a problem for any model engine that does not use some form of fuel pump. So with regular glow model engines moving the tank back more than about an inch from the rear of the firewall is almost certainly going to introduce fuel draw issues and lousy engine runs.

Remember that the CG with an empty tank is the most tail heavy condition, and what that means in terms of the way the plane handles.
Also consider a similar electric powered model CG measured with the battery in place. Despite rumors to the contrary, the battery weight does not change
between fully charged and discharged.
The traditional "cure" for a tank mounted away from the engine was first, a pressurized tank, usually using crankcase or exhaust pressure,
and later, a pump.

On my Venus II with the tank just behind the firewall, I notice the engine leaning out on long uplines once I've burned off half the tank of fuel. I need to set it at least 400 rpm rich on the ground to prevent this. And yes, I use muffler pressure and the line and fitting are open.

Things change because you don't have a regulator.
(I haven't used one in decades, so I don't even know who still makes such things.)

Yes, without a pump or regulator it is inevitable that your engine will lean out some on up lines, and it will be worse as the fuel burns off. Moving the tank back farther from the engine will not help, it will make it even worse because in the vertical up position the fuel has to be drawn up even farther than before.

Jim

Given a situation like this, I'd find the CG with the tank full, 1/2 full. and empty.
There will be a spread. Then, I'd compare that to any +/- tolerance given for the model.
If the 1/2 tank fell within the tolerances, I'd try it!
One afternoon not too long ago, I was flying an Advance 25 (electric power)
All of a sudden, it started acting quite squirrelly.
When It was safely back on the ground in one piece, I quickly discovered that the 4S 3300mah battery had been sliding back and forth
almost 1/2 the length of the battery. While the plane was still flyable, elevator trim was way off, and I think that the battery must have slid
at least partway back forward during the landing approach.

Venus II spec is for the balance to be at 6.5" and says it could be varied 1/2" either way if desired for a permissible range of 6.0" to 7". My balance point (empty) was at 6.45". With a full tank, the plane balances at 5.6" and, after a typical 8 minute flight, it balances at 6.0. So, the question is, should I do the empty balance at 7.0" which would shift the full to 6.1" and the landing to 6.5"? I do notice that I need to re-trim at the end of the flight.

I know, move the tank to the CG!!!!! Before that, though, I'd like to see how the plane handles at the 6.5" CG.

As you're finding out there's no absolute location for the balance point. If you can't live with the trim change that occurs from a full to empty tank then move it. The only issue will be the engine's ability to draw the fuel. If it's got a proper pump on the engine you may be fine. If it doesn't have a pump and only relies on the weak muffler pressure idea then you're stuck with the tank being as close to the engine as practical.