RC Aircraft Center of Gravity Calculator
Calculate CG position as a percentage of mean aerodynamic chord for RC aircraft.
Returns forward and aft limits for trainers, sport planes, and delta wings.
Why CG matters in RC aircraft:
The center of gravity (CG) is the single most critical setup parameter on any RC airplane. Too far forward and the plane is nose-heavy: it wants constant up-elevator to hold level, it stalls at a higher speed, and it arrives hard. Too far aft and it is tail-heavy: twitchy in pitch, oversensitive to every input, and close to a stall it will not come out of.
The two errors are not equally forgiving, which is the part beginners miss. Nose-heavy costs you performance. Tail-heavy costs you the airframe.
CG position formula:
CG is expressed as a percentage of the Mean Aerodynamic Chord (MAC), measured from the leading edge:
CG position (mm) = MAC leading edge position + (MAC length × CG percentage / 100)
Safe CG ranges by aircraft type:
| Aircraft Type | CG Range (% MAC) | Recommended Start | Notes |
|---|---|---|---|
| Trainer | 25–30% | 28% | Nose-heavy is forgiving |
| Sport aerobatic | 25–33% | 28% | Adjust for 3D vs precision |
| Pattern plane | 28–32% | 30% | Precision requires neutral |
| Warbird | 20–28% | 25% | Often need nose weight |
| Flying wing | 15–25% | 18% | Very sensitive to CG |
| Delta wing | 15–22% | 18% | Swept wings shift CG aft |
| Glider/sailplane | 28–35% | 30% | Adjust for thermal vs slope |
| 3D aerobatic | 30–40% | 33% | Aft CG for extreme maneuvers |
Finding the Mean Aerodynamic Chord (MAC):
For a rectangular wing, MAC = wing chord (root to tip same width).
For a tapered wing:
MAC = Root chord × 2/3 × (1 + Taper ratio + Taper ratio²) / (1 + Taper ratio)
Where Taper ratio = Tip chord / Root chord
Example calculation:
Sport aerobatic plane with:
- Root chord: 300 mm
- Tip chord: 200 mm (taper ratio = 0.667)
- MAC = 300 × 2/3 × (1 + 0.667 + 0.444) / (1 + 0.667)
- MAC = 200 × 2.111 / 1.667 = 253 mm
- CG at 28% MAC = 253 × 0.28 = 70.8 mm from MAC leading edge
How to check CG:
Support the plane on your fingertips at the calculated CG point, one finger under each wing. It should balance level, or with the nose slightly down. If the tail drops, the CG is too far aft and the plane needs nose weight.
On a straight, unswept wing you can measure that point back from the root leading edge. On a swept or strongly tapered wing you cannot, because the MAC sits some way out along the span and its leading edge is further back than the root. Fill in the semi-span and leading-edge sweep and the calculator works that offset out for you.
Adjusting CG:
Move the battery first. It is the heaviest single item you can slide, so it does the most work for the least effort. If the battery is already hard against the firewall and the tail still drops, add lead to the nose. As a rough guide, 10 grams at 100 mm ahead of the CG moves the balance point about 1 mm, scaled by how heavy the whole aircraft is.
The golden rule:
A nose-heavy plane flies poorly. A tail-heavy plane flies once. Always start at the forward end of the CG range and move aft gradually until handling feels neutral.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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