Balancing RC airplanes
Correctly balancing rc airplanes is so important for safe flying, because any deviation from the model's Centre of Gravity (CG) can potentially result in the model being quite uncontrollable.
Every rc airplane (and all other aircraft) has a specific CG position, it's the mean point where all gravitational forces act upon the plane and hence the point where the model balances fore-aft correctly. You can liken a plane's Centre of Gravity to the fulcrum of a see-saw, for example. The CG point is determined during the design stage of the airplane or aircraft and is typically shown on a plan as a disc split in to four quadrants, as shown right.
If you've built from a kit & plan the CG should be clearly marked on the plan but if you've bought an ARF or RTF plane then the instruction manual will likely give the CG position in terms of distance back from either the leading edge of the wing or from the nose.
Incidentally some model aircraft manufacturers specify a range that the Centre of Gravity can fall in to, rather than a single point. If you're unlucky the manual might not even mention anything about balancing rc airplanes and the CG!
Balancing an rc airplane correctly about its Centre of Gravity is so important because a very badly balanced rc airplane will, at best, be hard to control, this is especially true for tail-heavy planes. At worst, the plane will crash within seconds of getting airborne.
Methods of balancing RC airplanes
High wing trainers are the easiest airplanes 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 position according to the plan or manual. As a very 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, if there is one, often lies in this general area.
Again, this position is only a generalisation and in reality a CG point can be found anywhere from, say, 25% to 50% of the wing chord back from the leading edge. A CG point outside of that range is rare, but not impossible. An easy method of balancing rc airplanes is this: place the tips of your index or middle fingers under each wing, exactly on the line of the CG (i.e. the specified distance back from the leading edge of the wing or nose of the plane) and a couple of inches out from the fuselage sides. Gently lift the airplane up so it is clear of any surface and let it hang freely on your fingers.
By the way, your airplane must be 'flight ready' when you balance it i.e. battery pack in place or fuel tank empty. With the latter scenario, an IC plane is best balanced with an empty fuel tank otherwise the plane will become tail heavy as the tank empties during the flight.
A correctly balancing rc airplane, sitting on your fingertips, 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 that.
If the balance does need to be adjusted to get the correct Centre of Gravity, the first thing to do is try moving the battery pack and/or receiver further forward or backward inside the plane. By doing this you are adjusting the balance without adding extra 'dead' weight to the model in the form of ballast.
Obviously you need to move the component in accordance with how the plane hangs: if it's tail heavy then move the component forward and vice versa.
The motor/receiver battery pack is by far the best thing to move because it is the heaviest item and will have the greatest 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 completely secure and won't move from its new position.
If you can't reposition anything, which is sometimes the case in RTF 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 radio control 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 Centre of Gravity. Add only enough to make your plane balance correctly, nothing more.
Suitable ballast to add to an rc airplane is modelling clay or fishing shots, for example. Thin sheet lead, if you can get it, is perfect because you can cut it and bend it to shape and the pieces don't have to be big to have an effect on the balance.
Whatever ballast you do add, make sure it is secure to the plane and won't drop off in flight!
Roll balancing RC airplanes
This is an often overlooked balance and isn't as critical as fore-aft balance, but an rc airplane that has one side heavier than the other will have a tendency to naturally roll and turn to the heavier side, making your life on the sticks a bit harder. However, unless something is seriously wrong then the plane won't spiral out of control just because one wing is slightly heavier than the other, unlike a tail heavy plane that can go out of control very quickly.
For balancing your rc airplane side-to-side simply loop some thread around the propeller shaft, between the spinner and the fuselage, and then some around the rear of the fuselage as close to the tail as you can get. If possible insert a fine gauge pin or screw in to the very rear of the fuselage, this gives a more accurate result but often isn't possible because of the airplane's rudder.
Lift the plane up by both pieces of thread and let it hang freely - see if it wants to roll to one side or another. If it does then you need to add some small ballast to the wingtip of the lighter (higher) side. Add only enough to make the plane hang so that both wings are level.
Taping the weights to the wingtip is an easy method, although you might want to take the trouble to set them into the wingtip and cover over them to hide them. Bear in mind that adding larger bits of tape will actually add weight in addition to the ballast.
For rc airplanes with foam wings, pushing a small gauge nail or panel pin into the foam is a great way of adding any necessary weight and the ballast can be easily hidden with suitable paint or marker pen.
So there you have it... two easy methods of balancing rc airplanes. As already mentioned, the fore-aft balance is very critical if you want to keep your plane in one piece; the roll balance isn't so critical but is worth doing if you have the time.
A correctly balanced airplane will always be safer and easier to fly, and won't need as much trimming at the transmitter.