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Every radio control model airplane has a correct Center of Gravity, this is the point where the radio control model airplane balances front-rear correctly. If you've built from a plan, then the Center of Gravity should be marked on the plan. If you've bought an "Almost ready to Fly" (ARF) or Ready to Fly (RTF) radio control model airplane, then the manufacturer's instruction manual should tell you where the Center of Gravity lies for that particular model.
A badly balanced radio control model airplane will, at best, be hard to control, and this is especially true for tail-heavy RC airplanes, at worst, the plane will crash within seconds of getting airborne!
A way of balancing RC airplanes.
A high wing trainer radio control model airplane is the easiest airplane 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 Center of Gravity, according to the plan or user manual. As a general rule of thumb, the Center of Gravity will be about one-third of the way back from the leading (Front) edge of the wing. The main wing spar also often lies in this general area.
Place the tips of your index or middle fingers under each wing, on the line of the Center of Gravity, a couple of inches out from the fuselage. Gently lift the radio control model airplane up so it is clear of any surface, and let it hang freely on your fingers.
A correctly balanced radio control model airplane will either be level, or have the nose pointing only slightly downwards. If the tail points downwards, then the radio control model airplane is tail heavy and you need to do something about it.
If the balance of your radio control model airplane does need to be adjusted to get the correct Center of Gravity, the first thing to do is try moving the battery pack or any of the radio control equpment either further forward or further back inside the plane, as 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 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 forwards or backwards, 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 ready to fly model airplanes, you might have to add ballast to either the nose or the tail of the radio control model airplane to correct the Center of Gravity, 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 Center of Gravity, 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 Center of Gravity. Add only enough to make your plane balance correctly on your fingertips. Suitable ballast to add to an rc airplane is modeling clay or fishing shots, for example. Whatever ballast you do add, make sure it is secure to the plane, and won't drop off in flight!
Once you successfully have yur radio control model airplane balancing level and straight on your fingertips from its Centrew of Gravity point, your done, but do keep in mind that if you change anything on your model at all, such as a different battery or move a servo, even a bigger/smaller propellor, you will need to check and possibly rebalance your radio control model airplane.
