Its not as difficult as you may think to balance your plane. Thats good because R/C or free flight, you MUST balance. Its one of those things sound weird until you do it once.
With free flight, you must balance for level flight. The method I use is to glide the airplane, balanced where I think it might be best, and see how it flies. If it "noses in" your plane is nose heavy. If it "noses up" your plane is tail heavy. On some planes, you move the wing to adjust the location of CG. On most planes you add weight. [ Ballast] If you have a model of a jumbo jet, where the wing is pretty much in the middle of the plane, you probably won't need much ballast to balance it. A ww1 bi-plane fighter, with its short nose and comparatively long body will require a lot more weight in the nose to balance in the correct spot.
Here is a fairly/ sorta balanced glider. Remember, this is not a scale model so there is not as much drag involved. There is always some drag involved, but not much here. A model bi-plane with a million feet of wing and wire and landing gear ect, will have more drag. It will not glide as far, but still be a stable glide to earth.
http://www.youtube.com/watch?v=NeW9zJrg9U8 This plane is a bit more nose heavy than it should be. The trim is set to compensate. Trim is speed sensitive. You can see the effect of speed on the trim by watching the glide. Watch it again if you need to. When I first launch it it tends to go up because it still has the force from the throw. That extra speed causes extra trim to raise the the plane. Nose up, it bleeds off speed. As it slows down, the trim becomes less effective, she drops the heavy nose, and picks up speed as it heads towards earth. When it gets back up to a speed it likes, it levels out and glides into the grass.
If there had been more altitude, the plane would repeat the process over and over till it touches earth. Nose up, slow down, drop and glide, nose up, slow down, ect. Not the best glide, but it is close. If I readjust the CG, and re adjust the "trim", it will glide smoother and farther. That means bringing the CG and CL closer together. The result will be a long, stable, steady rate of decent to the ground.
On a side note, before I got the bugs out of it, I accidentally stepped on it. Some times that's how it goes, when reaching for the stars. All you can do is grin, and go build another one.
If I add a prop and rubber motor, the same thing happens but the speed is now affected more by the thrust of the prop than the strength of my toss. That is another subject that we will get to later. The goal for now is to obtain the best glide you can. Work with it and try balancing it in slightly different places. With practice, you will learn to see what it wants and putting the CG where it should be, will be a snap.
If its out of balance, it wont glide well.
If it wont glide, it wont fly.
Of course, there is more to flying than just that, but this will get you started. If you skip this step, the rest of it wont matter. You cant build a square house on a crooked foundation.