Centre of Gravity 

Setting Correct Centre of Gravity for RC Aeroplane

Correctly balancing your RC aeroplanes is so important for safe flying because any deviation from the model's Centre of Gravity (CG) can and probably will result in the model being quite uncontrollable.

Every RC aeroplane has a correct CG, this is the point where the model balances fore-aft correctly. If you've built from a plan, then the CG should be marked on the plan. If you've bought an ARTF or RTF plane then the manufacturer's instruction manual should tell you where the CG lies for that particular model.A badly balanced RC aeroplane will, at best, be hard to control, and this is especially true for tail-heavy planes. At worst, the plane will crash within seconds of getting airborne!

A way of balancing RC airplanes, High Wing trainers are the easiest aeroplanes to balance, and if it's your first plane then this is likely to be the case. Obviously the first thing you need to do is identify the correct Centre of Gravity, according to the plan or manual. As a general rule of thumb, the CG will be about one-third of the wing chord (width) back from the leading edge of the wing. The main spar often lies in this general area.
Place the tips of your index or middle fingers under each wing, on the line of the CG, a couple of inches out from the fuselage. Gently lift the airplane up so it is clear of any surface, and let it hang freely on your fingers.
A correctly balanced aeroplane will either be level, or have the nose pointing slightly downwards. If the tail points downwards, then the model is tail heavy and you need to do something about it.
If the balance does need to be adjusted to get the correct CG, the first thing to do is try moving the battery pack or any of the RC gear either further forward or further back inside the plane. By doing this, you are adjusting the balance without adding extra 'dead' weight to the model in the form of ballast.
The motor/receiver battery pack is by far the best thing to move, because it is the heaviest item and will have the most effect with the smallest amount of movement. Carefully try and reposition it fore or aft, carefully rechecking the balance of the plane after you've moved it. Once you're happy with the new balance, make sure that the battery pack is secure and won't move from its new position.
If you can't reposition anything, which is always a possibility in ARTF airplanes, you might have to add ballast to either the nose or the tail of the plane to correct the CG. You need to remember, though, that ballast adds dead weight to a model which is never good - the lighter a plane is, the better it performs. So if you do need to add ballast to correct the CG, you need to add as little as possible.
The way to do this is to add the ballast as far forward or as far back as you possibly can on the model. By doing this, the ballast will have the most effect on the CG. Add only enough to make your plane balance correctly on your fingertips.
Suitable ballast to add to an RC aeroplane is small self adhesive lead weights, for example. Whatever ballast you do add, make sure it is secured to the plane, and won't drop off in flight!