I am supposed to check the CG with an empty tank of fuel right? The instructions on my Sky Raider mach 1 say 3.15 inches back from the leading edgeof the wing, which looks to be about 1/4 of an inch in front of the main spar. That is assuming i'm right about where the main spar is. Thanks again!

Yep, tank empty

Yes, the fuel tank needs to be empty when balancing the plane.

Ken

The CG is usually right around the main spar, and there's usually 3/8" or so of variance (3/8" forward and back of that point).

The fuel tank should be empty for CG setup IF the fuel tank is forward of the CG.
On some airplanes the fuel tank is actually aft of the CG (like my mid-engine, tractor prop, biplane canard) which changes the rules.
Basically, you want to set the CG with the airplane in its most tail-heavy (or nose light) condition. On most airplanes, the fuel tank is forward of the CG so you'd want it empty to create the most "nose-light" condition in which you would ever fly the model.
Hope I didn't confuse the issue for anyone, but for full-size aircraft, the location and amount of luggage can establish the limit for the CG.

You don't have to guess or assume.

Get a ruler and measure. Then slap a strip of masking tape where the 3.15" would be right in the middle of the tape. Put a strip on each wing where you figure to hold with your "balancing fingers".

See the funny blue color on the wing in the picture. It's a strip of tape used to make balancing that Corsair a doable proposition. Trying to hold up, balance, and measure all at the same time is a waste of time if you want to be halfway accurate. I wanted the balance of that Corsair to be at 5.25". I put the tape so 5.25 would be right in the centerline of that strip. My fingers could easily tell where they were front-and-back on that tape. Yeah, I balanced the airplane upside down. And with an empty tank. It flies like gangbusters. Fast, slow, and once I adjusted the elevator throw to match the CG, with excellent pitch control and accuracy. Excellent airplane.

A small drop of epoxy at the balance point works well you can feel it with your fingers and also when you adjust it 1/4" back or forth you can still feel it. It is almost invisible.

I think the first time we balanced it, I had about a half tank of gas. It flew great, but I want to take the lead off the tail and get it right by moving the electronics if I can.

Or hot glue!

[link=http://home.mindspring.com/~the-plumber/Vanessa%20CG%20Machine.htm]The Vanessa CG Machine[/link]

I've balanced nearly anything you can think of on that thing. .40-sized Super Stick, an F6f .60 size, a Giant Aeromaster, a 33% Pitts S2S...you name it.

It's VERY simple to use. Just hang the airplane level in the loops, and the plumb bob will point directly at the current CG...simple as that. Leaves you both hands free to measure, or add/remove weight to either end, to see what the effect is.

By using that, do you adjust the weight till it points at the manufacturer's cg spot?

I'd try to get it right on then adjust it from there if needed. The Mach-1 is pretty forgiving though unless the CG is just way too far back.

The CG shown on the plans or given in instructions is only a starting point. You then fly it and make changes to the empty balance point to fine tune the model's response to a given control input.

CG is part of the puzzle. The mfg gives you a CG location he thinks will give a plane most people will enjoy flying AND he gives elevator throws that will match that CG location.

Yes.

Basically, the loops of rope that the airplane rests in are wrapped around a simple dowel. You rotate that dowel to make the airplane sit level...where the plumb bob points is current CG.

If, say, it's a bit nose heavy, add some weight to the back or move some things rearward, then rotate the dowel again until the airplane is level....plumb bob points at new "adjusted" CG.

Here's a shot of my big Pitts on the machine...might help clear things up a bit.

CG is checked with an empty tank, all radio equipment installed/rigged, wheels on, engine installed and covering finished. Pretty much as it will stand at the field before you fuel up. You might wonder what happens with a full tank. The model is a "nose heavy," which is way better than being tail heavy. What is most important to realize is that checking the CG with an empty tank makes sure close to empty or deadstick landing glides are smooth!

I got it almost dead on to the manufacturer's specs and used the string cg machine to do it, it worked great!