Let's Talk CG
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 mess with it?. 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.
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
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
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
After the first flight, the CG position is most often taken for granted,
but whereever it is, that is the starting point. Here's how to do it.
- Basement beam or similar, fitted with a hook or nail.
- String, which is long enough and strong enough to support your plane.
- A very small, about 4ins long and very lightweight spirit level,
readily available from any hardware store.
- Weights, something like pennies or quarters or washers, depending on
how much weight you will need. The weights should all be the same size.
- A plumb bob, another hardware item.
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.
If necessary, wind the string round the nail to stop it from slipping,
then place the spirit level on a flat and horizontal surface, 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 nail or hook and adjust
the bob string so that it its point is a quarter inch from the plane.
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 where the bob is pointing with a fine
Place some of the weights on the stabilizer to make the bob move 1/8 in.
to the rear. Mark the spot. Add that amount of weight again and the bob will
move rearward the same 1/8 in. Now you know how much weight to add to move
the CG rear ward 1/8ins.
Suspending the plane like this will also show you if the plane is out
balance side to side. Probably, one wing is dipping, showing that the plane
is out of balance. 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
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
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 speed. 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
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
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 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.
Posted: Jan. 14, 2004.