Center of gravity
 Fixed Wing
The center of gravity of a plane will modify its aerodynamic behavior in an extremely sensitive manner. A slight shift will put the elevator and wing surfaces at a different angle of attack and change the way they naturally rest in the air. A nose-heavy plane will not glide efficiently, it will require considerable elevator input and increased throttle, and it will have a tendency towards phugoid instability. Worse than this, a tail-heavy plane will have a tendency to have wandering, sensitive responses to small control inputs, go into frequent tip stalls, and in low-speed situations stall and fall out of the sky backwards, resistant to any control inputs.
A nitro plane has to have the dominant item, the engine, at a particular place on the aircraft. Due to this, achieving proper CG usually requires the use of lead weights placed at the extreme rear of the tail and the forward part of the nose.
In an RC plane, the battery will tend to be the heaviest part of the gear. Its placement is often left up to the user, so this is a requisite part of getting a plane in the air and keeping it there for any user, even on RTF models.
To balance a plane, a center of gravity finder is set up, and the battery is moved until a desired balance can be achieved. If this is not possible with the given cockpit area, builders will try to move anything else, and if this doesn't work, resort to lead weights on the tail and the nose.
 Optimal CG
The optimal CG range may be a few millimeters wide, and will be the same for any exact copy of the airframe, absent aerodynamic obstructions like exterior-bolted equipment. Usually the manufacturer will offer instructions or a manual indicating the location of the CG, but when this is absent, it is advisable to turn to the RC community to find out; If it must be derived experimentally, start with nose heavy and work your way backwards, as this is much less risky. A typical starting point on normal aircraft is 1/4 of the wing chord from the leading edge of the wing, but this doesn't work for tail-less aircraft.
- Flying Wing CG Calculator - RCGroups
- Calculator direct link - 3DZone
- CG Calculator - NASA and Palos R/C Flying Club
 Flight Testing
The acid test of a CG determination and trimming zero-point is whether the plane remains stable in hands-off free flight.
- "This is a nifty little trick us sailplane pilots use to check the CG, works really well. Must be done in no wind conditions, I do it just after sunrise or just before sunset, that's about the only time the wind is not blowing here. Get some altitude, cut the motor, get it gliding straight and level, put it in a 45 degree dive and let go of the sticks. If it balloons up you are nose heavy, if it tucks under you are tail heavy (and lucky its controllable at all If it stays on line you are good to go. Setting the CG like this will result in a plane that is very responsive and will thermal on a gopher fart. Beginners may want to have the plane a little nose heavy for extra stability" - garryarm1, RCGroups
One way this may be done is to take a test flight, adjust trim electronically in flight until the plane is stable, look at the zero points you've chosen for the control points on the ground, re-adjust CG and do another experimental flight, and so on until you've determined a CG where the control surfaces don't have to do much drag-inducing work to keep the plane stable.
A multirotor has to worry about three things:
- Making sure that the center of gravity is close to the horizontal center of pressure in order to maintain a reasonably even distribution of thrust across the motors (because the first one that 'maxes out' will limit the others).
- The vertical location of the center of gravity relative to the center of pressure, governed mainly by how many layers the central hub has, how big the motors & motor mounts are, and whether the battery is above or below the center of pressure. Counterintuitively for most, many people have remarked that their multirotors appear to be more stable with a CG at or above the center of pressure rather than below.
- Pendulum Rocket Fallacy and the Quadcopter - DIYDrones
- The degree to which a balanced CG spreads its mass out to the edges, versus centralizing it at the hub. The increased moments from increasing outer arm weight should make it more difficult to change yaw, pitch, and roll, which may be considered beneficial or harmful, while they reduce the necessary arm rigidity & strength.