by: Rupert Kosmala

An article aimed at the sport RC flier to encourage him/her to discover that a plane can be made to fly better by moving the Center of Gravity.
In the twenty or more years since I took up aeromodeling again, I have noticed that many sport R/C fliers view the Center of Gravity (CG) as something to be highly respected, a necessary evil, and definitely not something to be messed with. I have many times made the suggestion to my fellow modelers that moving the CG can greatly improve the way a model flies. Frequently, the response is along the lines, ‘the CG is perfect, it’s exactly where the plans say it should be’, or, ‘it’s at 30% and I’m not moving it’, or, the plane is flying OK, so why fix what isn’t broken?. It is often accepted as a cast-iron fact that moving the CG will make the plane uncontrollable. For those of you who are willing to experiment a little, here is an explanation of how you can tune the handling of your plane by adjusting the position of the CG.

Illustration of Stability

CG in front of NP, ball always rolls to the center.
Left to itself, the airplane will adopt a stable nose-down attitude.

CG at the NP, neutrally stable, ball doesn’t roll.
The airplane won’t change its attitude without a command from the pilot.

CG behind NP, unstable, ball will roll away.
The airplane will tumble like a leaf out of control.

The Center of Gravity is the point where the total weight of the plane acts; it lies somewhere inside the plane, somewhere in the cockpit of the typical high or low wing sport model. Weight is a force which is always pulling the plane straight down, regardless of the attitude of the plane. The Neutral Point (NP) on the other hand is the point where all the lift acts. The wing generates most of the lift of course, but the fuselage also contributes.

When a plane is stable in pitch, it will always go to a nose down attitude when the pilot stops controlling it, unless of course, the engine is producing enough power to overcome the stability. In order for a plane to be stable in pitch the CG has to lie somewhere in front of the NP. The further forward, the more stable, the more rearward the less stable. Some planes are so stable (nose heavy) they are difficult to fly. At the other end, if the CG is too far back and gets behind the NP, the plane is unstable and impossible to fly.

As the CG moves closer to the NP, the plane approaches neutral stability, and becomes neutrally stable when the CG is at the NP. When the pilot takes his hands off the sticks of a neutrally stable plane it will just continue flying along the same path, because there is no stabilizing force to make it change, until eventually, gravity wins. In this condition it is not unstable, but it has to be “flown” all the time. There is a trade-off between stability and maneuverability which is why pattern planes are neutrally stable.

The designers of kits and plans will always show a CG position where the plane will definitely be stable, and it is often assumed that it must not be moved from this spot. Modelers commonly do the ‘balancing on the finger tip’ test by placing the finger tips at some mystery point behind the leading edge and seeing if the plane “balances”. Some modelers even insist that the nose of the plane should dip a little “just to make sure”. Once a plane is known to be a flier, this test is as good as meaningless. The fact that the plane flies is all the modeler really needs to know about the position of the CG. Being certain about the actual position of the CG is only important before a first flight. It is not necessary to know the exact position of the CG before starting the process of adjusting it to get the best flying characteristics.

After the first flight, the CG position is most often taken for granted, but wherever it is, that is the starting point. Here’s how to do it.

Materials needed:-
a} Basement beam or some suspension point, preferably about 6ft high.
b} String, which is long enough and strong enough to support your plane.
c} A very small, about 4ins long, and very lightweight spirit level, readily available from any hardware store.
d} Weights, something like small coins or washers, depending on how much weight you will need. The weights should all be the same.
e} A plumb bob, another hardware item.

Step 1.
Tie a loop at each end of the string and fit one loop over the spinner/propeller, and the other over the fin/rudder and round the tail wheel if you have one, or you may need to insert a pin to stop the loop sliding off. The string should be long enough to suspend the plane about six inches off the ground.

Step 2.
Because the CG is closer to the nose the front part of the suspending string will be subject to greater tension than the rear part and the string may tend to slip around the suspension point, so loop the string around the suspension point to stop it from slipping. Now place the spirit level on a flat and horizontal surface on the plane, the stabilizer is usually a good place, adjust the string so that the plane hangs level. (see diagram) Then hang the plumb bob from the same suspension point and adjust the bob string so that it its point is a quarter inch from the plane.

Step 3.
Let the plane and the bob settle down and stop swinging. Approach the plane slowly so that you don’t create air movement to start it swinging again. Very carefully, mark the spot the bob is pointing at with a fine felt pen.

Step 4.
Place some of the weights on the stabilizer to make the bob (appear to) move 1/8 in. to the rear. Mark the spot. Actually the plane is swinging under the suspension point because the bob always points at the CG. Add that amount of weight again and the bob will move rearward again the same 1/8 in.

Now you know how much weight to add to move the CG rearward 1/8ins.

Suspending the plane like this will also show you if the plane is out of balance side to side. Probably, one wing is dipping, showing that the plane is out of balance laterally.

Putting the fin/rudder in the loop of the suspending string helps to keep the plane straight during these checks.

Now find some weights to add to the high, or light wing tip to bring it level. You can use screws in the wing tip, lead stick-on weights (but they tend to come unstuck) or insert some 1/8 dia solder pieces into the tip and apply some thin CA.

When a plane is out of balance laterally the heavy wing will always tend to drop. Adding some aileron trim to straighten it out will work only for one throttle setting. Slow down and the heavy wing will drop again, speed up and the other wing will drop. Because the correcting force from the aileron varies with speed, the plane will never fly straight at all speeds.

Weight always acts downwards and never changes with speed. It is quite rare for a plane to be perfectly balanced laterally, there are always differences in material and construction and mufflers, for example, which help to unbalance a plane.

Similarly, when the CG is in a very forward position it is always pulling the nose down. Trimming the elevator to apply a downward force on the other side of the CG, will only be effective for one throttle setting.

A plane so trimmed to fly straight and level at half throttle will climb when the throttle is opened and will dive when it is closed.

A forward CG makes a plane difficult to land because the balancing downward force of the elevator decreases as it slows down, requiring more up elevator. Sometimes the elevator runs out of travel before the plane has slowed down enough, and that can make for heavy “carrier” landings.

Moving the CG rearwards in small steps will reduce the pitch stability in small steps and this will let you creep up on the best CG position.

As you add the weights to the tail and move the CG a little bit at a time, the plane will not suddenly become difficult to fly, but it will gradually feel lighter and more responsive.

A nose heavy dog of a plane will gradually become more and more fun to fly, and easier to land. (If your plane is already very sensitive to elevator movement, you may want to do the opposite by adding some weights to the nose and then gradually removing them.)

When you find the plane becoming a bit too lively for comfort, take off that last weight from the tail and you will have found the best position for the CG for your style of flying.

It is really quite unimportant where the CG actually is in terms of %age of the wing chord, but it may be interesting to find out how the right CG position for you, compares to the one given on the plan.

If you end up with some unsightly weights on the stabilizer, try moving the battery or servos to regain the new CG position and do away with the weights. That’s an easy job, now that you know how to locate the CG accurately.

The correct CG position is determined by how the plane flies and not by a mark on a piece of paper, that’s only the starting point. Happy flying!